This strategy provides an optimal performance with no obvious degradation being observed after 100 h of high temperature operation and 10 redox cycles, suggesting a reliable electrochemical process for conversion of methane into valuable chemicals.Conversion of methane to ethylene with high yield remains a fundamental challenge due to the low ethylene selectivity, severe carbon deposition and instability of catalysts. Here we demonstrate a conceptually different process of in situ electrochemical oxidation of methane to ethylene in a solid oxide electrolyzer under ambient pressure at 850 °C. The porous electrode scaffold with an in situ-grown metal/oxide interface enhances coking resistance and catalyst stability at high temperatures. The highest C Conversion of methane into ethylene is important for chemical feedstocks, but is challenged by catalysts with low selectivity and performance degradation. Here the authors report electrochemical oxidation of methane in a solid oxide electrolyzer, achieving ethylene production with high selectivity and yield. Light olefins, mainly C2H4, as key chemical feedstocks and building blocks, are currently produced through multistage processes from syngas to methanol and then to olefins, although there is some ongoing research of direct conversion of syngas to olefins. This syngas-to-olefin route is dominant in current and near-term industry production5; however, the carbon-atom efficiency is normally below 50% in addition to the significant amount of CO2 emission. This mainly arises since the removal of oxygen in carbon monoxide consumes hydrogen and generates water.Conversion of natural gas, mainly CH4 to C2H4 is potentially more economical and environmental friendly, but it is a more challenging route. CH4 is a small and stable molecule that exists in nature, and it has strong C–H bonds, negligible electron affinity, large ionization energy, and low polarizability. This direct-conversion route can be realized through either oxidative coupling or direct nonoxidative conversion in a heterogeneous catalysis process1. It is effective to activate the C–H bond of CH4 with oxygen to generate C2H4; however, over-oxidation of CH4 remains unavoidable, leading to lower carbon atom efficiency. Furthermore, the low CH4 conversion with low selectivity of C2H4 is another huge challenge7. Nonoxidative conversion is a process of dehydrogenation of highly stable CH4 together with selective C–C coupling, and a recent work has represented the highest methane conversion (48.1%) and ethylene selectivity (48.4%) at 1090 °C 8. In terms of reaction mechanism, the cleavage of a C–H bond with C–C coupling is indeed an oxidation process for either oxidative coupling or nonoxidative conversion of CH4 according to the charge balance of CH4 → C2H4 transformation. This process would be ideally realized using a conceptually different approach in the form of an efficient electrochemical process with a reaction 4 to C2H4 or with a reaction 4 to C2H4 through a synergistic control of electrochemical oxidation with in situ catalysis on the electrodes.Direct conversion of CH9. The advantages, including long life, versatile scale, and low cost, have also demonstrated huge practical application potentials. They can exploit available high-temperature exhaust heat stream to maximize the electrical energy efficiency, and the high operation temperature of around 800–900 oC well fits the requirement of catalytic conversion of CH4 to C2H414. In an oxide-conducting SOE, the oxygen ions are transported from the cathode through the electrolyte membrane to the anode and form oxygen gas 2– ions from gaseous molecules that contain oxygen atoms under an externally applied potential. As shown in Fig. 2− ions to in situ electrochemically oxidize CH42H4 in the anode. In this case, the electrochemical conversion of CH4 into C2H4 would be successfully achieved, demonstrating economic and sustainable potential if this process exploits available exhaust heat stream and renewable electrical energy. Similar conversion of CH4 into C2H44 cracking might lead to coke formation in the electrode.Solid oxide electrolyzers (SOEs) have been attracting great interest because of their high efficiencies in converting electricity into fuels and chemicals using renewable electrical energy4 to C2H4 in an oxide-ion conducting solid oxide electrolyzer involves the key step of the first cleavage of C–H bonding in CH4 and then the selective coupling of C–C bonding for C2H4 generation through a gaseous electrochemical process in the porous anode. The electrochemical pumping of O2− ions from the cathode to the anode is indeed a process of generating active oxygen species for CH4 activation and oxidation in the presence of suitable catalysts. Electrochemical oxidation of CH4 with O2− ions is also a charge-rebalancing process while the transformation of O2− ions to the active interfaces in the anode would involve different types of oxygen species including O2− and O− ions to initiate the CH4 oxidation process19. Active metal–oxide interfaces at nanoscale would be ideal to accommodate these oxygen species especially in the form of O− ions. These active oxygen species are one kind of the key active radicals at the confined metal–oxide interfaces to activate CH4 while the electrochemical pumping of O2− ions would continuously oxidize CH4 in conjunction with gaseous selective coupling to generate C2H4 in the presence of suitable interface catalysis. Electrochemical pumping of active oxygen species to the porous anode would facilitate the oxidative removal of deposited carbon on the electrode scaffold and therefore enhance the coking resistance during the CH4 cracking process. The applied potentials would split the oxygen-containing molecules such as gaseous O2, H2O, or CO2 in the cathode to generate O2− and simultaneously transport them to the anode. This feature provides the unique advantage of directly electrolyzing H2O or CO2 to H2 or CO to generate oxygen species in conjunction with selective oxidation of CH4 to C2H4 in the anode.Electrochemical oxidation of CH4 to C2H4 and the typical metal nanocatalysts, such as iron nanoparticles are normally considered to be suitable to construct metal–oxide interfaces with strong interactions to facilitate the catalysis conversion21. These metal–oxide interfaces are usually prepared by directly loading metal nanoparticles on porous substrates through an impregnation method. However, long-term stability of metallic nanoparticles simply loaded on oxide surface remains a major challenge. The severe agglomeration and sintering of these nanoparticles lead to catalysis performance degradation especially at high temperature. An alternative method is to dope the iron element in the host lattice of oxide scaffold during the catalyst synthesis in air and then in situ grow the metallic nanoparticles on oxide surface through a phase decomposition process under reducing conditions24. The metal nanoparticles directly embedded on porous oxide backbones particularly exhibit enhanced high-temperature stability and coking resistance, which may be due to the strong metal–oxide interactions at the unique interface architectures at nanoscale. Strong interface interactions are also highly favorable for the transfer of oxygen species especially in the form of oxygen ions in this work. These active oxygen species are expected to in situ electrochemically activate and oxidize CH4, and even remove the deposited carbon if present at metal–oxide interfaces in the cathode.The incorporation of metal nanoparticles on oxide scaffold has been widely proven to be quite effective to enhance catalytic conversion of CH2Fe1.5Mo0.5Oδ6− (SFMO) anode to electrochemically oxidize CH4 to C2H4 in an oxide-ion-conducting solid oxide electrolyzer. Ceramic SFMO is a mixed conductor with rich oxygen vacancies and we have already shown its excellent activity and coking resistance toward CH4 oxidation27. We firstly in situ construct metal–oxide interfaces at nanoscale on porous SFMO scaffold by synergistic control of nonstoichiometry and doping of Sr2Fex1.5+Mo0.5Oδ6− (x = 0–0.1). We then tailor the amount of excess iron to grow iron nanoparticles with different metal nanoparticle distribution on the SFMO scaffold through a phase decomposition process in reducing atmospheres. We then investigate the electrochemical pumping of oxygen species from air in the cathode to the anode for the selective electro-oxidation of CH4 to C2H4. We finally study the electrochemical oxidation of CH4 to chemicals in the anode in conjunction with electrolysis of CO2 to CO in the cathode.Here, we utilize redox-reversible layered perovskite Sr2Fex1.5+Mo0.5Oδ6− (x = 0–0.1) samples while the excess iron element is doped into the oxide lattice to form a series of homogeneous solid solutions. The chemical formulas of Sr2Fe1.5Mo0.5Oδ6−, Sr2Fe1.525Mo0.5O6-δ, Sr2Fe1.55Mo0.5Oδ6-, Sr2Fe1.575Mo0.5Oδ6-, and Sr2Fe1.6Mo0.5Oδ6− are denoted as SFMO, 0.025Fe–SFMO, 0.050Fe–SFMO, 0.075Fe–SFMO, and 0.100Fe–SFMO, respectively. The presence of excess iron in the host oxide lattice would result in metallic iron nanoparticles on the SFMO scaffold after reduction, producing the electrochemically active metal–oxide interfaces through a phase decomposition process. The iron in the host lattice is mainly present in the form of Fe3+/4+ oxidation state as shown in the X-ray photoelectron spectroscopy (XPS) in Supplementary Fig. 2Fex1.5+Mo0.5Oδ6− (x = 0–0.1) after reduction, confirming the presence of metallic iron phase that arises from the exsolution of excess iron dopant in the oxide lattice upon reduction. We further determine the oxygen nonstoichiometry of Sr2Fex1.5+Mo0.5Oδ6− (x = 0–0.1) before and after reduction using iodometric titration and summarize the results in Supplementary Table x = 0.025–0.1) has been transformed into metallic iron nanoparticles while the left iron remains unchanged in a fully oxidized state. The exsolution of iron nanoparticles further leads to oxygen loss which shows a strong dependence of oxygen nonstoichiometry on the exsolution of iron metal as shown in Supplementary Table 4 pyrolysis.Figure 0.9Sr0.1Ga0.8Mg0.2Oδ3− (LSGM) electrolyte with dimension of 1 mm in thickness and 20 mm in diameter. LSGM is an excellent oxygen-ion-conducting electrolyte with high ionic conductivity at intermediate to high temperatures and we conduct CH4 oxidation at 850 °C to enhance the C2H4 yield. The electrochemical oxidation of CH4 in the anode is conducted in conjunction with O2 electrolysis from air in the cathode at 850 °C. The CH4/O2 fuel cell mode is a mode of electrochemical oxidation of CH4 with O2 gas toward CO2 generation driven by the oxygen gradient across two different electrodes in a concentration cell. Here we further apply voltages to boost the electrochemical CH4 oxidation process toward C2H4 generation in the presence of interface catalysis in the anode. Before operation, the anodes are pre-reduced using 5%H2/Ar to in situ grow the exsolved interfaces that are composed of iron nanoparticles anchoring on the SFMO scaffolds. Supplementary Fig. I–V) relationship clearly reveals the superior performance with up to 100% enhancement of the SFMO cathodes with nanoscale metal–oxide interfaces in comparison to the bare cathode. The optimum chemical composition of the anode is found to be 0.075Fe–SFMO. The growth of metal–oxide interfaces drastically improves the current densities to ~1 A cm−2 at 1.6 V with the presence of optimum length of metal–oxide interfaces in the porous SFMO scaffolds. Supplementary Fig. 2 for the different anodes at the applied voltages of 1.6 V, indicating the significantly enhanced electrode process at higher applied voltages. Figure 4 oxidation at different voltages, which again confirms the significantly enhanced electrochemical process with metal–oxide interfaces in the anodes. Figure 4 with different cathodes at 1.2–1.6 V at 850 °C. It is observed that the reduction of electrode polarization resistances strongly depends on the growth of metal–oxide interfaces in the electrode while higher applied voltage facilitates the electrochemical process and therefore enhances the electrode reaction kinetics. It should be noted that the overall electrode activity is enhanced with the growth of metal–oxide interfaces; however, more attentions need to be paid to the conversion of CH4 to C2H4 with less over-oxidation of CH4 to CO2 in the porous anodes.SOEs with porous SFMO anodes are constructed while the cell microstructures are shown in Supplementary Fig. 4 explain the exceptionally high molecule stability which therefore leads to the difficulties of CH4 activation and conversion. A limitation with most existing heterogeneous catalysts is that the initial C–H bond cleavage is the rate controlling step. It is widely accepted that the oxidation reaction of CH4 to C2H4 is usually initiated on the surface of solid catalyst by reacting CH4 with surface oxygen species to form methyl radicals with the first C–H bond cleavage, and then it continues in a gas-phase coupling step involving the coupling of methyl radicals to C2H6 followed by dehydrogenation to C2H428. In our work, the oxygen species are continuously produced and transported to the anode while the metal–oxide interfaces at nanoscale would be highly favorable to accommodate these active oxygen species to react with CH4 to initiate the first C–H bond cleavage under external applied potentials. Here the theoretical calculations are tentatively conducted to understand the cleavage of C–H bond in CH4 especially at the nanoscale metal–oxide interfaces. We firstly study the CH4 activation over the (Fe/Mo)O2 termination on the SFMO (001) surface as shown in Supplementary Fig. 4 molecule from the feed gas dissociatively adsorbs on a metal site and a neighboring oxygen atom, forming a hydroxyl group with O on the surface and releasing a methyl radical. As shown in Fig. 4 on one Fe site with the bond length of 3.263 Å is energetically unfavorable. After the C–H bond cleavage of CH4, the CH3 radical adsorbs on surface metal Fe with the bond length of 2.146 Å and the H atom binds with the O atom of SFMO surface forming O–H bond (0.987 Å). And the energy barrier of hydrogen abstraction reaction is up to 1.0 eV on the SFMO (001) surface, indicating an energetically unfavorable process for CH4 activation. In contrast, for the Fe/SFMO system, the CH4 mainly adsorbs at the interface between Fe clusters and SFMO surface. The adsorption energy of CH4 is −0.26 eV, where the CH4 binds with the Mo atom (3.734 Å) of the SFMO surface and together with the Fe atom (3.545 Å) of Fe clusters. After the first C–H bond cleavage of CH4, the CH3 radical adsorbs on the Fe atoms of Fe clusters with the bond lengths of 2.027 and 2.093 Å while the H atom binds with the O atom at the Fe/SFMO interface forming O–H bond (0.980 Å). The cleavage of C–H bond in CH4 at the metal–oxide Fe/SFMO (001) interface is highly favorable with the energy barrier as low as only 0.46 eV. At this stage, active oxygen at metal–oxide interfaces would be present in the form of new oxygen species (Oads) to initiate the cleavage of C–H bond in CH4. The energy barrier of oxygen transfer process through oxygen vacancy site is only 0.34 eV, as shown in Supplementary Fig. 4 activation and oxidation29. As we know, the charge transfer from adsorbed CH4 molecule to catalyst surface may be another key factor in CH4 activation1. In the anode, the adsorbed CH4 coupled with active oxygen species would continuously donate free electron to external circuit under electrochemical potentials and thus facilitates CH4 activation at the metal–oxide interfaces. It can be therefore concluded that the metal–oxide interface with active oxygen species would be effective to accommodate the reaction of C–H bond cleavage under externally applied potentials in the porous anode.The properties of CH4 with different composite anodes while the oxide ions are ionically pumped to the anodes from air in the cathode. CH4 with 100% concentration is supplied to the anode and the products from the anode exhaust stream are analyzed using an online gas chromatography. We firstly investigate the thermal splitting of CH4 to C2H4 without externally applied voltage and find that only ~0.4% C2H4/C2H6 is generated with H2 of around 1.6%, as shown in Supplementary Fig. 2H4 has been formed and its concentrations show a strong and positive dependence on the applied potentials, which confirms the successful conversion of CH4 to C2H4 using this electrochemical approach. C2H6 has also been formed through the oxidative coupling of CH4 in the electrochemical conversion process while its concentrations are ~1/3 of the C2H4 concentrations. The by-products are mainly H2 together with small amount of CO; however, their concentrations are in a close decreasing trend with the C2 (C2H4 + C2H6) product generations. We show that the C2 products reach 16.7% (11.5% C2H4 + 5.2% C2H6) with 82.2% C2 selectivity while the CH4 conversion ratio is approaching as high as 40.5% in Fig. 2H4 yields and C2 selectivity in the reported catalytic conversion of CH4 including non-oxidative and oxidative conversion processes. The C atom efficiency is reaching ~98–100% with negligible over oxidation of CH4 to CO2, which further indicates the exceptionally high efficiency for CH4 conversion in this electrochemical process. The SFMO itself is an oxygen-nonstoichiometric catalyst and the iron species with unsaturated oxygen coordination at the SFMO surface would deliver catalytic activity. In contrast, redox stable La0.75Sr0.25Cr0.5Mn0.5Oδ3− (LSCM) electrode without iron dopant in the lattice shows negligible catalysis activity toward C2H4 generation11. It is also observed that the metal–oxide interfaces enhance C2 product generation by ~50–100%, indicating the favorable cleavage of C–H bond at metal–oxide interfaces. Here the electrochemical processes are therefore dominated by the synergistic effect of the applied voltages and metal–oxide interfaces in the anodes. We further cycle the output gas for the electrochemical oxidation process and confirm that CH4 conversion has been enhanced to ~81.1% in Fig. 2 selectivity remains ~41%  relationship of CO2 electrolysis in the cathode in conjunction with CH4 oxidation in the anode in a simultaneous process. The growth of metal–oxide interfaces in the porous electrode scaffolds also improves the current densities while the optimum composition is 0.075Fe–SFMO in which the content of metal nanoparticle dominates the metal–oxide interface length. Figure 2 electrolysis both on metal–oxide interfaces and applied voltages, which indicates that the higher voltages facilitate CO2 reduction while optimum composition is the 0.075Fe–SFMO composite with proper interface length. Figure 4 conversion toward chemicals in the anode while the CO2 electrolysis is simultaneously performed in the cathode. Similar phenomenon has been observed for the performance dependence on the synergy of applied voltages and metal–oxide interfaces on the anode scaffold. The C2 products reach 2.04–5.08% (C2H4 + C2H6) with 75.6% C2 selectivity while the CH4 conversion ratio reaches 6.02–13.72% in the anode as shown in Fig. 2/CH4 conversion in contrast to O2/CH4 conversion in the electrochemical process would be attributed to the higher thermodynamic and kinetic barrier of CO2 electrolysis than O2 splitting in the cathode under applied potentials. However, these performances of CH4 conversion and C2 yields/selectivity are still much higher or comparable to the reported work in catalytic CH4 conversion including non-oxidative and oxidative conversion processes. We calculate the current efficiency and the results are shown in Supplementary Fig. 2 electrolysis in conjunction with CH4 oxidation in the anode. However, the current efficiency can only reach ~80% for CO2 electrolysis with simultaneous CH4 oxidation in the anode. Electrolysis of O2 is highly favorable and much easier than CO2 electrolysis even under the similar operation conditions. We believe that the current leakage may be present in CO2 electrolysis in the cathode, which therefore leads to lower CH4 conversion under the similar conditions. The loss of current efficiency may be due to the transport of impurities including p-type hole conduction in the LSGM electrolyte, which therefore limits the transport of oxygen species and accordingly degrades the CH4 conversion and hydrocarbon generation.This simultaneous electrochemical process involves ionically pumping oxide ions from the cathode, across the electrolyte, to the anode for CH4 conversion at high temperature remains a major challenge due to the nanoparticle agglomeration and carbon deposition leading to performance degradation8. In this work, the iron nanoparticles are exsolved and anchored on porous SFMO scaffold through a phase decomposition process, which is expected to possess enhanced high-temperature stability against nanoparticle agglomeration. For CH4 conversion, carbon deposition is another challenge that leads to severe deactivation of metal nanoparticles. The carbon coking normally originates from the disproportionation of CO and the pyrolysis of CH4. As shown in Fig. 2 hydrocarbons remain stable even after such a tough operation at high temperatures. The exsolved metal/oxide interfaces at nanoscale are expected to improve both long-term and redox stability by preventing nanoparticle coalescence driven by surface energy reduction. One reason may be that the strong interactions at the exsolved metal–oxide interfaces produce typical interfacial charge transfer to enhance coking resistance at high temperatures. The other important reason would be that the carbon removal at the interfaces is highly favorable with electrochemically provided oxygen species in the anode because the oxygen ions are continuously transported from the cathode to the anode under externally applied voltages, which would electrochemically oxidize the deposited carbon to carbon monoxide gas, and thus substantially enhance coking resistance. We further use SEM to observe the porous electrode before and after the redox cycling experiments over 100 h. In Supplementary Fig. Long-term stability of CH4 to C2H4 with exceptionally high C2H4 yield and C2 selectivity in a solid oxide electrolyzer. The growth of metal–oxide interfaces at nanoscale with strong interactions in the porous electrode scaffolds possesses not only improved activity of CH4 activation but also enhanced coking resistance and thermal stability. Electrochemical oxidation of CH4 in the anode in conjunction with O2 electrolysis in the cathode shows the highest C2 yields of 16.7% (11.5% C2H4 + 5.2% C2H6) with 82.2% C2 selectivity and exceptionally high durability even after 100 h of high temperature operation. Another efficient process with high C2 yields has also been demonstrated with electrochemical oxidation of CH4 in the anode in conjunction with CO2 electrolysis in the cathode. The active oxygen species electrochemically pumped to the confined metal–oxide interface in the anode scaffold enable both carbon removal and CH4 activation. This strategy provides a reliable and efficient electrochemical process for conversion of CH4 into valuable chemicals. The growth of the metal–oxide interface can give rise to an interfacial interaction that facilitates catalytic conversion using CeO2, SrTiO3, TiO2, and MnO2 substrates. The catalytically active Fe or Ni can be directly doped into the lattice of the oxide substrate during materials synthesis, and then the metal nanoparticles would be grown to anchor on the oxide surface after reduction. These metal–oxide interfaces with enhanced stability and coking resistance could have broad potential applications in many other catalysis fields.In conclusion, we demonstrate a conceptually different approach of highly efficient conversion of CH2Fex1.5+Mo0.5Oδ6−; x = 0–0.1) were carried out by a microwave-assisted combustion method25. The Ce0.8Sm0.2Oδ2− (SDC) powders were synthesized using a combustion method14. LSGM was prepared by using a traditional solid-state reaction method25.Synthesis of SFMO, 0.025Fe–SFMO, 0.050Fe–SFMO, 0.075Fe–SFMO, and 0.100Fe–SFMO (Sr2Fex1.5+Mo0.5Oδ6− (x = 0–0.1) before and after reduction was determined using iodometric titration30. The microstructures of the samples and the exsolution of nanoparticles were analyzed using scanning electron microscopy  and high-resolution transmission electron microscopy .XRD  and XPS  were used to analyze the phase formation of samples and elemental states before and after reduction, respectively. The oxygen nonstoichiometry of Sr2Fex1.5+Mo0.5Oδ6− (x = 0–0.1) and SDC powders were mixed with a weight ratio of 65:35 and a suitable amount of cellulose and α-terpineol were added to prepare electrode slurry. The 1-mm-thick LSGM disks were used as the electrolyte support to assemble single cells with different electrodes (1 cm2) using a brush printing method followed by heat treatments at 1100 °C for 3 h in air. The silver current collector with 0.18 mm in diameter was coated on the electrode surfaces and then heat-treated in air at 550 °C for 0.5 h. For electrochemical tests of CH4 oxidation, the in situ AC impedance, long-term stability tests and the current density–voltage curve (I−V curve) were recorded using an electrochemical workstation . The CH4, C2H4, C2H6, CO, and H2 in the output gas were analyzed uisng an online gas chromatograph .The Sr29. The generalized gradient approach (GGA) was used including Perdew–Burke–Ernzerhof (PBE) functional to describe exchange and correlation. The interaction between core and valence electrons was described with the projector augmented wave (PAW) method. The energies and residual forces were converged to 10–6 eV and 0.02 eV Å−1, respectively. The plane wave cut-off used for energy calculations was set to 800 eV and the U–J value for Fe was set to 4.0 eV while no U–J parameter was used for Mo. The Sr2Fe1.5Mo0.5O6 cubic supercells of bulk SFMO with different distributions of Mo atoms were optimized with a 4 × 4 × 4 k-point grid and only the (Fe/Mo)O2 terminated surfaces were considered because they were expected to be more catalytically active than the SrO termination. By comparison of stability, the (Fe/Mo)O2 terminated surface model constructed from the plane diagonal-Mo bulk structure  was used to simulate the (001) surface. A p(2 × 2) superstructure with four layers (160 atoms) of the (001) surface of SFMO was used to simulate the periodic slab model. The vacuum region is 15 Å. The Fe segregation on the (001) surface slab of SFMO was mimicked by a cluster containing with 9 Fe atoms laying on the (001) surface of SFMO. A 2 × 2 × 1 k-point grid was used for Brillouin zone sampling of (001) Fe/SFMO surface system. The adsorption energy of CH4 with van der Waals Correction was calculated using the equation of 4 in gas phase, respectively10. The climbing-image nudged elastic band (CI-NEB) method was employed to calculate the energy barrier of oxygen vacancy transfer and the C–H bond cleavage in CH4 reaction processes14.Spin-polarized DFT+U theory calculations were performed using a plane wave basis set Vienna Ab-initio Simulation Package (VASP) codeSupplementary Information


PLOS ONE Editors. Specific concerns include similarities between bands in the following panels:Following publication of this article , a numbeFig 1A Ctrl 0’ and CΔE 0’Fig 1E Ctrl 0’, Rv 0’, ΔE 0’ and CΔE 0’Fig 1I LPS 0’, Rv 0’, and ΔE 0’Fig 1I Ctrl 60’ and LPS 60’Fig 1I Pan SAPK/JNK 0’ and Pan SAPK/JNK 60’Fig 1C Pan ERK 1/2 Ctrl and Fig 3B Total Erk CtrlFig 1C Pan ERK 1/2 LPS and Fig 3B Total Erk DMSO and CΔEFig 1C Pan ERK 1/2 Rv and Fig 3B Total Erk LPSFig 1C Pan ERK 1/2 ΔpknE and Fig 3B RvFig 1C Pan ERK 1/2 CΔE and Fig 3B ΔpknEFig 3A Ctrl and DMSOThe authors have provided the full blots underlying the panels in which possible duplications were observed; however, the raw uncropped blots underlying the other Western blot time points in Figs 1 and 3 are now unavailable.The authors acknowledge duplication between bands shown in the Fig 3B Total Erk panel and those shown in Fig 1C Pan Erk 1/2, and indicate this arose due to an unintentional error in image selection for Fig 3. The authors stand by the accuracy of the underlying findings and other figure panels presented in the article.PLOS ONE Editors have assessed the available data and files, and have consulted with a member of the Editorial Board. While the concerns regarding Fig 3B Total Erk were addressed in follow up, concerns remain that there are discrepancies between other figure panels and the uncropped blot images provided by the authors.The Additional issues were raised regarding the methodology and interpretation of the data. Specifically,It is unclear whether loading control blots were carried out for all experiments/time points.It is unclear how the Western blot data were quantified and what steps were taken in data normalization.Separate Western blots were run for each time point in the time course experiments shown in Fig 1, raising concerns about the validity of comparisons made between signaling responses at different timepoints.Some data points in Fig 2 are accompanied by a *, indicating p<0.05 by two-way ANOVA. The figure legend states “The results from three independent experiments are shown. The O.D values denote standard error of the means”; however, there are no error bars, and the data used for the statistical analysis are unclear.In Fig 4, some of the statistically significant differences have very small changes in value, and it is difficult to assess whether this is biologically significant.PLOS ONE Editors retract this article.In light of these concerns, which call into question the integrity and reliability of the published data, the DKP, LEH, SN did not agree with retraction.


The oxide–nitride/graphite fibers have the lowest overpotential requirements of 71 and 183 mV at 10 mA cm−2 for HER and OER in alkaline medium, respectively. These values are comparable to those of commercial Pt/C (20 wt%) and RuO2. The electrodes also show a response to HER and OER in both neutral and acid media. Furthermore, the 3D structure can be highlighted by all‐round electrodes for overall water splitting. The calculations on the changes in electrons transfer and the Femi level from oxides to oxides/nitrides reveal that the observed superb electrocatalytic performance can be attributed to the presence of Ni3N and CoN derived from the in situ nitridation of NiCo2O4.Many efforts have been made to develop bifunctional electrocatalysts to facilitate overall water splitting. Here, a fibrous bifunctional 3D electrocatalyst is reported for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with high performance. The remarkable electrochemical performance is attributed of the catalysts to a number of factors: the metallic character of the three components (i.e., Ni Electrocatalytic water splitting is regarded as a promising approach to a sustainable, secure, and eco‐friendly source of hydrogen‐fuel energy.− to facilitate HER and OER while the metal nitride core, compared to metal oxides, can expedite electron transport from the conductive support to the catalyst surface.2O4 hollow spheres with metallic properties, which exhibited high capacities as supercapacitors, indicating that NiCo2O4 has a spinel structure and possess good conductivity.Recently, rational improvements have been made by increasing the number of active sites through nanostructuring of catalysts, dispersing the catalysts on supports with high surface areas, and developing shape‐controlled catalysts with specific crystalline faces.In this regard, 3D materials with porous structures have been widely used as electrode materials. The compact pore structure gives the materials high surface areas, which expands the contact area between the electrode and electrolyte compared with that of bulk solid structures.[[qv: 11b]] This structure further facilitates electron transport from the catalyst to the substrate and promotes the release of the evolved gas bubbles to maximize the catalytic performance.2O4 nanoflakes/graphite fibers (denoted as NiCo‐nitrides/NiCo2O4/GF) by assembling nitride–oxides hetero‐nanostructures onto graphite fibers. We used a simple electrochemical deposition, followed by in situ nitridation. The resultant NiCo‐nitrides/NiCo2O4/GF can act as bifunctional electrocatalysts for HER and OER over a broad pH range. The graphite fibers were more desired alternative compared with other conductive substrates due to their more conductive nature associated with the higher degree of graphitization, as evidenced by Raman spectrum . Our results revealed synergistic effects between the metallic NiCo‐nitrides nanoparticles and NiCo2O4 nanoflake hybrid structures. The electric field at the interface of the NiCo‐nitrides/NiCo2O4 formed on the graphite fibers provides a pathway for electron transfer. Furthermore, NiCo‐nitrides offer more exposed active sites for electrocatalytic water splitting. As a result, our NiCo‐nitrides/NiCo2O4/GF shows excellent electrocatalytic activity and stability towards overall water splitting over a wide range of pH values. The newly developed 3D structure could provide a large surface area to act all‐round electrode for overall water splitting.Herein, we develop a facile method to fabricate 3D NiCo‐nitrides nanoparticles/NiCoScheme2O4 by a pyrolysis process (denoted as NiCo2O4/GF). Finally, NiCo2O4 was partially converted into NiCo‐nitrides by thermal ammonolysis, to form hetero‐nanostructured NiCo‐nitrides/NiCo2O4/graphite fibers (denoted as NiCo‐nitrides/NiCo2O4/GF) .As shown in Figure2O4 . Although the interconnected nanostructures remained tightly attached to the surface of the graphite fibers after nitridation, the morphology of nanostructures was altered from that of the non‐nitrided sample. A large number of nanospheres with diameters in the range of 30–50 nm assembled on the surface of the interconnected nanoflakes to form a unique 3D beaded fabric‐like nanostructure. This structure offers a pathway for rapid electron‐transport from the interface of the electrodes and electrolyte to the intrinsic electrically conductive graphite fibers  plane of CoN. Lattice fringes with lattice spacings of 0.245 and 0.469 nm can be indexed to the (311) and (111) plane of NiCo2O4, respectively. We observed clear interfaces between NiCo2O4 and CoN or Ni3N. TEM mapping images of NiCo‐nitrides/NiCo2O4 in Figure The graphite fiber is a continuous filament with a high graphitization degree and 7–9 µm in diameter  analysis, the survey spectra of both the NiCo2O4 supported on graphite fibers demonstrate that the Ni, Co, O, C elements are the main components in NiCo2O4. After nitridation, the intensity of O1s decreases, the peak of N1s appears, which further confirms the partial transformation from NiCo2O4 to NiCo‐nitrides . The peaks of NiCo2O4 around 796.5 and 780.4 eV were well indexed to Co3+, whereas those at 798.1 and 782.3 eV related to Co2+  the characteristic peak was located at around 399 eV, which can be assigned to the nitrogen in a metal nitride.We then carried out X‐ray diffraction (XRD) measurements to identify the phase of the samples. As shown in 2O4/GF, ex situ X‐ray absorption fine structure (XAFS) spectroscopy has been carried out. The Fourier transformed extended XAFS (FT‐EXAFS) data for NiCo‐nitrides/NiCo2O4/GF and NiCo2O4/GF are shown in Figure2O4/GF showed a tendency for the pre‐edge feature to shift to lower energy bonding compared with that of NiCo2O4/GF  loaded on a glassy carbon electrode were also performed. Linear sweep voltammetry curves in Figure2O4/GF exhibits the highest HER activity with the lowest overpotentials (η) of 71 and 180 mV to obtain a current density of 10 and 50 mA cm−2, respectively, compared with NiCo2O4/GF electrode and graphite fibers, which is close to that of commercial Pt/C (20 wt%). The NiCo2O4/GF electrode featured an overpotential (η) of 307 mV, which is higher than that of the NiCo‐nitrides/NiCo2O4/GF. The bare graphite fibers without any active materials show a small response. The remarkable activity for HER of the NiCo‐nitrides/NiCo2O4 was further assessed by its inherently higher current density over the potential ranges measured. We evaluated the catalytic kinetics of the samples are evaluated from their Tafel plots  and the results was also close to that of Pt/C (35 mV dec−1), indicating that the NiCo‐nitrides/NiCo2O4/GF shows promise as a HER electrocatalyst. Electrochemical impedance spectroscope (EIS) tests on NiCo2O4/GF and NiCo‐nitrides/NiCo2O4/GF were conducted at the same applied potential (−0.38 V vs RHE) to further investigate the kinetics with catalytic activity  and −0.369 V (vs RHE), respectively, are shown in Figure S10 in the Supporting Information. Almost no decrease in current density occurred over 40 h for the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF via continuous chronoamperometric response (i–t) under alkaline condition. The morphology of the NiCo‐nitrides/NiCo2O4/GF changed little . These results confirmed that NiCo‐nitrides/NiCo2O4/GF featured remarkable HER catalytic activity with low overpotential, high current density, and low charge transfer resistance in addition to good chemical stability. The main electrochemical properties arose from the 3D bead‐curtain‐like nanostructures formed by the unique porous and ultrathin nanoflakes decorated with nitride nanospheres. This structure provides many accessible active sites for the adsorption of H2O and short diffusion pathways for charge transfer to a charge collector. The hybrid structures of NiCo2O4, Ni3N, and CoN play a vital role in contributing to the high performance.The electrocatalytic HER activities of NiCo2O4/GF and NiCo‐nitrides/NiCo2O4/GF electrodes should feature good OER activity owing to their graphite substrate and the metallic characteristics of the active nanostructures on the graphite fibers. The OER activities of NiCo2O4/GF and NiCo‐nitrides/NiCo2O4/GF electrodes were evaluated in O2‐saturated 1.0 m KOH with a scan rate of 5 mV s−1. According to the polarization curves in Figure 2O4/GF exhibits the highest OER activity with the lowest overpotentials (η) of 183 and 344 mV to obtain current densities of 10 and 50 mA cm−2. By comparison, 300 and 366 mV overpotentials at a current density of 10 mA/cm−2 are required for NiCo2O4/GF and RuO2. Furthermore, the Tafel slope for NiCo‐nitrides/NiCo2O4/GF (56 mV dec−1) was the smallest among all the was the smallest among all the electrocatalysts tested including NiCo2O4/GF (61 mV dec−1) and RuO2 (66 mV dec−1) . The stability test of the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF in Figure S13 (Supporting Information) was also tested by continuous chronoamperometric response (i–t) measurements under alkaline condition at the applied potential of 1.41 V (vs RHE) and 1.60 V (vs RHE), respectively. The results show almost negligible degradation during 40 h of continuous operation, which confirms the excellent durability. It is generally accepted that transition metal oxides on the surface of transition metal nitrides as electrocatalysts serves as actual catalytic sites during OER process.[[qv: 11b]] Notably, in our system, NiCo‐oxides can directly work as actual electroactive sites for OER. Compared to the generated oxides or hydroxides on the surface of transition metal nitrides, NiCo‐oxides based on the in situ formed NiCo‐nitrides/NiCo2O4 provide larger active area. The enhanced performance of NiCo‐nitrides/NiCo2O4/GF should be ascribed to the NiCo‐nitrides/NiCo2O4 structures with electric field effect to contribute to electron transfer and expanded electroactive area. A comparison list in Table S1 (Supporting Information) shows the HER and OER activities of previously reported non‐noble bifunctional electrocatalysts and the NiCo‐nitrides/NiCo2O4/GF synthesized in this work. These data illustrate that both the HER and OER activities of the NiCo‐nitrides/NiCo2O4/GF are better activity than that of most other non‐noble electrocatalysts reported to date.An ideal bifunctional electrocatalyst should possess bifunctional activities toward HER and OER, which could simplify water splitting systems and lower total costs. We expected that NiCo2O4/GF acted as bifunctional electrocatalysts with high catalytic performance towards both HER and OER in alkaline media. The linear sweep voltammetry (LSV) curves of the NiCo‐nitrides/NiCo2O4 displayed overpotentials of 170 and 280 mV (vs RHE) at 20 mA cm−2 for HER and OER in a three‐electrode system, respectively, indicating that the overall potential window can be 1.68 V at 20 mA cm−2 in Figure2O4 electrodes to assemble a water splitting electrolyzer with a two‐electrode system, which was tested in 1 m KOH. The applied potential was larger than 1.5 V, and hydrogen and oxygen bubbles were sustainably released as the potential was increased . To confirm the improved performance of synthesized samples, we compared the electrochemically active surface area (EASA) of the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF by measuring the electrochemical double‐layer capacitance (Cdl). The NiCo‐nitrides/NiCo2O4/GF delivered a higher Cdl (114.4 mF cm−2) than that of NiCo2O4/GF (90.13 mF cm−2) and these values are both related to EASA . Previous reports have proved that the activity increases owing to the greater electrochemical surface area.2O4/GF possess a notably greater exposure of active sites. Furthermore, we could operate the electrolyzer with a single AAA battery of 1.5 V  as shown in Figure 2O4/GF and NiCo2O4/GF as simultaneous cathode and anode reach ≈100%, respectively in the 1 m KOH. Photographs and videos show that the fibrous electrodes consisted of a bunch of NiCo‐nitrides/NiCo2O4/GF continuously released small bubbles owing to full use of substrate to achieve all‐round water splitting . In general, the electrocatalytic overall water splitting activities of the NiCo‐nitrides/NiCo2O4/GF were comparable to that of other recently reported overall water splitting electrocatalysts .According to the above studies, we clarified that the NiCo‐nitrides/NiCo2O4/GF does not only perform excellent HER and OER performance in alkaline electrolyte but shows a response towards HER and OER in both acid and neutral media. The HER activities of the NiCo2O4/GF and the NiCo‐nitrides/NiCo2O4/GF electrode in acid were evaluated in N2‐saturated 0.5 m H2SO4 with a scan rate of 5 mV s−1. The NiCo‐nitrides/NiCo2O4/GF exhibits a lower overpotential of 432 mV than that of NiCo2O4/GF (573 mV) at a current density of 10 mA cm−2 , the Tafel plots of the NiCo‐nitrides/ NiCo2O4/GF (68 mV dec−1) and the NiCo2O4/GF (88 mV dec−1) are close to that of Pt/C (32 mV dec−1). Moreover, Figure 2O4/GF has the smaller semicircular Nyquist plots, indicating a smaller charge transfer resistance than that of NiCo2O4/GF. We also examined the electrocatalytic of the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF for OER in O2‐saturated 0.5 m H2SO4 at a scan rate of 5 mV s−1. The NiCo‐nitrides/NiCo2O4/GF displays a higher electrocatalytic activity with a small overpotential of 460 mV at a current density of 10 mA cm−2 than that of NiCo2O4/GF (490 mV), which is comparable to commercial RuO2 (390 mV). The Tafel plots of the NiCo‐nitrides/NiCo2O4/GF (73 mV dec−1) and the NiCo2O4/GF (82 mV dec−1) are close to that of RuO2 (67 mV dec−1) . The HER activities of the NiCo2O4/GF and the NiCo‐nitrides/NiCo2O4/GF electrodes were evaluated in N2‐saturated 1 m PBS (pH = 7) at a scan rate of 5 mV s−1. The comparison in Figure 2O4/GF exhibits inferior HER activity with overpotential of 418 mV to afford current density of 10 mA cm−2. Conversely, the NiCo2O4/GF requires an overpotential of 674 mV, which indicates the NiCo‐nitrides/NiCo2O4 compounds features improved catalytic activities. The corresponding Tafel plots of 78 and 89 mV dec−1 for the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF are shown in Figure −1 for Pt/C is inferior to that of the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF. The EIS curves confirm that the improved electron transfer for the NiCo‐nitrides/NiCo2O4/GF with smaller semicircular arc compared with that of NiCo2O4/GF. We further investigated the OER performance of the NiCo2O4/GF and the NiCo‐nitrides/NiCo2O4/GF electrodes compared with that of commercial RuO2 in O2‐saturated 1 m PBS (pH = 7) at a scan rate of 5 mV s−1. To attain a current density of 10 mA cm−2, the NiCo‐nitrides/NiCo2O4/GF and the NiCo2O4/GF require an overpotential of 673 and 703 mV, which rivals that of the commercial RuO2 (520 mV) .The development of OER and HER electrocatalysts operating in neutral medium is of critical importance owing to safety consideration and to reduce the costs for the water splitting. Despite of many efforts, the current density is still worth enhancing and the overpotentials need to be lowered. For the purpose, we investigated the HER and OER activities of the NiCo‐nitrides/NiCo2O4/GF has remarkable OER and HER activities and superior stability over a wide pH range. We attribute the remarkable OER and HER activities of NiCo‐nitrides/NiCo2O4/GF to the interfaces formed by the nitridation of NiCo2O4. To obtain further insight, we performed the first‐principles density function theory (DFT) calculations to determine the Fermi level and the spin resolved total electron density of states (TDOS) for Ni3N, NiCo2O4, and CoN, respectively using Vienna ab initio simulation package known as the VASP code. All three compounds are metallic in character and the filled area shows the occupied state and the energy of the highest occupied state is the Fermi level. The metallic character ensures the efficient electron transfer. Their Fermi level are 0.44, 0.16, and 2.4 eV respectively  of ≈2.24 V across the interface. Similar processes also occur at the NiCo2O4/Ni3N interface with a CEP of ≈0.28 V. This CEP acts as an additional electric potential versus the reversible hydrogen electrode, which offers interface electric field that facilitates electrochemical reactions .The aforementioned data indicates that NiCo‐nitrides/NiColy Figure7a. At 2O4/GF exhibited distinct activities toward HER and OER over a wider pH range. These bifunctional electrodes for HER and OER were fabricated by in situ electrodeposition and in situ modification. Metallic NiCo2O4 with bifunctional activity for HER and OER was selected as the basic precursor. After nitrogenation, CoN and Ni3N were formed in situ and enhanced the OER or HER performance. The metallic properties of these materials, and the valence and electronic states of the double‐metal nitrides, promoted electrostatic adsorption of H+ or OH− ions and H2O molecule, and reduced the energy barrier for transitions of Ni and Co from lower to higher valence states. These findings contrast with those of single‐metal nitrides and pure metal oxides as previously reported by Zhang et al.[[qv: 12b]] Of particular interest, the p orbitals of N are continuous, producing a covalent interaction between Ni, Co, and N in the metal nitrides. This type of metallic behavior is different from that in metals and can help to resist erosion from acid media. The interface electric field effect at the highly‐coupled interfaces of the oxides and nitrides also drives electron transfer, inducing electron flow until work function equilibrium is reached, further lowering the overpotentials of the electrocatalysts. The many edges of the nanoflakes, the walls, and the pore structures give the nanostructures a large surface area and many electroactive sites for redox reactions. A large number of nanospheres assembled on the surface of the interconnected nanoflakes, forming a unique 3D beaded fabric‐like nanostructure, which offered a pathway for rapid electron‐transport from the interface of the electrodes and electrolyte to the excellent intrinsic electrically conductive graphite fibers. The unique 3D structures, complex combination of components, and metallic behavior, give our NiCo‐nitrides/NiCo2O4/GF catalysts bifunctional electrocatalytic activities toward HER and OER with remarkable performance, superior stability, and good reaction kinetics. The above advantages for electrocatalytic activities make our NiCo‐nitrides/NiCo2O4/GF effective all‐round electrodes, which are particularly effective for overall water splitting over a wide pH range.The NiCo‐nitrides/NiCo2O4/GF as pH‐universal bifunctional electrocatalysts toward HER and OER for overall water splitting. The NiCo‐nitrides/NiCo2O4/GF displayed an overpotential of 71 and 183 mV at 10 mA cm−2 for HER and OER in 1 m KOH, respectively, as well as superior stability. Furthermore, for electrocatalytic overall water splitting of the NiCo‐nitrides/NiCo2O4/GF in 1 m KOH, the overall potential window was 1.68 V at 20 mA cm2 and the Faradaic efficiency reached to ≈100%. The NiCo‐nitrides/NiCo2O4/GF exhibits a response in acid and neutral media. First‐principles simulations reveal that the metallic behaviors of NiCo2O4 and NiCo‐nitrides/NiCo2O4, and the interface electric field effect at the interface of NiCo‐nitrides/NiCo2O4 are the key factors for improving the electron transfer efficiency. This work opens new possibilities for the use of double‐metal nitrides/oxides heterostructures for overall water splitting.We have developed a 3D electrode configuration composed of metallic NiCo‐nitrides/NiCoThe authors declare no conflict of interest.SupplementaryClick here for additional data file.SupplementaryClick here for additional data file.


Extraordinarily poor recruitment of Japanese eels in East Asia has been generally reported during extreme El Niño years. However, the scenario failed to take place during the 2015–16 extreme event. In this study, we examined possible factors responsible for differing eel abundance in East Asia during the two strongest recent extreme El Niños, which occurred in 1997–98 and 2015–16. Numerical tracer experiments were carried out to determine why the impacts on eel catches seen in 1997–98 were not repeated in 2015–16. Among physical factors, two scenarios are likely responsible for extremely poor recruitment in East Asia: southward migration of the North Equatorial Current (NEC) or southward movement of eel spawning grounds. Comparing the latitudinal shift of NEC locations between the 1997–98 and 2015–16 El Niños, we conclude that NEC migration may be a factor, but is not chiefly responsible, for lower eel catches. Our findings pointed to southward movement of spawning grounds as the dominant factor. The northward movement of spawning grounds during 2015–16 meant that eel larvae were preferentially transported into the NEC-Kuroshio system, which resulted in a higher rate of recruitment success. The distinct evolution and dynamics of these two El Niño events led to different spawning ground locations, impacting eel abundance in East Asian countries. Eel larvae in their early life stages are poor swimmers and fundamentally carried by ocean currents, especially the North Equatorial Current (NEC) and Kuroshio, to their freshwater habitat  were carried by ocean currents during El Niño years. Kimura et al.2 further found that extraordinarily reduced eel catches in East Asian countries usually occur during the strongest El Niño events, such as the 1997–98 extreme event.Japanese eels traverse thousands of kilometers over several months to spawn around 12–15°N, south of the salinity front in the North Pacific (34.5 PSU)Comparable to the 1997–98 El Niño event, the recent extreme El Niño of 2015–16 has a similar strength Fig. , with maFigure 8 or southward movement of eel spawning grounds7. Chang et al.8 suggested that spatial variation of the NEC is an important determinant of the distribution and migration of eel larvae. A northward-shifted NEC is capable of covering almost the entire spawning ground (about 12–15°N) and transporting a large number of eel larvae into the Kuroshio, which may increase eel catches in East Asian countries. In contrast, eel larvae cannot be transported by the NEC when it migrates southward, resulting in smaller eel catches. Through comparison of the latitudinal shift of NEC locations between the 1997–98 and 2015–16 El Niños, we conclude that southward migration of the NEC is not solely responsible for differences in eel recruitment, as demonstrated previously8.Among physical factors, collective wisdom points to two factors most likely responsible for extremely poor Japanese eel recruitment in East Asia: southward migration of the NECThe variability of the NEC bifurcation latitude (NECBL) off the Philippines is a useful proxy for meridional migration of the NEC. NECBL variability was compared between the 1997–98 and 2015–16 El Niños Fig. . During 9. The location of the salinity front identified by the sea surface salinity (SSS) pattern is chiefly associated with the precipitation distribution in the equatorial Pacific Ocean. Previous studies have suggested that spawning grounds of Japanese eels are not confined to a fixed latitude, but are instead modulated by the location of the salinity front10. Figure Aside from migration of the NEC itself, meridional migration of the salinity front has been demonstrated to impact eel abundance in East Asian countries12, and each release was staggered at a 5-day interval. Approximately 1,350 v-larvae were released each year.We conducted two simulated tracer experiments with the same ocean currents to better understand the effect of a latitudinal shift in spawning locations on the recruitment success of the Japanese eel. These tracer experiments were based on a data-assimilation ocean model (JCOPE2). Experiment 1 (EX1) was carried out using the predetermined locations of eel spawning grounds in 1997, with the particles (v-larvae) released from locations spread throughout the region of 140 to 142°E along 12°N, as shown in Fig. The simulated particles (eel larvae) were carried by the NEC to the eastern Philippine coast, where most of the particles were then transported into the southward-flowing Mindanao Current (MC); only a few (7.8%) entered the northward-flowing Kuroshio  matched that of EX1, except it was carried out using the predetermined locations of eel spawning grounds in 2015. The v-larvae were released at locations spread through the region of 140 to 142°E along 16°N, as shown in Fig. 14. To further investigate SST distribution in the west Pacific warm pool, composites were created for the extreme El Niños of 1997–98 and 2015–16  and EP El Niño types, where the distinguishing characteristic of CP-El Niño affected the 2015–16 event and made it distinct from the 1997–98 event16. In 1997–98, the anomalous slope of the equatorial thermocline was set up rapidly and was proportional to the wind stress17, resulting in a deeper thermocline and warming in the eastern equatorial Pacific, along with a shallower thermocline and cooling in the western equatorial Pacific. On the other hand, the strong CP-El Niño dynamics in 2015–16 sustained large positive SST anomalies in the central Pacific16, and thus no intense cooling of the warm pool occurred during the 2015–16 event. Furthermore, the SST anomaly in the Pacific warm pool preceded the precipitation anomaly by 0–4 months7, and the precipitation distribution indicated that the location of the salinity front is an indication of possible spawning grounds. The distinct evolution and dynamics of the two types of El Niño  led to differing locations of eel spawning grounds, impacting eel abundance in East Asian countries.The distinct SST anomalies of the 1997–98 and 2015–16 El Niños indicate that the two strongest El Niño events differed greatly in the light of their underlying dynamics and associated climate influences. The 1997–98 extreme event is representative of the canonical Eastern-Pacific El Niño (EP-El Niño)The 2015–16 extreme El Niño did not result in extremely poor recruitment of Japanese eel in East Asia, as have other extreme El Niños. Rather than southward migration of the NEC, the present study demonstrated that southward movement of eel spawning grounds was mainly responsible for lower eel catches. During the eel spawning season (May–July) of 1997–98, Japanese eels spawned beyond the range of the NEC as the salinity front was shifted to a lower latitude (south of 12°N). The eel larvae drifted to the eastern coast of the Philippines around August-October when the NCEBL reached its northernmost position (~16°N). The larvae consequently were not capable of being successfully carried by ocean currents (the NEC and Kuroshio), which resulted in extraordinarily diminished eel catches in East Asia  during the period from 1995 to 2015 are gathered from the Japan Aquaculture Information News .Ocean surface velocity data are adopted from Satellite Oceanographic data . The AVISO data include sea surface height anomalies and surface geostrophic current velocities that are gridded on a horizontal region of 0.25 degree with an interval of one day.Salinity data are adopted from World Ocean Atlas 2013 version 2  and Japan Coastal Ocean Predictability Experiment 2 18. The WOA13 dataset is based on in-situ measurements in climatological fields on global grids (0.25° × 0.25°) and 102 vertical layers. The JCOPE2 model domain covers the western North Pacific Ocean from 10.5°N to 62°N latitude and 108°E to 180°E longitude with a resolution of 1/12° in horizontal and 46 sigma levels in vertical.The 1/4° daily SST data are the product of the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation Sea Surface Temperature (OISST) version 2 (http://www.ncdc.noaa.gov/oisst), which is distributed by NOAA’s National Climatic Data Center (NCDC).Precipitation data are adopted from the version 2.2 of the Global Precipitation Climatology Project (GPCP V2.2). The product combines in-situ measurements and satellite precipitation data into 2.5° × 2.5° global grids19. Monthly averages since 1979 are available at the Physical Sciences Division (PSD) of the NOAA’s Earth System Research Laboratory .20. In-situ measurements of temperature and salinity, together with satellite remote sensing data were assimilated into the JCOPE2 model18.The tracer experiments were executed to gain an understanding of the possible passive behavior of eel larvae, and based on a data-assimilation ocean model (known as JCOPE2). The JCOPE2 model domain covered the western North Pacific Ocean from 10.5°N to 62°N latitude and 108°E to 180°E longitude with a resolution of 1/12° in horizontal and 46 sigma levels in vertical. The formulation of the JCOPE2 followed that of the sigma-coordinate Princeton Ocean Model (POM)1 (about 140–142°E), the particles (tracers) were released at locations spread over the region of 140°E to 142°E, along either 12°N (1997) or 16°N (2015). Furthermore, Eel larvae likely exhibit diel vertical migration (DVM)21 behavior, remaining in the surface layer at night and swim to a deeper layer during the daytime. In this study, thus, the particles were set to stay at a fixed depth of 50 m during the nighttime, and a fixed depth of 150 m during the daytime. The releasing time was over a span of three months starting on May 1 to the end of July and each release was staggered at a 5-day interval.Since the eel spawning area was particularly located between 12°N and 15°N and along the western edge of the West Mariana RidgeSupplementary Info


The method is validated on crystalline GB1 and then applied to a >300 kDa precipitated complex of GB1 with full length human immunoglobulin G (IgG). The addition of a paramagnetic agent increased the signal to noise ratio per time unit by a factor of 5, which allowed full relaxation dispersion curves to be recorded on a sample containing less than 50 μg of labelled material in 5 and 10 days on 850 and 700 MHz spectrometers, respectively. We discover a similar exchange process across the β-sheet in GB1 in crystals and in complex with IgG. However, the slow motion observed for a number of residues in the α-helix of crystalline GB1 is not detected in the complex.Solid state NMR is a powerful method to obtain information on the structure and dynamics of protein complexes that, due to solubility and size limitations, cannot be achieved by other methods. Here, we present an approach that allows the quantification of microsecond conformational exchange in large protein complexes by using a paramagnetic agent to accelerate  Applying these methods to slowly tumbling proteins and protein complexes above a few tens of kDa in solution becomes increasingly difficult due to the enhanced T2 relaxation resulting in size dependent broadening of NMR lines. In solid-state NMR, however, line broadening is independent of the size of the system. This allows the study of biomolecules in assemblies of several hundred kDa and beyond, provided the intrinsic challenges of sensitivity and resolution can be addressed successfully11. In solids, just as in solution, chemical exchange processes can be followed by observing the effects of modulation of isotropic chemical shift. In addition, they can be monitored through the effects of modulating anisotropic interactions, e.g. modulation of dipolar couplings in near-rotary resonance relaxation dispersion experiments (NERRD)13. The combination of the two approaches can yield a detailed view of microsecond motions12. Previously, we have shown that we can access the protein backbone dynamics of GB1 in a >300 kDa asymmetric complex with full length immunoglobulin G (IgG) by measuring 15N R1 and R1ρ relaxation rates3. Proton detection at >50 kHz magic angle spinning (MAS) frequencies provided sufficient sensitivity enhancement to obtain data for a miniscule quantity of GB1 (~50 μg in a 1.3 mm rotor) in a reasonable timespan4. The strong spinning frequency dependence of 15N R1ρ14 suggested the presence of extensive slow μs motions of GB1 in the complex with IgG that are not present in GB1 crystals3. Since methods such as R1ρ relaxation dispersion15 rely on recording numerous 2D (or 3D) spectra to measure R1ρ relaxation rates at several different spin lock fields, they require unpractically long experimental times for large complexes . This forced us to resort to more approximate, qualitative methods to detect the presence of microsecond exchange. However, the microsecond exchange detected in this way could not account for the observed spinning frequency dependence of R1ρ and suggested the presence of an overall motion of GB1 in the complex3. To permit quantification of microsecond exchange through relaxation dispersion measurements on large protein complexes in more realistic time frames, we propose to add paramagnetic relaxation enhancement agents to the samples to accelerate 15N R1ρ relaxation dispersion measurement. The addition of a paramagnetic agent such as copper ethylenediaminetetraacetate (CuEDTA) or gadolinium diethylenetriaminepentaacetic acid bismethylamide (Gd(DTPA-BMA)), shortens 1H T1’s to allow fast recycling of the experiments. This effect is frequently used in solid state NMR to speed up acquisition for chemical shift assignments and structure calculations30.Protein dynamics play an important role in many biological processes such as enzymatic catalysis, ligand binding, or molecular recognition. Many motions implicated in these processes occur on microsecond or slower time scale and can be probed using chemical exchange based methods such as relaxation dispersion1H and 15N R2’s (R2 = 1/T2), its concentration needs to be adjusted to avoid excessive line broadening 15, is not affected by the paramagnetic relaxation enhancement (provided it does not induce paramagnetic shifts). Thus, in principle, quantitative relaxation dispersion can still be measured in the presence of paramagnetic agents. The only difference between a relaxation dispersion experiment in the presence and absence of paramagnetic dopants should be a different base/plateau relaxation rate . While this manuscript was in preparation, a similar approach was used to characterize exchange in crystalline SH332 but to the best of our knowledge, no extensive validation of it has been performed. Thus, to validate the proposed accelerated relaxation dispersion method, we first compared 15N R1ρ relaxation dispersion experiments with and without paramagnetic agent on crystalline GB1 . Subsequently, we applied the validated technique to obtain site specific information on microsecond conformational exchange of GB1 in the >300 kDa complex with IgG.Since the paramagnetic agent also affects 34 or even disrupt assembly of certain systems (T. Polenova – private communication). The absence of any chemical shift changes in either crystalline GB1 or precipitated GB1:IgG complex upon addition of Gd(DTPA-BMA) indicates that the dopant does not interact specifically with our samples  as paramagnetic dopant over the, in solid state NMR, more popular CuEDTA because Gd(DTPA-BMA) is a more inert probe. The overall negatively charged CuEDTA has in some instances been reported to bind specifically to proteins1H T1 and resolution was determined experimentally for each system. The overall broadening seems to be dependent on the specific assemblies (or rather the specific pattern of solvent accessibility). For example, 5 mM Gd(DTPA-BMA) induced levels of broadening acceptable for site specific analysis in the GB1:IgG complex but caused too much broadening in crystalline GB1 . The optimal concentration of paramagnetic compound that results in the best compromise between shortening of 35. Consequently, in order to obtain quantitative relaxation dispersion measurements, the experiments need to be performed under conditions where coherent effects are suppressed36. For 15N measurements, this is typically achieved by deuteration of the studied protein and partial re-protonation at exchangeable sites and application of fast, >40 kHz, spinning15. For larger concentrations of remaining protons in the sample, faster spinning frequencies need to be employed. Here, we apply 60 kHz MAS to 100% back-exchanged perdeuterated GB1 samples. As can be seen by the flat dispersion curves for residues not undergoing conformational exchange , the dipolar dephasing is sufficiently suppressed under these conditions. Generally, residues that show clear relaxation dispersion in experiments with Gd(DTPA-BMA), exhibit the same behavior in control experiments without paramagnetic dopant  and strands 2–4 of the β-sheet and the loops connecting them . For some residues, raised rates were observed near spin locks ~5 kHz (3.14 × 104 rad s−1), e.g. in D46  are obtained for the same kex values regardless of whether the α helix and β sheet regions are fitted individually or together. However, if the α region of GB1pre is fitted alone  of 0.9993 and 0.9440 for GB1dia  and a good correlation between φex values for most residues  to the two-site exchange Bloch-McConnell formalism  yielded an increase in SNR per time unit of about 5 in the complex 40. Extrapolated to room temperature, this exchange process would occur on the order of hundreds of nanoseconds, which is too fast to induce relaxation dispersion in the here considered regime. Indeed, in the crystalline sample of GB1, we do not observe relaxation dispersion for any residues in loop 1, with the exception of Leu12. This is consistent with extended model free analysis of 15N and 13C’ relaxation rates in crystalline GB1 at room temperature, which places the slow backbone motions in loop 1 in the range of hundreds of nanoseconds41. Thus, the available solution NMR relaxation dispersion data do not help to shed light on the potential origins of the microsecond motions detected in the solid state. The most likely explanation for the motions observed in the solid-state is that intermolecular interactions lead to increased energy barriers between subsets of interconverting conformations. Higher energy barriers would cause the concerned motions to be slower than in an isolated molecule in solution. The influence of intermolecular interactions on slow motions has been observed previously: in ubiquitin, the main slow conformational exchange process is present in solution and in crystals but appears much slower in the latter42. In the case of GB1, different intermolecular interactions seem to lead to a slowing down of a subset of motions that are too fast to be picked up by relaxation dispersion in solution. A combination of RDC measurements and molecular dynamics simulations of GB3 and GB1 in solution has suggested a presence of slow correlated motions across the β-sheet45. These motions are the most likely to be slowed down by intermolecular interactions to yield patterns observable through relaxation dispersion in the solid state. The characteristics of the motions in the β-sheet region in crystalline GB1 and the GB1:IgG complex are the most similar near the β2 strand. There, the pattern of intermolecular hydrogen bonds between two GB1 molecules in the crystal is very similar to the hydrogen bonds present between GB1 and the Fab fragment of IgG in the complex. The differences in the pattern of microsecond motions increase towards the β3 strand, where a set of intermolecular hydrogen bonds is present between GB1 molecules in the crystal but a different interaction interface is formed with the Fc fragment in the complex. The microsecond exchange observed for residues in the α-helix in crystalline GB1 and the apparent absence of similar motions in the complex could also be linked to different intermolecular interactions. In the crystals, the helix is packed more densely than in the complex with IgG47, which likely leads to increased energy barriers for helix motions in the crystal. Finally, we observe differences between φex values in crystalline GB1 and GB1 in complex with IgG. These parameters are linked to the populations of the exchanging states and the chemical shift difference between the states. In crystalline GB1 the φex values are between 25–140 × 103 rad2 s−2, with the lower values in the α-helix and higher values in β strands and loop regions. The φex values for GB1 in complex with IgG are significantly larger, 90–300 × 103 rad2 s−2, suggesting that either the population of the minor state or the chemical shift differences between the states (or both) are larger in the complex than in the crystals.Somewhat unexpectedly, the results of R1ρ14. Whereas previously we observed a very strong spinning frequency dependence of R1ρ rates in the GB1:IgG complex, most residues in crystalline GB1 showed, within the experimental error, weak or no spinning frequency dependence  with the time scale fixed to the inverse of the average of kex for GB1dia and GB1pre (1/(15 × 103 s−1) = 67 μs), the best-fit order parameters for the slow motions are between 0.998–0.983. To obtain such high order parameters, either the amplitude of the motion needs to be very small when the population of the minor state is relatively large or the amplitude of motion can be larger when the population of the minor state is small48. For example, in a two-site jump model with unequal populations, those order parameters correspond to a jump angle between 7–20° for a minor population of 5% and a jump angle of 3–9° for equal populations of the two states  and thus would not lead to relaxation dispersion in the regime considered here3. On a side note, the bulk 15N NERRD data obtained on crystalline GB1 at a lower spinning frequency (20 kHz) and lower sample temperature (293 K) was previously attributed to ultralow amplitude overall rocking of the molecule49. However, under conditions employed in ref.3 (40–60 kHz MAS and 300 K) such a motion would result in negligible spinning frequency dependence of 15N R1ρ  cells transformed with pGEV250 were grown in LB until they reached an OD600 > 1.0. The cells were washed once with PBS before resuspension in M9 with D2O, deuterated [U-13C]glucose and 15NH4Cl and expression was induced with 0.5 mM IPTG after 1 h incubation at 37 °C. The protein was expressed for 4 h at 37 °C before the cells were harvested by centrifugation (4000 × G for 20 min at 16 °C) and lysed by sonication . After heat treatment at 75 °C for 10 min, the lysate was cleared by centrifugation (12000 × G for 50 min). GB1 was precipitated over night with 80% ammonium sulfate and collected by centrifugation (15000 × G for 50 min). The pellet was resuspended in buffer , and purified on a 16/600 Sephadex pg75  gel filtration column. Fractions containing GB1 were collected, desalted, freeze-dried and stored at −20 °C.Isotope labelled GB1 was dissolved in buffer  to obtain a protein concentration of 10 mg/ml and crystallized with the aid of 2:1 2-methyl-2,4-pentanediol (MPD):Isopropanol51. The GB1:IgG complex was formed by mixing GB1 and IgG  solutions in a 2:1 molar ratio4. Crystalline GB1 and precipitated GB1:IgG complex were packed into NMR rotors using the following procedure: The crystals/precipitate were collected by centrifugation (1 min at 20 000 × G using a bench top centrifuge), and resuspended in a small volume of the supernatant containing 2% DSS and Gd(DTPA–BMA) at the desired concentration. The samples were transferred into 200 μl pipette tips, which were then attached to the rotors, put into 1.5 ml Eppendorf tubes and centrifuged (20 000 × G) in 1–4 minutes intervals until the rotors were full. The rotor caps were sealed with a silicone-based glue to prevent leakage.Freeze-dried [U-1H Larmor frequency with the GB1:IgG complex, 10% D2O was added to the sample buffer before packing the rotors and deuterium locking was used in the same way as in solution NMR. 15N-1H 2D correlation spectra were recorded using a proton-detected heteronuclear correlation sequence. Double quantum cross-polarization (CP) contact times were between 0.5–1.5 ms and individually optimized for each sample. Recycle delays between 0.2–2 s were used depending on the amount of paramagnetic agent and magnetic field  with the varying spin-lock power as the 4th dimension. In all solid-state experiments, hard pulses were applied at nutation frequencies of 100 kHz for 1H and 83.3 kHz for 15N. 10 kHz WALTZ-16 decoupling was applied on protons during 15N evolution, and on the 15N channel during direct 1H acquisition, while quadrature detection was achieved using the States-TPPI method. Suppression of the water signal was achieved by saturation with 50–200 ms of slpTPPM 1H decoupling applied at an amplitude of ¼ of the MAS frequency52 or 100–140 ms MISSISSIPPI53 at an amplitude of ½ the MAS frequency on resonance with the water signal. R1ρ relaxation curves were sampled using 7–10 points for all experiments. 1–25 kHz nutation frequencies, measured by nutation experiments, were used for the spin-lock fields in the R1ρ experiments , GB1dia at 700 MHz 1H Larmor frequency and 1.5 kHz spin-lock frequency , GB1pre at 700 MHz 1H Larmor frequency and 1 kHz spin-lock frequency  GB1pre at 700 MHz 1H Larmor frequency and 1.5 kHz spin-lock frequency , GB1pre at 600 MHz 1H Larmor frequency and 1.2 kHz spin-lock frequency , GB1 in complex with IgG at 850 MHz 1H Larmor frequency and 1.85 kHz spin-lock frequency , GB1 in complex with IgG at 850 MHz 1H Larmor frequency and 2.45 kHz spin-lock frequency , GB1 in complex with IgG at 700 MHz 1H Larmor frequency and 2 kHz spin-lock frequency . All spectra were processed using TopSpin 3.2. GB1 resonances in the complex with IgG were previously assigned on the basis of 3D H(H)NH, CONH, CO(CA)NH and CANH experiments4. Peak integrals were calculated in TopSpin 3.2. OriginPro 2016 and MatLab R2014a were used to analyze the relaxation data.Solid state NMR spectra were recorded at 600 MHz Bruker Avance II+, 700 MHz Bruker Avance III HD and 850 MHz Bruker Avance III spectrometers, using Bruker 1.3 mm triple resonance probes at 60 kHz magic angle spinning. A Bruker BCU-X cooling unit was used to regulate the internal sample temperature to 27 ± 2 °C . For experiments recorded at 700 MHz R1ρ values received from the fits for each residue were used as errors.Peak integrals from TopSpin were exported to MatLab where a mono-exponential function was used with the fminsearchbnd function to fit the relaxation data. Average values calculated from integral regions containing only noise were used as input errors. Fit errors were calculated by Monte Carlo error estimations. A random number between 0 and 1 was multiplied with the integral error and added to the recalculated integrals. The fitting was then repeated 2000 times with a new random number between 0 and 1 generated each time. Two times the standard deviations of the kex), populations of the minor state and major sites (pApB), and difference in chemical shifts between the two states (Δδ) were obtained by fitting the R1ρ values obtained at two different B0 fields simultaneously to a Bloch-McConnell two-site exchange Eq.  by Eq. R1 rates used are published elsewhere39 and the angle θ was calculated from Eq. Exchange coefficients  and 15N chemical shift anisotropy  was calculated for each model as:Supplementary Information


The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH‐dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE‐pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink‐casted LiMn 2O4 oxidizes by delithiationLiMn in LiOH at pH values below 14, as confirmed by X‐ray absorption spectroscopy. This chemical modification reduces the catalytic activity for oxygen evolution and induces a pH dependence on the reversible hydrogen electrode scale. The results indicate that vulnerability to transition‐metal redox reactions may lead to increased overpotentials, which is important for the design of stable electrocatalysts. We discussed the electrocatalytic mechanism of LiMn2O4 for the OER in the context of photosynthesisPreviously, we studied LiMn2O4 was first reported as a positive electrode material for non‐aqueous batteries by Thackeray et al.2O4 is its previous use as an active material for aqueous batteries,2O4 delithiation under standard conditions is above the thermodynamic potential in alkaline electrolytes, which can be illustrated by aligning the potential scale relative to the Li/Li+ redox  to the scale of the standard hydrogen electrode (SHE).2O4 in KOHHere, the battery aspects of the material are discussed further from the viewpoint of electrocatalysis. LiMn2O4 in LiOH electrolytes with pH between 12 and 14 using rotating ring disk electrodes (RRDE) and X‐ray absorption spectroscopy (XAS). Disks of LiMn2O4 show pronounced redox peaks below pH 14 in LiOH, which was not previously reported in other hydroxide electrolytes.2O4. A simple model for the reversible potential of delithiation is derived, which matches the data well. Furthermore, the onset of oxygen evolution is determined using the ring of the RRDE. Finally, we construct an ERHE‐pH diagram based on the model and our experimental data. Predictions regarding the oxidation stability are verified using additional ex situ XAS measurements.In this report, we investigate the oxygen evolution reaction on LiMn22O4 nanopowder for our investigations in LiOH as was also used in our previous studies2O4 (space group Fd3m)1Mn2O4.3+Mn4+)O4 but we will not make this distinction because XAS can only measure averages. Overall, these analyses demonstrate that the pristine powder has the expected size, bulk crystal structure, bulk composition and bulk valence.We used the same batch of commercially available LiMnth cycle, which was selected because initial Mn loss had ceased before the 5th cycle in NaOH.th cycle . Our previous studies showed no evidence of morphological changes of the LiMn2O4 nanoparticles in NaOH).2O4 preceding OER occurring only in 100 mM LiOH.The traces of the voltammogram in LiOH evidently differed from our previous studies in NaOH.2O4 in 10 mM LiOH as compared to our previous studies in 100 mM NaOH.We investigated the CV of 10 mM LiOH further as it showed a clear exponential rise in current density and emphasized the aforementioned differences to previous studies in NaOH2O4 3.5+ to Mn3.9+).We gained insight into the manganese redox using hard XAS at the Mn−K edge Figure d. In thi1‐xMn2O4 as a battery material in similar aqueous electrolytes.1‐xMn2O4, i. e., x is twice the absolute valence change. However, the lithiation of x ≈0.8 estimated from XAS after 1 h at 1.55 V We propose that the process associated with Mn oxidation is delithiation of the bulk. This proposal is supported by the use of Li+ cation into the electrolyte must be compensated by creation of an OH− anion (assuming water as the only anion source). In our hydrodynamic experiments, they are coupled because the bulk solution contains equal molarities of Li+ and OH−. The thermodynamic activity is 0.96 in 1.0 M OH and approaches 1 below pH 13.In our experiments, both the molarity of Li and the pH changed. Both are intimately coupled in stagnant aqueous solutions as releasing a LiThe pH in this report is calculated from the molarity of the prepared LiOH solutions and not measured using conventional glass electrodes as they are inaccurate in LiOH.We performed additional RRDE experiments in LiOH electrolytes prepared with pH values between pH 12 (10 mM) and pH 14 (1000 mM) that are shown in Figure − concentration of a factor 10. This shift is the Nernst slope ∂ERHE/∂pHi=conston the RHE scale. The solubility of O2 in hydroxides changes little with concentration in hydroxides,2O4, which is also supported by differences in the redox peaks. The most likely chemical modification is a change of the lithiation x in the bulk of Li1-xMn2O4as also supported by XAS.The simultaneously measured ring currents revealed that the onset and kinetics of oxygen evolution likewise depended on the pH Figure b. The ri+ will no longer be extracted from LiMn2O4Based on the hypothesis of changes in bulk lithiation, we can derive the expected reversible potentials as function of the pH. The derivation is based on earlier work by Li et al.2 forms at the (Pt) counter electrode in its standard state (denoted by superscript 0), i. e. 1 bar. Furthermore, water can be considered in its standard state for the used hydroxide concentrations.We assume that HThe positive and negative charges in the electrolyte must be balanced, i. e.+]≫[H+]), the proton concentration in Eq. (4) can be neglected. Assuming full dissociation and no interactions, the chemical potentials of the Li and OH in solution can now be obtained from the Nernst equations using the concentrations rather than activitiesSince we study basic solutions (i. e. [LiEq. (6) can also be rewritten using the definition of the pH in Eq. (1) as+]=[OH−], givesCombining Eq. (3) with Eq. (7) and using that [Li2O4) and anode (Li)The voltage in an intercalation battery is given by the difference between the cathode  becomeswhere 2O4, ELiMn2O40, varies to some extend and depends on the synthesis.ELiMn2O40,low=3.996V vs. Li/Li+ (i. e. the anode) for lithiation above x≈0.5 in Li1‐xMn2O4 and a second plateau ELiMn2O40,high=4.138Vvs. Li/Li+, which correspond to a one‐phase reaction (that of pristine Li1Mn2O4) and a two‐phase reaction.2O4 cathode is thusThe experimental standard potential of LiMnELiMn2O40,lowand ELiMn2O40,high. As the duration of delithiation is rather short during a CV at 10 mV/s, we expect that ELiMn2O40,lowis the relevant potential.The values of the pH‐independent term are −0.111 V and −0.253 V for 6 PC/EC electrolyte and 1 M LiOH electrolyte are nearly identical. The half‐cell potential can thus be expressed against the RHE without considering further reactions. Finally, LiMn2O4 delithiation occurs on the anode in our application and the hydrogen redox on the cathode and so the polarity of the cell must thus be opposite as derived aboveThis half‐cell voltage is expressed relative to the reversible hydrogen electrode (RHE), as we considered both the standard potential and the pH dependence of the solution in Eq. (7), which is the very definition of the RHE scale. Moreover, Li et al.2O4 will delithiate in electrolytes with equal molarities of Li+ and OH−; delithiation occurs for all applied voltages Eappl>ELiMn2O4RHE.This equation can now be used to predict whether LiMnRHE‐pH diagram  diagram, that is also used in previous work of aqueous battery work,2/OH− equilibrium, on the working electrode (black line) is a horizontal line in the ERHE‐pH diagram and the experimental overpotential is also a horizontal line if the Nernst slope vanishes, i. e. ∂ERHE/∂pHi=const=0, which is the expected pH dependence based on common mechanisms with proton‐coupled electron transfers.RHE‐pH diagram is often preferable in electrocatalysis as it allows to read overpotentials with respect to the OER more directly from the plot. Even minor changes of the overpotential are clearly visible in this diagram, whereas they are difficult to read from the classical ESHE‐pH diagram.We used Eq. (12) and the reversible potential of the OER (1.23 V vs. RHE) to construct an Em Figure a. It is RHE‐pH diagram to compare to the predicted thermodynamic lines  is often attributed to the so‐called scaling relationsThe kinetic data from the CVs in Figs. 3 and S2 was overlaid onto the Es Figure a. The ov2O4 was also determined experimentally. For this, we determined the peak potentials as the zero‐crossings of the second derivative of the measured data . The analysis was performed separately for the anodic and cathodic shoulder/peaks and their average potentials are shown in Figure The reversible potential of delithiation of LiMnRHE‐pH diagram can be used to predict whether LiMn2O4 will retain its bulk valence of +3.5 during electrolysis in LiOH electrolytes, which is important for the stability and activity of the electrocatalyst. The fit line to the experimental oxygen evolution data of the ring electrode intersects that of the (de/–)lithiation reaction (Eq. 12) slightly below pH 13.5. Had we read the voltages at a lower ring current, then the intersection would occur at a slightly lower pH value. This intersection is highly significant because oxygen evolution (blue symbols and line) should be performed at potentials where Li cannot be extracted from LiMn2O4 (red symbols and lines). Otherwise, the Mn on the surface and in the bulk of LiMn2O4 oxidize. The ERHE‐pH diagram suggests that measurements at pH 12 in LiOH lead to significant Mn oxidation (as discussed above), while those at pH 14 should retain their valence.Our E2O4 electrodes had to be made in fluorescence yield (FY) mode due to the comparably low loading required by the electrocatalytic measurements. The FY mode only approximates an X‐ray absorption spectrumRHE‐pH diagram. In particular, the LiMn2O4 sample measured at pH 14 retained its bulk valence.The expected bulk valences were again investigated by hard XAS at the Mn−K edge Figure b. A densg occupancy has been proposed to correlate with activityg orbital points towards the absorbing oxygen, which is a key step in the mechanism.2O4 and several closely related spinels, the (bulk) eg occupancy of the octahedral site has been proposed to correlate with the overpotential.2g orbitals and the eg orbital fills up (with spin up) electrons from 0 (Mn4+) to 1 (Mn3+) to 2 (Mn2+).g occupancy, Mn3+ (eg occupancy of 1) is desirable in an electrocatalyst for the OER. However, a single parameter does not fully describe the catalytic properties of a material as complex as an oxide.3+ and Mn4+ result in the highest activity, i. e. lowest overpotential, in many reports.3.5+ valence of the pristine LiMn2O4 is crucial for sustained electrolysis with unchanged activity.The Mn valence has been correlated with the activity for the OER previously.3+ have higher activity than those with lower or higher manganese valence.2O4 above average valences of +3.5.The observed increase of the OER overpotential from pH 14 to pH 12 in LiOH can now be explained in the context of changed manganese valence. The available literature data suggests that oxides with Mn valences slightly above Mn∂ERHE/∂pH). The same pH dependence was reported for materials that (de/–)intercalate oxygen.SHE‐pH (Pourbaix) diagram predicts valence changes for many used oxides, e. g. simple manganese oxidesIt is often assumed, especially in theoretical work,32O4 as an electrocatalyst for the OER in LiOH electrolytes with pH between 12 and 14. We found pronounced redox peaks in LiOH electrolytes with pH≤13.5 that were not observed at pH 14 and in all previous reports where NaOH and KOH electrolytes were used.2O4 oxidized at pH 12, which we used to assigned the observed peaks and shoulders to the Mn3.5+‐Mn3.5+δ redox. As the XAS measurement was bulk sensitive, we argued that the most likely process of charge compensation was delithiation of the bulk. The current due to oxygen evolution could not be determined from the total disk currents due to interference of these Mn redox peaks. Yet, the ring of the used RRDE setup qualitatively measured the onset of oxygen evolution, which occurred at voltages higher than that of the Mn redox. The onset of the OER was pH‐dependent on the RHE scale with a Nernst slope of −40 mV/pH at −5 μA  ring current. We derived a simple model for the expected reversible potentials of delithiation for the used electrolytes with equal molarities of Li+ and OH−. The calculation of the expected delithiation potentials needs the standard potential of LiMn2O4 delithiation, which we determined experimentally in a common battery electrolyte as 3.996 V vs. Li/Li+ below x=0.4 in Li1‐xMn2O4. The predicted Nernst slope of delithiation of 118 mV/pH was identical to the experimental Nernst slope of 116(25) mV/pH within error. The model and experimental data of both delithiation and oxygen evolution were used to construct an ERHE‐pH diagram. The lines given by the Nernst slopes of the onset of the OER and that of delithiation intersect near pH 13.5. The ERHE‐pH diagram illustrates that delitihation occurs at voltages below that of the onset of OER at pH below about 13.5 while the onset of OER has a lower onset at higher pH. This is significant because the average bulk valance of Mn3.5+ will only be retained at pH above about pH 13.5, which we verified experimentally using XAS on a LiMn2O4 electrode operated at pH 14. Overall, the model and experimental data strongly support bulk delithiation of LiMn2O4 below a pH of about 13.5. We discussed Mn oxidation due to delithiation in the context of the eg orbital descriptor, where oxidation above Mn3.5+ should increase the overpotential for the OER. Therefore, we concluded that bulk delithiation and the concomitant oxidation of Mn3.5+ to Mn3.5+δ increased the overpotential of the OER and were the origin of the pH dependence on the RHE scale. The ERHE‐pH diagram in our work provides an intuitive graphical tool to gauge the stability of electrocatalyst against redox changes when they are not measured under standard conditions, i. e. 1 M LiOH. While it is most straightforwardly extended to predict the pH dependence of other electrocatalysts that are also common battery materials, e. g., LiCo1‐xMxO2 and LiCoPO4,2/3+ redox couple differs from that of the Co3/4+ redox couple. Therefore, pH dependence on the RHE scale (i. e. non‐Nernstian behavior) should be expected when the electrocatalyst is not stable against redox changes in the investigated electrolyte.We investigated the effect of bulk oxidation on the overpotential of LiMn2O4 (>99 %) catalyst powder was purchased from Sigma‐Aldrich. Tetrahydrofuran (THF) was purchased from VWR (≥99.9 % stabilized). For the electrolyte Lithium hydroxide powder (99 %) purchased from Merck was dissolved in ultrapure water (Milli‐Q R≥18.2 MΩ). Argon (5.0) to purge the electrolyte was purchased from AirLiquide Alphagaz. Acetylene carbon black was purchased from Alfa Aesar and was acid‐treated.LiMnα radiation in a two theta range of 15°–85° in 0.05° steps. For this, the LiMn2O4 powder was glued using rubber cement  on a microscope slide made of glass. The particle distribution was determined by Nova Nano SEM 650 in high vacuum mode at 15 kV. Both results were in good agreement with our previous publicationsThe pristine powder was also characterized by a XRD Bruker D8 Discovery with monochromatized Cu‐K2) and a concentric platinum ring electrode with 5 mm inner and 7 mm outer diameter separated by a Teflon spacer. Both working electrodes were separately polished to a mirror finish with Al2O3‐polish on separate polishing pads and cleaned afterwards with isopropanol. After this cleaning procedure the RRDE was assembled from both parts. This procedure reduces a possible contamination of the electrodes. A RHE  was used to calibrate the SCE to RHE scale.For our electrochemical experiments we were using an OrigaFlex (OrigaLys SAS) system of three OGF500 potentiostats in bipotentiostat configuration. The RRDE‐setup was composed of an RRDE‐3 A rotator (ALS Japan Co Ltd.) and a custom‐made cylindrical PTFE cell that was used in a three‐electrode configuration, consisting of a saturated calomel electrode  and a platinum counter electrode, which were arrange radially around the working electrodes. The distance between the RRDE‐electrode and counter and reference electrode was 17 mm. We used a RRDE‐electrode made by ALS Japan Co Ltd containing a removable glassy carbon electrode 4 mm in diameter area 0.126 cm cm2 and 2O4 (83 % of solid part) and carbon black (17 % of solid part) in THF. The Teflon spacer of the RRDE assembly prevents the ink from contacting the ring electrode. If it happened by accident, the electrode was discarded. The electrolyte was saturated with argon gas. For RRDE‐measurements, we used a rotation speed of 1600 rpm. The detection potential for the detection of oxygen was determined in our previous work.For catalytic experiments we realized a loading of 50 μg active material on a glassy carbon electrode by drop coating. For this, we applied 10 μL of an ink containing of LiMn2O4 using BN. After these powders have been homogenized by a mortar and pestle, a pellet of 10 mm in diameter was pressed at 20 bar. This pellet was transferred to the beamline.For XAS measurements the catalytic ink was applied on a graphite foil and a CA at 1.55 V vs. RHE was performed for one hour in a LiOH electrolyte with pH 12 or pH 14. Afterwards the electrode was washed off with Milli‐Q water, dried and transferred to the beamline. To characterize the pristine powder by XAS, we diluted the powder to 1‐wt % of Mn in LiMnXAS measurements of post mortem samples were performed in fluorescence mode (detector: Bruker X‐Flash 6|60) at the KMC‐2 beamline of the BESSY II synchrotron in Berlin. The beamline energy resolution is 1/4000. The acquisition time for one EXAFS scan takes around 90 min. The used beam size was around 2 mm×4 mm (hor. x vert.). The first inflection point in the XANES of a manganese foil was used for the energy calibration by setting it to 6539 eV. XAS measurements of the pristine powder were performed in transmission mode at CLÆSS beamline of ALBA synchrotron in Barcelona. The used beam size was around 1 mm×1 mm. We also used a manganese foil for energy calibration. All XANES spectra were normalized by subtraction of a straight line before the edge and division of a polynomial after the edge.The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


Czarnecka K, Girek M, Maciejewska K, et al. New cyclopentaquinoline hybrids with multifunctional capacities for the treatment of Alzheimer’s disease. J Enzyme Inhib Med Chem 2018;33:158-170.“Kinetic characterisation of AChE inhibition” and “Kinetic characterisation of BuChE inhibition” on page 161 have to be replaced by the following paragraph.In the “Materials and methods” section the two paragraphs Characterisation of AChE and BuChE inhibition3b against AChE and BuChE was further characterized in kinetic measurements using the Ellman’s method,3b, and with BuChE (4 U/ml in assay) at inhibitor concentrations of 0.2 µM, 1.1 µM and 2.2 µM. The reciprocal activities were plotted as function of the reciprocal substrate concentrations at the indicated inhibitor concentrations in form of Lineweaver-Burk plots.The inhibitory potency of compound Tables 2 (page 164) and 3 (page 165) in the “Results and discussion” section have to deleted.Furthermore, The mechanism of EeAChE inhibition …) and page 164  has to be replaced by the following paragraph:The text between page 163  its potency was further investigated. The obtained Lineweaver–Burk plots for the inhibition of AChE  and PAS , which leads to a mixed-type of inhibitionDue to the relatively strong inhibitory potency of compound  of AChE  and BuCh of AChE  by compo