Here we seek to re-characterize the activity, selectivity,and stability of the five most utilized transition metal catalystsfor CO2RR  at elevated reactionrates through electrochemical operation, physical characterization,and varied operating parameters to provide a renewed resource andpoint of comparison. As a basis, we have employed a common cell architecture,highly controlled catalyst layer morphologies and thicknesses, andfixed current densities. Through a dataset of 88 separate experiments,we provide comparisons between CO-producing catalysts , highlighting CO-limiting current densities on Au and Pd at 72and 50 mA cm–2, respectively. We further show theinstability of Sn in highly alkaline environments, and the convergenceof product selectivity at elevated current densities for a Cu catalystin neutral and alkaline media. Lastly, we reflect upon the use andlimits of reaction rates as a baseline metric by comparing catalyticselectivity at 10 versus 200 mA cm–2. We hope thecollective work provides a resource for researchers setting up CO2RR experiments for the first time.Continued advancementsin the electrochemical reduction of CO Renewableelectricity generated from solar or wind and sustainable feedstockssuch as air and water is needed to replace fossil fuels and reducegreenhouse gas emissions in the production of important chemicalsand fuels. One promising approach can directly utilize atmosphericCO2 reduction reaction (CO2RR) has receivedincreasing attention due to its potentialto supplant fossil fuels in the production of base chemicals and fuels.The field has built upon pivotal work in the 1990s and 2000s by Hori,which categorized the activity of transition metal catalysts for CO2RR under well-controlled conditions at a current density of5 mA cm–2.1 These studiesprovided a solid foundation for exploratory catalyst development intoeach metal, giving the research field a fixed current density pointof comparison. For CO2RR to be both economically feasibleand environmentally impactful, however, significant progress is nowneeded to make the process efficient and stable at scale. In particular,large-scale facilities (> MW), high current densities (>100mA cm–2), and long-term stability (>1000 h) withhigh energyefficiency and single pass conversion efficiency are needed to achievethese goals, while retaining near-uniform selectivity to reduce downstreamseparation processes.2 The necessity forprocess intensification in particular has now led to the rapid increasein current densities to the realm of 100–1000 mA cm–2, which significantly affects the local reaction environment,3 system design,4 catalyticbehavior,5 and overall stability.6 The original controlled experiments characterizingmaterials at 5 mA cm–2 did not experience theseconsequences of process intensification, motivating the need for anupdated reference of base performance of transition metal catalyststhat reflect practical industrial conditions.In the past decade,the CO–2)by having the catalyst supported on a microporous substrate at a gas–liquidinterface.11 As the CO2RR community begins to use such electrode architecturesthat allow concentrated gas-phase CO2 to be fed close tothe cathode, greater emphasis has been placed on understanding theinterconnected factors, which govern the electrocatalytic performanceas the scale and intensity of the system increases. While the electrodepotential is ultimately the driving force that allows surface reactionsto occur, the reaction environment is heavily influenced by currentdensity and mass transport. For example, recent studies on catalystsdeposited on GDEs have shown that an increase in current density13 and the use of different electrolytes17 have effects on product selectivity by varying the local reactionenvironment. The importance of current density dependent effects suchas mass transport and homogeneous reactions is also observed in bicarbonate(KHCO3) electrolysis systems, where bicarbonate plays adual role as a proton and CO2 source. A study on the directconversion of a bicarbonate electrolyte (KHCO3) to CO forexample showed that CO production was largely retained on a GDE whilefeeding nitrogen gas instead of CO2.18 These examples highlight the importance of the catalyst’ssurrounding reactor configuration on the measured performance anduse fixed current densities to support previous work performed atfixed cathode potentials.The use of gasdiffusion electrodes (GDEs) has shown the abilityto achieve high current densities . Furthermore,few works present the data for multiple materials within the samepaper as was previously done by Hori et al. at 5 mA cm2 reductionperformance of Au, Ag, Pd, Sn, and Cu catalysts deposited on GDEsat fixed current densities within a representative reactor configuration(3 and KOH) and at four applied current densities . Constant current densities were choseninstead of constant potentials in order to control the overall catalyticconversion rate, thus keeping the total diffusion and migration ofthe involved species comparable in each case. The dataset then supplementswork using fixed cathodic voltages. During electrolysis at the appliedcurrent densities, the products were collected for analysis and thenegative voltage was measured with respect to a Ag/AgCl referenceelectrode. The combined work provides a wide dataset for comparisonwith the literature, highlighting features of each of the metals whichcannot be elicited from low current density experiments alone.Inthis work, we compare the baseline COguration1 over a 27 and such a system acts as a direct comparative baselinefor research assessing changes in the type of gas diffusion layer,catalyst morphology, catalyst loading, electrolyte type, electrolyteconcentration, and operating conditions . The chosen operating conditions for our comparisonsspan a range of current densities  for the two most commonly used electrolytes (1 M KHCO3 and 1 M KOH), thus encompassing common testing conditions in theliterature.For characterizing the electrochemicalperformance of the fivetransition metals, we have chosen to use a fixed cell architectureand catalyst morphology, which represents a recognizable baselinefor the field. This entails a nanoparticle-based catalyst layer witha nominal catalyst thickness of 100 nm deposited onto a carbon-basedgas diffusion layer with a flowing catholyte configuration 1a. Such Supporting Information (SI).While the configuration and operating conditionsthat are chosenfor the dataset are important, we must make sure that their implementationis extremely well controlled to ensure both a high level of repeatabilityof the experimental data, as well as reproducibility of the resultsby external users. Without providing such regulation and documentation,the baseline cannot function as well as intended. Here, we providelarge control over both the utilized catalyst, and the testing infrastructureas described below and in detail in the SI A). The deposition thickness of the unit was confirmed throughprofilometry for each individual material. The as-deposited samplesthen resulted in a nanoparticle layer on the top of the GDL, whichwas similar for each base material as confirmed though scanning electronmicroscopy (SEM) , high resolution SEM imaging , and atomic force microscopy (AFM) . The five materials are visualized in Figure S21, exhibiting a similar porous structure. Becauseof the roughness of the GDE and the catalyst layer porosity, the thicknessis greater than the deposited 100 nm. The elemental composition ofthe catalyst surface was examined ex situ by X-ray photoelectron spectroscopy(XPS)  before and after electrolysis toidentify the surface species present on the electrolyte side of thecatalyst layer. Since XPS is performed ex situ, a measure of oxidationfrom air is expected for surface species for all samples. SEM andXPS analyses allow for the stability of the catalyst layer to be examinedfrom a morphology and contaminant perspective. In order to minimizethe influence of residual electrolyte species on the ex situ SEM andXPS results, a rinsing protocol with DI water and drying was included(see SI A). All catalysts were depositedhomogeneously on a 4.4 cm2 square electrode area, witha geometric active electrode area of 2.25 cm2 exposed tothe electrolyte while placed in the assembled electrochemical cell.Lastly, a new sample was used for each electrochemical experiment.To create a repeatable nanoparticle-basedcatalyst we depositedour five metal transition catalysts  ontoa Sigracet 39BB gas diffusion layer (GDL) using magnetron sputtering to deposit ∼100 nm thick metal catalystsmeasurements. Here, we demonstrate a system, which incorporates back-pressureregulation (to prevent gas/liquid crossover) and mass flow meters to maintainthe gas–liquid environment as consistently as possible duringexperiments. GC measurements every 5 min lead to some gas pressureincrease and gas escaping through the liquid phase, but only afterinjection of the product gas stream. To improve the confidence inthe presented results, duplicates of each experiment were performed.While control over the catalyst deposition and morphology is ofcritical importance, so too is the robustness of operating the electrochemicalsystem itself. Operating GDE systems for COFigure S57–S60). Theelectrochemical setup consists of external liquid bottles containing80 mL each of the respective anolyte and catholyte connected to aperistaltic pump to recirculate the catholyte and anolyte chambersat 10 mL min–1. It is important to note that therecirculation of the electrolyte could induce transient pH effectsdue to a combination of continuous acidification by CO2,g reacting with hydroxyls and the production of hydroxyls at the cathode.In general, a KHCO3 bulk pH shift from 7.8 to 8.5–8.8 was measured for KHCO3. For KOH, the dissolution of CO2 was a more significantfactor reducing bulk pH from 13.8 to 13.0–12.8. CO2 was provided using a pure CO2 bottle and regulated bya mass flow controller to feed the cathode gas compartment at 30 mLmin–1. The electrochemical measurements were performedutilizing a ParSTAT MC potentiostat (Ametek SI) to perform 1-hourchronopotentiometry on each sample. The electrochemical cell includesa Ag/AgCl reference electrode, positioned in the catholyte chamberto measure cathodic potential. A liquid trap at the gas outlet ofthe cell is used to protect the GC in case of flooding. All outletsare connected to a back-pressure regulator and enable the balancingof gas and liquid pressures at 1220 mbar, hereby promoting gas/liquidseparation. The quantity of gas entering the GC was measured againusing a mass flow meter (Bronkhorst EL-FLOW Select), since the conversionand dissolution of CO2 can lead to great disparity betweenthe in- and outflow. The products of electrochemical CO2 reduction over 1 h were measured using online GC .Electrochemical experiments were performed in a three-compartmentGDE system as shown in 4, C2H4, and H2) were measured by online GC, postexperimentalanalysis of the accumulated liquid products in the catholyte  were performed using an Infinity 1260 II HPLC(Agilent Technologies). A Nafion-212 membrane was deployed to preventanionic products from crossing over to the anolyte. As validation,anolyte samples were taken from experiments in which large quantitiesof formate were expected to be formed. During analysis of the anolyte,product signals were less than 0.5% of the catholyte signal and werethus discarded. Data of product analysis and the electrochemical experimentswere combined to show the faradaic efficiency (FE) and partial currentdensity of the products as a function of applied current density,providing two different perspectives of the same data.While gas products .The electrodepotentials versus a Ag/AgCl reference were also recordedduring experiments and converted to the RHE, but were not iR-corrected.In the system configuration, a large ohmic drop exists, which reducesthe accuracy of the iR-correction, particularly because of changesin the electrolyte conductivity with current density,SI A. Protocol. The following sections thenprovide the detailed structural properties and electrochemical performanceof the five metal catalysts  deposited onGDEs over the examined operating range.More detailed informationon the fabrication of electrodes, measurementequipment, and followed procedures can be found in SI B. Characterization Data, categorized by characterization technique, foruse in further studies and comparisons.Here,we present the material and electrochemical characterizationfor the five most commonly investigated transition metal catalysts for CO2R. For each of the five metals andtwo electrolytes and duplicates of four current densities were tested.In some cases, extra experiments were added to extend observed trends(for Au/Pd) and where stability issues were observed (for Sn). Inthis work, 88 GDE samples were then fabricated and characterized withchronopotentiometry, product analysis, SEM imaging, XPS, and withthat a substantial dataset was obtained. For the sake of brevity,only the most relevant data are presented in the following sections,with the most critical findings given greater emphasis. All obtaineddata are available in While all electrochemicalexperiments were run for 1 h, the datapresented here use the selectivity versus current density after thefirst 20 min of operation, averaged over the duplicate experiments.This time was chosen as it simultaneously allowed for the stabilizationof product curves from the GC, and does not conflate catalyst stabilityover time with the selectivity of the original catalyst and configuration. The stability of the catalyst overthe full-length of experiments is, however, discussed.2 to CO and has previouslybeen studied in H-cells34 as well as in GDE architectures.16 The selectivity of Ag to produce CO from CO2 is largelydue to the weak binding energy that CO has with Ag surfaces, thoughthere are minor differences with facet/site composition and coordination.A recent study found that 20–30% of the selectivity of Ag towardCO can be tuned toward formate (HCOO–) by increasingthe pressure and electrolyte alkalinity without affecting the catalyststability.36 When this work was comparedto other Ag-GDE studies, it showed that CO/HCOO– selectivities and energy efficiencies at equal current densitieswere nonuniform across separate studies, implying the presence ofunique parameters for each configuration.Silver (Ag) is a promising electrocatalyst forthe selective conversion of CO– formation increasing at higher current densitieshas been previously reported to be an effect of high local pH, whichfavors HCOO– formation at the expense of CO.37 Despite the high selectivity of these electrodes,the surface morphology exhibited significant changes in both electrolytesafter 1 h of electrolysis. SEM images (SI B: HR-SEM). Although the selectivity was minimallyaffected during the 1 h experiments, continuation of electrolysisunder these conditions will likely lead to large salt crystals formingon the surface, eventually blocking gas flow and/or rupturing thesubstrate. A recent paper on the surface coverage and the effectsof electrolyte concentration on K-salt growth has shown similar featureswhich are directly attributed to be potassium carbonate (K2CO3) and resulted in a rapid decrease of selectivity toCO after 50% of the surface was covered.38In our work, for alltested current densities, Ag shows >80–90% selectivity towardCO with a gradual shift toward formate as the current density increased3a,b. TheM images 3c–e show2 caused the local reaction conditions to becomemore alkaline over time, promoting the production of formate.From the performed experiments,it can be concluded that Ag isan effective CO producing electrocatalyst with high selectivity andlow overpotential compared to other catalyst materials presented here.Such a result is not unexpected given silver’s prevalence inGDE-based CO2R. Overall, the selectivity for Ag to CO was retainedover the evaluated current density range. Increasing the current densityto 200 mA/cm2RR and high selectivitytoward CO.42 Although Au has shown the ability to lower the initial energy barrierin the CO2RR, increasing current densities above the H-cell regimeshow a continuous loss of selectivity toward CO while H2 evolution is promoted. Subsequently, gold is comparatively un-utilizedin GDE configurations compared to H-cell systems. In examples wheregold has been used in GDEs, low partial current densities toward COare observed before the hydrogen evolution reaction begins to dominate.43Gold (Au) has historically been shown to be thebest performing CO reduction catalyst in aqueous based H-cells dueto its low onset potential for the COjCO = 72 mA cm–2. In experiments conducted in KHCO3, COalso begins to plateau in the tested range. Comparing material characterizationbefore and after the reaction, XPS scans  show no changes in Au peak intensity, and SEMimages show no mesoscopic changes to the surface. However, post-experimentalXPS results do show peaks for potassium (K 2p) and oxygen (O 1s),due to the formation of (bi)carbonate on the catalyst surface similarin nature to the peaks observed for the Ag catalyst, but in lowerquantities.Within our experimental dataset, we observea similar limitation from the gold catalysts. In particular, the 1M KOH experiments depict a clear downward trend in CO selectivitywith increasing current density which occurs earlier than the masstransport limited currents achievable. Plotting the same data as apartial current density instead 4c, it ca–2 in 1 M KOH. To assess whether the limitation wasdue to surface site availability, we doubled the sample thicknessto 200 nm nominal thickness and tested over the same range of currentdensities. At this thickness, the entire catalyst layer should stillhave ample access to CO2. However, these 200 nm samplesshowed near identical results to the thinner 100 nm samples  show a wide variety of drasticmorphological surface changes.Palladium (Pd) has beenstudied as a singlecrystal electrode for CO51 Here, at theelevated operating potentials, it is, however, unclear if this islimiting its performance. From the results here, the overall highlevel of H2 formation and the relatively large requiredoverpotentials make pure Pd nanoparticles an inadequate catalyst forlarge-scale utilization in its present form. Alternatively, Pd mightfind its use as a bimetallic co-catalyst, as past studies have shownit to be an interesting metal to tune dimerization to multicarbonproducts53 due to its strong binding with CO.Unlike Au, previous literatureperformed at lower current densitiessuggests that Pd experiences a plateau current density for CO dueto surface poisoning by CO at lower overpotentials.59 Finding a highly selective formate (HCOO–) catalystcan be helpful for the development and implementation of CO2 reduction technologies. Alongside CO, formate is another chemicalbuilding block that can be used as a reactant in further downstreamsynthesis, but can also be used as a renewable feedstock in biosynthesistoward fine chemicals.54 Sn does sufferfrom poor stability, leading some researchers to investigate alloyingand adding ionomers and binders to protect the Sn catalyst.63Tin (Sn) is a catalyst studied for its highly selectiveformation of formate.–2, the system lost selectivity toward carbon containing products,reflected by the increase in hydrogen evolution over the durationof the experiment. An explanation for this is provided by observingthe XPS spectra, where a scan of the Sn 3d peaks shows a significantdecrease of Sn after the KHCO3 experiment and near-completedisappearance after 1 h operation in KOH, indicating the loss of Snduring electrolysis due to dissolution in the highly alkaline conditions,as described by the Sn Pourbaix diagram. The O 1 s peaks also showdecreased signal, following the trend of Sn 3d. Potassium uptake isrelatively low for these samples, as is displayed by less prominentK 2p peaks . Crystals (different from the earlier seen (bi)carbonate)were also found in KOH experiments. These crystals were likely formedby a combination of the dissolution of Sn in the highly alkaline environment,while the applied potential caused localized redeposition in a morestable agglomerated form.Here, GDEs deposited with Sn show high selectivity towardformate throughout all experiments across the entire applied currentrange. At an applied current density of 200 mA cm– throughout many years of priorresearch, and this trend is confirmed here. However, the lack of stabilityat elevated current densities of a sputter deposited Sn catalyst showedthat it is vital to find techniques to stabilize the catalyst andprevent the Sn dissolution through the use of nanoparticles, bindingagents, co-catalysts, or ionomers in order to ensure long-term stability.Overall Sn has shown to be an effective catalyst for theselectiveproduction of HCOO2 reduction researchers due to its unique abilityto convert CO2 into at least 16 different products.64 Numerous studies focused on improving the activityand selectivity of Cu through morphological enhancements,66 facet-dependent behavior,68 and local environmentcontrol.70 Some of the mechanistic pathways behindthe formation of various products are still debated,72 but it has become clear that the specific binding strength of Cuto CO allows for the dimerization of adsorbed CO* and CHO* species,resulting in multicarbon (C2+) product formation. In GDEexperiments, Cu and Cu-alloys have shown promising selective behaviortoward prominently ethylene at elevated current densities.73Copper (Cu) has received significant attentionby CO–2, the Cu GDEs produce a mixture of H2,CO, and formate at low overpotentials. At an applied current densityof 50 mA cm–2, methane, ethylene, and ethanol aredetected as well. Further increasing the current density shows a shiftin the product distribution toward ethylene while CO production plateaus.Comparing XPS results before and after experiments show that, besidesa slightly higher degree of oxidation and the presence of potassiumin the case of KOH , the compositionremains consistent. In contrast, SEM imaging does show significantrestructuring of the surface in most experiments. The 200 mA cm–2 case shows that the specific conditions and appliedpotential resulted in the formation of Cu nanoneedles and cubes. Thepost-electrolysis HR-SEM image of KHCO3 (S37) shows that the Cu catalyst has restructured under theapplied potential, favoring cubic shapes.In our work, sputter-deposited Cu GDEsshow highly varied product selectivities with changing current densities,as reported elsewhere. At an applied current density of 10 mA cm2 reduction products over a broadcurrent density range, while Au, Sn, and Pd tended toward H2 as a primary product as current densities were increased. Cu maintainedits total CO2 reduction selectivity, with product distributionsgrowing at higher reaction rates. These results highlight how lowversus high current density testing conditions change the observedproduct selectivities through variations in the local reaction environment,changes to catalyst stability, and the increased applied potentialswhich influence the relative activity of each product at differentcurrent densities.Throughthe presented experiments, we were able to observe trendsfor the different catalysts as a function of applied current density.Of the five assessed transition metals, only silver maintained itsselectivity toward CO2 on Ag remains low up to 200 mA cm–2, allowing for high CO selectivities to be maintained. Converselyon Au, hydrogen formation increases with current density while COplateaus. A detailed study into the intrinsic limit of convertingCO2 could help determine which of these metals can effectivelybe used for industrial purposes, and why gold is a less favorableCO2R catalyst at higher potentials and current densities.One observation that needs to be highlightedis the limiting current density of Au and Pd toward CO, whereas Agdid not exhibit such a limit under the same conditions. Here the productionof H2 to formate.We found that Sn experiments in KHCO3 lead to mild catalystrestructuring, while in KOH structural instabilities damaged the catalystsurface irreversibly. During the 1 h electrolysis the effects of restructuringwere not clearly expressed through product distributions yet, butit became apparent that increasing current densities led to enhancedsurface reformation and more frequent flooding issues. These trendshighlight the necessity of applied current density and electrolytecomposition when comparing or benchmarking the electrochemical performanceof catalysts on GDEs.For Sn the effect of electrolyte composition was more impactfulthan current density in the conversion of CO–2) and higher(200 mA cm–2) current density as a reflection tothe baseline work previously performed in an H-cell.1 As shown in 3. The differences between the two electrolytes that are prevalentat lower current densities  up to the masstransport capabilities of the GDE regime (200 mA cm–2). Emphasis was placed on comparability of metals by producing 100nm thick samples, and performing electrochemical and material characterization,and assessing collected data according to a detailed protocol. Althoughsuch a protocol allows for correlating catalysts without bias, a downsideof this approach is that only a singular experimental configurationis screened. None-the-less we have strived to perform this analysisin a well described and controlled testing environment for the confirmationand reproducibility of new and existing research within the field.Changes in performance due to varying catalysts, configurations, andoperating conditions are then grounded by a common point of comparison.The main focus of this work is to provide a comparison of elementalcatalysts by creating a controlled system and identifying the effectof current density on activity, selectivity, and stability while movingfrom the H-cell regime (10 mA cm


Over the past decade polyelectrolyte multilayer (PEM)-basedmembraneshave gained a lot of interest in the field of nanofiltration (NF)as an alternative to conventional polyamide-based thin film compositemembranes. With great variety in fabrication conditions, these membranescan achieve superior properties such as high chemical resistance andexcellent filtration performance. Some of the most common polyelectrolytesused to prepare NF membranes are weak, meaning that their charge densitydepends on pH within the normal window of operation relevant for potentialapplications (pH 0–14). This might cause a dependency of membraneproperties on the pH of filtered solutions, as indicated by otherapplications of PEMs. In this work, the susceptibility of membranestructure (swelling and surface charge) and performance  toward the pH of thefiltration solution was studied for four fundamentally different PEMsystems: poly (PDADMAC)/poly(sodium-4-styrenesulfonate)(PSS) (strong/strong), poly (PAH)/poly(acrylicacid) (PAA) (weak/weak), and PAH/PSS (weak/strong) and PAH/PSS+PAH/PAA(asymmetric). Slight variations in structure and performance of thePDADMAC/PSS-based membranes were observed. On the contrary, structureand performance of PAH/PAA-based membranes are very susceptible tofeed solution pH. A continuous change in charge density with variationin pH significantly affects salt retention. An increased swellingat pH 9 translates to variation in permeability and molecular weightcutoff of the membrane. The susceptibility of PAH/PSS-based membranesto pH is less pronounced compared to the PAH/PAA-based membranes sinceonly one of the polyelectrolytes involved is weak. No structural changeswere observed, indicating additional specific interactions betweenthe polyelectrolytes other than electrostatic forces that stabilizefilm structure. A combination of the PAH/PSS and PAH/PAA system (8+ 2 bilayers) also displays a clear dependency of both membrane structureand performance on solution pH, where PAH/PSS is dominating due toa higher bilayer number. By means of interfacial polymerization, a thin selectivefilm is fabricated on top of a porous support membrane resulting inRO and NF type membranes,6 a process whichis well established for flat sheet membranes but is more difficultto perform for membranes with a hollow fiber (HF) geometry.7In recent years nanofiltration (NF) typemembranes have shown greatpotential in numerous fields and applications such as hardness removal,8 anddrug delivery.9 Also in the field of membranescience PEMs have been shown to have great potential.11 First introduced by Decher et al. in 1992,12 it was shown that exposing a charged substrate to polycations andpolyanions in an alternating fashion results in the self-assemblyof a multilayered thin film due to the gain in entropy by the releaseof counterions upon layer formation. By employing the layer-by-layer(LbL) technique,13 PEMs can be fabricatedonto a HF support membrane. For this, a charged UF HF membrane isalternatingly exposed to polycations and polyanions resulting in athin PEM which allows for increased selectivity.A very promising and less challenging alternativeto polyamide-basedHF NF membranes is the use of polyelectrolyte multilayers (PEMs).Over the last years, PEMs have found their way into a broad rangeof applications, among others, optics14 or the removal of organic micropollutants.15 Bruening and co-workers have shown that it is possible to have selectiveion rejections using poly (PDADMAC)/poly(sodium-4-styrenesulfonate)(PSS) multilayer NF membranes, reporting phosphate rejection of 86–98%(depending on pH) with a chloride/phosphate selectivity of 48.16 Similarly, Wang et al. have used PDADMAC/PSSto fabricate NF membranes with, compared to commercial NF membranes,similarly high retention toward organic micropollutants (around 90%),slightly lower pure water permeability, but at the same time low retentiontoward divalent cations (below 20%).17 Theseproperties result in a lower scaling potential compared to commercialNF membranes. In a recent study by Elshof et al.,18 it was shown that the filtration performance of NF membranesbased on the frequently applied PEs PDADMAC and PSS remained stableafter more than two months of exposure to basic (pH 14) and acidic(pH 0) conditions. This outstanding pH stability allows for potentialapplications of these membranes under harsh conditions found in industrialprocesses such as treatment of mining effluent,19 treatment of dairy cleaning solutions,20 sulfate removal in vacuum salt production,21 and hemicellulose removal in viscous fiber production.22 The high chemical resistance of these membranes23 also allows the use of common cleaning methodsto mitigate membrane fouling.By carefulchoice of polyelectrolytes (PEs) and fabrication conditions,great control over PEM fabrication and thus final properties is obtained.The tuneability of PEMs allows for targeting specific applications,e.g., water softening and desalination15 wherethe concept of an asymmetric structure within the PEM layer was utilizedto fabricate high-performance NF membranes for micropollutant removal.These membranes were based on PSS/poly(PAH) (as a more open support layer) and poly(acrylic acid) (PAA)/PAH(as a dense separation layer).One way to exploit the versatilityof the LbL method was recentlydisplayed by te Brinke et al.,28 and postassembly45) on PEM properties and (postassembly) on the stability of PEM NFmembranes18 have been widely studied, theinfluence of pH (within the range of stability) on the performanceon NF PEM membranes has to the best of our knowledge not been studied,yet.Even though the influence ofpH  significantlyaltered ion transport (and likely excess charge of the PEM film).The expected influence of pH on PEM NFmembranes based on currentknowledge can be divided into three categories that will be summarizedin the following.53 Here pH can affect filtration performance by influencing both membraneproperties and solution properties. Second, the influence of solutionpH during LbL coating on final PEM properties (with at least one weakPE involved) has been shown in multiple studies,27 also for the application of NF.28 The dissociation behavior of functional groupsattributing to the overall charge density depends on solution pH.Here we can distinguish between weak  and strong PEs. Strong PEs are characterized by a chargedensity that is independent of solution pH (here defined for a rangeof pH 0–14). Weak PEs, on the other hand, exhibit a dependencyof charge density as a function of pH within this range. The groupof Rubner has contributed a lot to clarifying the influence of PEcharge density on PEM formation.27 The charge density ofweak PEs (controlled via pH) influences the conformation in whichthey are incorporated into the multilayer. As a result, propertiessuch as layer thickness, composition, surface wettability, and layerinterpenetration can be controlled. Third, the responsiveness of PEMs(postassembly) toward external stimuli, such as pH, has been shownin multiple studies.45 These properties are especially interesting for loading and releasingof target molecules, either within a capsule37 or the PEM31 itself, withpotential applications in the biomedical field. Here, the buildupof excess charge as a result of pH changes can cause swelling, changesin film morphology, or even break down PEMs.41 To prevent the PEM from falling apart, attractive forces  mustoutweigh the repulsive forces .41First, for conventional polyamide-based membranesused in NF processesthe influence of solution pH on filtration performance has alreadybeen studied intensively.15Thus, solution pH might have a severeinfluence on the membraneperformance under certain conditions and has to be considered in NFapplications. In addition, investigating the behavior of PEM membranesunder different pH conditions might offer a different perspectivecompared to conventional characterization methods and improve ourunderstanding of the fundamental mechanisms of PEMs. In this work,we systematically study the influence of feed solution pH on the performanceof PEM NF membranes. The PEM systems under investigation are frequentlyused for the fabrication of NF membranes and cover the three fundamentallydifferent configurations weak/weak (PAH/PAA), weak/strong (PAH/PSS),and strong/strong (PDADMAC/PSS). Through a combination of membraneperformance measurements with swelling and surface charge measurements,intrinsic PEM properties are inferred. Lastly, we extend our investigationto asymmetric PEM membranes developed by te Brinke et al. (PAH/PSS+ PAH/PAA).46 Therefore, we did expect thefiltration performance of the chosen PEM NF membranes to be sensitiveto pH within the chosen range. Lastly, we expect any effects on PEMstructure within this moderate pH range to be intensified when goingto more extreme pH values.The sensitivity of PEM NF membranestoward solution pH was investigatedby experimentally studying these films under slightly acidic (pH 4),neutral (pH 6), and slightly basic (pH 9) conditions. Moderate variationsin pH were chosen to ensure the stability of the PEM membranes underrepeated and long-term operation but also to remain in the pH windowrelevant to typical water treatment applications. Also, it was shownpreviously that already small changes in pH (from pH 6.5 to 8.3) cansignificantly affect ion transport through PEM layers as ion exchangemembranes.Mw = 150.000 g mol–1, 40 wt % in water) was obtained from Nittobomedical CO., LTD, Japan. Poly(acrylic acid) , poly(sodium-4-styrenesulfonate) , poly , glycerol solution (86–89%),MgCl2 hexahydrate (purity 99%), sodium hydroxide , and hydrchloric acid  were purchasedfrom Sigma-Aldrich. H2SO4 (96% in H2O) was purchased from Acros Organics B.V.B.A. NaCl (purity 99.9 wt%) was kindly provided by Nouryon Industrial Chemicals. H2O2 (30 w/w% solution in H2O), Na2SO4 decahydrate, ethylene glycol, and polyethyleneglycol (PEG) of various Mw  were obtained from Merck. Diethylene glycol waspurchased from Sigma-Aldrich. All chemicals were used without anypurification.Modified poly(ethersulfone) HF UF membraneswere kindly provided by NX Filtration B.V., Enschede, The Netherlands.The provided membranes have an approximate 90% MWCO of 10 kDaand an inner diameter of 0.7 mm. Poly ; symmetric [PDADMAC/PSS]6/6.5, [PAH/PAA]8/8.5, [PAH/PSS]8/8.5, and asymmetric [PAH/PSS]8+[PAH/PAA]2/2.5.It should be mentioned that even though the performed dip coatingleads to layer deposition on both the inside and outside of the HFUF membrane, an asymmetric pore structure with the dense layer beinglocated on the inside of the HF results in defect-free layer formationonly on the inside of the HF.The obtained HF UF membranes weremodified with multiple combinations of the 4 types of PEs. Modificationwas done by dipcoating based on the LbL principle. The HF UF membraneswere alternately exposed to polycation and polyanion, starting withthe polycation because of the negative surface charge of the supportmembranes. Both aqueous PE solutions were set at pH 5.5 and an ionicstrength of 50 mM NaCl. The HFs were immersed in the PE solutionsfor 15 min. In between PE solutions, the HFs were rinsed using aqueoussolutions of 50 mM NaCl, to remove loosely bound PEs withoutinfluencing the salt concentration of the PEM. The rinsing step wasperformed 3 times, for 5 min each. One bilayer was obtained when theHFs had been exposed to polycation and polyanion, with 3 rinsing stepsin between. Depending on the combination of polycation and polyanion,a specific amount of bilayers was coated onto the HF support membranes.Dipcoating resulted in PEM membranes of the following composition of the tubing were potted using a 2component polyurethane glue. Since the HFs were operated in an inside-outflow configuration, a hole was cut at the midpoint of the tubing enablingone to collect permeate. An example of a membrane module can be foundin Pure water permeability had been obtainedfor all membranes which were made as mentioned under section 2– h–1 bar–1) was calculated for all samples. Where mp is the permeate mass in g, ρw equals the density of water (1000 g L–1 at 20 °C), A is the active membrane area in m2, t is the duration of the experiment in h, and ΔP is the transmembrane pressure in bar.The permeate was collected and weighed,and with the use of The permeabilitiesat pHs 4 and 9 were obtained together with saltretention measurements. The reason for this is the ability to maintaina more stable pH value in salt solutions. Similar to the permeabilitymeasurements at pH 6, the permeate was weighed. Due to the salt concentrationof 5 mM, it was necessary to compensate for osmotic pressurein calculations for permeability. 3PaK–1 mol–1), T istemperature in Kelvin, cf the salt concentration in thefeed and cp the salt concentration in the permeate in molm–3. The value for osmotic pressure, calculated using 2SO4 and MgCl2 a van ’t Hoff coefficient of 3 was used.In 2, and Na2SO4. Feed solutions of the desired salt were preparedusing Milli-Q water and a salt concentration of 5 mM. The pHof the feed solutions was set using 1 M solutions of eitherHCl or NaOH, and pH values were measured using a FiveEasy benchtoppH meter from Mettler-Toledo B.V.Salt retention experiments have beenperformed at pHs 4, 6, and 9 for NaCl, MgClcp is the conductivity of the permeateand cf is the averaged conductivity ofthe feed solution.Retention was determined bymeans of conductivity using a WTW ProfiLine portable conductivitymeter. The conductivity of the feed solution was measured at the startand end of the experiments. The permeate conductivity was measuredonce at the end of the experiment. With the use of –1 for eachPEG. Permeate and feed samples were collected and analyzed using gelpermeation chromatography . The GPC was set up with two columns in series obtained fromPolymer Standards Service GmbH. The columns: Suprema 8 × 300mm-1000 Å, 10 μm followed by 30 Å, 10 μm wereoperated at a flow of 1 mL min–1. Columns were followedby a refractive index detector. The refractive index variation dueto PEG depends linearly on concentration and is therefore directlytranslated into retention. A feed sample was collected at the startand end of every experiment to determine the average signal intensity.From the obtained signal intensities the MWCO, defined as the molecularweight of PEG with 90% retention, was determined. Although the likelihoodof the MWCO exactly matching the molecular weight of one of the PEGin solution is low, the overlap of peaks  leadsto a continuous dependency of retention as a function of molecularweight (effectively interpolating). To illustrate this, a typicalsignal of a feed and permeate sample as well as the resulting sievingcurve is displayed in Figure S3.To study the relative changein pore size, MWCO measurements were performed. Again, these measurementswere done at pHs 4, 6, and 9. A mixture of ethylene glycol, diethyleneglycol, and PEGs varying in size from 200–1000 Da wasused at a concentration of 1 g LTo obtain the surface charge of thevarious polyelectrolyte multilayer membranes (PEMMs), zeta potentialmeasurements have been performed using the as prepared membranes.For this, an electrokinetic analyzer for solid surfaces  was used. Sample modules have been prepared using tubingwith an outer diameter of 8 mm to allow the placement of theHF PEMMs into the apparatus. Approximately 6–8 cm longpieces of single HF PEMMs were placed into this tubing and glued inplace with a two-component polyurethane glue. The entire tubing aroundthe HF PEMMs (shell side) was filled with glue such that there wasonly flow through the HF PEMMs without permeation. Measurements havebeen performed in pH titration mode in the range of pH 4 until pH9 with an increment of 0.3 using a 5 mM KCl solution. Adjustmentsof pH were done using a 0.1 M solution of NaOH and HCl. Duringthe measurements, the electrolyte solution containing 5 mMKCl is forced through the HF. Upon the flow of electrolyte throughthe HF, charge separation takes place at the HF inner surface. Thischarge separation leads to electrokinetic effects, among which thestreaming potential is measured by the electrokinetic analyzer. Usingthe streaming potential and U is the streamingpotential in V, p is the applied pressure in Pa,η is the electrolyte viscosity in Pa s, ε is the dielectriccoefficient of the electrolyte, ε0 is the permittivityof vacuum in A2s4kg–1m–3, κB is the electrolyte’sspecific electrical conductivity in A2 s3 kg–1 m–3.Here, 2SO4 (30  w/w%):H2O2 of 2:1) for1 h after which they were treated with oxygen plasma for 15 min. First,the dry thickness was measured in N2 atmosphere. Waferswere mounted in a cell and exposed to a N2 flow of 400 mLmin–1 for at least 30 min or until a stablesignal was obtained (Mk-2000 X). Subsequently, the wet thickness wasmeasured using a heated liquid cell with a volume of 5 mL (Mk-2000V). Both dry and wet measurements were performed under an angle of75° and a wavelength range of 380–1000 nm underambient conditions. In the heated liquid cell, the wafers were exposedto Milli-Q and salt solutions of 5 mM NaCl with pHs 4, 6, and9. After being exposed to Milli-Q, the wet cell was filled with thesalt solution at different pH values. This was done in the followingorder: pHs 6–4–6–9–6.PEM swelling under different pH conditionswas investigated by means of ellipsometry using a rotating compensatorellipsometer . Multilayerswere coated on model surfaces, in our case silicon wafers, under thesame conditions as the membranes (mentioned earlier). To ensure thesilicon wafers were properly cleaned before coating, the wafers weretreated with piranha solution . The SiO2 thicknesswas determined to be 83 nm prior to film deposition. Layer thicknessand refractive index of the PEM were fitted using a standard Cauchymodel, shown in n is the refractive index of the layer, A andB are the Cauchy coefficients, and λ is the wavelength.For ellipsometrymeasurements, Si wafers with thermal SiOBoth the A and B parameters were determinedfrom dry thicknessmeasurements and kept constant in the following. For wet thicknessmeasurements, the model was extended using a Bruggeman effective mediumapproximation (BEMA).n is the refractive index,and the indices indicate thetwo phases  and the medium representingthe mixture of them (“eff”).Here Φ is the volume fractionof the respective medium, dwet is the wet thickness of the PEM obtainedunder the various pH conditions, and ddry is the PEM thickness obtained under N2 atmosphere.The swelling ratio(SR) of the PEMs on model surfaces was calculatedusing The applied characterizationmethods give insight into the structuralproperties of PEMs under different solution pH conditions. We divideour discussion of results into two major parts: indirect characterizationof the selective PEM layer (selective layer not applied in a filtrationprocess) and direct characterization of the membrane performance (selectivelayer is applied during filtration). These two approaches can be seenas complementary techniques to study thickness, mesh size, and chargeof the PEM systems.54 In this study, wehave determined the dry layer thickness of the 4 PEM systems undera humid-free N2 atmosphere, to prevent any swelling dueto humidity. To allow direct comparison of the systems and at thesame time ensure a minimum film thickness of at least 10 nm,a total of 10 bilayers of each system (8 + 2 for asymmetric) was chosen.It is known that the sensitivity of ellipsometry toward external influences(such as angle offsets of the measurement cell) can deteriorate measurementaccuracy at very low film thicknesses.55 In 26 and results from exponential layer growth.56Ellipsometry is a powerful toolto determine the layer thickness of thin films and subsequently swellingof these thin films.After dry film thickness determination, wet thicknessesunder variouspH conditions were measured. With the use of Ka between8 and 9)25 and the resulting excess ofnegative charge.For PAH/PSS films, no change in swelling ratiowas observed asa function of pH, although this would have been expected at pH 9 dueto the deionization of PAH  is around 6.5.25In contrast to PAH/PSS films, the PAH/PAA filmsshow a clear influenceof pH on the swelling ratio at pH 9. Similar results with regard tothe swelling ratio have been obtained by Tanchak et al. for PAH/PAAfilms fabricated at pH 5.Ka when PEs are incorporated in multilayers,a phenomenon that has previously been observed.58 An additional explanation is the fact that the stabilityof PEMs is not solely determined by electrostatic interactions butrather by a balance of the sum of attractive and repulsive forces,which are specific for each PE combination.One explanation for the lack of swellingfor the PAH/PSS systemat pH 9 and the PAH/PAA system at pH 4 is a shift in effective pFigure S4) as the swelling ratio during the third cycle of pHs 6–4–6–9–6drops significantly compared to the first cycle as can be seen in It also needs mentioningthat the multilayer seems to undergo reorganizationduring swelling measurements S4 as the59 slight intermixing of the systems could influencethe overall swelling degree.The asymmetricPEM system displays a clear combination of the swellingratio for PAH/PSS and PAH/PAA films with a slight increase in theswelling ratio at pH 9. We mainly attribute the minor increase inswelling compared to the pure PAH/PAA system to the dominant amountof PAH/PSS layers (8 bilayers). While it was shown that these kindof asymmetric films indeed form rather distinct layers,60 zeta potential does allow for a first indication of potentialretention behavior.61 By determining thezeta potential at various pH values (ranging from 4 to 9), a clearinfluence of pH on the surface charge of the prepared membranes wasobserved, as shown in To obtain more information about thecharge at the surface of the prepared membranes, streaming potentialmeasurements were performed. Although retention behavior is not onlydetermined by membrane surface charge, especially considering potentialnonhomogeneous charge distributions inside PEMs,62 a similar drop in zeta potential with increasing pH was observedfor PDADMAC/PSS-based membranes. One potential explanation could bea slight variation in surface charge caused by the adsorption of hydroxideions with increasing pH value.63 As expected,the zeta potential of the membranes ending on PSS is more negativeover the entire pH range.Due to large variations in measurement values forthe negativeending PDADMAC/PSS membrane, no statistically sound conclusion onzeta potential variation with pH can be made. By careful examination,one can however detect a minor decrease in the zeta potential of thepositive ending PDADMAC/PSS films toward pH 9. This decrease is unexpectedas the PDADMAC/PSS film should be fully ionized across the investigatedpH range. In a recent work by Wang et al.,64For PAH/PSS, two clearly distinctbehaviors are observed for filmsending on PAH  and PSS .At a pH of 4, the zeta potential of positive ending films is lessnegative. However, at a pH of 9, the zeta potential of both negativeand positive ending films match. This results in a steeper slope ofzeta potential versus pH for the positive ending PAH/PSS films comparedto the negative ending ones, as well as the PDADMAC/PSS films. Tounderstand this behavior, it is important to keep the typical structureof a PEM in mind. While the charged groups of PE inside the bulk ofthe multilayer are compensated by mostly oppositely charged PEs (intrinsic)or ions (extrinsic), the region close to the film–solutioninterface consists of an excess of the final PE where the charge isnot fully compensated.Ka valuein solution (around 8–9). The PAH inside the film that formsionic bonds with PSS is less affected by the increase in pH, mostlikely due to a higher effective pKa value,as discussed previously. While the variation of zeta potential withpH for the PAH ending film is now a combination of change in the bulkregion and top layer, the film ending on PSS is only affected by thevariation in bulk, since PSS is strong and therefore fully chargedover the whole pH range.For thePAH ending film, the surface layer is deprotonated withincreasing pH, according to its pKa for PAA to lower values, which couldexplain that no swelling was observed under these conditions.Out of the symmetric PEM membranes,the PAH/PAA films display thestrongest dependency of zeta potential as a function of pH, with bothpositive and negative ending films behaving similarly. Since for thissystem both PEs are weak, the difference in the top layer that wasobserved for the PAH/PSS system (weak/strong) is no longer present.We attribute the stronger variation in zeta potential to both PEs,varying their charge density as a function of pH. The higher absolutezeta potential at pH 9 compared to pH 4 again indicates a shift ofpK values of the PEs due to the influence of thebottom layer of PAH/PSS. When ending on PAH , the zeta potential variation also matches that of the symmetricPAH/PAA system, although with the inflection point of the function shiftedto a high pH value.The zeta potential of the asymmetric PEMs feature quite distinctbehavior depending on the ending PE. The asymmetric layer is builtfrom a bottom layer of 8 bilayers PAH/PSS and a top layer of 2/2.5bilayers PAH/PAA. When ending on PAA ,the zeta potential variation with pH matches quite well with the oneobserved for the symmetric PAH/PAA system. This matches again theassumption that the zeta potential is predominantly affected by thelast layer of PE, in this case, PAA. At a pH of around 5 to 6, a shortplateau is observed, which could be caused by a slight shift in effectivepEven though out of the studied symmetricsystems, the zeta potentialof the asymmetric PEMs matches the PAH/PAA system best, shifts inzeta potential as a function of pH indicate an influence of the bottomlayer of PAH/PSS.59 While zetapotential is dominated by the top layer of PAH/PAA (2 bilayers), theoverall swelling behavior of the film is dominated by PAH/PSS (8 bilayers),due to the higher relative layer number.Both swelling and streaming potential measurementsreveal the fundamentaldifferences between weak and strong PEs and their incorporation inPEMs. Due to the dependency of charge density on the pH of weak PEs,the excess and surface charge of multilayers containing these is susceptibleto variations in solution pH. When reaching a critical charge density,repulsive electrostatic interactions can lead to strongly enhancedswelling. For the asymmetric system, these characterization methodssuggest rather distinct layers of PAH/PAA and PAH/PSS, which has beenindicated in the literature.In the following section,the results of membrane performance characterization will be discussed.It should be kept in mind that the order in which these results arepresented does not match the order the measurements were conductedin. Measurements where conducted in the pH order, 6–4–6–9–6.For each pH, the complete set of characterization measurements wasconducted. This way, it was possible to monitor the reversibilityof membrane performance, as will be discussed in the last part ofthis section.The pure water permeabilityof a membrane is a measure of the amount of water passing throughthe membrane per bar of applied pressure, thus a direct indicatorof the energy requirement of the filtration process. The ambitionto maximize the permeability of a membrane and minimize energy consumption,however, is often limited by a decrease in solute selectivity. A somewhatsimplified approach to picture this relation is by distinguishingthickness and available effective pore area as the determining factorsfor the pure water permeability. With a low thickness and large availableeffective pore area, one can achieve a high permeability. If the highavailable effective pore area is achieved by large pores, the membraneselectivity toward solutes is reduced.In the case of PEM membranes,the water uptake (swelling) of the film is expected to influence boththe thickness and the available effective pore area.The ellipsometrymeasurements conducted in this study display anincrease in swelling for PEM membranes that contain PAH/PAA (weak/weak).Therefore, one might expect an influence of solution pH on the purewater permeability of those membranes.Figure S5.In our study, we determinepure water permeability directly atthe natural pH of Milli-Q water (around 5.8) and indirectly at pH4 and 9 from salt retention measurements by accounting for osmoticpressure. For reasons of clarity, in 2– h–1 bar–1. The difficulties in reproducing the exactmembrane properties originate from the high sensitivity of PAH/PAAmultilayer growth on solution pH in the range from pH 5 to 6.26 In the range of pH 4 to 9, we do not observe a sharp increase inpure water permeability as a function of pH. This indicates that allPEM systems do have a stable film structure in that range. Overall,there is a slight trend of decreasing permeability with increasingpH. This could be caused by long-term aging effects of the PEM layersleading to densification of the film . However, an alternative explanationcould be slight fouling over the course of the filtration measurements.The much bigger effect on the PAH/PAA membranes might be caused byincreased swelling of the film at high pH values, which was also observedat pH 9 for PAH/PAA films as shown in First, it should be mentioned here that due tolimited long-termstability of membranes, the displayed permeability of PAH/PAA at asolution pH of 9 was measured for a different batch of membranes thanat pH 4 and pH 6. The new batch has a slightly lower permeabilityat pH 6, around 1 L m2– h–1 bar–1), about one magnitudelower compared to the other systems, which is mostly related to themuch higher film thickness  is slightly lower than theone of PDADMAC/PSS (11 L m2– h–1 bar–1). Even though PDADMAC/PSS forms thickerlayers on a model surface  is higher compared toPDADMAC/PSS (0.063 ion pairs in the complex/carbon atoms inthe complex), therefore the system is likely to form more dense layers.2– h–1 bar–1) has a reduced permeability compared to PAH/PSS,caused by the extra 2 bilayers of PAH/PAA coated on top.The asymmetric PEM system based on PAH/PSSand PAH/PAA .Similar to pure water permeability, one might expect an influenceof solution pH on size-based exclusion due to swelling. An increasingamount of water inside the film might, in addition to increasing thethickness of the film, lead to an increase of the mesh size and withthat a reduced steric hindrance for solutes.Figure S6. Furthermore, sieving curves of the initial measurement at pH 6,after exposure to pH 4 and after exposure to pH 9 can be found in Figure S7.The determinedMWCO of the negative ending films at solution pH values of 4, 6, and9 are shown in For the PAH/PAA system, the graph again distinguishesbetween thefirst and second batch of membranes, as discussed previously. Thereis no significant change in MWCO for the three PEM systems PDADMAC/PSS,PAH/PSS, and the asymmetric multilayer as a function of solution pH.This is in accordance with the minor changes in swelling and purewater permeability observed for these systems.Figure S4), we suspect the polyelectrolyte chainsto restructure in an energetically more favorable and more dense configuration.One can additionally imagine that the induced mobility at pH 9 forthe membrane promotes “filling” of defects within thelayer (which has been observed even for high bilayer numbers of PAH/PAA15) due to the expansion and reorganization ofthe PE chains at high pH. However, further research is needed to confirmthis theory.Similar to thepure water permeability, the MWCO of PAH/PAA decreasesat pH 9. This indicates a “densification” of the membrane,meaning a reduction in effective pore size. This is counterintuitiveto the idea of swelling of the multilayer, as observed for pH 9 in 65 [PDADMAC/PSS] < [PAH/PSS] < [PAH/PAA].For the asymmetric system, one would expect a MWCO that is similarto the symmetric PAH/PAA system. The fact that the MWCO is betweenthe symmetric systems of PAH/PSS and PAH/PAA suggests that the PAH/PAAlayer is not fully developed, yet. Since the MWCO would typicallyslightly decrease with an increasing flux, the MWCO of the PAH/PAAfilm at comparable flux would be even lower. However, this would notchange the observed order in MWCO between the systems.Although the MWCO is a process parameter of a membrane(meaningit is dependent on parameters such as the flux through the membrane),it gives a good estimate of the mesh size of the membrane, especiallyfor similar fluxes. Comparing all films at pH 6, the films are inthe order, [PDADMAC/PSS] > [PAH/PSS] > [asymmetric] > [PAH/PAA],wherea high MWCO represents a large mesh size. For the symmetric systems,this matches nicely considering the charge density of PE complexes:The retention of salts by NF is mostlydetermined by electrostatic interactions between the ions and themembrane. Here, commonly two effects are distinguished: Donnan exclusiondescribes the interaction between charged groups of the membrane andthe ions, and dielectric exclusion describes the interaction betweeninduced charges at the interface of two media with different dielectricproperties (here the aqueous and membrane phase). Those two effectscan be distinguished by their effect on salt retention. Donnan exclusioncauses salts that contain co-ions (same charge as the membrane) withhigher valence to be repelled to a larger extent, which because ofelectroneutrality also leads to high retention of the counterion.Dielectric exclusion causes any salt with ions of high valency, independentof sign, to be retained to a larger extent. Therefore, salt retentionmeasurements of binary mixtures with different combinations of mono-and divalent ions can give qualitative information on membrane chargeand exclusion mechanisms .2, and Na2SO4) determined forthe negative ending PEM systems at different pH. Retention valuesare based on conductivity measurements of permeate and feed samples,where the influence of HCl and NaOH on the conductivity is neglecteddue to the at least 50-fold higher concentration of salt comparedto added acid/base. Again for reasons of clarity, the retention valuesfor positively ending PEM systems are presented in Figure S8. It should be mentioned, that there is a clear differencein salt retention for the PEM system with a different top layer atthe same pH value. Here, salt retentions indicate more positive effectivecharge values for positive ending layers. However, the trend withvariation as a function of pH is again similar.In 2 retention, increasing NaCl as well asincreasing Na2SO4 retention with pH. This indicatesa slight variation in effective membrane charge with pH, althoughno charge reversal is observed. This matches qualitatively to theobserved decrease in zeta potential with increasing pH  and PAH(8–9), meaning at pH 4, PAA is deionized whereas PAH is fullyionized.25 Again, the reduced MgCl2 retention compared to pH 6 is surprising as one would expecta much higher value . The exactopposite can be seen at pH 9, where the PAH/PAA film displays a morenegative charge due to the partial deionization of PAH and the retentionfor Na2SO4 is higher than that for MgCl2. It also stands out that the retention for all salts at pH9 is very high. This matches to the sudden densification of the membranethat was observed at pH 9 .Like in pH 9 see 3 and 4. 2SO4 retention is lower (approximately10%) and NaCl retention is higher compared with the PAH/PSS film;this is believed to be caused by the presence of the 2BL PAH/PAA film.From this it can be concluded that the more open bottom layer (PAH/PSS)still plays a big role with regard to salt retention of the asymmetricsystem. This clearly displays the difference in the two measurementmethods , again, and stronglysuggests a distinct layer structure of these multilayers. While thetop layer of PAH/PAA clearly dominates the zeta potential . Both display significantchanges in salt retention with varying pH and low reversibility ofretention behavior at pH 6 after exposure to pHs 4 and 9. Consideringstreaming potential results and the fact that both PDADMAC and PSSare considered strong PEs (so fully charged over the whole pH range)only minor fluctuations in salt retention were expected.One system that displayed unexpected and partiallyinexplicableresults is PDADMAC/PSS, both the positive and negative ending membrane or accelerated(lower pH permeate side), respectively. Knowing the diffusion coefficient,charge, and concentration of the ions that are present, the influenceon electrical conductivity can be estimated using the following formula:68–1, F the Faraday constant, R the ideal gas constant, T the temperature(297.15 K), zj the charge of the ion, Dj the diffusion coefficient of the ion (m2 s–1), and Cj the concentration of the ion (mol m–3). Usingthis equation, one can evaluate the influence of hydrogen or hydroxideions on the overall conductivity used to estimate the retention.Tocheck for potential retention of hydrogen and hydroxide ionsand the associated influence on the retention measured via conductivity,the pH of permeate samples was monitored for filtration measurementsat pH 4 and pH 6. As measurements were conducted in open atmosphere,at pH 9, the solution pH would drop slowly over time due to the incorporationof carbon dioxide from the atmosphere to form bicarbonate. The solutionwas therefore manually titrated with NaOH to keep the pH stable. Theaddition of a buffer was avoided since it might influence the retentionbehavior. Caused by the length of the filtration measurement, thepH of permeate samples would already drop quite severely, making itimpossible to estimate the transport of hydroxide ions through themembrane in this case. The difference of pH between permeate and feedphase is shown in 2 retention at pH 4 compared to pH 6. The first caseis a positive membrane with a high retention of 90% for MgCl2 at pH 4. As a consequence, the pH in the permeate will decrease. Neglecting the influence ofpH on conductivity, this would correspond to a conductivity of around1320 μS cm–1 in the feed and 130 μScm–1 in the permeate. Considering the influenceof pH, the feed conductivity is around 1360 μS cm–1, and the permeate conductivity is around 270 μScm–1. This results in an observed retention of only80%. This matches well with the observed drop in retention of MgCl2 when going to pH 4 for almost all PEM systems. Consideringthe uncertainty in permeate pH measurements during MgCl2 retention, one can estimate a range for the retention variationfrom −30% to −10% for a permeate pH of 3 and 3.5, respectively.The second case considers the retention of NaCl at pH 4. Here we assumea retention of 30%. As also shown in the measurements, no significantvariation in pH of the permeate is expected. With the same calculationas before, the observed retention only shows a minor drop to around28%. The third case considers a negative ending membrane with a highretention of 90% for Na2SO4 at pH 9. On thebasis of the observed pH variations, the permeate pH is expected tobe around pH 10. With the use of, again, Here we show the potential influence for three extreme cases, oneof which partially explains the unexpected behavior of decreasingMgClThe evaluation of performance of NF membranesbased on the studiedPEMs allows the characterization of PEMs from a different point ofview. Complementing these studies with the presented ellipsometryand streaming potential measurements, a more complete insight intothe PEM structure is gained, as well as, strength and weaknesses ofdifferent methods are revealed. Ellipsometry measurements clearlyindicate changes in the cross-linked network structure of the PEMcaused by variation in charge density of PEs. However, direct translationto membrane properties in terms of steric retention and hydraulicresistance can be difficult. Not only is the support structure ofthe film different (silicon wafer vs UF membrane), but also can theeffect of swelling and increased chain mobility be counterintuitive.This is illustrated in the fact that the PAH/PAA film swells alot at a pH of 9 and ambient pressure, while the membrane permeabilityand MWCO both decrease, illustrating a densified structure of theselective layer .K value, salt retention measurements reveal the quantitativeoffset of the method. In addition, the zeta potential is clearly misleadingfor the asymmetric PEM, as the salt retention is dominated by thesupport layer of PAH/PSS while the zeta potential is dominated bythe top layer of PAH/PAA.Streaming potential measurements display the variation in chargedensity of weak PEs with pH but are limited to indicating the surfacecharge of the PEM. It should be mentioned that the typical odd–eveneffect  was not observed in the performed study. As we arenot sure about the cause for the very little shift of absolute zetapotential when ending on a positive PE, we focus only on the quantitativevariation of zeta potential with pH. Although, the variation of zetapotential matches qualitatively to the ending PE and its pTo monitor for irreversible changes tothe membrane performance after exposure to pHs 4 and 9, measurementsat these pH values are followed up by measurements at pH 6 (wholesequence pHs 6–4–6–9–6). Since PEMs arenot necessarily in their energetically most favorable conformation, one might expect reorganization of chainsgiven sufficient mobility, for example at a pH that reduces ionicinteraction by partial deionization.As well, PEMs based onweak PEs might fall apart in certain pH conditions, but this is notexpected for the studied pH range.2, and Na2SO4) as well as MWCO at pH6 for negative ending PEMs is displayedin Figures S10, S11, and S12.Salt retention , resultingin densification of the PAH/PAA film and/or closing of possible defectswhich are known to be present in PAH/PAA films.15Also, PAH/PAAdisplays good reversibility after pH 4. For reversibilityafter pH 9, we have to take into account the fact that a new batch(#2) of PAH/PAA membranes was used. These had a slightly higher Na2SO4 retention, decreasing NaCl and MgCl2 retention). We attribute this to fouling, which is known to be moresevere for positive membranes.71 Here, negatively charged foulantsare expected to deposit at the membrane surface and therefore reducethe effective charge density.Finally, the asymmetric system shows reversible behaviorafterboth pH 4 and 9. It has to be mentioned here that the positive endingmembranes of the displayed systems, especially considering salt retention,display worse reproducibility than the negative ending ones. However,they also display a clear trend of decreasing effective membrane chargedensity over the run of the measurement series  are surprisingly enough to a small degree susceptibleto solution pH. As both PEs are fully charged over the studied pHrange, one would not expect an influence of pH on PEM properties.Relatively, minor decrease in charge density with increasing pH (thathave been previously observed in literature) are attributed to hydroxideion adsorption and affect salt retention behavior. Still these membranesstay negatively charged over the whole pH range.Exchangingone strong PE (PDADMAC) for a weak PE (PAH), resultsin a severe dependency of PEM charge as a function of solution pH.This is due to the partial deprotonation of PAH at increasing pH values,which leads to an excess negative charge. One consequence is the variationof single salt retention of membranes with solution pH. The networkstructure of the PEM, however, is unaffected by pH in the studiedrange. Going to a system that is based on two weak PEs (PAH/PAA),an even stronger variation of PEM properties is observed. Since, thecharge densities of both PEs are affected by solution pH, the variationof excess charge of the PEM is larger. In this case, an additionalreorganization of the PEM network structure is observed. Ellipsometrymeasurements on a model surface reveal that excessive swelling atfirst exposure to pH 9 is followed by permanently reduced swellingin the studied pH range (including repeated exposure to pH 9). Consideringthe permanent decrease in MWCO and pure water permeability of themembrane after exposure to pH 9, we conclude an effective densificationof the selective PEM layer.Lastly, an asymmetric PEM that isbased on PAH/PSS as a supportlayer and PAH/PAA as a dense top layer was studied. Also, this PEMis susceptible to solution pH. One can distinguish effects of thetop layer, which influences zeta potential, from the bottom layer,which dominates salt retention. The MWCO for this system does notvary significantly with solution pH. The fact that the MWCO lies betweenthe one that is observed for the symmetric systems suggests that thePAH/PAA layer is not fully developed, yet.