Catalyst Ionomer Research Articles

The Effect of Water Quality on PEM Electrolyzer Performance

There has been growing interest in the use of removable, clean, and sustainable energy sources as a solution to eliminate environmental problems caused by fossil fuels. In particular, the electrolysis of water to produce green hydrogen is one of the most promising technologies. Among various electrolysis techniques, proton exchange membrane water electrolyzer (PEMWE) stands out as the most promising for mobility and large-scale energy-storage applications (1). Hydrogen production by PEMWEs requires approximately 45 kg of high-quality water to produce 5 kg of hydrogen. It has been estimated that about 22% of the total cost of a state-of-the-art PEMWE system is for water deionization treatment (2). This water purity requirement is not only an economic barrier, but also a constraint for the use of this system in areas where high-quality water is not accessible or in areas where water for human consumption is a scarce resource. Therefore, studying the effect of water quality on PEMWE performance is an important first step in identifying key degradation mechanisms on the path to discovering new PEMWE architectures that are more tolerant of poor-quality feed waters.In this work, the effect of common impurities, such as Na+ and Mg2+, in feed water on the performance of common PEMWE architectures is studied. The main goals of the work are: 1) to understand the tolerance range for long term operation, 2) to identify major degradation mechanisms when the failure occurs. Experiments were carried out in an electrolyzer equipped with platinized Ti electrodes, IrO2 and Pt/C as anodic and cathodic catalysts, respectively, and a 115 Nafion® membrane. Durability studies of 100-hour duration and electrochemical characterization of the electrolyzer at different time intervals (polarization curves and impedance spectroscopy) were performed. The membrane/electrode assembly was probed by SEM /EDX before and after the experiments and cation concentrations in the water were monitored by ICP. An electrochemical model is proposed to explain the observed behavior of the electrolyzer with DI water and in the presence of impurities.The results show that the presence of cations affects the performance of the electrolyzer at different levels depending on the concentration of the cation and its valence (Mg2+ > Na+ >DI water). Mg+2 had the same effect as Na+ even at 5 times lower concentration. Cations have higher affinity than protons for sulfonic acid groups (-SO3 -), leading to a displacement of protons and a consequent reduction in the membrane proton conductivity. However. according to the polarization curves, there are effects beyond the ohmic potential due to the increase in membrane resistance. The major effect of cations is on the reaction kinetics, which is the primary reason for the observed increase in cell voltage. The presence of cations within the membrane blocks the transport of protons, which creates a lack of protons at the cathode, resulting in the kinetic change from proton reduction to reduction of water. Moreover, the EDX results showed that Na+ and Mg2+ ions transfer from anode to cathode and deposit on the membrane and catalyst layers, which may cover the active catalyst sites, block the proton transport inside the membrane and degrade the catalyst ionomer layers.References Sazali N. Emerging technologies by hydrogen: A review. International Journal of Hydrogen Energy. 2020 (45): 18753-18771Becker H, Murawski J, Shinde DV, Stephens IEL, Hinds G, Smith G. Impact of impurities on water electrolysis: A review. Sustainable Energy & Fuels. 2023;7(7):1565-603.

(Invited) Overview of Ion-Conducting Polymer Membranes and Membrane Separators for Water Electrolysis

Traditional liquid alkaline electrolyzer uses a porous diaphragm to allow diffusion of hydroxide ion from a feed of concentrated KOH solution while separating anode and cathode and suppressing mixing of produced H2 and O2 gases. Although liquid alkaline electrolyzer is considered as a matured technology, recent renaissance of new electrode separators suggests there is significant potential for improvement in efficiency and cost reduction by lowering area specific resistance and increasing operation current density.Ion-conducting polymers (e.g., proton or hydroxide) are used as a key component of polymer electrolyte membranes, such as acidic proton exchange membrane (PEM) and alkaline anion exchange membrane (AEM), in electrochemical energy conversion and storage technologies including water electrolyzers. For example, the state-of-the-art PEM electrolyzers have used Nafion for a PEM and ionomer catalyst binder for decades, though it is not ideal proton-conducting membrane material for those applications. Recent pressure on perfluoroalkyl substrates (PFASs) from government and environmental activist groups due to their environmental and health hazard has attracted the development of alternative polymer electrolyte membranes based on hydrocarbon polymers within electrochemical society. Compared to perfluorosulfonated Nafion, hydrocarbon ion-conducting polymers (both PEMs and AEMs) can offer advantages of flexible synthetic tunability, lower gas permeability, and importantly lower production cost.In this tutorial, an overview of recent progress in the development of advanced porous membrane separators, PEMs and AEMs, their key membrane properties, and the state-of-the-art performance in applications of water electrolyzers will be discussed. Figure 1

The difference of the ionomer–catalyst interfaces for poly(aryl piperidinium) hydroxide exchange membrane fuel cells and proton exchange membrane fuel cells

The difference of the ionomer–catalyst interfaces for poly(aryl piperidinium) hydroxide exchange membrane fuel cells and proton exchange membrane fuel cells

Electrically Insulated Catalyst–Ionomer Anode Interfaces toward Durable Alkaline Membrane Electrolyzers

Electrically Insulated Catalyst–Ionomer Anode Interfaces toward Durable Alkaline Membrane Electrolyzers

Regulating the selectivity through ionomer–catalyst interactions for high-efficiency electrocatalytic CO2 reduction

The surface morphology and microenvironment of GDEs could be altered through the addition of ionomers to electrodes. It was crucial to consider the potential interactions between the ionomer and the catalyst for better performance.

Molecular Additives at the Catalyst Ionomer Interface

The growth and viability of deep decarbonization through the hydrogen economy is dependent upon scientific advances that will address the existing political, regulatory, and technological barriers that currently hinder electrochemical energy technologies. The strong correlation between device-level performance and catalytic electrode activity/durability highlights the criticality of identifying the limiting processes and developing strategies to address these limitations. The goal of our group is to use fundamental insight derived from the study of well-defined systems to develop unique mitigation strategies, aimed at improving device-level performance, that go beyond simply varying electrocatalyst material properties. The interface between catalyst and ionomer in polymer electrolyte membrane fuel cells (PEMFC) greatly limits device performance through detrimental impacts to local reactant transport and reaction kinetics. Optimization of the catalyst/ionomer interface to not only mitigate these detrimental impacts, but to also beyond existing performance is a challenging task. In this presentation we will present our work in developing molecular interfacial modifying species to optimize the interaction between catalyst and ionomer. Integrating molecular components such as ionic liquids or caffeine both enhances the kinetics of the electrode reactions and limits the detrimental impacts of ionomer on the catalyst surface. We will present our fundamental understanding of why these molecular additives improve catalyst activity and limit the impacts of ionomer. We will also present our integration of these molecular moieties into next generation ionomers and their performance in PEMFC membrane electrode assemblies (MEA).

(Poster Award - Honorable Mention) Probing the Catalyst-Ionomer Interface in Catalyst Layers with Sulfo-Phenylated Polyphenylene Ionomers Using CO-Displacement and Stripping Techniques

Sulfo-phenylated polyphenylene ionomers (sPPP) are a class of hydrocarbon Ionomer being explored as a replacement for the perfluorinated sulfonic acid (PFSA) ionomer in fuel cell membranes and catalyst layers because they do not contain fluorinated compounds. Integration of sPPP ionomers into catalyst layers is challenging, as they have significantly different morphology compared to PFSA-based ionomers. Furthermore, little is known about the resulting ionomer-catalyst interface and whether characterization techniques developed for catalysts layers with PFSA-based ionomers will perform well for catalyst layers containing an sPPP ionomer. In this work, we examine the usefulness of CO displacement and stripping techniques to probe both the electrochemically active surface area (ECSA) of Pt/C in the catalyst layer and adsorbed species on the Pt catalyst sites. For catalyst layers containing PFSA, we measure a strong CO displacement reduction peak at potentials above potential of zero charge, when CO is introduced to the system, corresponding to sulfonic acid groups being displaced from active sites. The range of sulfonic acid groups adsorbed on the surface is 15- 25 % depending on the carbon support used. When the catalyst layers contain an sPPP ionomer instead of PFSA, the CO displacement peak is almost entirely suppressed, resulting in less than 10 % of sulfonic acid group adsorption. This is indicative that sPPP ionomer orientation near the Pt surface is very different compared to PFSA-based ionomer and that sulfonic acid groups do not “poison” Pt when in contact with the sPPP ionomer. However, mass activity increase is not observed for the sPPP ionomer-based catalyst layers, indicating there might be different “poisoning” mechanisms present, perhaps associated with the phenyl rings adsorption [1]. ECSA measurements for MEAs containing the non-fluorinated ionomer catalyst layers align with those measured for PFSA MEAs with the same catalysts type, loading, and comparable ionomer content, indicating that CO stripping is still a useful tool for assessing the ECSA for MEAs.[1] Zhai, Y., Baturina, O., Ramaker, D., Farquhar, E., St-Pierre, J., & Swider-Lyons, K. (2015). Chlorobenzene Poisoning and Recovery of Platinum-Based Cathodes in Proton Exchange Membrane Fuel Cells. Journal of Physical Chemistry C, 119(35), 20328–20338. https://doi.org/10.1021/acs.jpcc.5b06362

Understanding How Anion Exchange Membranes and Cationic Polymer/Catalyst Interactions Behave with Time in Alkaline Electrolysis

We are leveraging our existing success with anion exchange membranes (AEM) to develop new highly stable and highly conductive membranes and ionomers for AEM electrolysis. Our novel and scalable tri-block co-polymers ABA or BAB, where the A block can be functionalized with an advanced chemically stable cation and the B block is an unsaturated polymer that when hydrogenated results in high anion conduction membranes with exceptional mechanical strength and durability analogous to polyethylene (PE). We have also developed a novel immobilization technology for processable catalyst inks. Synergistically the A and B block lengths and chemistries are tuned to achieve all the properties desirable for an anion exchange membrane for electrolyzer applications. Random polymers of the same A and B chemistries can also be formulated for additional fidelity in chemically compatible ionomers for the membrane electrode assemblies (MEA). These systems are unique in that they achieve very high dissociation of anions beyond OH-, giving CO3 2- σ<100 mS cm-1 ca. 60°C and enhanced chemical stability as the water is separated from the high MW polyethylene backbone that additionally gives the materials unprecedented strength. This will allow us to fabricate thinner membranes for long durability pressure differentiated operation. Because of the unusual and exceptional CO3 2- conductivity that we have designed into our membrane, we are able to optimize these for a carbonate electrolyte electrolyzer. While we lose a 100 mV in activity compared to operation in OH-, the advantages including high current densities enabled by our novel membrane outweigh that disadvantage. Chemical stability of the membrane is not a concern, and the system is less complicated as we do not need to use expensive components to protect against caustic corrosion or protect the hydroxide electrolyte from carbon dioxide present in air to avoid its inevitable conversion to CO3 2-.Not all proposed AEM polymer chemistries are scalable, many involve too many synthetic steps, suffer from low overall yields, and are highly exothermic reactions (thus only safely executed on a small scale), or do not produce sufficiently high molecular weight for film formation. The polymer systems proposed here have already been demonstrated on the 1 kg scale. This novel AEM intellectual property, jointly invented by Mines and UMass, has been licensed to Spark Ionix who are commercializing an early version of this polymer technology. The next generation of tailored polymers proposed are increasingly robust, scalable, efficient and more amenable to high volume manufacture to produce inexpensive hydrocarbon polymer membranes for use in electrolyzers. The block lengths, overall molecular weight and chemistries are completely tunable to fabricate AEMs with all desired properties or compatible ionomer chemistries that allow high levels of catalyst utilization, through both ionic conductivity and chemical transport. We have been investigating this polymer system in 1M K2CO3 at 50°C with great success and have shown good performance and durability in 5cm2cells. We are developing the system for >60°C and 25 cm2 cells. We have begun to understand the science behind ionomer catalyst interactions and initially used a commercial Ag nano-catalyst as a model system. Few ave studied the effect of ionomer chemistry and catalyst or catalyst type and loadings on performance. We have shown durability of > 600 h at 0.5 A cm-2 and continue to see improvements. It should be noted that the voltage is dropping in theses tests, at end of test at –ve 60 µV h-1 is achieved, while this may be interpreted as evidence of membrane thinning when the cell was dissembled the membrane thickness was ca. the same as at the beginning of test (80 µm), indicating slow, but long term conditioning, the cell did not reach the point at which a +ve degradation rate was observed. Further investigation of cell conditioning indicated that these cells currently take 40-50 hs under load to reach steady state operation (this break in time will be understood and significantly shortened) giving 400MV improvement at 1 A cm-2. With a MnO2 catalyst the voltage at 0.5 A cm-2drops to 1.5 V indicating that the target of 2 A cm-2 at 1.8 V will be easily achieved with further investigation.

Membrane Electrolyzers for Hydrogen Production: From Catalyst Fundamentals to Devices

Water electrolysis powered by renewable sources such as wind or solar produces clean hydrogen gas, which is used for many industrial processes and will be essential in a future clean energy economy. Commercialized membrane electrolyzers use acidic proton exchange membranes (PEMs), which offer high performance but require the use of expensive precious-metal catalysts such as IrO2 and Pt that are nominally stable under the locally acidic conditions. Anion exchange membrane (AEM) electrolyzers in principle offer the performance of PEM electrolyzers with the ability to use earth-abundant catalysts and inexpensive bipolar plate materials. I will present our fundamental work in understanding the chemical and electrochemical processes in earth-abundant water-oxidation catalysts, including the use of integrated reference-electrode device architectures and cross-sectional materials analysis, as well as progress in building high-performance AEM electrolyzers. Baseline systems operate at 1 A·cm-2 in pure water feed at < 2 V at a moderate temperature of ~70 °C using either IrO2 or Co3O4 anode catalyst layers, Versogen TP-85 alkaline ionomers, and stainless-steel porous transport layers. These devices, however, degrade rapidly compared to PEM electrolyzers. The voltage profile corresponding to degradation has initial fast (~10 mV/h) and steady-state slow components (~1 mV/h), which we link to chemical and structural changes in the ionomer catalyst reactive zone. We further discover that dynamic Fe-based OER catalysts – that have world record performance in traditional liquid alkaline electrolyzer systems – perform poorly with enhanced degradation rates in alkaline membrane electrolysis, illustrating fundamentally different chemical design principles for OER catalysts. Lastly, I will discuss these processes in electrolyzer devices and in model systems, as well as present promising new chemical strategies we have developed to mitigate degradation and enhance performance using novel ionomers and electrode catalyst compositions and architectures.

Impact of the binding ability of anion exchange ionomer on the initial performance degradation of anion exchange membrane water electrolyzers

A durable catalyst layer (CL) is essential for the successful commercialization of anion exchange membrane water electrolyzers (AEMWEs). However, AEMWEs often exhibit a large performance drop in the early stage of operation, which has not been clearly understood. Herein, we unravel the root cause of the initial degradation for a typical anode CL composed of a quaternized polycabozole-trimethyl amine (QPC-TMA) anion exchange ionomer and IrO2 nanoparticles. Post-mortem and electrochemical analysis reveal that the detachment of the IrO2 nanoparticles from the CL and the interfacial delamination of the CL from the porous transport layer (PTL) surface are responsible for the initial degradation. Nafion (ionomer) as a co-binder prevents the initial degradation by increasing the mechanical integrity and interfacial adhesion of the CL, thereby indicating a strong binder effect on the initial degradation. Molecular dynamics and density functional theory calculations elucidate that the binding energy of QPC-TMA on IrO2 or PTL surface is much smaller than that of the Nafion ionomer, because the ionomer–catalyst or ionomer–PTL interaction is sterically hindered for the bulky TMA group. This study motivates structural designing of anion exchange ionomers with strong binding abilities to address the initial degradation problem.

Proton Exchange Membrane Water Electrolysis Incorporating Sulfo-Phenylated Polyphenylene Catalyst Coated Membranes

Herein, fully hydrocarbon-based ionomer catalyst coated membranes (CCM)s based on sulfo-phenylated polyphenylene biphenyl (sPPB-H+) are evaluated in a proton exchange membrane water electrolyzer cell at 70 °C. In conjunction with catalyst layers incorporating Nafion® D520 ionomer, sPPB-H+ membranes require a lower applied potential than reference Nafion® NR112 recast membranes of equal thickness, e.g., 1.66 V vs 1.75 V at 1 A cm−2. Initially, sPPB-H+ membranes yield substantially lower gas crossover compared to the CCMs containing NR112 membrane, however, increasing gas crossover and a higher rates of voltage evolution is measured during a stability test at 1 A cm−2. A reinforced and commercialized membrane of similar chemistry (Pemion®) is also evaluated, demonstrating smaller changes to the initial gas crossover owing to higher mechanical stability. Employment of sPPB-H+ as the catalyst layer binder is also investigated, where the highest efficiency for all-sPPB-H+ CCMs is achieved when the anode and cathode ionomer content are 20 wt%, which from electrochemical impedance spectroscopy is predominately attributed to changes in the proton conductivity within the catalyst layer. Despite this advancement, high ionomer swelling and catalyst delamination led to shorter electrolyzer lifetimes for the all hydrocarbon and non fluorinated membrane electrode assembly, informing future ionomer development.

Molecular-Level Control over Oxygen Transport and Catalyst–Ionomer Interaction by Designing Cis–Trans Isomeric Ionomers

We here demonstrate a monographic study to investigate the effects of cis–trans conformation on the properties of anion exchange ionomers for the first time. Twisted cis-conformation possessed of abundant subnanoscale free volume cavities facilitates oxygen permeability and water adsorption. Moreover, its nonplanar conformation can alleviate the undesirable absorption of ionomer on Pt and Pt–Ru surfaces, thus improving the electrochemical performance of the catalyst layers. Theoretical and electrochemical investigations are mutual corroborations, demonstrating our proof-of-concept. The catalyst layer equips cis-ionomer showing 51% and 54% increases in limiting current density and electrochemically active surface area. The H2/O2 single-cell peak power density of the cis-membrane electrode assembly (c-MEA) is increased by 56% (1.0 vs 0.64 W cm–2) compared with that of the t-MEA at 100% relative humidity (RH). Moreover, the performance of c-MEA is particularly prominent at low humidity (increased by 114% and 113% at 80% and 60% RH, respectively).

(Invited) Controlling Charge Transfer and Ion Transport in Electrodes for the Oxygen Evolution Reaction

Anion Exchange Membrane (AEM) based energy conversion devices offer the possibility of utilizing non-precious metals as the catalyst and the advantage of more facile electrochemical conversions. While many AEMs are showing practical performance and durability’s, their potential will only be realized if the catalyst particles are modulated by the cationic ionomer for maximum performance. The catalyst layer in the electrode provides the triple-phase boundary between the anion-conducting polymer, supported catalyst particles, and reactants where the electrochemical reactions take place. In proton-exchange membrane fuel cells (PEMFC), the electrode catalyst layer was developed empirically over many decades. State of the art AEM devices could be developed with new scientific insight and methodology facilitating the opportunity to be much more quickly developed as well as to exhibit maximum performance and durability. Also for PEMFC to improve Pt catalyst utilization, high surface area carbon-supported Pt catalysts were developed, followed by the development of catalyst inks and the implementation of electrode spraying techniques commonly in use today. However, carbon supported catalysts do not have the necessary durability for the oxygen evolution reaction (OER) necessitating the use of Iridium oxide in acid systems.We hypothesize that the crucial component of an electrode is the ionomer that modulates the transport of ions and neutrals to the catalyst surface and by its interaction with the surface of the catalyst can also modulates charge transfer in solid state electrodes. So, by understanding the ionomer chemistry we can design ionomer catalyst interfaces for maximum performance. For the OER we use porous Ni electrodes that we imbibe with catalyst ionomer inks. In these inks we can not only vary the catalyst to ionomer ratio, but the ionomer chemistry and IEC. To date we have studied nano-structured silver1 or cobalt oxide catalysts. The ionomers are either random or block co-polymers consisting of polyisoprene or polycyclooctene hydrophobic components as the hydrophobic component that can be hydrogenated to polymethylbutylene or polyethylene. The other component is a quaternizable polychlorostyrene that can we functionalize with trimethyl amine or methylpyrolidine. These combinations give a large experimental space in which to probe the effect of ionomer chemistry on electrode performance. We test the electrodes in a separated anode experiment where linear sweep voltammetry is used to get kinetic information and cyclic voltammetry is used to determine the double layer capacity that we use as a measure of electrochemical surface area. Using silver the ionomer chemistry clearly effcts charge transfer and transport of ions and neutrals in the electrode.1 Optimized electrodes are them used to construct 5 cm2 electrolysis cells where the electrolyzer performance and durability is evaluated. In these cells we use a Pt on carbon anode with a cationic ionomer of fixed composition and high performance triblock AEM2 that we developed that is currently being commercialized under license to SparkIonix as Tuffbrane™. Our studies to date use pumped carbonate as the water source, but we are also extending this work to distilled water or hydroxide solutions. Removing the supporting anions from the water feed by ionomer/catalyst interface engineering would dramatically improve these AEM based water electrolysis systems. “Evaluating the Effect of Ionomer Chemical Composition in Silver-Ionomer Catalyst Inks toward the Oxygen Evolution Reaction by Half-Cell Measurements and Water Electrolysis.” N.C. Buggy, I. Wu, Y. Du, R. Ghosh, M.-C. Kuo, M.S. Ezell, J.M. Crawford, S. Seifert, M.A. Carreon, E.B. Coughlin, A.M. Herring,* Electrochimica Acta., 2022, 412, 140124.“A Polyethylene-based Triblock Copolymer Anion Exchange Membrane with High Conductivity and Practical Mechanical Properties.” N.C. Buggy, Y. Du, M.-C. Kuo, K.A. Ahrens,S. Wilkinson, S. Seifert, E.B. Coughlin,* and A.M. Herring*, ACS Applied Polymer Materials, 2020, 2, 1294 – 1303.

(Invited) Alkaline Membrane Electrolyzers: Catalysts, Degradation Mechanisms, and Materials Engineering for Performance and Durability

Commercialized membrane electrolyzers use acidic proton exchange membranes (PEMs). These systems offer high performance but require the use of expensive precious-metal catalysts such as IrO2 and Pt that are nominally stable under the locally acidic conditions of the ionomer. Alkaline-exchange-membrane (AEM) electrolyzers in principle offer the performance of commercialized PEM electrolyzers with the ability to use earth-abundant catalysts and inexpensive bipolar plate materials. I will present our fundamental work in understanding earth-abundant water-oxidation catalysts, including the use of integrated reference-electrode device architectures and cross-sectional materials analysis, as well as progress in building high-performance AEM electrolyzers. Baseline systems operate at 1 A·cm-2 in pure water feed at < 1.9 V at a moderate temperature of ~70 °C using either IrO2 or Co3O4 anode catalyst layers, PiperION alkaline ionomers, and stainless steel porous transport layers. These devices, however, degrade rapidly compared to PEM electrolyzers. The voltage profile corresponding to degradation has intial fast (~10 mV/h) and steady-state slow components (~1 mV/h), which we link to chemical and structural changes in the ionomer catalyst reactive zone. I will discuss these processes in electrolyzer devices and in model systems that are correlated with this performance loss, as well as present promising new strategies we have developed to mitigate degradation using novel ionomers and electrode catalyst compositions and architectures. Figure 1

Anion Exchange Membrane Water Electrolysis from Catalyst Design to the Membrane Electrode Assembly

Anion exchange membrane (AEM) electrolysis aims to combine the benefits of alkaline electrolysis, such as stability of the cheap catalyst and advantages of proton‐exchange membrane systems, like the ability to operate at differential pressure, fast dynamic response, low energy losses, and higher current density. However, as of today, AEM electrolysis is limited by AEMs exhibiting insufficient ionic conductivity as well as lower catalyst activity and stability. Herein, recent developments and outlook of AEM electrolysis such as cost‐efficient transition metal catalysts for hydrogen evolution reaction and oxygen evolution reaction, AEMs, ionomer, electrolytes, ionomer catalyst–electrolyte interaction, and membrane‐electrode assembly performance and stability are described.

Structural stability of catalyst ink and its effects on the catalyst layer microstructure and fuel cell performance

At present, the further development of proton exchange membrane fuel cell (PEMFC) is in urgent need of commercialization. Therein, the storage of catalyst ink is a neglected but crucial topic. Undoubtedly, a structurally stable catalyst ink is more resistant to storage and suitable for commercialization. In this work, the structural stability of catalyst inks containing short-side-chain (SSC) ionomer and long-side-chain (LSC) ionomer after storage and their effects on the catalyst layer (CL) microstructure and the cell performance are investigated. The result demonstrates that the catalyst ink with LSC ionomer has better structural stability than the catalyst ink with SSC ionomer. The adsorbed ionomers on Pt/C in SSC ionomer catalyst ink increase after ink storage, resulting in increased ionomer coverage and deteriorated ionomer distribution in the CL, which makes the cell subject to higher mass transport resistance. As a result, the LSC ionomer catalyst ink after storage can achieve the initial ink properties after redispersion, while the SSC ionomer catalyst ink fails. Therefore, to promote the practical application of SSC ionomer, the ink storage should be considered. • The effect of catalyst ink storage on structural stability is studied. • The LSC ionomer catalyst ink has structural stability. • The adsorption of ionomer on Pt/C in SSC ionomer catalyst ink is time-dependent. • Storage leads to increased ionomer coverage and deteriorated ionomer distribution. • SSC ionomer ink storage causes higher mass transport resistance and proton conduction.

(Invited) Electrocatalytic Conversion of CO2 to C2 Products

Renewably powered CO2 electrocatalysis presents an opportunity to de-carbonize fuel and chemical production. Commercial application of CO2 reduction will require electrocatalytic systems that provide reactants, electrons, and products at high rate and efficiency, and that are compatible with established upstream and downstream processes. I will outline our progress on membrane electrode assembly based cells to meet this challenge. To accommodate O2 impurities from upstream processes we develop a hydrated ionomer catalyst coating that selectively slows O2 transport and stabilizes the copper catalyst. To increase reaction rate and energy efficiency we develop an adlayer catalyst strategy that increases local CO2 availability and tunes intermediate adsorption for ethylene production. For ethanol production we focus on minimizing product cross-over to the anode, targeting ethanol production in excess of the 10wt% - comparable to bio-ethanol production and compatible with downstream processes. I will highlight learnings, challenges and opportunities arising from scaling our membrane electrode assembly based cells in a pilot plant.

Characterization of Solvent-Dependent Ink Structure and Catalyst Layer Morphology Based on Ink Sedimentation Dynamics and Catalyst-Ionomer Cast Films

Ink structure is of great research interest because it influences the catalyst layer microstructure and ultimately determines fuel cell performance. To improve understanding of the ink structure, particle size distribution and ink sedimentation dynamics were characterized in this study by performing acoustic spectroscopy (AS) and analytical centrifugation (AC) analyses, respectively, using actual concentrations of Pt/C inks (10.4 wt% of solid content) without dilution. For the catalyst inks prepared with 1-propanol–water bi-solvents of different compositions, the particle size distributions of the inks were similar when a small amount of ionomer (I/C 0.2) was included in the ink. However, the ink sedimentation dynamics showed significantly different results depending on the solvent composition. The different ink dynamics can be explained in terms of the ink structure, and these solvent-dependent ink structures were further studied through the fabrication of catalyst–ionomer cast films. The catalyst agglomeration size and the catalyst–ionomer interfacial structure were effectively observed using scanning electron microscopy (SEM) images obtained from the cast films. The present study reveals the effects of the solvent properties on the structures of ink and catalyst agglomerates. It also demonstrates that the AS and AC techniques can be useful for ink studies.

Tuning Ni-MoO2 Catalyst-Ionomer and Electrolyte Interaction for Water Electrolyzers with Anion Exchange Membranes.

Tailoring catalyst–ionomer and electrolyte interaction is crucial for the development of anion exchange membrane (AEM) water electrolysis. In this work, the interaction of Ni–MoO2 nanosheets with ionomers and electrolyte cations was investigated. The activity of Ni–MoO2 nanosheets for the hydrogen evolution reaction (HER) increased when tested in 1 M NaOH compared to 1 M KOH; however, it decreased when tested in 0.01 M KOH compared to 1 M KOH electrolyte. The capacitance minimum associated with the potential of zero free charge (pzfc) was shifted negatively from 0.5 to 0.4 V versus RHE when KOH concentration increased from 0.1 mM to 1 M KOH, suggesting a softening of the water in the double-layer to facilitate the OH– transport and faster kinetics of the Volmer step that lead to improved HER activity. The catalyst interaction with cationic moieties in the anion ionomer (or organic electrolytes) can also be rationalized based on the capacitance minimum, because the latter indicates a negatively charged catalyst during the HER, attracting the cationic moieties leading to the blocking of the catalytic sites and lower HER performance. The HER activity of Ni–MoO2 nanosheets is lower in benzyltrimethylammonium hydroxide (BTMAOH) than in tetramethylammonium hydroxide (TMAOH). Anion fumion ionomer and electrolytes with organic cations with benzyl group adsorption (such as BTMAOH) lead to decreased HER activity in comparison with TMAOH and Nafion. By utilizing Ni–MoO2 nanosheet electrodes as a cathode in a full non-platinum group metal (PGM) AEM electrolyzer, a current density of 1.15 A/cm2 at 2 V cell voltage in 1 M KOH at 50 °C was achieved. The electrolyzer showed exceptional stability in 0.1 M KOH for 65 h at 0.5 A/cm2.

Effect of N-cyclic cationic groups in poly(phenylene oxide)-based catalyst ionomer membranes for anion exchange membrane fuel cells

Herein, as a binder (or catalyst ionomer) for AEMFCs, we investigated the effect of two different cationic copolymers based on poly(phenylene oxide) (PPO) with N-cyclic quaternary ammonium (QA) groups, including six-membered dimethyl piperidinium (DMP) and bis-six-membered azonia-spiro undecane (ASU). An earlier report on the same polymers for membranes in AEMFCs indicated the better electrochemical performance of PPO-ASU compared with PPO-DMP. Therefore, we would like to investigate these two polymers for catalyst ionomers. The outcome in this study using these two copolymers as catalyst ionomers indicates the opposite result; the electrochemical performance of the PPO-DMP ionomer is much better than the PPO-ASU ionomer. The commercial Fumion ionomer was used for the qualitative comparison. The density functional theory (DFT) calculation of the adsorption energy according to different orientations of the cationic groups on the catalyst surface shows that there is no difference between the adsorption energy of DMP and ASU cations, in compliance with the orientations of the cations. Although the PPO-ASU ionomer membrane has the highest hydroxide conductivity at 60 °C in liquid water, the hydrogen oxidation/reduction (HOR) activity of PPO-DMP and PPO-ASU showed similar values with the Fumion ionomer. While the PPO-DMP ionomer membrane shows relatively large fuel gas (hydrogen) permeability in dry and wet conditions, due to the chain flexibility and the presence of two methyl groups compared to the single methyl groups and lower flexibility of the PPO-ASU and Fumion ionomers. The electrochemical performance of a membrane electrode assembly (MEA) using the PPO-DMP ionomer exhibited an exceptional peak power density of 335 mW cm−2 compared to lower peak power densities the of PPO-ASU and Fumion ionomers under 60 °C and a fully humidified condition (H2/O2). The SEM images of MEAs after testing supports the conclusion that the PPO-DMP ionomer forms a uniform catalyst interface that is very well bound between the electrode and membrane, unlike the PPO-ASU and Fumion ionomers. The PPO-DMP ionomer membrane also showed better tensile strength and elongation at break than the PPO-ASU ionomer membrane. Therefore, we conclude that the well-prepared three-phase boundary structure played a critical role for the catalyst ionomer in each electrode, overcoming one of the critical performance-limiting factors.