Andrew M. Herring

As renewable electricity becomes less and less expensive and it's use becomes more widespread, there is a need for electrical energy-to-chemical energy conversion for both storage and for the use of electrons as reagents. Current electrolysis applications focus on large-scale alkaline water electrolysis (\ or proton exchange membrane water electrolyzers, featuring well-developed technology and industrial equivalent current densities. However, significant drawbacks, such as high overpotential, poor design, inherent environmental risks, reliance on hard-to-access chemicals, and poor understanding of membrane and electrode properties, have constrained opportunities for improvement. We have been working on anion exchange membranes for a number of years and we are now addressing the cationic ionomers and electrocatlysts needed for device fabrication. To address this we are investigating the interaction of ionomers on metal substrates and the use of structured ionomers as templates to produce nano-structured metal particles as heterogeneous electrocatlayt/ionomer compose materials . We use a series of random and structured block co-polymers with unsaturated hydrophobic components that can be hydrogenated to rubbery polymethylbutylene or semi-crystalline polyethylene. Co-polymerized with these components we use vinyl benzyl chloride which once polymerized can be quaternized with a large variety of amines to systematically vary the size and nature of the organic cation in the polymer. Initial studies using commercial Ag nano-particles or flat Ag substrates indicated that we could vary the interaction of the ionomer with the metal surfaced by changing the polymer chemistry. Unsaturated unquaternized polymers showed very strong orthogonal alignment on flat Ag surfaces. When quaternized the material became less ordered but still maintained vertical transport channels to the catalysts surface. Ag nano-particle effected the crystal city of the polyethylene component of the block co-polymers via interaction with the cations. We have recently been extending these studies to Nickel foam electrodes and studying the effect if ionomer and ionomer/Ag composites on electrochemical processes important to electrolysis.

Practical proton exchange membrane (PEM) fuel cells still use PFSA materials for durability and performance reasons and are limited operating temperatures of 80°C at reasonable pressures to ensure they have sufficient hydration for high proton conductivity. PFSA materials have become undesirable due to the use of forever chemicals in their synthesis and low temperature PEM limits heat transfer and the use of these devices as direct fuel cells for fuels beyond hydrogen. We were inspired to create membranes using polyoxometlates (POM) by John Turner over 25 years ago when he was inspired by a paper he attended where aqueous solutions of POMs showed promise as an alternate electrolyte to PFSA membranes. In collaboration with NREL students we showed that POM membranes in the solid state had high performance in low temperature fuel cells and could even function as fuel cell membranes at >200°C. The POMs are extremely soluble in water and so we use lacunary POMs functionalized with chemistry that can lead to the functionalized POM being a monomer in a variety of insoluble polymer systems. Initially we used a polyacrylate system that produced fuel cell ready membranes with very high proton conductivity's measured to 120°C under relatively dry conditions. Inevitably we came to the conclusion that the ester linkage in these interesting conditions might not be fuel cell stable. So we next turned our attention to functionalizing dehydroflourinated PVDF based materials. This perflourinated solution gave materials with superior performance under standard operating conditions and exceptional durability results in accelerated stress tests. Currently we are investigating a polyamide solution that continued research will result in a fluorine free membrane that has promise to operate at hotter and drier conditions than are currently accessible enabling heavy duty fuel cell applications. One chance encounter with John has led to a continuing 25 year effort that is challenging what is possible in PEM membranes.

The need for a consistent, well-understood electrical energy-to-chemical energy conversion method is currently being observed. It is further necessary as predictions show increased electrical energy within the next century. Electrolytic processes are a possibility to address the need for this conversion due to being highly efficient at low temperatures; however, there are limitations in applying electrolytic processes to everyday settings, which include the limited understanding of the electrochemical process and scalability. Current research focuses on large-scale alkaline water electrolysis (AWE) and proton exchange membrane water electrolyzers (PEMWE), featuring well-developed technology and industrial equivalent current densities. However, significant drawbacks, such as high overpotential, poor design, inherent environmental risks, reliance on hard-to-access chemicals, and poor understanding of membrane properties, have constrained opportunities for process scalability and demonstrate the need for more in-depth knowledge of the relation between physio-chemical properties and their effects on the electrochemical properties. To address the knowledge gap challenge, we will conduct basic scientific research on silver-anion exchange membrane composite materials in water electrolysis to enhance our knowledge of electrochemical processing when observing the hydrogen evolution reaction. To carry out this research, we employ a cation ionomer as an electrochemical template for developing silver nano-array electrodes. We will systematically vary parameters within the membrane to gain insights into the physical and chemical properties that occur while optimizing electrochemical performance. This research utilizes a polyvinylbenzylchloride-b-polyethylene-b-polyvinylbenzylchloride triblock polymer, quaternized either in trimethylamine or methyl piperidinium, as a cationic ionomer and an electrochemical interface template to create structured silver nano-array electrodes. Polymer membranes will be dip-coated in a basic silver salt solution and a reducing solution to synthesize the silver-membrane composite material. Controlled variations in tri-block polymer concentration, polymer morphology, and silver deposition enable systematic adjustments to be observed within the system, resulting in optimal electrochemical properties. The initial tri-block polymer’s chemical and physical properties will be assessed using gel permeation chromatography, attenuated total reflection-Fourier-transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Confirmation of silver nanoparticle synthesis will be achieved through elemental analysis, high-resolution transmission electron microscopy with energy-dispersive X-ray spectroscopy, with X-ray diffraction and small-angle X-ray scattering as determining the membrane morphology. The electrochemical properties of the polymer membranes, such as resistance and conductivity, will be evaluated using electrochemical impedance spectroscopy, cyclic voltammetry, ion exchange capacity, and electrochemical atomic force microscopy. We will establish initial design rules to optimize the electrochemical double layer and further electrocatalytic processes by systematically developing silver-membrane composite material.

Proton-exchange membrane water electrolysis (PEMWE) is one of the most pivotal technologies in searching for clean and sustainable hydrogen production. It has the ability to produce high-purity hydrogen at high current densities within a compact system design, which makes it a great promise as a solution for future hydrogen-based energy systems[1, 2]. Developing high-performing ionomers for electrode integration is crucial for optimizing the performance and long-term durability of PEMWE systems [3]. Currently, perfluorosulfonic acid (PFSA)--based ionomers are regarded as standard choices for electrochemical devices due to their chemical durability and excellent proton conductivity. However, their widespread use is limited on a daily basis for several key drawbacks, such as environmental concerns related to their fluorinated nature, high cost, and the possibility of degradation by reactive oxygen species like hydroxyl radicals generated from hydrogen peroxide byproducts[4]. This PFSA limitation accelerated the search for alternative fluorine-free ionomers, such as hydrocarbon-based ionomer materials, which provide low cost, improved performance, and reduced environmental impact. Polyoxometalates (POMs), especially lacunary heteropolyacids (HPAs), are potential candidates for their strong proton mobility and ability to reduce chemical degradation through scavenging hydroxyl radicals[5]. However, HPA's usages are still limited due to their high water solubility, which, if improperly immobilized within a solid matrix, might lead to induced migration, clustering, or leaching. In response, this study investigates the synthesis of next-generation proton-conducting ionomers using polymer polyoxometalates (PolyPOMs). In order to create a stable hybrid ionomer matrix, a novel synthetic technique is employed in which a lacunary HPA—more precisely, 11-silicotungstic acid, a Keggin-structured POM—is covalently immobilized at maximal loading onto a polymer backbone and then copolymerized with an organic co-monomer. By efficiently immobilizing the ordinarily water-soluble POM inside the ionomer, this technique allows it to serve as a radical scavenger as well as a proton-conducting moiety. A higher concentration of HPA should theoretically improve proton transport and chemical degradation resistance against oxygenated radicals like •OH and •OOH without introducing the leaching problems often seen with mobile radical scavengers like cerium or manganese [6]. Additionally, the inclusion of the polymer backbone offers superior chemical and thermal stability, which qualifies the ionomer for the demanding water electrolysis operating conditions. This study primarily focuses on the synthesis, structural optimization, and assessment of its essential transport characteristics so that the synthesized ionomer can be incorporated into membranes and electrodes in PEMWE systems. A suite of different advanced analytical techniques is used to determine the structural integrity of the synthesized ionomers. A detailed understanding of the ionomer's molecular architecture and morphology is thoroughly obtained via Fourier Transform Infrared Spectroscopy (FTIR), Small Angle X-ray Scattering (SAXS), Wide Angle X-ray Scattering (WAXS), and Solid-State Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), the electrochemical performance of the produced ionomers will be investigated. This study seeks to identify key structure-property correlations that govern proton conductivity and stability. The findings will guide the logical design of advanced ionomers for PEMWE systems that meet the current and future operational and regulatory requirements. References: [1] G. A. Lindquist, Q. Xu, S. Z. Oener, and S. W. Boettcher, “Membrane Electrolyzers for Impure-Water Splitting,” Joule , vol. 4, no. 12, pp. 2549–2561, Dec. 2020, doi: 10.1016/j.joule.2020.09.020. [2] J. E. Park et al. , “High-performance proton-exchange membrane water electrolysis using a sulfonated poly(arylene ether sulfone) membrane and ionomer,” J. Membr. Sci. , vol. 620, p. 118871, Feb. 2021, doi: 10.1016/j.memsci.2020.118871. [3] “Advanced Science - 2023 - Kim - Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers.” [4] S. Siracusano, V. Baglio, F. Lufrano, P. Staiti, and A. S. Aricò, “Electrochemical characterization of a PEM water electrolyzer based on a sulfonated polysulfone membrane,” J. Membr. Sci. , vol. 448, pp. 209–214, Dec. 2013, doi: 10.1016/j.memsci.2013.07.058. [5] J. L. Horan et al. , “Copolymerization of Divinylsilyl‐11‐silicotungstic Acid with Butyl Acrylate and Hexanediol Diacrylate: Synthesis of a Highly Proton‐Conductive Membrane for Fuel‐Cell Applications,” ChemSusChem , vol. 2, no. 3, pp. 226–229, Mar. 2009, doi: 10.1002/cssc.200800237. [6] A. R. Motz et al. , “Heteropoly acid functionalized fluoroelastomer with outstanding chemical durability and performance for vehicular fuel cells,” Energy Environ. Sci. , vol. 11, no. 6, pp. 1499–1509, 2018, doi: 10.1039/C8EE00545A.

At the electrode-electrolyte interface, a phenomenon known as the Electric Double-layer (EDL) occurs, where the accumulation of anions and cations allow for electrochemical reactions at the electrode surface. The EDL is crucial to overall cell performance, as the structure of the double layer influences the rate of electrode processes. By adjusting the local environment of the EDL, the kinetics of these surface reactions can be altered [1]. In Anion Exchange Water Electrolysis (AEMWE) research, studies on how the ionomer’s structure at the electrode-ionomer interface affect the EDL remain limited. This project investigates how the size of an ionomer’s the ion-transfer groups can impact the performance at the EDL by analyzing the double-layer capacitance (C _dl ), electrochemically active surface area (ECSA), specific capacitance (C _s ), and onset potentials seen with varying quaternized ionomers. To study this question, we have synthesized five ionomers all with identical backbones, poly(methyl butylene)-ran-poly(vinyl benzyl chloride) (PMB-r-PVBCl), followed by the quaternization with varying quaternary amines (QAs): methylpiperidinium (MPRD), triethyl amine (TEA), trimethyl amine (TMA), tripropyl amine (TPA), and tributyl amine (TBA). These ionomers are then dip-coated onto porous Ni foam and placed in a FlexCell to perform half-cell tests of the oxygen evolution reaction (OER). Using this set-up, cyclic voltammograms (CVs) are taken at varying scan rates (ν) where the equation, C _dl = Δj(j _a -j _c )2ν, provides the double-layer capacitance. The C _dl represents the charge accumulation and dissipation at the interface and is directly correlated to the ECSA, providing insight into the electrode’s health in terms of charge capability [2]. Along with varying the ionomer size, this half-cell setup allows for adjustments to the local environment to identify optimal conditions for the anodic reaction. Parameters including temperature, pH, electrolyte concentration, and ionomer loading can all be altered and observed [3]. The goal of this work is to connect the physical properties of anion-exchange ionomers with the kinetics of water electrolysis through use of voltammetric techniques. The outcome of these tests will give guidance on choosing which QA group is best suited for the OER in this ionomer system to enhance AEMWE performance. With support from collaborators within the Center of Ionomer-based Water Electrolysis (CIWE), future modeling of this data will provide a portfolio of information on ionomer-electrode interactions at the EDL.

Anion Exchange Membrane (AEM) electrolyzers hold much promise in that they can be composed entirely of environmentally benign hydrocarbon polymers and potentially need no precious metal catalysts for operation. One of the barriers to commercialization of these devices is the effective utilization on non-precious mortal catalysts and demonstration that practical durability’s can be obtained. We have shewn previously thtat cobalt oxide is an excellent non-precious metal catalyst for the oxygen evolution reaction (OER) in AEM systems. However, it is hard to optimize cobalt oxide loadings in conventional catalyst ink formulations because it is a poor electronic conductor. One way to overcome this is to electrodeposit the cobalt oxide on the electrode, ensuring that all of the catalyst is electrochemically wired. We do this on a conventional Ni foam electrode and use these electrodes to study catalyst ionomer interactions. We probe both ionomer backbone and head group chemistry. The three polymers families we study are a rubbery random co-polymer of isoprene and benzylvinyl chloride, the homopolymer of benzylvinyl chloride and a triblock copolymer with benzylvinyl chloride outer blocks and a polyethylene mid-block. We quaternate these to give cations ranging in size from benzyltrimethylammonium to benzyltri-n-butylammonium, as well as the sterically stabilized benzylmethylpyrolidinium (MPRD) cation. The polymers are then coated on to the electrodes from organic solvents at various loadings. We characterize the electrodes with IR, XPS, and SEM. Electrochemistry of the separated anode is then used to find the double layer capacitance as a measure of surface area, the onset potential and Tafel slope of the OER in carbonate, and hydroxide electrolyte. For the more active combinations we run durability testing to assess the effect of currents >1 A/cm ² and the effect of bubble evolution from the electrode surface. Finally, we build electrolyzers using a membrane derived fro the triblock co-polymer tuned for high performance and chemical stability using the benzylmethylpyrrolidinium cation. The machine-built membranes from this polymer electrolyte are 2 for quality control allowing us to study electrolyzer electrodes for these AEMs in multiple 5 cm ² membrane electrode assemblies (MEAs). The cathode is composed of Pt/C with random co-polymer of isoprene and benzylvinyl chloride quaternized with MPRD. These electrolyzes are characterized for performance and studied for durability at 1 A/cm ² .

Anion Exchange Membrane (AEM) fuel cells hold much promise in that they can be composed entirely of environmentally benign hydrocarbon polymers and potentially need no precious metal catalysts for operation. We have developed a triblock co-polymer with a mechanically robust polyethylene mid-block and quaternizable outer blocks that can be tuned for high performance and chemical stability using the benzylmethylpyrrolidinium cation. We machine build membranes from this polymer electrolyte that are 2, allowing us to study fuel cell electrodes for these AEMs in multiple 5 cm² membrane electrode assemblies (MEAs). Although these membranes are 4 times thicker than commercially desirable and have 4 times lower performance initial fuel cell studies in H₂/O₂ cells show that the performance is limited by flooding. In proton exchange membrane fuel cells excellent performance is observed using an ionomer in the electrode of the polymer with the same chemistry as the membrane, but with higher ion exchange capacity. In our AEM fuel cells when we use the same polymer chemistry as the membrane in the electrode we observe very poor fuel cell performance. This is because water is both a reactant and a product in AEM fuel cells and so water balance and transport across the MEA is important especially if we want to achieve the ultimate goal of utilizing dry fuel and air inputs and stable enough fuel cell operation that the device can be used in the transient operation that would be required for automotive applications. For these reasons we have developed a new ionomer for the electrodes in these cells. The ionomer is based on a random co-polymer of rubbery methybutylene and quaternized vinylchlorobenzne. This polymer has proven to be an excellent electrode binder when used in catalyst inks consisting of Pt on carbon. We can vary the cation by quaternizing with the smaller trimethyl amine or the chemically more stable methylpyrolidine. Careful understanding of the kinetics of the quaternization allows us to vary the degree of functionalization to give ion exchange capacities (IEC) from 1 – 4 meq g^-1. This not only allows us to control the ionic conductivity of the ionomer, but also its water content. We study the water balance and transport in these cell by varying the chemistry, IEC, and ionomer content in the each electrode. Initial studies used symmetrical cells from 50 – 85 °C and showed that while cell resistance decreased the onset of the flooding potential increased. Asymmetrically loaded MEAs allowed us to better understand the water flux through the MEA during fuel cell operation. In this paper we will show how such studies can be used to optimize fuel cell operation. Initial work on understanding durability and the use of non-precious metal catalysts will also be presented.

Headway in research focused on renewable energy sources, specifically in hydrogen, has grown significantly over the past fifteen years. A bibliometric analysis of green-hydrogen-related publications revealed a multifold increase in these works just from 2016 to 2021, with major themes including hydrogen production and storage, the hydrogen economy and water electrolysis. Water electrolysis, a process to produce molecular hydrogen through the splitting of water, has garnered attention in research focused on the scalability and material choices for industrial-level, sustainable hydrogen production. Yet, studies to further explore such research into the specific ionomer interfacial characteristics within in the cell remain limited. Within water electrolysis the ionomer acts as an ion-conducting film binding to the catalyst enabling high current densities. This project focuses on studying the ionomer-interface interactions through observing how the size of the ionomer’s charge transfer groups effect electrochemical kinetics specifically, at the double layer interface. This study begins with characterizing the physical and electrochemical properties poly(methyl butylene)-ran-poly(vinyl benzyl chloride) (PMB-r-PVBCl) ionomers with varying sized quaternary ammonium (QA) groups. Once synthesized, the PMB-ran-PVBCl copolymer will be quaternized with trimethylamine (TMA), triethylamine (TEA), tripropylamine (TPA), tributylamine (TBA) and, methyl piperidinium (MPRD). Techniques such as Gel Permeation Chromatography (GPC), Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR), and Nuclear Magnetic Resonance spectroscopy (NMR) will be used to analyze the composition and physical characteristics of the ionomers. To follow, Electrochemical Impedance Spectroscopy (EIS) and Ion-exchange capacity (IEC) will be used to evaluate the ionomers’ electrochemical properties, such as conductivity and resistance. Additionally, EIS will provide data on the double layer capacitance of the cell with each ionomer. This capacitance provides insight on the electroactive interface at between the ionomer-catalyst later and the electrode. Throughout this investigation, we will be working in collaboration with our partners in the Center of Ionomer-based Water Electrolysis (CIWE) at Lawrence Berkeley National Lab (LBL), Oak Ridge National Lab (ORNL), Texas Tech University, and the University of California at Berkeley, Irvine and Merced. By investigating the effect of different QA group sizes on the same ionomer backbone we can better pinpoint the influence of functional group size on the electrochemical characteristics of the ionomer. Furthermore, the study will investigate how those effects manifest at the ionomer-catalyst interface through changes in the double layer capacitance.

Of all the emerging electrolyzer technologies that can produce low carbon hydrogen, AEM electrolysis coupled with wind, solar or other renewable electricity has the greatest potential as it combines the contactless and rapid response advantages of PEM electrolysis without the need for rare or precious metal catalysts, or per fluorinated hydrocarbons, as demonstrated in the less scalable less responsive, lower power density liquid alkaline electrolysis. Indeed, AEM electrolyzer technology has the potential to meet the DOEs hydrogen earth shot goal of $1/kg H₂. Practical AEM electrolysis with pure water has so far proved elusive. It has been easily shown that adding a small amount of supporting electrolyte, 0.5 - 1 M hydroxide or carbonate, to the circulating aqueous feed facilitates practical current densities. While adding 0.5 M hydroxide gives the highest performance, the use of hydroxide necessitates the use of higher cost, more caustic stable components, and the isolation of the hydroxide from air. Using 0.5 M potassium or sodium carbonate electrolytes only increases the over potential by 100 mV versus hydroxide allowing excellent performance to be achieved and saves significant system costs while excellent durability is simultaneously obtained. We utilize a scalable and versatile triblock co-polymer, ABA, polychloromethylstyrene-b-polyethylene-b-polychloromethylstyene, pCMS-pE-pCMS, polymer system that can be quaternized to give high performance and highly durable AEMs or tunable ionomers. Typically OH^- conductivities are > 0.1 S cm^-1 > 40°C and > 0.1 S cm^-1 for all other common anions, Cl^-, carbonate, bicarbonate > 70°C. In the laboratory this could easily be fabricated at high MW with complete control of A:B ratios on the 10 g scale and in the hands of a third-party it has been scaled up to >1 kg. The polymer can be processed on a tape caster to give sheets > 500 cm² with controllable thicknesses from 10-100 ± 1μm. Further improvements to the polymer chemistry are being investigated that will affect its performance and durability and allow us to cast higher quality thinner films. Furthermore, we are beginning to optimize the ionomer chemistry to allow ultra low loadings of Pt on the Hydrogen side and non-precious metal catalysts, such as MnO₂ and Co₃O₄, on the Oxygen side. To achieve this we can swap the structural polyethylene for the more rubbery polymethylbutylene in our block polymers as well as make random polymers that act as excellent electrode binders. Finally we have begin to probe the effects of changing the aromatic hydrophilic group for a completely hydrocarbon polymer brush. We will present the results of testing these materials for performance and durability in an ambient pressure 5 cm² electrolyzer at 60°C.

While there has been much interest in the proton exchange membrane (PEM) fuel cell, its wide spread use has been restricted by cost, durability, fuel versatility issues, and the environmental consequences of using perfluorosulfonic acid polymers as the membrane and ionomer. This is primarily because catalyst based on Pt are nessecary in a PEM fuel cell on both the anode and the cathode and reactive oxygen species chemically degrade the cell. Alkaline catalysis in fuel cells has been demonstrated with non-precious metal catalysts, and a variety of fuels beyond H₂ and methanol. Alkaline fuel cells (AFCs), based on aqueous solutions of KOH, have serious drawbacks associated with system complexity, low system power densities and carbonate formation. Anion exchange membrane (AEMs) fuel cells have a number of advantages over both PEM fuel cells and traditional AFCs. Great strides have been made in ionic conductivity and durability in AEMs and fuel cells with reduced precious metal loadings have been demonstrated. However, AEM fuel cells have significant water management issues, as water is both a reactant and a product leading to flooding and control issues. We have developed a triblock copolymer Poly (vinylbenzyl-N-methylpiperidinium carbonate)-b-polyethylene-b-poly(vinylbenzyl-N-methylpiperidinium carbonate)as a robust, high performance, durable, and scalable AEM. We have modified the block ratios and ion exchange capacity of this material to show excellent performance and durability in both electro-desalination and water electrolysis. Despite the materials potential, we have only occasionally tested it as a fuel cell membrane. We have now begun to throughly evaluated the material in 5 cm² H₂/O₂ fuel cell. Initial experiments showed good potential performance but were hampered by severe flooding of the electrodes. We will present a materials approach to solving this problem by investigating various related ionomers in the electrodes. Both variations of the polymer used to fabricate the membrane and a random co-polymer derived from polymethylbutylene and poly(vinylbenzyl-N-methylpiperidinium) were investigated. The effect of loading and ionomer ion exchange capacity on the performance of the fuel cell and the relationship between these properties will be reported.