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Self-Adhesive Ionomers for Alkaline Electrolysis: Optimized Hydrogen Evolution Electrode
Hydrogen produced through low-temperature water electrolysis using anion exchange membranes (AEM) combines the benefits of liquid-electrolyte alkaline electrolysis and solid-polymer proton exchange membrane electrolysis. The anion conductive ionomers in the oxygen-producing anode and hydrogen-producing cathode are a critical part of the three-dimensional electrodes. The ionomer in the hydrogen-producing cathode facilitates hydroxide ion conduction from the cathode catalyst to the anode catalyst, and water transport from the anode to the cathode catalyst through the AEM. This ionomer also binds the catalyst particles to the porous transport layer. Here in this study, the cathode durability was improved by use of a self-adhesive cathode ionomer to chemically bond the cathode catalyst particles to the porous transport layer. It was found that the cathode ionomers with high ion exchange capacity (IEC) were more effective than low IEC ionomers because of the need to transport water to the cathode catalyst and transport hydroxide away from the cathode. The cathode durability was improved by using ionomers which were soluble in the spray-coated cathode ink. Optimization of the catalyst and ionomer content within the cathode led to electrolysis cells which were both mechanically durable and operated at low voltage.
Evaluating the effect of ionomer chemical composition in silver-ionomer catalyst inks toward the oxygen evolution reaction by half-cell measurements and water electrolysis
The effect of anion exchange ionomer (AEI) chemistry on the kinetics of the oxygen evolution reaction (OER) was systematically studied in both half-cell and single-cell electrolysis experiments. OER was studied in 1 M K 2 CO 3 at 50°C using an array of ionomer-silver catalyst inks deposited on Ni foam. Different ionomer modifications were investigated to optimize the OER performance. The AEI used in this work features a block copolymer backbone of polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene which was functionalized with either benzyltrimethylammonium (TMA) or benzylmethylpiperidinium (MPRD) quaternary ammonium cations. Here, using an MPRD quaternary ammonium cation shifted the rate determining step at high overpotentials, ultimately providing enhanced performance. Increasing the catalyst ink dilution was also found to substantially improve mass activity and catalyst utilization (increase from 35 to 88 A g –1 Ag), likely by reducing the ionomer thickness and decreasing transport resistances. In the final ionomer modification, the Ni foam substrate was leveraged to partially hydrogenate the polycyclooctene midblock to be polyethylene-like at the Ni interface. A significant increase in the electrochemical surface area (Cdl increased from 8.8 to 20.8 mF cm –2 ) and performance (by 47 mA cm –2 or 29 A g –1 Ag) was observed with the incorporation of polyethylene. Kinetic results from half-cell experiments were validated via single-cell anion exchange membrane electrolysis experiments, where the optimized electrode displayed enhanced performance in both 1 M K 2 CO 3 and DI H 2 O.
Platinum–Nickel Nanowires with Improved Hydrogen Evolution Performance in Anion Exchange Membrane-Based Electrolysis
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Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers
Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K 2 CO 3 are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH – ), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO 3 2– /HCO 3 – anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. Furthermore, the insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.
Effect of stoichiometry and hydration level on water domain size and transport in poly(aryl piperidinium) alkaline anion-exchange membranes
Alkaline water electrolysis holds promise in decarbonizing the global economy by enabling renewable hydrogen production with non-precious group metal catalysts. Anion exchange membranes are an important component of alkaline water electrolyzers and would ideally be durable while allowing for high hydroxide conductivity. The poly(aryl piperidinium) (PAP) class of polymers has attracted recent interest due to their good mechanical robustness and high ionic conductivity. Here, in this work, we perform atomistic molecular dynamics (MD) simulations of several PAP polymers at experimentally relevant hydration levels and polymer ion exchange capacities (IECs) to gain nanoscale insight into their properties and to help elucidate the trade-offs that result from tuning the IECs through the polymer stoichiometry. Our MD-predicted macroscopic polymer properties were found to be in good agreement with experimentally available polymer swelling ratios, water-occupied volumes, X-ray scattering, and ionic conductivities. The models show that for hydration levels greater than 8H 2 O per cation a single water cluster will form that percolates through the system. The growth in water cluster size results in large polymer swelling, the creation of larger channels with widths of 7 Å or larger, and nanophase separation between the hydrophilic domains and the polymer with characteristic length scales of approximately 20–30 Å. The experimentally observed lack of a strong X-ray scattering peak at low wavevectors can be explained by a cancellation between the polymer-polymer/water-water and polymer-water correlations and not a loss in nanophase separation. The overlap in coordination environments of the hydroxide oxygen and polymer nitrogen atoms implies that vehicular diffusion between cationic groups could play a role in hydroxide transport. The polymers' hydroxide and water diffusion constants increase by approximately an order of magnitude between hydration levels of 8 and 20H 2 O per cation. However, there are diminishing returns in hydroxide diffusion constant once the IEC exceeds 2.4 meq/g.
A Review of the Use of Immobilized Ionic Liquids in the Electrochemical Conversion of CO2
This paper is a review on the application of imidazolium-based ionic liquids tethered to polymer backbones in the electrochemical conversion of CO2 to carbon monoxide and formic acid. These tethered ionic liquids have been incorporated into novel anion ion exchange membranes for CO2 electrolysis, as well as for ionomers that have been incorporated into the cathode catalyst layer, providing a co-catalyst for the reduction reaction. In using these tethered ionic liquids in the cathode catalyst composition, the cell operating current increased by a factor of two or more. The Faradaic efficiencies also increased by 20–30%. This paper provides a review of the literature, in addition to providing some new experimental results from Dioxide Materials, in the electrochemical conversion of CO2 to CO and formic acid.
Functionalized Embedded Monometallic Nickel Catalysts for Enhanced Hydrogen Evolution: Performance and Stability
Viable anionic exchange membrane (AEM) electrolysis for H 2 production requires highly active hydrogen evolution reaction (HER) catalysts that are also robust and cost-effective. Traditionally juxtaposition of two transition metals with one being more oxophilic is necessary for easier access to water in the inner Helmholtz plane as a source of protons in the crucial Volmer step for enhanced HER activity. However, they are prone to passivation at anodic potentials or deactivation via the formation of sub-surface metal hydrides at high cathodic overpotentials. Here we report a method of enabling close juxtaposition of Ni-Ni-oxide surfaces by functionalizing Ni-based catalysts via the careful thermal treatment of carbon-supported Ni nano-particles chelated with a select organic chelating compound referred to as cupferron. This functionalization results in an embedded Ni surface with a clear graphitic coating engendering long-term stability for passivation and subsurface hydride formation. The stability and activity of the functionalized Ni catalyst are demonstrated in half-cell RDE and full-cell AEM membrane-based hydrogen pump experiments. In the case of the latter, the functionalized Ni outperforms carbon-supported Pt.
Reports from the Frontier: Understanding Voltage Losses in Anion Exchange Membrane Water Electrolyzers
With the growth of renewable energy sources, hydrogen is attracting significant attention worldwide as an effective medium for energy storage. “Green hydrogen” is currently produced primarily by water electrolysis in which water is split into hydrogen and oxygen using power from low-carbon energy sources such as wind, solar, and nuclear. Among the low temperature water electrolysis technologies, anion exchange membrane water electrolyzers (AEMWEs) have recently emerged as a promising competitor to traditional alkaline water electrolyzers (AWEs) and proton exchange membrane electrolyzers (PEMELs) due to their potential stack cost reduction in various cell components. In conclusion, favorable aspects of AEMWEs include the use of PGM-free electrocatalysts as well as low-cost membranes, bipolar plates (BPs), and porous transport layers while offering high voltage efficiency and durability.
Ionomer Optimization for Water Uptake and Swelling in Anion Exchange Membrane Electrolyzer: Oxygen Evolution Electrode
Water electrolysis using an anion conductive, solid polymer electrolyte is an attractive method for point-of-use hydrogen production. Recent advances in catalysts and anion exchange membranes (AEM) have made alkaline devices increasingly competitive with their acidic counterparts. However, less attention has been paid to the anion conductive ionomers (ACI) used in the fabrication of electrodes for AEM electrolyzers. The ACI contributes to ion conduction between the catalyst and bulk electrolyte and serves as a binder for adhering the catalyst to the gas diffusion layer and AEM. Ionic conductivity, water uptake and ionomer swelling are critical properties for electrode performance. High ion exchange capacity (IEC) in the ionomer is desired for reduced electrode resistance, however, it can lead to excess water uptake (WU) and disruptive ACI swelling. In this study, a series of poly(norbornene)-based ionomers were synthesized, characterized and used to fabricate oxygen evolving anodes for low-temperature AEM water electrolysis. The IEC of the ionomers (0 to 4.73 meq g −1 ) was adjusted by controlling the ratio of ion conducting to non-ion conducting norbornene monomers in the ACI tetrablock copolymers. Low conductivity ionomers are shown to yield the best-performing oxygen evolution electrodes, in the absence of ACI polymer cross-linking because they do not experience excessive water swelling. Light cross-linking within the anode ACI was used as a means to independently lower WU of the ionomer without compromising ionic conductivity. This control over water swelling allows higher ionic conductivity within the ACI to be used in water-fed electrolyzer applications. Other methods of water management were compared including the use of hydrophobic additives and adjustment of the ionomer concentration in the electrode. As a result, it was shown that the cell performance greatly benefits from a highly conductive ionomer in the oxygen evolution reaction electrode if the WU is managed.
Aryl ether-free polymer electrolytes for electrochemical and energy devices
Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ethercontaining AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels–Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure–property–performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO 2 electrolysers, along with the current status of scale-up synthesis and commercialization.
Electrically Insulated Catalyst–Ionomer Anode Interfaces toward Durable Alkaline Membrane Electrolyzers
Anion-exchange-membrane water electrolysis (AEMWE) is an emerging technology for scalable hydrogen production. AEMWE has poor durability when operating without supporting electrolyte due to the oxidation of ionomers and membranes in contact with the anode oxygen evolution reaction (OER) catalyst. We report a new “passivated” anode architecture for AEMWE where the OER catalysts and ionomers are physically separated with a thin film amorphous oxide coating that is electrically insulating but conductive to hydroxide ions. We find that 2–3 nm of HfO x passivation layers show sufficient hydroxide ion transport to minimally limit the cell performance while suppressing ionomer degradation with both Ir (500 mA·cm –2 for 40 h) and CoO x (1.0 A·cm –2 for 100 h) model porous-transport-layer-supported catalysts in AEMWE. As a result, this interfacial engineering approach guides electrode design to improve the durability of AEMWE, particularly for systems operating with pure-water feed.
Recent progress in understanding the catalyst layer in anion exchange membrane electrolyzers – durability, utilization, and integration
This review discusses recent insights in catalyst layer design strategies for anion exchange membrane water electrolyzers, including electrode design, catalyst/ionomer integration, operational variables, in situ diagnostics, and cell durability.
The ionomer as an oxygen evolution reaction promoter: piperidinium’s impact on mechanistic pathways on NiO, IrO 2 , and Fe–NiO
The commercial viability of anion exchange membrane (AEM) electrolysis requires optimization of various stack components, with specific catalyst-ionomer combinations often yielding higher current densities, lowered Tafel slopes, and improved mass activity. In this joint theoretical-experimental study, theoretical calculations detail the impact of Versogen’s piperidinium functional group on the complex, kinetically limiting oxygen evolution reaction, finding that the functional group can act as a promoter of specific steps (O*/O 2 * formation; H 2 O/O 2 desorption with reaction enthalpies ranging between 0.2–0.6 eV at higher coverages of O x H y intermediates) on NiO and NiFeO x catalysts. In particular, Fe sites on the NiFeO x catalyst facilitate concerted mechanisms of O*/O 2 * formation and H 2 O desorption with a low enthalpy of 0.5 eV; O 2 desorption alone requires only 0.3 eV. In contrast, Versogen-IrO 2 results in stronger Ir–O bonds, where the enthalpies for bond breaking (Ir–OH 2 and Ir–O 2 ) are considerably higher (1.4 eV and 1.6 eV, respectively). Rotating disk electrode studies utilized commercially available NiO and IrO 2 and synthesized 7.5 wt % Fe in NiFeO x catalysts in combination with Versogen, a common AEM ionomer, and Nafion, an alternative binder. Electrochemical testing validated the impact of these mechanistic changes on ionomer-catalyst combinations, finding that Versogen particularly activates NiO and NiFeO x compared to IrO 2 . Following a 13.5 h hold at 1.8 V, mass activities and Tafel slopes improved to 34 ± 13 A g −1 and 79 ± 2 mV dec −1 (NiO) and 82 ± 4.9 A g −1 and 72 ± 2 mV dec −1 (NiFeO x ). In contrast, Versogen-IrO 2 only reached 17 ± 2.9 A g −1 and 81 ± 3 mV dec −1 . Optimization of the ionomer-catalyst can yield significant increases in performance from initial activity and after an electrochemical conditioning procedure: this enhancement to the mass activity resulted in a 200.9 ± 106.1% improvement for Versogen-NiFeO x and 1284.2 ± 260.5% for Versogen-NiO. In contrast, Nafion-NiFeO x and -NiO offered moderate improvements of 39.1 ± 30.5% and 120.9 ± 59.1%, respectively.
Chalkboard 2 - How to Make Clean Hydrogen
Clean hydrogen is a carbon-free energy carrier that can be produced from water and sustainable energy sources such as wind, solar, and nuclear. Therefore, clean hydrogen is one of the best ways to not only decarbonize the energy supply system, but also address the zero-emission challenges specific to large-carbon emitting industries that are difficult to separate from fossil fuels. To help achieve the Biden Administration's goal of a 100% clean energy economy and net-zero emissions by 2050, several tens of millions of metric tons of clean, low-cost hydrogen will be needed annually. The HydroGEN Advanced Water Splitting Materials (AWSM) Consortium was established in 2016 as part of the Energy Materials Network (EMN) under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office to enhance the performance, improve the durability, and reduce the cost of clean hydrogen production technologies, and it is helping to advance the H2@Scale vision.
Long-term durability test of highly efficient membrane electrode assemblies for anion exchange membrane seawater electrolyzers
Direct seawater electrolysis faces fundamental electrochemical challenges, such as suppression of chlorine adsorption and/or evolution reactions that poison the current state-of-the-art anode catalysts and accelerate the degradation of the anion exchange membrane (AEM). The technically favored solution path for addressing these issues includes development of novel selective catalysts and implementation of limited operating conditions. Therefore, development of highly active, selective, cost-effective, and stable seawater-splitting catalysts is required for a successful commercialization of the AEM water electrolyzers (AEMWEs). In this work, we report for the first-time an extended operation for over 1000 h of advanced AEMWEs, operating directly with seawater. To fabricate the membrane electrode assemblies (MEAs), catalyst coated electrodes (CCEs) are assembled with Sustainion® X37-50 grade T anion exchange membranes. The anode CCEs are fabricated by spraying an ink prepared from nanostructured NiFe-layered double hydroxide (LDH) catalyst that are synthesized by the solvothermal method and deposited directly onto platinized titanium porous transport layers (PTLs). The cathode CCEs are prepared by spraying an ink prepared from commercial Renay-Nickel catalyst onto a nickel fiber felt. As prepared AEM electrolyzer cells operated with seawater and demonstrated better than previously reported in the literature performance of over 1000 h of operation at a constant current density of 300 mA cm –2 .
Next-generation anion exchange membrane water electrolyzers operating for commercially relevant lifetimes
Alkaline anion exchange membrane (AEM) water electrolysis has gained increasing attention due to its potential to achieve low-cost, high performance hydrogen production. However, most existing membranes are not durable in industrial settings. In this work, we demonstrate good performance relative to industrial parameters using Sustainion® anion exchange membranes. Long-duration tests showed stable performance of 1 A/cm 2 at 1.85 V with a degradation rate of less than 1 μV/h over 10,000 h. The projected lifetime is thus over 20 years. Daily on/off cycling performance over the course of 30 years was simulated experimentally through accelerated voltage shock tests, resulting in a performance loss of only 0.15 μV/cycle over 11,000 cycles. As shown through impact and crossover testing, an improvement in performance is achieved by the addition of zirconia to the polymer matrix and mechanically reinforcing the membrane.