Article

# (Plenary) Challenges in Going from Laboratory to Megawatt Scale PEM Electrolysis

Authors:
• Proton OnSite
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## Abstract

Renewable hydrogen is becoming an increasingly important component of the transition away from fossil fuel use and towards reduction in carbon dioxide production. Hydrogen is the intermediary between primary energy sources and end products in many chemical processes such as ammonia generation, refining, and biogas processing, and is currently mainly produced by reforming of natural gas. Hydrogen from electrolysis can both make a strong environmental impact on these industries and also improve utilization of intermittent renewable energy sources such as wind and solar by leveraging otherwise stranded resources. Proton exchange membrane (PEM) electrolysis is especially well suited to energy capture because of the dynamic range and ability to quickly ramp up and down from near zero output to full capacity. This paper will discuss the challenges in continued scale up, translating laboratory scale findings to commercial PEM systems as well as some of recent advancements and impact on cost.

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... Typical catalysts for commercial electrodes have iridium oxide loading from 1 to 3 mg cm −2 [14]. This level of catalyst loading is too high to meet the long term cost targets for energy markets [13,15,16]. Furthermore, while using current electrolysis technology, the translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of catalyst cost and stability [15]. ...
... This level of catalyst loading is too high to meet the long term cost targets for energy markets [13,15,16]. Furthermore, while using current electrolysis technology, the translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of catalyst cost and stability [15]. ...
Article
The balance of catalyst loading, activity and stability remains a challenge for the anode of proton exchange membrane (PEM) water electrolyzers. Here we report a nano-size IrOx/Nafion® composite catalyst that exhibits both outstanding activity for oxygen evolution reaction (OER) and stability in a PEM water electrolyzer. The IrOx/Nafion® catalyst layer is fabricated using a flame-based cost-effect process, reactive spray deposition technology. The IrOx/Nafion® catalyst shows >10 times improvement in OER mass activity compared to IrOx nanoparticles synthesized using the wet chemistry method. The IrOx/Nafion® catalyst also achieved ∼4,500 h of stable operation in MEA electrolyzer at 1.8 A cm⁻² and 80 °C with ultra-low iridium loading of 0.08 mg cm⁻². Analysis of the IrOx structure and the electrochemical performance revealed three key factors for balancing high stability and activity: (1) high ratio of Ir (IV) to Ir (III) species and high content of hydroxide on the surface; (2) high anodic charge and surface area due to nano-size IrOx particles that are well-dispersed in the Nafion® ionomer electrolyte; (3) homogeneous anode catalyst layer morphology.
... Typical catalysts for commercial electrodes have IrOx loading from 1 to 3 mg cm À2 [24]. This level of catalyst loading is too high to meet the long-term cost targets for energy markets [23,25,26]. Furthermore, while using current electrolysis technology, the translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of catalyst cost and stability [25]. ...
... This level of catalyst loading is too high to meet the long-term cost targets for energy markets [23,25,26]. Furthermore, while using current electrolysis technology, the translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of catalyst cost and stability [25]. ...
... But, up to now, hydrogen production using PEM-electrolyzers is not yet economically, as hydrogen production by steam reforming is much more cost effective. In order to build and operate PEM-electrolysis systems on the same level as steam reformers (concerning costs per kg hydrogen output), large scale (> MW) and longterm stable (several years) systems are required [3], [4]. On the other hand, the conversion of the stored chemical energy back to electrical energy seems to be rather settled, as fuel cell technology is already commonly used [5]. ...
... On the other hand, the conversion of the stored chemical energy back to electrical energy seems to be rather settled, as fuel cell technology is already commonly used [5]. As on the field of PEM-electrolysis, huge progress has been achieved in recent years [6] and largescale systems have been built already [4]. Regardless, further fundamental research and development on a laboratory scale is a necessity. ...
Article
The technology of polymer electrolyte membrane (PEM) electrolysis provides an efficient way to produce hydrogen. In combination with renewable energy sources, it promises to be one of the key factors towards a carbon-free energy infrastructure in the future. Today, PEM electrolyzers with a power consumption higher than 1 MW and a gas output pressure of 30 bar (or even higher) are already commercially available. Nevertheless, fundamental research and development for an improved efficiency is far from being finally accomplished, and mostly takes place on a laboratory scale. Upscaling the laboratory prototypes to an industrial size usually cannot be achieved without facing further problems and/or losing efficiency. With our novel system design based on hydraulic cell compression, a lot of the commonly occurring problems like inhomogeneous temperature and current distribution can be avoided. In this study we present first results of an upscaling by a factor of 30 in active cell area. © 2017, Engineering and Technology Publishing. All rights reserved.
... A similar example (top view) of the anodic side of a CCM after 100 h of electrolyzer operation including cracks caused by excessive cell compression is shown by Millet et al. 100 Performance deterioration is mostly related to problems with the purity of feed water, and is a reversible degradation mechanism. 101 Metallic cations are dissolved in the de-ionized water that is circulated through the system components made of stainless steel. The cations contaminate the membrane by exchanging with protons in the PFSA material, which leads to decreased proton conductivity. ...
Article
Full-text available
Although polymer electrolyte water electrolyzers (PEWEs) have been used in small-scale (kW to tens of kW range) applications for several decades, PEWE technology for hydrogen production in energy applications (power-to-gas, power-to-fuel, etc.) requires significant improvements in the technology to address the challenges associated with cost, performance and durability. Systems with power of hundreds of kW or even MWs, corresponding to hydrogen production rates of around 10 to 20 kg/h, have started to appear in the past 5 years. The thin (∼0.2 mm) polymer electrolyte in the PEWE with low ohmic resistance, compared to the alkaline cell with liquid electrolyte, allows operation at high current densities of 1-3 A/cm² and high differential pressure. This article, after an introductory overview of the operating principles of PEWE and state-of-the-art, discusses the state of understanding of key phenomena determining and limiting performance, durability, and commercial readiness, identifies important 'gaps' in understanding and essential development needs to bring PEWE science & engineering forward to prosper in the energy market as one of its future backbone technologies. For this to be successful, science, engineering, and process development as well as business and market development need to go hand in hand.
... However, their MEAs feature an IrRuOx catalyst at the anode side and are based on thinner, 90 μm Aquivion ® membranes, which renders a quantitative comparison with our data in Fig. 3 difficult. Also Danilovic et al. 53 show averaged cell voltage data for a long-term electrolysis test (60,000 h) of two different industrial stack design evolution levels, whereby the cell voltage change rates are highest during the first 1000 h. However, there are also results in the literature where such an initial increase or nonlinear change is not immediately visible. ...
Article
Lowering the iridium loading at the anode of proton exchange membrane (PEM) water electrolyzers is crucial for the envisaged GW-scale deployment of PEM water electrolysis. Here, the durability of a novel iridium catalyst with a low iridium packing density, allowing for low iridium loadings without decreasing the electrode thickness, is being investigated in a 10-cell PEM water electrolyzer short stack. The anodes of the membrane electrode assemblies (MEAs) of the first five cells utilize a conventional iridium catalyst, at loadings that serve as benchmark for today’s industry standard (2 mg Ir /cm ² ). The last five cells utilize the novel catalyst at 8-fold lower loadings (0.25 mg Ir /cm ² ). The MEAs are based on Nafion ® 117 and are tested for 3700 h by load cycling between 0.2 and 2.0 A/cm ² , with weekly polarization curves and impedance diagnostics. For both catalysts, the performance degradation at low current densities is dominated by an increase of the overpotential for the oxygen evolution reaction (OER), whereby the OER mass activity of the novel catalyst remains ≈4-fold higher after 3700 h. The temporal evolution of the OER mass activities of the two catalysts will be analyzed in order to assess the suitability of the novel catalyst for industrial application.
... Water electrolyzers in industrial applications have operating lifetimes exceeding 20,000 h under harsh conditions [43,44]. The field of CO 2 electrolysis is much less mature. ...
... The total amount of deposited catalyst per electrode corresponds to a loading of 0.2 mg cm −2 , and a total PGM loading of 0.4 mg cm −2 , which is one order of magnitude lower than a conventional PEM electrolyzer MEA corresponding to 3-5 mg cm −2 . 3,22 Photovoltaic.-Commercially available triple-junction GaAs solar cells (Type: TJ Solar Cell 3T34C) from AZUR SPACE Solar Power GmbH with InGaP/GaAs/Ge sub-cells on a Ge substrate were used to drive the electrolyzer or PEC cell, the electrical data for that solar cell is available on the manufacturer's website. ...
Article
Full-text available
Photoelectrochemical (PEC) water splitting has the potential to significantly reduce the costs associated with electrochemical hydrogen production through the direct utilization of solar energy. Many PEC cells utilize liquid electrolytes that are detrimental to the durability of the photovoltaic (PV) or photoactive materials at the heart of the device. The membrane-electrode-assembly (MEA) style, PEC cell presented herein is a deviation from that paradigm as a solid electrolyte is used, which allows the use of a water vapor feed. The result of this is a correspondent reduction in the amount of liquid and electrolyte contact with the PV, thereby opening the possibility of longer PEC device lifetimes. In this study, we demonstrate the operation of a liquid and vapor-fed PEC device utilizing a commercial III-V photovoltaic that achieves a solar-to-hydrogen (STH) efficiency of 7.5% (12% as a PV-electrolyzer). While device longevity using liquid water was limited to less than 24 hours, replacement of reactant with water vapor permitted 100 hours of continuous operation under steady-state conditions and diurnal cycling. Key findings include the observations that the exposure of bulk water or water vapor to the PV must be minimized, and that operating in mass-transport limited regime gave preferable performance.
... Proton exchange membrane water electrolysis (PEMWE) is considered as one of the most promising technologies for this purpose [4]. However, the widespread application of PEMWE is limited mainly by high cost and low efficiency of the electrocatalyst materials [5]. Materials that catalyze water decomposition should provide high reactivity and maintain stability throughout long-time operation. ...
Article
Full-text available
The anodic oxygen evolution reaction has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up to date Iridium is known as the best catalyst material for the oxygen evolution reaction in acidic media due to its relatively high activity and long-term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One of the promising ways would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the oxygen evolution reaction of an oxidized Ir–Ti thin film material library covering the composition range from 20 – 70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and X-ray photoelectron spectroscopy. Overall, our data show that Ir-Ti mixtures can be promising OER catalysts with both high activity and high stability.
... ll CO 2 Hydrogenation Coupled with Water Electrolysis Direct CO 2 electroreduction is a potentially promising single-step route for methanol production from CO 2 ; however, CO 2 hydrogenation combined with electrolysis has proven to be the most scalable and implemented technology for renewable CO 2 conversion to alcohols. Electrolysis of water occurs in standard commercial alkaline, PEM, or solid oxide systems that are commercially available on the scale of several megawatts, 38 whereas exothermic hydrogenation of CO 2 (Equation 5) occurs in fixed-bed flow reactors that typically use a catalyst comprising copper oxide, zinc oxide, and alumina (CZA) similar to that of the syngas process. ...
Article
Production of renewable alcohols from air, water, and sunlight present an avenue to utilize captured carbon dioxide for the production of basic chemicals and store renewable energy in the chemical bonds of liquid fuels. Of the technologies that utilize CO2 directly, CO2 electrolysis, as well as CO2 hydrogenation coupled with H2O electrolysis, have the benefit of requiring only CO2, H2O, and renewable electricity as inputs with O2 as a sole byproduct. Among alcohols, renewable methanol has seen the most development and analysis in the chemical industry because it is currently a syngas-derived product that could be adapted for direct CO2 utilization. In this perspective, we compare renewably powered CO2 electrolysis and CO2 hydrogenation with the incumbent methanol production method from syngas from a cost and CO2 life cycle perspective by analyzing recent literature to identify the research goals that enable further scale-up. Survey of the industry shows that CO2 hydrogenation is among the closest CO2 utilization technologies to large-scale deployment. We further discuss these CO2 hydrogenation systems and the catalysts that drive them, with recommendations to drive further development and scale-up.
... owing to stabilization of the polymer endgroups, 13 so that membrane thinning is likely less of an issue when using state-of-the-art PFSA membranes. 14 Another membrane related degradation effect is the contamination of the ionomeric membrane with cations, 11,[15][16] typically introduced by improperly treated feed-water which is the major cause for PEM-WE failures in the field, [16][17] Sun et al. showed the operation of a 9-cell PEM-WE stack for 7800 h at constant current, and recorded a gradual decrease in performance that they attributed to cationic contamination, since the initial performance was mostly recovered by boiling the degraded MEA (membrane-electrode-assembly) in sulfuric acid. 18 Apart from degradation of the membrane in the membraneelectrode-assembly via chemical degradation and cationic contamination, gradual passivation of the titanium porous transport layer (PTL) at the high potentials experienced by the anode electrode of an electrolyzer increases the internal ohmic resistance and, hence, leads to a decrease of performance. ...
... Experimental tests have been reported on material analysis (e.g., [2,3]), operating conditions (e.g., [4,5]), cell design (e.g., [6,7]), or the analysis of systematic phenomena (e.g., [8,9]), which are sometimes difficult to generalize without knowledge of the existing system constraints. Additionally, a well-known key issue for PEMWE is the scalability of these results from the microscopic to the system level (e.g., [10]). The deduction from microscopic phenomena with differential cells via spatially distributed cells of different geometries to the stack or the system level are of particular interest. ...
Article
Full-text available
In the field of polymer electrolyte membrane water electrolysis (PEMWE), a significant amount of excellent scientific results has been generated during the past decades. However, the comparability and reproducibility of these results between different cell types and different laboratories is not always straightforward. In this contribution, an exemplary ring experiment on the single-cell level compares the performances of three cell types: the differential cell (4cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${4}{\text { cm}^{2}}$$\end{document}) and two integral cells: an elongated cell (50.4cm×0.45cm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${50.4}{\text { cm}}\times {0.45}{\text { cm}}$$\end{document}) and a circular cell (63.5cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${63.5}{\text { cm}^{2}}$$\end{document}). Therefore bi- and trilateral experiments were carried out with differently prepared catalyst-coated membranes (CCMs) and porous transport layers (PTLs) as well as with an alternative catalyst-coated electrode (CCE) concept in three laboratories. This contribution aims to evaluate the grade of systemic inequality, which still permits a comparison of individual parameters. The comparison of CCM preparation methods showed no significant influence on the initial electrochemical characteristics. An HCl etching of the anode PTLs in two different cells confirmed to be a useful treatment for the reduction of Ohmic losses in PEMWE cells. Self-made CCEs could not serve as an alternative concept, owing to their inadequate contact between the electrode and the membrane, which was observed in three laboratories as well. The general compatibility between the different cells was proven by the observation of a phenomenon in one laboratory that could be reproduced in one or two other laboratories. In this context, the size and geometry of the single cells did not influence the performance, indicating that up to the present measuring range and with sufficient water feed rates, the different single cells were functioning comparably. Graphical Abstract
... ll CO 2 Hydrogenation Coupled with Water Electrolysis Direct CO 2 electroreduction is a potentially promising single-step route for methanol production from CO 2 ; however, CO 2 hydrogenation combined with electrolysis has proven to be the most scalable and implemented technology for renewable CO 2 conversion to alcohols. Electrolysis of water occurs in standard commercial alkaline, PEM, or solid oxide systems that are commercially available on the scale of several megawatts, 38 whereas exothermic hydrogenation of CO 2 (Equation 5) occurs in fixed-bed flow reactors that typically use a catalyst comprising copper oxide, zinc oxide, and alumina (CZA) similar to that of the syngas process. ...
Article
Full-text available
Production of renewable alcohols from air, water, and sunlight present an avenue to utilize captured carbon dioxide for the production of basic chemicals and store renewable energy in the chemical bonds of liquid fuels. Of the technologies that utilize CO2 directly, CO2 electrolysis, as well as CO2 hydrogenation coupled with H2O electrolysis, have the benefit of requiring only CO2, H2O, and renewable electricity as inputs with O2 as a sole byproduct. Among alcohols, renewable methanol has seen the most development and analysis in the chemical industry because it is currently a syngas-derived product that could be adapted for direct CO2 utilization. In this perspective, we compare renewably powered CO2 electrolysis and CO2 hydrogenation with the incumbent methanol production method from syngas from a cost and CO2 life cycle perspective by analyzing recent literature to identify the research goals that enable further scale-up. Survey of the industry shows that CO2 hydrogenation is among the closest CO2 utilization technologies to large-scale deployment. We further discuss these CO2 hydrogenation systems and the catalysts that drive them, with recommendations to drive further development and scale-up.
... Currently, nanoparticle iridium oxide/metallic blends are the main OER catalysts used in PEM electrolyzers. [11][12][13][14] Other acid-stable catalyst combinations of platinum group metals have been reported, but these metals are exceedingly rare. 15 Ir is ~10,000 times less abundant than Ni, Co, or W. 16 Even with affordable minimal catalyst loadings, it is unclear if production of these noble metals could be scaled up to meet global energy needs. ...
Article
Acid-based electrolysis has many advantages, but to achieve simultaneous activity and stability, commercial water oxidation catalysts rely on noble metal oxides that are expensive and too rare for the global scale. Here, earth-abundant tungsten was used as a structural metal to dilute the noble metal iridium content while maintaining high activity and stability in acid. Mixed-metal oxide catalysts were synthesized using rapid plasma oxidation in which the non-equilibrium reaction environment permitted better formation of a homogenous W1-xIrxO3-δ phase. With an Ir metal content as low as 1%, a competitive and durable overpotential for oxygen evolution was achieved. Relative to high Ir content, low Ir compositions consisted of a more highly crystalline, phase-pure iridium polytungstate which was more catalytically active per Ir content. Moreover, the plasma-synthesized material had a sharp electrocatalytic improvement over an equivalent composition synthesized via standard thermal oxidation, demonstrating the value of non-equilibrium synthesis to find new catalysts.
... The technologies necessary for the electrification of transportation, the generation of green hydrogen at scale [1] for transport and manufacturing [2], and the grid-scale energy storage to accommodate the intermittency of renewable electricity generation [3,4], all require new materials solutions [5]. The performance and service life time of these materials are inherently related to their microstructure, chemistry, physics and interaction with their environment. ...
Preprint
Full-text available
The search for a new energy paradigm with net-zero carbon emissions requires new technologies for energy generation and storage that are at the crossroad between engineering, chemistry, physics, surface and materials sciences. To keep pushing the inherent boundaries of device performance and lifetime, we need to step away from a cook-and-look approach and aim to establish the scientific ground to guide the design of new materials. This requires strong efforts in establishing bridges between microscopy and spectroscopy techniques, across multiple scales. Here, we discuss how the complementarities of X-ray- and electron-based spectroscopies and atom probe tomography can be exploited in the study of surfaces and sub-surfaces to understand structure-property relationships in electrocatalysts.
... While electrolysis is an established commercial technology, much of the product development has focused on scale and assembly, with less focus on specific material and process development optimized for electrolysis. 13 The companies and component suppliers in this technology area are also primarily smaller, with limited resources, which extend commercialization timelines even further. Electrolysis, therefore, lags far behind fuel cells in material optimization, though lab and subscale experiments have shown an enormous potential for cost and performance improvements. ...
Article
Sustainable, carbon-free methods of large-scale hydrogen production are urgently needed to support industrial processes while decreasing carbon dioxide emissions. The realities of product development timelines dictate that existing commercial technologies such as low-temperature electrolysis will have to serve the majority of this need for at least the next 20 years. At the same time, even a cursory understanding of device design principles and real-world constraints can help to inform basic research. Accelerating the impact from fundamental material discoveries in related technologies therefore requires improved collaboration between academic, government, and industry sectors. Renewable hydrogen is a key component to global decarbonization and reduction in carbon dioxide emissions. A common misconception is that the need for greener sources of hydrogen is dependent on whether fuel cell vehicles significantly penetrate the automotive market. However, hydrogen is a critical feedstock for many industrial processes, with an annual demand of 65 million metric tons globally. The large majority of this hydrogen is made via steam methane reforming, which represents the major carbon dioxide contribution for industrial processes such as ammonia production. Sustainable manufacturing of hydrocarbons also requires a sustainable source of hydrogen. Deep decarbonization and meeting 80% reduction targets for carbon dioxide emissions thus requires carbon-free sources of hydrogen. Based on the technology readiness levels, the reality is that existing commercial technologies will dominate the market for the next 20 years and beyond. To accelerate the impact of fundamental work in long-term technologies, improved collaboration between researchers across academic, government, and industry sectors is essential, to inform basic research as well as to leverage technology breakthroughs in the near term.
... Electrode stability, however, should not be sacrificed. Considering acidic environments as the most promising ones, nowadays only Irbased anodes meet these strict requirements [4][5][6][7]. Scarcity and the high price of Ir, however, are serious hindrances in the way of its universal application. Hence, optimization of the catalyst utilization becomes of very high importance. ...
Article
Full-text available
Owing to their superior electrocatalytic performance, non-stoichiometric mixed oxides are often considered as promising electrocatalysts for the acidic oxygen evolution reaction (OER). Their activity and stability can be superior to those of the state-of-the-art IrO2 catalysts, although the exact nature of this phenomenon is not yet understood. In the current work, a Ir0.7Sn0.3O2-x thin-film electrode is taken as a representative example for a thorough evaluation of OER activity of the non-stoichiometric oxides. Complementary activity and stability analysis of Ir0.7Sn0.3O2-x electrodes is achieved using a setup based on an electrochemical scanning flow cell and ICP-MS. The obtained ICP-MS data presents an unambiguous proof of the preferential dissolution of the less noble Sn from the mixed oxide during OER. While less than a monolayer of Ir is dissolved after a prolonged electrolysis of 1400 min during which its dissolution rate drops to near zero, the amount of Sn lost is ten monolayers. The latter finding is confirmed by XPS analysis, which besides showing Ir surface enrichment also indicates a gradual transformation of Ir⁰ to IrIII species. This transition is beneficial for electrode activity, as the overpotential for OER at j = 5 mA cm⁻² was decreasing up to 300 mV. The increase in electrode activity is attributed to several mechanisms including generation of IrIII active sites and overall surface area increase. A generalized description of OER catalysis by Ir-based materials is given, including data from the current work as well as from other Ir-based mixed oxides, such as Ir-Ru-O and Ir-Ni-O. Graphical Abstract
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We present a platinum wire micro-reference electrode (Pt-WRE) suitable for recording individual electrochemical impedance spectra of both the anode and the cathode in a proton exchange membrane water electrolyzer (PEM-WE). For this purpose, a thin, insulated Pt-wire reference electrode (Pt-WRE) was laminated centrally between two 50 μ m Nafion® membranes, whereby the potential of the Pt-WRE is determined by the ratio of the local H 2 and O 2 permeation fluxes at the tip of the Pt-WRE. Impedance analysis with the Pt-WRE allows determination of the proton sheet resistance of the anode, the anode catalyst layer capacitance, and the high-frequency resistance (HFR) of both electrodes individually, using a simple transmission-line model. This new diagnostic tool was used to analyze performance degradation during an accelerated stress test (AST), where low and high current densities were alternated with idle periods without current (i.e., at open circuit voltage (OCV)), mimicking the fluctuating operation of a PEM-WE with renewable energy. Our analysis revealed that the increasing HFR that was observed over the course of the OCV-AST, which is the main cause for the observed performance decrease, can unequivocally be assigned to an increasing contact resistance between the anode electrode and the porous transport layer.
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This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.
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Low-temperature water electrolysis using an anion conductive polymer electrolyte has several potential advantages over other technologies, however, the fabrication of durable alkaline electrodes remains a challenge. Detachment of catalysts results in the loss of electrochemical surface area. Simple mixtures of ionomer and catalyst can suffer from poor catalyst adhesion because only physical adhesion is used to bind the components together. A family of chemically bonded, self-adherent, hydroxide conducting ionomers were synthesized and tested under alkaline electrolysis conditions with nickel ferrite anode electrocatalysts and platinum-nickel cathode catalyst. The ionomers are based on hydroxide conducting poly(norbornene) polymers used as the solid polymer electrolyte in alkaline fuel cells and electrolyzers. The synthesized terpolymer ionomers have been functionalized to provide pendant sites for covalent chemical bonding of bis(phenyl)-A-diglycidyl ether to the ionomer, catalyst, and porous transport layer. The electrodes show excellent adhesion between the catalyst particles, porous transport layer and ionomer, as determined by adhesion measurements and electrolysis performance. The AEM electrolyzer had stable voltage performance under high current density (1 A/cm² at 1.83 V (67% voltage efficiency)) for extended time periods (>600 h) without degradation.
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Hydrogen is an important chemical feedstock for many industrial applications, and today, more than 95% of this feedstock is generated from fossil fuel sources such as reforming of natural gas. In addition, the production of hydrogen from fossil fuels represents most carbon dioxide emissions from large chemical processes such as ammonia generation. Renewable sources of hydrogen such as hydrogen from water electrolysis need to be driven to similar production costs as methane reforming to address global greenhouse gas emission concerns. Water electrolysis has begun to show scalability to relevant capacities to address this need, but materials and manufacturing advancements need to be made to meet the cost targets. This article describes specific needs for one pathway based on proton exchange membrane electrolysis technology.
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