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/cm 2 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. In 2015, the global primary energy consumption was 153 PWh, 1 corresponding to an average rate of energy conversion of 17 TW. About 30% (∼6 TW) of this is used for electricity generation, which yields around 2.8 TW of electrical power (24 PWh per year). Around two thirds of the electricity is generated from fossil fuels, hydropower contributes 16%, nuclear power 11%, and other renewables (such as solar and wind) only 6.7%.1 Solar (photovoltaics) and wind power have a combined installed capacity of about 660 GW (in 2015).2 Electricity supply based on a significant share of these "new renewables" is associated with large discrepancies between supply and demand, owing to the intermittent nature of these primary energy sources. Hence, solutions for the grid-scale storage of electricity need to be developed and implemented. The electrochemical splitting of water (electrolysis) is a clean and efficient process offering interesting prospects to store large amounts of excess electricity in form of chemical energy ('power-to-gas' concept). 3,4 The produced hydrogen and oxygen can be used to regenerate electricity in periods of low production and high demand or serve as clean transportation fuel for fuel cell electric vehicles. Therefore, water electrolysis is a key technology in future sustainable energy scenarios, since hydrogen as an energy 'vector', i.e., as a universal energy carrier, could promote the decarbonization of the energy economy, or even become its backbone in the context of a 'hydrogen economy'.5 Moreover, the produced hydrogen can be used to methanate CO 2 from suitable sources, such as biogas plants, to produce synthetic natural gas (SNG), which can be injected and stored in the natural gas network. 6...

Formation of radicals, such as HO , H and HOO , in the membrane of the polymer electrolyte fuel cell and their attack on perfluoroalkylsulfonic acid (PFSA) and poly(styrenesulfonic acid) (PSSA) ionomers was simulated based on a kinetic framework with H 2 O 2 as "parent" molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10 À19 M for H , 10 À16 M for HO and 10 À10 M for HOO . H is formed via the reaction of HO with H 2 dissolved in the membrane. The attack of the PFSA ionomer was assumed to proceed via weak carboxylic end-groups. The corresponding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test conditions. The predicted FER under fuel cell operating conditions was underestimated by 2-3 orders of magnitude. It is likely that degradation via side-chain attack is prevalent during open circuit voltage hold tests. The oxidative degradation of PSSA ionomer follows an entirely different pathway, because, in addition to a-hydrogen abstraction by HO , the aromatic ring effectively scavenges HO to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product.The proton conducting membrane in the polymer electrolyte fuel cell (PEFC) is exposed to considerable oxidative stress due to the presence of reactive intermediates formed in the membrane electrode assembly (MEA), which attack the electrolyte membrane and lead to chain scission, loss of polymer constituents, membrane thinning and, eventually, failure of the cell. 1,2 The chemical breakdown of the polymer additionally causes loss of mechanical integrity with ensuing mechanical failure of the membrane due to pinhole or crack formation. 3 The nature of the reactive intermediates formed during electrochemical H 2 and O 2 conversion in the PEFC has been discussed for many years in many articles. The insight that H 2 O 2 is involved in membrane degradation was already gained in the 1960s. 4,5 The observation that iron or other redox-active metal ions appeared to accelerate the degradation led to the suggestion that oxygen radicals are produced through the metal-ion-catalyzed decomposition of hydrogen peroxide (Fenton reaction). The working hypothesis that hydroxyl radicals (HO ) and hydroperoxyl radicals (HOO ) are the responsible species for membrane degradation has been accepted for a long time. Direct detection of radical intermediates in fuel cells was accomplished only recently by introducing spin-trapping agents into the MEA and placing a miniature fuel cell into an electron spin resonance (ESR) spectrometer. Using this approach, Roduner et al. proposed that HO and polymer derived carbon centered radicals are formed. 6 More recently, Schlick et al., using a similar approach, proved the presence of carbon centered radicals, HO , HOO as well as H in an operating fuel cell. 7 The reaction mechanisms leading to the formation of radical intermediates have been the subject ...

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The cost of polymer electrolyte fuel cell (PEFC) components is crucial to the commercial viability of the technology. Proton exchange membranes fabricated via the method of radiation grafting offer a cost‐competitive option, because starting materials are inexpensive commodity products and the preparation procedure is based on established industrial processes. Radiation grafted membranes have been used with commercial success in membrane separation technology. This review focuses on the application of radiation grafted membranes in fuel cells, in particular the identification of fuel cell relevant membrane properties, aspects of membrane electrode assembly (MEA) fabrication, electrochemical performance and durability obtained in cell or stack tests, and investigation of failure modes and post mortem analysis. The application in hydrogen and methanol fuelled cells is treated separately. Optimized styrene / crosslinker grafted and sulfonated membranes show performance comparable to perfluorinated membranes. Some properties, such as methanol permeability, can be tailored to be superior. Durability of several thousand hours at practical operating conditions has been demonstrated. Alternative styrene derived monomers with higher chemical stability offer the prospect of enhanced durability or higher operating temperature.

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A novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.

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The attack by HO • on the ionomer in the polymer electrolyte fuel cell (PEFC) can be mitigated by the incorporation of regenerative radical scavengers, such as cerium and manganese, into the membrane. The influence of the presence of the Ce and Mn ions on the reaction of HO • with the perfluoroalkylsulfonic acid (PFSA) ionomer was studied in the framework of a kinetic model. Scavenging of HO • by Ce 3+ and Mn 2+ leads to a decrease in the steady-state concentration of HO • and, therefore, the rate of ionomer attack, where Ce is more effective than Mn. The HO • scavenging yield was found to decrease with increasing reactive end-group concentration of the PFSA ionomer. The oxidized metal ions are rapidly reduced by both and HOO • , which restores the scavenger and ensures that the majority (>99.99%) of metal ions is in the reduced, "HO • -scavenging-active" oxidation state. This stabilization mechanism is not expected to be operational in hydrocarbon membranes, because HO • reacts with aromatic units far too rapidly. Also, the effectiveness of the mitigation mechanism is limited under the conditions of an ex situ Fenton test due to the abundance of H 2 O 2 , which is confirmed by experimental data reported in the literature.