Critical Review—Identifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development

Abstract

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...

Model-supported characterization of a PEM water electrolysis cell for the effect of compression

“…One explanation is to connect the deviation from what the Tafel equation predicts at higher currents to mass transport phenomena [28]. However, in this work mass transport problems are excluded due to high over-stoichiometry, interdigitated feed water channels, a small cell geometry, and a high anode PTL porosity [56]. Additionally, no evidence for mass transport problems were found in the recorded IV curves or impedance spectra.…”

Influence of Operating Conditions and Material Properties on the Mass Transport Losses of Polymer Electrolyte Water Electrolysis

“…7 This two-phase flow was visualized by visible light, neutron and X-ray imaging with focus of the PTL/flowfield interface and the flow regime in the channels. [20][21][22][23] Based on operando in-plane neutron radiography data, Seweryn et al 24 reported a gas/water ratio in the PTL independent of current density, verified up to 2.5 A · cm −2 .…”

“…Ohmic and kinetic overpotentials, related to the movement of protons in the polymer electrolyte and the finite rates of the electrochemical reactions are reasonably well understood for PEWE. [6][7][8] In most cases, at least at current densities above about 1 A · cm −2 , losses in addition to ohmic and kinetic sources are observed and related to mass transport resistances in the porous structures of the catalyst and porous transport layers (PTLs), though their nature and origin in PEWE are not well understood.6 Nevertheless, as long as a sufficient water supply is guaranteed, no transport limitations occur in the form of a turning point in the current/voltage characteristics (i/E-curves), as demonstrated by Lewinski et al 9 up to 19 A · cm −2 . In polymer electrolyte fuel cells (PEFC), a technology similar to PEWE, the effect of mass transport in the micro-porous structures of the electrodes and PTLs on the performance of the cells is reasonably well studied.…”

Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings

“…Hence, a growing concern in the context of large-scale PEM water electrolysis applications is the availability of Ir, which is one of the rarest materials on earth with an estimated annual production of only ≈4 tons. 4 In a recent study by Babic et al, the authors estimate that if 25% of the annually produced Ir were to be used for PEM water electrolysis and considering that current PEM water electrolyzers require ≈0.5 g Ir kW −1 , the annual PEM water electrolyzer installation would be limited to 2 GW/year. 4 This currently estimated maximum Ir-supply limited annual PEM water electrolyzer installation capacity may be compared to the water electrolysis capacity which would be needed, for example, if a large fraction of the currently used fossil fuels in the transportation sector were to be replaced by hydrogen.…”