Polymer Electrolyte Water Electrolysis Research Articles

Communication—Deuterium Isotope Separation by Solid Polymer Electrolyte Water Electrolysis

Electrolytic deuterium (D) separation from protium-containing solution has been investigated. Proton (PEM) and anion exchange membrane (AEM) electrolysis were performed at constant current. The mass analysis of hydrogen gas and water vapor was carried out. For both PEM and AEM electrolysis, the D concentration in the evolved hydrogen gas was lower than that of the initial electrolytic solution. The D concentration in the water vapor was unchanged by PEM electrolysis, but was increased by AEM. The separation factor of PEM was smaller than that of AEM, which was attributed to the humidification state on platinum catalyst.

Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis

In the first paper of this series the bulk and surface structural properties of Ti-fiber based porous transport layers (PTL) were characterized and described. In this second part the correlation of structure to the performance in polymer electrolyte water electrolysis cells is analyzed by determination of the three main overpotentials of ohmic, kinetics and mass transport losses. The strongest correlation between the PTL bulk transport properties and cell performance is obtained for heat transport. The current density dependent temperature gradients show good agreement with ex situ determined heat conductivity from part one. However, surface properties of the PTL materials, have a stronger influence on cell performance than the bulk properties. Catalyst layer utilization and ohmic interfacial resistances correlate with the interfacial contact areas reported in part one and performance increases with increasing contact area. This is due to a local mass transport resistance decreasing with increasing catalyst layer utilization.

Synthesis and activities of IrO2/Ti1−xWxO2 electrocatalyst for oxygen evolution in solid polymer electrolyte water electrolyzer

Novel IrO2/Ti1−xWxO2 electrocatalysts for the oxygen evolution reaction (OER) in a solid polymer electrolyte (SPE) water electrolyzer were synthesized using the Adams fusion method and the evaporation-induced self-assembly method. Physical and chemical characterizations of the sample—including Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and polarization curves for single cells—were performed. The W-doped TiO2 support with various doping amounts (0, 5, 10, and 20at.%) exhibited a single-phase anatase structure and a large BET surface area. Furthermore, with annealing at a reducing atmosphere, a phase change from anatase to rutile occurred, trace α-W was precipitated from the crystal lattice of the Ti0.8W0.2O2 sample, and the BET surface area decreased accordingly. On the other hand, the conductivities of the W-doped support exhibited an increasing trend, indicating a significant impact of the W dopant on the enhancement of the TiO2 conductivity. The voltammetric charge of 40IrO2/Ti0.9W0.1O2 (1754mC (cm2mg)−1) was higher than that of unsupported IrO2, and 40IrO2/Ti0.9W0.1O2 exhibited enhanced electrochemical activity for the OER process. A single-cell performance of 1.79V was achieved by using 80IrO2/Ti0.9W0.1O2 as an anode for the minimum charge-transfer resistance and ohmic resistance. Electrocatalysts with the Ti0.9W0.1O2 support exhibited good durability. Thus, Ti0.9W0.1O2 is a promising anode electrocatalyst support for SPE water electrolysis.

Hydrogen production from industrial wastewaters: An integrated reverse electrodialysis - Water electrolysis energy system

This work presents a novel approach combining reverse electrodialysis (RED) and alkaline polymer electrolyte water electrolysis (APWEL) for renewable hydrogen production. APWEL is fuelled by salinity gradient power (SGP) extracted from sulfate (SO42−)-rich industrial wastewater. The performance of a pilot-scale RED unit (200 cells, active area: 31.5 × 63.5 cm2), using salt solutions mimicking sulfate-rich waste streams (0.01–0.3 M Na2SO4), was evaluated. An open circuit voltage (OCV) of 12.3 V, a maximum power density of 0.22 W/m2MP (MP: membrane pair) and internal area resistance of 43.2 Ωcm2/cell were recorded by using 0.01 M/0.3 M Na2SO4 solutions at 35 °C. The APWEL stack (6 cells, active area: 5 × 5 cm2), equipped with Ni foam electrodes and heterogeneous anion-selective membranes, was tested with varying concentrations of liquid electrolyte (0.85–2.5 M KOH) and varying temperatures (28–48 °C). The APWEL stack attained a maximum current density of 110 mA/m2 at 1.85 V/cell (i.e. 11 V per stack), 2.5 M KOH and 48 °C. Under these conditions, the integrated system exhibited a maximum hydrogen production rate of 50 cm3/h·cm2. This study opens up a new perspective on renewable hydrogen production fuelled by non-intermittent SGP from SO42--rich industrial effluents.

Degradation Mechanisms and Accelerated Stress-Test Protocols for Polymer Electrolyte Water Electrolyzers

If polymer electrolyte water electrolysis (PEWE) is to be deployed to serve as an energy storage technology in the grid, the potential challenges related to the component durability need to be understood and addressed. The US Department of Energy (DOE) targets a cost of 2 $/kg H2 for the year 2020. Maintaining stack efficiency is key to a competitive end-cost of H2, as the main contribution is projected to come from the cost of electricity (1). The development of tailored PEWE components therefore needs to go hand-in-hand with the development of accelerated stress-tests that trigger specific degradation mechanisms. In this study, different degradation mechanisms were induced by subjecting commercial catalyst-coated membranes (CCMs) to specific operating conditions. PEWE cells were operated for 300 h to estimate the effects of the imposed stressors, and the cell performance was compared to the 300 h non-stressed benchmarks. The cell response was analyzed using electrochemical impedance spectroscopy (EIS). Individual catalyst layer (CL) characteristics were probed in the H2/N2 regime, and the cyclic voltamograms were recorded. Finally, samples were post-test investigated using scanning electron microscopy (SEM). The effect of the cell compression on the CL performance was stessed using cell assemblies with Ti-sintered porous transport layers (PTLs) of different morphological surface properties. Rigid PTL-surface features have induced local variation in CL porosity, and increased CL proton transport resistance (RCLa H+). In the case of PTLs with large voids, CCM creep led to the formation of thin spots and an increase in measured H2 in the O2 outlet. We have found that the increased gas crossover with mechanically stressed and locally thinned CCM results in a higher fluoride release rate over 300 h. The effects of start-stop operation on the CL stability were quantified over 300 h by periodically analyzing the anodic water using ICP-mass spectroscopy for traces of precious metals. Additionally, elemental mapping was done on the post-test CCM cross-sections to deterime if potentially dissolved species had deposited in the PEM (2). As water impurities are an important factor determining the electrolyzer lifetime (3), we have designed Ir2+ doping experiments to simulate the accelerated reverisble contamination of the CCM. A detalied overpotential analysis was done to determine the effect on the ohmic, activation and mass-transport overpotential. A specially designed cell was used to conduct operando neutron imaging of the contaminated CCM, and track the distribution of metal cations in the CCM cross-section under various operating conditions. EIS was done in paralell to correlate the impact of contaminants on the cell performance with their cross-sectional position. We established an comprehensive degradation matrix based on the PEWE degradation-inducing experiments, highlighting the stressors, degradation mechanisms and the affected aspects of PEWE performance.

Highly Active Pt‒M (M=Co, Fe) Alloy Catalysts for the Hydrogen Evolution Reaction in Polymer Electrolyte Water Electrolysis

Conventional polymer electrolyte water electrolysis (PEWE) requires a large amount of noble metal catalysts such as Pt and/or Ir. Platinum (ranging from 0.5 to 2 mgPt cm−2) has been used as the cathode catalyst for the hydrogen evolution reaction (HER).1, 2 Development of highly active and durable catalysts for the PEWE is essential for the efficient production of hydrogen with reduced amounts of noble metals.3 While some Pt alloys have been examined for the HER in the PEWE, the durability was not sufficient due to dealloying.2 Recently, we have found that PtxAL–PtM (M=Fe, Co, Ni)/C (with stabilized Pt skin of 1 to 2 atomic layers, PtxAL) nanoparticles on carbon supports exhibited superlative activity for the hydrogen oxidation reaction (HOR) in H2-saturated 0.1 M HClO4 at 70 and 90ºC.4 In this presentation, we demonstrate that the PtxAL–PtM and PtM/C catalysts also exhibited higher activity for the HER, compared with that of Pt. Pt3Co/C catalyst was prepared by the nanocapsule method,5 followed by heat-treatment in 5% H2 (denoted as Pt3CoHT/C). The PtxAL–PtM/C catalysts were prepared in the same manner as that described previously.6, 7 Two commercial catalysts, c-Pt/C (d TEM =2.6±0.9 nm) and c-Pt3Co/C (d TEM =5.1±1.8 nm), were used for comparison. To measure the HER activities of the catalysts in H2-saturated 0.1 M HClO4 solution, the channel flow electrode (CFE) cell4 was used to minimize the ohmic loss by H2 bubble evolution. Each catalyst was uniformly dispersed on an Au substrate as the working electrode in the CFE cell with a constant loading of carbon support of 11 µg cm−2, which corresponds to approximately two monolayers in height of the carbon particles. Nafion film was coated on the catalyst layer with an average thickness of 0.10 µm. It was found for the Pt3CoHT/C by TEM, XRD and ICP that Pt3Co nanoparticles with narrow size-distribution (d TEM =2.9±0.5 nm) were uniformly dispersed on the carbon support. The values of electrochemically active area (ECA) of all catalysts examined accorded well with those estimated from d TEM with the assumption of spherical particles. Figure 1(a) shows iR-free polarization curves for our Pt3CoHT/C and commercial catalysts at 80ºC in H2-saturated 0.1 M HClO4 with the mean flow rate U m = 111 cm s−1. Apparent area-specific activities j S for the HER at −20 mV vs. reversible hydrogen electrode (RHE) are summarized in Fig. 1(b). The value of j S on the Pt3CoHT/C was about 1.6 times higher than that of c-Pt3Co/C and 3 times higher than that of c-Pt/C. The value of apparent mass activity MA at −20 mV on the Pt3CoHT/C was as high as 4.3 A mgPt −1 (see Fig. 1(a)), indicating the possibility of reduction of the Pt cathode loading to a level as low as 0.23 mgPt cm−2 for operation at 1 A cm−2 with an overpotential of −20 mV. Evaluation of performance and durability of PEWE cells with these cathode catalysts is now in progress. This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid No. 17H01229 for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture, Japan. References M. Carmo, D. L. Fritz, J. Mergel, and D. Stolten, Int. J. Hydrogen Energy, 38, 4901 (2013).M. K. Debe, S. M. Hendricks, G. D. Vernstrom, M. Meyers, M. Brostrom, M. Stephens, Q. Chan, J. Willey, M. Hamden, C. K. Mittelsteadt, C. B. Capuano, K. E. Ayers, and E. B. Anderson, J. Electrochem. Soc., 159, K165 (2012).H. Ohno, S. Nohara, K. Kakinuma, M. Uchida, A. Miyake, S. Deki, and H. Uchida, J. Electrochem. Soc., 164, F944 (2017).G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal., 7, 267 (2017).H. Yano, J-M. Song, H. Uchida, and M. Watanabe, J. Phys. Chem. C, 112, 8372 (2008).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016). Figure 1

Insight into Oxygen Evolution Reaction Kinetics from Gas Fed Polymer Electrolyte Water Electrolysis

In polymer electrolyte water electrolysis (PEWE) oxygen evolution reaction (OER) kinetics are a major efficiency limitation. At least with thin membranes, the anode kinetic overvoltage is the main voltage loss. The reaction mechanism for IrO2 in the most stable surface (110) configuration in acid environment was investigated by DFT calculation in literature and postulated according to reactions R1 to R4 [1]. The concept of gas feed electrolysis enables the identification of the rate determining step in the OER mechanism by using the additional freedom of water concentration variation. Therefore the activity of water was varied between 0.6 to 0.95 at 80°C. The determined reaction order is compared to proposed reaction mechanisms in literature. The used materials were validated by determination of the IrO2 activity as function of the loading. CCMs were prepared by automated spray coating process using IrO2/TiO2 catalyst. The loadings were varied from 1mg/cm2 to 3mg/cm2. In analogy to liquid PEWE, the CCMs were tested in cell configuration by using commercial Ti sinter based porous transport layers. The loading trends of exchange current densities estimated from Tafel fitting were compared to specific capacity determined by cyclic voltammetry at 1.0V. The linear dependence of specific capacity with loading exemplifies a constant roughness factor as shown in Figure 1. The catalyst layer utilization doesn’t show any impact on loading/catalyst layer thickness. Reference [1] J. Rossmeisl, Z.W. Qu, H. Zhu, G.J. Kroes, J.K. Nørskov, Electrolysis of water on oxide surfaces, J. Electroanal. Chem., 607 (2007) 83-89. Figure 1

Insight in Mass Transport of Polymer Electrolyte Water Electrolyzer By Systematic Analysis of Liquid and Gas Fed Water

For a broader commercialization of polymer electrolyte water electrolysis (PEWE), it is essential to gain a fundamental understanding of the different components and related losses to increase the overall system efficiency to further decrease the operating cost. Mass transport resistances are the least understood loss category. The use of water feed in the vapor state was utilized to gain insight in the origin of mass transport losses. Vapor feed amplifies governing transport parameters. Limiting current densities were determined and mass transport resistances calculated as a function of temperature (60°C-80°C) and relative gas humidity (60% to >100% rH i.e. oversaturation). The studied configurations with symmetric and asymmetric cell humidification exemplify the major impact of water drag and diffusion on the performance. In addition, a relation between mass transport limitation and high frequency resistance (HFR) measured by electrochemical impedance spectroscopy was established. The overall mass transport resistance is composed of the sub-resistances of the catalyst layer and the PTL. Partial pressure and PTL type variation were used to isolate these main resistances. The catalyst layer was identified to be the governing resistance in the system such that gas feed electrolysis features two order of magnitude lower current densities in comparison to liquid water electrolysis [1] (see also Figure 1). In the diffusion limited regime of the gas feed cells the limits of liquid water electrolysis are emphasized. The origin of the significant liquid/vapor feed differences was investigated by using different PTL types and IrO2 loadings. The required CCMs were produced by an automated spray coating process where IrO2 loading was varied between 1.0 mg/cm2 and 3.0 mg/cm2. references [1] J.M. Spurgeon, N.S. Lewis, Proton exchange membrane electrolysis sustained by water vapor, Energy & Environmental Science, 4 (2011) 2993-2998. Figure 1

Carbon-Free Connected Ru, Ir Based Nanoparticle Catalysts for Polymer-Electrolyte Water Electrolysis

Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) suffered from their low surface area because carbon support cannot be used for OER operating at high potential. Connecting metal nanoparticles enable catalysts to conduct electrons through nanoparticle networks without any support materials while keeping their high surface area. Thus, connected nanoparticle catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Pt-Fe nanoparticle catalysts with hollow capsule structure for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).[1] In this study, we proposed Ru and Ir nanoparticle catalysts for OER in water electrolysis because Ru and Ir have high OER activity. Ru and Ir nanoparticle catalysts were synthesized as follows. First, Ru and Ir nanoparticles were synthesized on silica template (PDDA/SiO2) via polyol method using tetraethylene glycol as reducing agent and Ru(III) acetylacetonate or Ir(III) acetylacetonate as the metallic precursors (M/PDDA/SiO2, M is Ru or Ir). Then, these nanoparticles were treated in supercritical ethanol at 330ºC for 90 min (Ru) or 60 min (Ir) (sc-M/SiO2). Dissolution of SiO2 in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (M capsule). OER performance were evaluated by electrochemical measurement in 0.1 M HClO4 solution. Fig. 1 (B) shows TEM image of Ir capsule. Capsule and networks structure of each catalysts were observed by TEM images. All Ru catalysts inactivated for OER at the first CV cycle. OER activity of Ru/PDDA/SiO2 did not improve by heat oxidation treatment in air at 400ºC. Fig. 1 (C) shows OER curves of Ir catalysts. All Ir catalysts showed stable OER. Among them, Ir/PDDA/SiO2 had the highest OER activity. Mass activity at 1.48 V of Ir/PDDA/SiO2 was higher than RuO2 [2], IrO2 [2] and Ir/C­[3] in previous reports as shown in Fig.1 (D). Heat treatment of Ir/PDDA/SiO2 at 200ºC further improved. In conclusion, we successfully synthesized connected Ru and Ir nanoparticle catalysts. Among them, Ir/PDDA/SiO2 showed the highest OER activity and the activity was further improved by heat oxidation treatment. The results showed that connected Ir nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis. [1] T. Tamaki, H. Kuroki, T. Yamaguchi et al., Energy Environ. Sci., 8, 3545-3549 (2015). [2] Y. Lee et al., J. Phys. Chem. Lett., 3, 399-404 (2012). [3] HN. Nong et al., Chem. Sci., 5, 2955-2963 (2014). Figure 1

Comparing the kinetic activation energy of the oxygen evolution and reduction reactions

The reduction of the activation or kinetic overpotential for the oxygen evolution and reduction reactions (OER/ORR) is of central significance to optimize conversion efficiency of polymer electrolyte water electrolysis and fuel cells (PEWE/PEFC). In both cases increasing temperature seems to be an option. In this work, similarities and differences of the temperature-dependence of the kinetic overpotential of the oxygen evolution and reduction reactions are investigated. While both overpotentials decrease with temperature, only in PEWE a significant cell performance gain is obtained at relevant current densities due to the decreasing reversible thermodynamic cell voltage with temperature. Furthermore, for both reactions, the corresponding activation energies at constant overpotentials and constant cell voltages are determined according to the Butler-Volmer equation and transition state theory. At constant overpotentials a similar decrease in the activation energy from equilibrium conditions (approx. 67 kJ/mol) to an overpotential of about 0.3 V (approx. 40 kJ/mol) is observed and considered as a coincidence. That is, the activation energy of the supposed rate determining steps behaves similar with increasing the OER/ORR overpotential. Considering the technically important situation of constant cell voltage, the activation energy for PEFC is decreasing more rapidly due to the temperature induced decrease of the reversible thermodynamic cell voltage.

Synthesis and Evaluation of Iridium Oxide Nanoparticle Catalysts Supported on Nitrogen-Doped Reduced Graphene Oxides

Recently, progress toward sustainable societies with renewable energies is a critical issue to overcome environmental and social problems, for example, global warming and depletion of fossil fuels. To utilize effectively renewable energies such as solar and wind energy, conversion of renewable energies to hydrogen via water electrolysis is one of the most attractive approaches for energy storage. However, for the practical application of water electrolysis systems, its activity, durability, and catalysts cost are insufficient due to high over-potential and low kinetics of electrode reactions, especially, oxygen evolution reaction (OER) at the anode [1,2] for polymer electrolyte water electrolyzer (PEWE). As anode catalysts, iridium oxide (IrO2) particles have been used because of its high activity and durability for OER. However, iridium is one of the platinum group metals and reducing loading amount of iridium is required to reduce catalyst cost. Recently, we have successfully synthesized novel catalysts, IrO2 nanoparticle catalysts supported on nanocarbon materials with improved specific surface area, surface area per weight, of anode catalysts for PEWE. In this study, we focus on modification of IrO2 catalysts by doping of hetero atom (N) on supporting materials and characterization its electronic state condition and catalytic activity for OER. Novel catalyst of IrO2 nanoparticles supported on nitrogen-doped reduced graphene oxide (N-rGO) was prepared by hydrothermal synthesis. Graphene oxide (GO) prepared by modified Hummers method [3] was mixed with urea and hydrothermally treated at 180 ˚C for 12 h to synthesize N-rGO. For the synthesis of IrO2 / N-rGO catalysts, mixture of H2IrCl6 complex and N-rGO dispersed in ethanol/water mixture solution was stirred at 80 ˚C for 6 h in a flask to attach iridium oxide colloid to N-rGO and heated at 150 ˚C for 4 h in a hydrothermal autoclave to form IrO2 nanoparticles. The IrO2 / N-rGO catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersed X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The OER activity of the IrO2/ N-rGO was examined in sulfuric acid solution using rotating disk electrode system. Figure 1 shows TEM image of synthesized IrO2 / N-rGO catalysts. The IrO2 nanoparticles with an average particle size of 1.2 nm are uniformly dispersed on surface of the N-rGO substrate. The loading amount of the IrO2 nanoparticles and doping weight of nitrogen atom of the IrO2 / N-rGO catalysts are approximately 9 and 7 wt%, respectively, which was estimated from EDX measurements. Figure 2 shows the results of XPS investigation on GO, N-rGO, and IrO2 / N-rGO. The XPS data reveal nitrogen doping on GO by hydrothermal synthesis of GO with urea. After the reaction between Ir complexes and N-rGO, Ir 4f peak appeared on the spectrum and intensity of O 1s was decreased due to reduction of GO during the hydrothermal reaction. Deconvolution analysis of the N 1s peak of N-rGO indicates doped nitrogen has three different types, pyridinic, pyrrolic, and graphitic type. The binding energy of the Ir 4f peak was shifted to a lower energy state. In addition, XRD pattern of the IrO2 / N-rGO catalyst suggests that the interlayer distance of graphene sheets decreased during hydrothermal reaction because of reduction of GO. The XRD data suppot the results of the XPS measurement.Figure 3 shows linear sweep voltammogram of the IrO2 / N-rGO catalyst obtained in 0.5 M H2SO4 solution. Electrochemical measurement reveals high activity of the IrO2 / N-rGO catalyst for OER, onset potential of the reaction is ca. 1.43 V and mass activity at 1.60 V is ca. 1000 A g-1. The overvoltage of the IrO2 / N-rGO catalysts is 70 mV lower than that of conventional IrO2 powder catalysts. High activity of the IrO2 / N-rGO catalyst indicates that the IrO2 / N-rGO catalyst is a promising candidate of anode material for PEWE. Acknowledgements This work was supported by JSPS KAKENHI Grant number 17K05969. Reference[1] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 38 (2013) 4901.[2] E. Antolini, ACS Catal. 4 (2014) 1426.[3] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. Figure 1

Communication—Contribution of Catalyst Layer Proton Transport Resistance to Voltage Loss in Polymer Electrolyte Water Electrolyzers

Mass transport losses (ηmtx) play a significant role at high current densities in polymer electrolyte water electrolyzers (PEWEs). Previous work has shown that ηmtx depends on the porous transport layer (PTL) structure, although a clear correlation between the material morphology and ηmtx has not been established. In this work, we experimentally determine the overpotential ηH +CLa associated with the proton transport in the anodic catalyst layer by measuring the ionic resistance in the catalyst layer using electrochemical impedance spectroscopy (EIS) and the transmission-line model. We found that overpotentials, including ηH +CLa, depend on the PTL morphologic surface properties.

Correlations between the Porous Transport Layer Properties and the Hydrogen Crossover in Polymer Electrolyte Water Electrolyzers

Water electrolysis is a key technology in future sustainable energy scenarios, since hydrogen 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’. Polymer electrolyte water electrolysis (PEWE) is a key technology in energy applications, where the possibilities of rapid start-up and dynamic operation with large differential pressures in compact units are of importance. The most prominent challenge for electrolyzers with improved performance and lower cost so far are the durability targets. (1, 2) In this work we compare the effect of dynamic operating conditions (start-stop vs. steady-state) on the component aging in 250 h experiments using Nafion 115-based CCMs. The main mechanism of performance decay in 250 h experiments, independent of the type of galvanostatic cycle, is the cation poisoning of the ionomer. It is, however, reversible by CCM re-protonation in sulfuric acid (3). More interestingly, on-line monitoring of the anodic gas purity indicates an increase in the hydrogen crossover (HC) during both experiments. Post test SEM analysis suggests that the cause is the creep of the membrane material into the voids of the titanium porous transport layer (PTL), with concomitant local thinning of the membrane. This effect could represent a bottleneck for implementing thinner membranes towards more efficient PEWE. The increase of HC over time is alleviated by changing to the less-expensive cathodic carbon PTLs. Being more compressible, the carbon based cathode PTL deforms under the cell clamping pressure, minimizing the creep of the polymer. The mechanical deformation of the CCM and the material creep into the PTL pores is much less pronounced according to post test SEM analysis. Moreover, carbon cathode PTLs lead to improvements in overall cell performance and lower HC (Figure 1). Both benefits are postulated to stem from a more uniform contact between the cathode catalyst layer (CL) and the PTL structure. The lower cell potential is related to the decrease in the contact resistance. Lower HC is thought to be a result of the more uniform current distribution over the CL and PTL interface. Hydrogen evolution reaction is thought not to take place on the catalytic sites situated in the voids of the PTL (nominally 30 - 40 µm). Due to the limited electrical in-plane CL conductivity, areas of the CL which are not in direct electronic contact to the PTL material remain inactive. Thus, the active regions need to sustain a higher local current density, which might result in local hot-spots and higher gas pressures in the CL. To test this hypothesis, we introduced a cathodic micro-porous layer (MPL) to further improve the current distribution. The presence of the MPL leads to further improvements in both HC and cell performance (Figure 1). We conducted X-ray tomography of PEWE PTL materials to characterize the interface and further investigate the correlations between the PTL morphology and the hydrogen crossover.

Computational fluid dynamics study on the anode feed solid polymer electrolyte water electrolysis

A steady-state two-dimensional model for the anode feed solid polymer electrolyte water electrolysis (SPEWE) is proposed in this paper. Finite element procedure was employed to calculate the multicomponent transfer model coupled with fluid flow in flow channels and gas diffusion layers and electrochemical kinetics in catalyst reactive surface. The performance of the anode feed SPEWE predicted by this model was compared with the published experimental results and reasonable agreement was reached. The results show that oxygen mass fraction increases because of the water oxidation when water flows from the import to the export on the anode side. On the cathode side, hydrogen mass fraction varies little since hydrogen and water mix well. The flux of water across the electrolyte increased almost linearly with the increase of the applied current density. Since the ohmic overpotential loss increasing as the solid polymer electrolytes’ thickness increasing, the performance of the anode feed SPEWE with Nafion 112, 115, 117 decreases at the same applied current density.

High pressure polymer electrolyte water electrolysis: Test bench development and electrochemical analysis

A test bench for a polymer electrolyte water electrolysis (PEWE) cell for high pressure operation of up to 100 bar in differential and balanced pressure mode is described. Important aspects referring to the design, safety and operability of the test bench and the design of a small scale electrolysis cell are described. The electrolyzer cell comprises a special compression mechanism which allows accommodating porous transport layers of different thickness and setting of the compression pressure independent of the clamping pressure. In order to analyze the electrochemical results with respect to the overpotentials, a power source with integrated high frequency resistance (HFR) as well as electrochemical impedance spectroscopy (EIS) measurement capabilities is implemented. The versatility of the test environment is demonstrated by comparing the DC, HFR and EIS data as a function of operating pressure, temperature (up to 70 °C) and current density (up to 4 A/cm2). With respect to pressurized operation of PEWE cells, only the differential pressure mode (hydrogen pressurized) shows the expected isothermal compression behavior, for balanced pressure operation a different characteristic is observed.

Investigation of V-doped TiO2 as an anodic catalyst support for SPE water electrolysis

A modified evaporation-induced self-assembly (EISA) method was employed to prepare titania samples doped with different amounts of vanadium (0, 10, 20, 30 at.%), which were further evaluated as catalyst supports for the oxygen evolution reaction (OER) catalyst IrO2 in the solid polymer electrolyte water electrolyzer (SPEWE). The effects of V dopant on titania supports are proved to be twofold: i) enhancing the simplification of phase composition and consequently improving the homogeneity of porous morphology; ii) introducing redox couple V (IV)/V (V) to the surface of titania. As a result, the catalyst's OER activity is improved with the increase of V dopant in the titania support after loading IrO2 via Adams fusion method. In single cells, the OER performance gradually increases with V dopant from 0 to 20 at.%, followed by a performance deterioration with V amount reaching 30 at.% due to the corrodible V2O5 precipitate.

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

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.

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

Three different porous transport layer (PTL) structures, based on titanium sintered powders, were characterized using X-ray tomographic microscopy to determine key geometric properties such as porosity, pore and particle size distributions as well as effective transport properties. The mass transport through the PTL contributes to the voltage losses in the polymer electrolyte water electrolysis cell. Therefore, influence of the PTL structure on the mass transport overpotential is investigated as function of current densities (≤ 4 A·cm−2), operating pressures (1–100 bar) and temperatures (40–60°C), respectively. A decrease of transport losses was observed with increasing pressure and temperature for all investigated PTLs. At around 100 bar balanced pressure, the transport losses for all PTLs converge to about 40 mV per applied A·cm−2, suggesting that other parts of the cell such as the catalyst layer or their interface contribute to these remaining losses. The performance loss, induced by the different PTL structures, shows a stronger correlation with geometric parameters such as pore and particle size distributions than transport properties like effective diffusivity and permeability. The finest materials with d50 pore and particle diameters of 40–48 and 68 μm, respectively, are performing better than the coarsest material with diameters roughly twice the sizes.

Electrochemical Hydrogen Compression: Efficient Pressurization Concept Derived from an Energetic Evaluation

Electrochemical hydrogen compression is a potentially high efficient, low-maintenance and silent technology to produce high pressure hydrogen. A new pressure concept with increased compression efficiency, termed intermediate differential pressure polymer electrolyte water electrolysis is proposed. With slightly pressurized oxygen and a much higher hydrogen pressure, this pressure concept profits from the advantages of the lower gas crossover of differential (only hydrogen compressed) pressure water electrolysis and the improved oxygen evolution reaction kinetics with increasing pressure of balanced pressure operation. Data for gas pressures up to 100 MPa is modeled, based on experimental results up to 5 MPa, of the three pressure concepts and validated with literature data up to 70 MPa. While differential pressure electrolyzer operation, following ideal isothermal compression, can be more efficient than today's best mechanical compressors up to 40 MPa, the intermediate pressure concept shows higher compression efficiency up to 90 MPa.

Vertically Aligned FeOOH/NiFe Layered Double Hydroxides Electrode for Highly Efficient Oxygen Evolution Reaction.

Employing a low-cost and highly efficient electrocatalyst to replace Ir-based catalysts for oxygen evolution reaction (OER) has drawn increasing interest in renewable energy storage. In this work, a vertically aligned FeOOH/NiFe layered double hydroxides (LDHs) nanosheets supported on Ni foam (VA FeOOH/NiFe LDHs-NF) is prepared as a highly effective OER electrode in alkaline electrolyte. The VA FeOOH/NiFe LDHs-NF represents nanosheet arrays on nickel foam with some interspace among them. The vertically aligned and interlayer-structured architecture is binder-free and contributes to facile strain relaxation, relieving the exfoliation of the catalysts layer caused by the oxygen evolution process. The as-prepared electrode shows current densities of 10 and 500 mA cm-2 at overpotentials of 208 and 288 mV, and good stability in a half-cell electrolyzer. Besides, the alkaline polymer electrolyte water electrolyzer (APEWE) with this electrode showed 1.71 V at 200 mA cm-2, and 2.041 V at 500 mA cm-2, exhibiting the corresponding energy efficiency of 86.0% and 72.0% (based on the lower heating value of hydrogen), which is better than the typical commercial alkaline water electrolyzer.