Water Electrolysis Performance Research Articles

Ether-free polymer based bipolar electrolyte membranes without an interlayer catalyst for water electrolysis with durability at a high current density

In this study, bipolar membranes (BPMs) were fabricated employing the sulfonated poly(fluorine biphenyl indole) based proton exchange membrane (PEM) and the poly(arylene piperidinium) based anion exchange membrane (AEM) for the production of green hydrogen by water electrolysis. As those polymers comprise similar aromatic backbone structures without ether groups, excellent chemical durability of the constituent membranes was expected for the long-term energy efficient operation in a wide pH range. Three BPMs with different ionic conductivity pairs between PEM and AEM were designed to explore its effect on water electrolysis performance. At the fixed anionic conductivity of AEM of 0.0370–0.0923 S cm−1 at 30–70 °C, the proton conductivity of PEMs was tuned as 0.0379–0.1196 S cm−1, 0.0909–0.1598 S cm−1, and 0.1104–0.1739 S cm−1 at 30–70 °C. BPMs were steadily operated at low voltages below 1.5 V in the wide current density range of 0–1500 mA cm−2. Among them, the water electrolytic cell assembled with BPM 80/20 showed the best water electrolysis performance, as the cell voltage of BPM 80/20, BPM 70/30, and BPM 60/40 was 0.75 V, 0.98 V, and 1.08 V, respectively, under the current density of 700 mA cm−2 at 70 °C. It was much lower than that of AEM, 2.24 V and PEM 80/20, 1.81 V even in the absence of interlayer catalyst. Also, the synthesized BPM 80/20 was not only thermally and mechanically stable up to 298 °C and 19.15 MPa (tensile strength), but also chemically stable in the concentrated acidic and basic environment, as its weight loss was less than 5.4 % and 4.62 % in 4 M H2SO4 and 4 M KOH solutions for 4 weeks, respectively.

Adenosine triphosphate induced transition-metal phosphide nanostructures encapsulated with N, P-codoped carbon toward electrochemical water splitting

Transition metal phosphides (TMPs) have emerged as the most potential non-precious metal catalysts thanks to their high abundance, low cost and superior catalytic activity. Although numerous efforts have been devoted to gain TMPs, most of the synthetic processes are dangerous and complex as the utilization of flammable and toxic phosphorus sources. By using a novel green phosphorus source-adenosine triphosphate (ATP), we synthesized various N, P doped carbon-sealed TMPs nanostructures (TMPs@NPC) via a facile and simple solid-phase method. Benefiting from the porous nanostructure and in situ derived heteroatom-doped carbon encapsulation, as-synthesized TMPs@NPC exhibit good bifunctional catalytic performance. Notably, the Co2P@NPC demonstrates the superior activity and durability toward HER and OER. The Co2P@NPC, meanwhile, displays robust overall water electrolysis performance. Therefore, our work presents a simple, ecofriendly, and general strategy for the synthesis of TMPs encapsulated with N, P-codoped carbon nanostructures, which are also expected to show a bright future in other electrochemical applications.

Novel side chain functionalized polystyrene/O-PBI blends with high alkaline stability for anion exchange membrane water electrolysis (AEMWE).

We report the synthesis of a polystyrene-based anion exchange polymer bearing the cationic charge at a C6-spacer. The polymer is prepared by a functionalized monomer strategy. First, a copper halide catalyzed C-C coupling reaction between a styryl Grignard and 1,6-dibromohexane is applied, followed by quaternization with N-methylpiperidine and free radical polymerization. The novel polymer is blended with the polybenzimidazole O-PBI to yield mechanically stable blend membranes representing a new class of anion exchange membranes. In this regard, the ratio of the novel anion exchange polymer to O-PBI is varied to study the influence on water uptake and ionic conductivity. Blend membranes with IECs between 1.58 meq. OH- g-1 and 2.20 meq. OH- g-1 are prepared. The latter shows excellent performance in AEMWE, reaching 2.0 A cm-2 below 1.8 V in 1 M KOH at 70 °C, with a minor degradation rate from the start. The blend membranes show no conductivity loss after immersion in 1 M KOH at 85 °C for six weeks indicating high alkaline stability.

Surface and near-surface engineering design of transition metal catalysts for promoting water splitting.

Transition metal catalysts are widely used in the field of hydrogen production via water electrolysis. The surface state and near-surface environment of the catalysts greatly affect the efficiency of hydrogen production. Therefore, the rational design of surface engineering and near-surface engineering of transition metal catalysts can significantly improve the performance of water electrolysis. This review systematically introduces surface engineering strategies, including heteroatom doping, vacancy engineering, strain regulation, heterojunction effect, and surface reconstruction. These strategies optimize the surface electronic structure of the catalysts, expose more active sites, and promote the formation of highly active species, ultimately enhancing water electrolysis performance. Furthermore, near-surface engineering strategies, such as surface wettability, three-dimensional structure, high-curvature structure, external field assistance, and extra ion addition, are thoroughly discussed. These strategies expedite the mass transfer of reactants and gas products, improve the local chemical environment near the catalyst surface, and contribute toward achieving an industrial-level current density for overall water splitting. Finally, the key challenges faced by surface engineering and near-surface engineering of transition metal catalysts are highlighted and potential solutions are proposed. This review offers essential guidelines for the design and development of efficient transition metal catalysts for water electrolysis.

NiFeOx and NiFeCoOx Catalysts for Anion Exchange Membrane Water Electrolysis

Hydrogen production using an Anion exchange membrane (AEM) electrolyzer allows the use of non-platinum group metal catalysts for oxygen evolution reaction (OER). Nickel and Cobalt-based oxides are active in an alkaline environment for OER and are relatively inexpensive compared to IrO2 catalysts used in Polymer electrolyte membrane (PEM) electrolysis. Mixed metal oxide catalysts NiFeOx and NiFeCoOx catalysts were synthesized by the coprecipitation method using NaOH. X-ray diffraction results showed mainly NiO diffraction peaks for the NiFeOx catalyst due to the low concentration of Fe, for the NiFeCoOx catalyst, NiCo2O4 diffraction peaks were observed. NiFeCoOx catalysts showed a higher Anion exchange membrane water electrolysis (AEMWE) performance compared to NiFeOx and commercial NiO, the highest current density at 2 V was 802 mA cm−2 at 70 °C using 1 M KOH as an electrolyte. The effect of electrolyte concentration was studied by using 0.01 M, 0.1 M and 1 M KOH concentrations in an electrolysis operation. Electrochemical Impedance spectroscopy was performed along with the equivalent circuit fitting to calculate ohmic and activation resistances, the results showed a decrease in ohmic and activation resistances with the increase in electrolyte concentration. Commercially available AEM (Fumasep FAA-3-50 and Sustainion dioxide membrane X-37-50 grade T) were tested at similar conditions and their performance was compared. EIS results showed that X-37-50 offered lower ohmic resistance than the FAA-3-50 membrane.

Self-supported molybdenum nickel oxide catalytic electrode designed via molecular cluster-mediated electroplating and electrochemical activation for an efficient and durable oxygen evolution reaction.

Efficient and durable nonprecious catalysts for the oxygen evolution reaction (OER) are crucial for practical water electrolysis for hydrogen production. A self-supported OER catalytic electrode with sufficient exposure of the catalyst and tight anchoring onto the current collector is vital for the catalytic activity and stability, and is therefore deemed to be a preferable tactic to enhance water electrolysis performance. Herein, a polyoxometalate (POM) molecular cluster-mediated electroplating and activation tactics are proposed to design a self-supported molybdenum nickel oxide (MoNiOx) catalytic electrode for the OER. The MoNiOx active layer can anchor tightly onto the Ni foam current collector with sufficient surface exposure and high structural stability, therefore enabling high alkali OER catalytic efficiency (222mV at 10mAcm-2) and robust durability (only slight decay in catalytic efficiency upon 12days of chronopotentiometry (V-t) test). Moreover, the easily processable electroplating and active protocol can serve as a general approach to prepare other OER catalytic electrodes by altering the reactants and current collectors. The current work paves a facile and universal way to design a highly active and durable molybdenum (Mo) based hybrid catalytic electrode for OER via molecular cluster-assisted electroplating and activation treatment.

Correction: Mao et al. Controllable Construction of IrCo Nanoclusters and the Performance for Water Electrolysis. Catalysts 2022, 12, 914

The authors wish to make the following corrections to this paper [...]

Exergy and Exergoeconomic Analysis for the Proton Exchange Membrane Water Electrolysis under Various Operating Conditions and Design Parameters

Integrating the exergy and economic analyses of water electrolyzers is the pivotal way to comprehend the interplay of system costs and improve system performance. For this, a 3D numerical model based on COMSOL Multiphysics Software (version 5.6, COMSOL, Stockholm, Sweden) is integrated with the exergy and exergoeconomic analysis to evaluate the exergoeconomic performance of the proton exchange membrane water electrolysis (PEMWE) under different operating conditions (operating temperature, cathode pressure, current density) and design parameter (membrane thickness). Further, the gas crossover phenomenon is investigated to estimate the impact of gas leakage on analysis reliability under various conditions and criteria. The results reveal that increasing the operating temperature or decreasing the membrane thickness improves both the efficiency and cost of hydrogen exergy while increasing the gas leakage through the membrane. Likewise, raising the current density and the cathode pressure lowers the hydrogen exergy cost and improves the economic performance. The increase in exergy destroyed and hydrogen exergy cost, as well as the decline in second law efficiency due to the gas crossover, are more noticeable at higher pressures. As the cathode pressure rises from 1 to 30 bar at a current density of 10,000 A/m2, the increase in exergy destroyed and hydrogen exergy cost, as well as the decline in second law efficiency, are increased by 37.6 kJ/mol, 4.49 USD/GJ, and 7.1%, respectively. The cheapest green electricity source, which is achieved using onshore wind energy and hydropower, reduces hydrogen production costs and enhances economic efficiency. The growth in the hydrogen exergy cost is by about 4.23 USD/GJ for a 0.01 USD/kWh increase in electricity price at the current density of 20,000 A/m2. All findings would be expected to be quite useful for researchers engaged in the design, development, and optimization of PEMWE.

Transition-metal-free preparation of polyethylene-based anion exchange membranes from commercial EVA

Anion exchange membranes (AEMs) bearing polyethylene (PE) backbones have general advantages of outstanding chemical stability, balanced rigidity and toughness, good compatibility with various ionic functional groups, and aromatic-free structures. However, previous preparation of aromatic-free PE-based AEMs relies on complicated monomer synthesis and noble transition-metal catalysts (for metathesis, hydrogenation, coordinative polymerization, etc.). We herein propose a novel approach using commercial poly(ethylene-co-vinyl acetate) (EVA) to conveniently produce a series of trimethylammonium- and piperidinium-functionalized PE-based AEMs. The synthetic process involves Mitsunobu derivatization to form amine-functionalized PE intermediates without the use of any transition-metal catalysts. The resultant AEMs have high hydroxide conductivities (up to 129 mS/cm at 80 °C), good alkaline stability in 5 M KOH at 60 °C for 30 days, and promising AEM water electrolysis performance.

High Performance Anion Exchange Membrane Water Electrolyzer Using Monolayer Nickel-Iron Layered Double Hydroxide As Anode Catalysts

Much efforts have been devoted to developing electrocatalysts applicable to anion exchange membrane water electrolyzer (AEMWE). AEMWE operates in basic condition, which allows non-noble metal-based catalysts to be used, while its membrane electrode assembly (MEA) design allows higher current density compared to conventional alkaline water electrolyzer (AWE). Among many candidates for oxygen evolution reaction (OER), NiFe layered double hydroxide (LDH)-based electrocatalysts show the highest activity in an alkaline medium. Unfortunately, the poor electrical conductivity of NiFe-LDH limits its potential as an electrocatalyst, which was often solved by hybridization with conductive carbonaceous materials. However, we find that using carbonaceous materials for anode has detrimental effects on the stability of AEMWE at industrially relevant current densities.In this work, a facile monolayer structuring is suggested to overcome low electrical conductivity and improve mass transport without using carbonaceous materials. Bulk NiFe-LDH (B-NiFe-LDH) with multiple cationic layers was synthesized by following a conventional co-precipitation method, while monolayer NiFe-LDH (M-NiFe-LDH) was prepared using a similar method, but with formamide present in solvent. While Fe 2p and O 1s did not show noticeable difference, M-NiFe-LDH had larger Ni3+ peak than B-NiFe-LDH, indicating that M-NiFe-LDH has more Ni species in NiOOH environment rather than Ni(OH)2 environment. Here, more NiOOH phase in M-NiFe-LDH caused higher conductivity, leading to higher specific activity.The effect of electrical conductivity was further investigated by mixing the catalysts with various amounts of carbon materials. When carbon black (Vulcan XC-72) was loaded together with B-NiFe-LDH, the OER activity increased from the absence of carbon up to carbon-to-catalyst weight ratio of 0.1. Further addition of carbon did not increase the current density. On the other hand, the OER activity barely changed upon carbon addition for M-NiFe-LDH. As only Ni3+/4+ species are known to have OER activity, facile electron transfer from Ni2+ sites to Ni3+/4+ is important. Mixing carbon materials with B-NiFe-LDH provided conductive networks into previously “electron-unreachable” regions, which was also confirmed by the increase in the size of Ni oxidation peaks upon carbon addition. On the other hand, the contact between glassy carbon and M-NiFe-LDH catalyst was sufficient to have efficient electron transfer without carbon.The M-NiFe-LDH deposited on Ni foam (NF) showed much better AEMWE performance than B-NiFe-LDH, due to better electrical conductivity and higher hydrophilicity. When the M-NiFe-LDH was loaded on carbon paper (CP) instead of Ni foam, water electrolysis performance was enhanced, but stability was greatly lowered. Similar to the M-NiFe-LDH, the CP substrate showed the higher initial performance than the NF substrate for B-NiFe-LDH, but the B-NiFe-LDH/CP stopped operating eventually even at milder conditions. Overall, using M-NiFe-LDH/NF electrode, the high energy conversion efficiency of 72.6% and an outstanding stability at a current density of 1 A cm-2 over 50 h could be achieved without carbonaceous material.This work highlights electrical conductivity and hydrophilicity of catalysts in membrane-electrode-assembly (MEA) as key factors for high performance AEMWE.

Sputtering Pt on Low-Loading Iridium-Based Anode to Improve Both Cell Performance and Gas Permeation Issue in PEM Water Electrolysis

Polymer electrolyte membrane (PEM) water electrolysis is one of the most promising technologies which can integrate with renewable sources to produce sustainable hydrogen energy. Thin membranes were preferred for making membrane electrode assembly (MEA) for the PEM electrolyzer to increase cell efficiency. However, these membranes suffered from significant gas permeation consequently resulting in an explosion by the combination of hydrogen and oxygen, especially in the high-pressure operation. Many publications employed platinum as a recombination catalyst for reducing hydrogen concentration in the oxygen product stream to ensure safe operation [1-4]. Beside, Pt could be coated on oxidated Ti porous transport layer (PTL) to improve contact resistance between Iridium-based electrode and PTL [5]. In this work, a thin layer of platinum was sputtered on the surface of a low-loading anode (0.5 mgIr/cm2). Three MEAs prepared from commercial hydrous IrOx catalyst with 3 different loadings (1 mgIr/cm2, 2 mgIr/cm2and 3 mgIr/cm2) were used as reference. SEM images of Pt-sputtered MEA showed that the Pt layer had a thickness of approximately 80 nm corresponding to a loading of 0.4 mgPt/cm2. The electrochemical performance of Pt-sputtered MEA was significantly higher than that of bare MEA with 1 and 2 mgIr/cm2. The better performance of MEA with the presence of Pt is due to the improvement in contact resistance between the catalyst layer and PTL, in which lower HFR can be seen. In addition, its performance was comparable to high-loading MEA, while the total amount of precious metal was much lower (0.9 and 3 mg/cm2, respectively). The sputtered MEA also exhibited a slight difference in kinetic resistance compared to pristine MEA. On the other hand, the MEA with the addition of Pt shows a lower concentration of hydrogen in the oxygen product stream compared to pristine MEA at ambient pressure.[1] C. Klose et al., "Membrane interlayer with Pt recombination particles for reduction of the anodic hydrogen content in PEM water electrolysis," Journal of The Electrochemical Society, vol. 165, no. 16, p. F1271, 2018.[2] N. Briguglio, F. Pantò, S. Siracusano, and A. Aricò, "Enhanced performance of a PtCo recombination catalyst for reducing the H2 concentration in the O2 stream of a PEM electrolysis cell in the presence of a thin membrane and a high differential pressure," Electrochimica Acta, vol. 344, p. 136153, 2020.[3] S. Garbe, E. Samulesson, T. J. Schmidt, and L. Gubler, "Comparison of Pt-Doped Membranes for Gas Crossover Suppression in Polymer Electrolyte Water Electrolysis," Journal of The Electrochemical Society, vol. 168, no. 10, p. 104502, 2021.[4] A. Stähler, M. Stähler, F. Scheepers, W. Lehnert, and M. Carmo, "Scalable Implementation of Recombination Catalyst Layers to Mitigate Gas Crossover in PEM Water Electrolyzers," Journal of The Electrochemical Society, vol. 169, no. 3, p. 034522, 2022.[5] M. Bernt et al., "Effect of the IrOx Conductivity on the Anode Electrode/Porous Transport Layer Interfacial Resistance in PEM Water Electrolyzers," Journal of The Electrochemical Society, vol. 168, no. 8, p. 084513, 2021. Figure 1

Role of Ionomer Dispersion in the Design of Microstructure of Catalyst Layers: Oxygen and Hydrogen Evolution Reactions

Green hydrogen production is importance in the upcoming hydrogen economy era. Proton exchange membrane (PEM) water electrolysis is one of the most important technology to produce the green hydrogen that requires only water and extra electricity supplied from renewable energy. Main component, i.e., membrane electrode assembly (MEA), is a key part consisting of polymer electrolyte membrane and two electrodes which are oxygen evolution reaction (OER) electrode and hydrogen evolution (HER) electrode. Both electrode should be coated on the surface of membranes for better performance and mass production. Catalyst layers for OER and HER electrodes are composed of an electrocatalyst (Pt/C) and proton conducting ionomer. In the previous literature, proton conducting ionomer directly affects their performance and durability. Thus, the optimized design of the microstructure of the catalyst layers is essential. In this study, for the reduction of hydrogen production cost, highly dispersed ionomers were introduced to develop the optimized catalyst layers for OER and HER. The effect of dispersing solvents for ionomers on the performance and durability of catalyst layers was mainly investigated. Developed ionomer dispersions showed higher performance and durability in PEM water electrolysis, which result was made by the electrochemical characterization such as I-V polarization, voltage increasing rate during durability test, and so on as well as the microscopic characterization such as SEM and TEM were carried out to evaluate the effect of ionomer dispersions on the performance and durability of HER electrode in PEM water electrolysis. Acknowledgments This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT ＆ Future Planning (NRF-2019M3E6A1063677) and by 2022 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

The Impacts of Membrane Pinholes on Performance and Hydrogen Crossover in PEM Water Electrolysis

Polymer electrolyte membrane (PEM) water electrolysis is a promising energy storage technology to produce highly pure hydrogen with high efficiency 1. However, to implement it on a larger scale, it is essential to understand the effects of defects and the required tolerances and quality controls during manufacturing or subsequent handling. Inhomogeneities in membrane electrode assembly (MEA) component materials caused by the manufacturing process or subsequent handling can lead to performance loss and failure 2. Defects such as membrane pinholes, cuts, tears, abrasions, or perforations can all potentially impact MEA performance. In terms of impact, a loss of integrity of the membrane can allow bulk gas transport (crossover) and/or electrical shorting 3.In particular, hydrogen crossover is a crucial issue for PEM water electrolysis in terms of safe operation and efficiency losses, especially at increased hydrogen pressures. Crossover reduces the overall efficiency of the system and poses flammability hazards under certain conditions 4. Pinholes that form during the manufacturing process or during operation through degradation of the membrane lead to dramatically increased hydrogen crossover.In this study, the impacts of membrane pinholes on the performance and hydrogen crossover of PEM water electrolyzer cells are studied. Pinholes with different sizes are artificially introduced to the MEA to investigate the effects on the initial cell performance and hydrogen crossover rate under various backpressure and current density conditions. Reference Liu, C.; Shviro, M.; Gago, A. S.; Zaccarine, S. F.; Bender, G.; Gazdzicki, P.; Morawietz, T.; Biswas, I.; Rasinski, M.; Everwand, A., Exploring the Interface of Skin‐Layered Titanium Fibers for Electrochemical Water Splitting. Adv Energy Mater 2021, 11 (8), 2002926.Ulsh, M.; DeBari, A.; Berliner, J. M.; Zenyuk, I. V.; Rupnowski, P.; Matvichuk, L.; Weber, A. Z.; Bender, G., The development of a through-plane reactive excitation technique for detection of pinholes in membrane-containing MEA sub-assemblies. Int J Hydrogen Energ 2019, 44 (16), 8533-8547.Phillips, A.; Ulsh, M.; Mackay, J.; Harris, T.; Shrivastava, N.; Chatterjee, A.; Porter, J.; Bender, G. J. F. C., The effect of membrane casting irregularities on initial fuel cell performance. 2020, 20 (1), 60-69.Martin, A.; Trinke, P.; Stähler, M.; Stähler, A.; Scheepers, F.; Bensmann, B.; Carmo, M.; Lehnert, W.; Hanke-Rauschenbach, R. J. J. o. T. E. S., The Effect of Cell Compression and Cathode Pressure on Hydrogen Crossover in PEM Water Electrolysis. 2022, 169 (1), 014502.

Pyrochlore Oxide Decorated with Exsolved Metal Nanoparticles for Enhanced Water Splitting Reaction

Recently, we reported hybrid catalysts consist of metal nanoparticles on a metal oxide support for water splitting reaction, which is synthesized through an in situ exsolution process of metallic nanoparticles on pyrochlore oxide material.1,2 Specifically, we prepared Ni substituted lead ruthenate pyrochlore oxide (Pb2Ru1.6Ni0.4O6.5) via sol-gel crystallization and reduce the samples at different temperatures. Ni nanoparticles supported pyrochlore oxide catalyst significantly accelerate the conversion of Ni to NiOOH due to favorable electron transfer during the oxygen evolution reaction. The hybrid catalyst of pyrochlore oxide with NiRu alloy nanoparticles exhibits superior hydrogen evolution reaction performance, resulting from the bifunctional reaction mechanism, modified electronic structure, and efficient electron transport. Consequently, remarkable water electrolysis performance with superior long-term stability was obtained at the hybrid catalysts.

Tailoring 3D-Printed Electrodes for Enhanced Water Splitting.

Alkaline water electrolysis, a promising technology for clean energy storage, is constrained by extrinsic factors in addition to intrinsic electrocatalytic activity. To begin to compare between catalytic materials for electrolysis applications, these extrinsic factors must first be understood and controlled. Here, we modify extrinsic electrode properties and study the effects of bubble release to examine how the electrode and surface design impact the performance of water electrolysis. We fabricate robust and cost-effective electrodes through a sequential three-dimensional (3D) printing and metal deposition procedure. Through a systematic assessment of the deposition procedure, we confirm the close relationship between extrinsic electrode properties (i.e., wettability, surface roughness, and electrochemically active surface area) and electrochemical performance. Modifying the electrode geometry, size, and electrolyte flow rate results in an overpotential decrease and different bubble diameters and lifetimes for the hydrogen (HER) and oxygen evolution reactions (OER). Hence, we demonstrate the essential role of the electrode architecture and forced electrolyte convection on bubble release. Additionally, we confirm the suitability of ordered, Ni-coated 3D porous structures by evaluating the HER/OER performance, bubble dissipation, and long-term stability. Finally, we utilize the 3D porous electrode as a support for studying a benchmark NiFe electrocatalyst, confirming the robustness and effectiveness of 3D-printed electrodes for testing electrocatalytic materials while extrinsic properties are precisely controlled. Overall, we demonstrate that tailoring electrode architectures and surface properties result in precise tuning of extrinsic electrode properties, providing more reproducible and comparable conditions for testing the efficiency of electrode materials for water electrolysis.

Effects of Artificial River Water on PEM Water Electrolysis Performance

Hydrogen, a clean and renewable energy source, is a promising substitute for fossil fuels. Electricity-driven water electrolysis is an attractive pathway for clean hydrogen production. Accordingly, the development of electrolysis cells has drawn researchers’ attention to capital costs related to noble catalyst reduction and membrane degradation by the contaminations. In the literature, polymer electrolyte membranes (PEMs) have been studied on single cations contamination. In this study, we investigated the performance of a PEM on monovalent and divalent cation contamination by feed water. Artificial river water, called soft water, was used to analyze the effect of impurities on the PEM. The results demonstrated that the operating voltage drastically increased and induced cell failure with increasing Mg2+ and Ca2+ concentrations; however, it did not increase for Na+ and K+ after increase in voltage. Therefore, divalent cations have a stronger affinity than monovalent cations to degrade PEM and should be effectively excluded from the feed water.

Controllable Construction of IrCo Nanoclusters and the Performance for Water Electrolysis

Finding a suitable catalyst is an important research direction in hydrogen (H2) production from water electrolysis. We report a synthetic method to obtain IrxCo/C clusters by polyol reduction. The catalyst is small in size and can be evenly distributed. The Ir3Co/C cluster catalyst had very good activity under acidic conditions. The overpotential of the best-performing Ir3Co/C cluster for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is only 290 mV and 91 mV when 10 mA cm−2 and 100 mA cm−2. The catalyst performance may be improved because of the synergistic effect and the small size of the prepared catalyst, which accelerates proton transfer. This approach offers a strategy to reduce costs while improving catalytic activity.

Tailoring the oxide surface composition of stainless steel for improved OER performance in alkaline water electrolysis

There is a strong desire to replace bulk nickel electrodes for oxygen evolution reaction (OER) with a cheaper and more active steel material in alkaline water electrolyser (AWE). However, implementing an activation procedure to optimize the steel electrode surface is necessary. In this work, we activated 316 stainless steel (SS316) in KOH electrolytes of increasing concentration by applying an optimized potential of +1.70 V vs RHE for 18 hours. The electrocatalytic activity increased significantly with increasing the KOH concentration of the activation electrolyte, which is linked to a beneficial change in the surface composition during activation. The surface was analysed using X-ray Photoelectron Spectroscopy (XPS), Glow Discharge - Optical Emission Spectroscopy (GD-OES) and Scanning Electron Microscopy (SEM). XPS showed an increasing Ni content (41% to 73%) in the surface with simultaneous removal of Cr (3.4% to 0%) and Fe (55% to 27%) with increasing KOH concentration of the activation electrolyte. Activation in minimum 7.5 M KOH gave the best performing OER electrode with an electrode potential (E) of +1.525 V vs. RHE at a current density (j) of 10 mA cm-2. Tafel slopes of around 40 mV dec-1 were found for all activated samples, indicating the same OER mechanism independent of the resulting surface composition. Reaction orders of 1.5 and 1.3 were found in the low and high potential regions, respectively. A negligible performance change was observed for the activated electrodes over a 48 h test at 10 mA cm-2 in 1.0 M KOH. This work shows that the electrochemical activation of 316 stainless steel in high pH KOH electrolytes improves the OER activity significantly and that the surface composition can be tailored with KOH concentration up to 73% Ni. Through optimizing the surface composition, steel materials offer a cost-effective alternative to bulk nickel electrodes as OER electrodes in commercial AWE.

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation.

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO2 electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO2 surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO2 and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance.

Characterisation of the Operational Parameters of the Laboratory-Scale Membrane Alkaline Water Electrolysis Stack

Alkaline water electrolysis utilising porous diaphragm type separator of the electrode compartments represents industrially well-established technology of hydrogen production. This is due to the durable and stable operation utilizing abundant materials as iron, nickel, or stainless steel. However, many of the designs, construction solutions and approaches originate in the historical demands on the process (like mentioned durability rather than flexibility) and do not reflect up to date demands. Current demands on water electrolysis are given by the expected connection of water electrolysis technology with renewable sources of energy. The role of the water electrolysis is to use the surplus electrical energy in periods of the high output and/or low need for production of hydrogen. Hydrogen in such scheme represents an energy vector, which can be stored and used for electricity production in periods of the insufficient output of the renewable sources and/or high demand. Alternatively, hydrogen can be used as a fuel in fuel cell-based cars or as feedstock in industry. Due to connection with unpredictable renewable source, it is necessary for water electrolysis technologies to be flexible, highly efficient and be able to produce hydrogen of high purity. Current alkaline water electrolysis failed in several of these demands.Nowadays demands on water electrolysis are met by process utilizing proton exchange membrane (PEM) as separator of the electrode compartments. Unfortunately, PEM water electrolysis is limited by the need of the materials like titanium, platinum and iridium, which are rare, to be involved as electrodes, or catalysts of electrode reactions. The significant effort is paid to the possibility to combine advantages of both, alkaline and PEM processes by replacing porous diaphragm by dense anion-selective polymer membrane (ASM). Such change would allow to reduce the interelectrode distance to the thickness of the ASM used as separator of the electrode compartments, which would increase the performance of the membrane alkaline water electrolysis process. Utilisation of the ASM would also allow to decrease concentration of circulating electrolyte from commonly used 25 – 30 wt.% KOH to 1 – 5 wt.% KOH, increasing thus safety and flexibility. Another important beneficial impacts of ASM application are related to the improved current efficiency at low current densities and purity of the produced gasses, again especially at low current densities.Significant development was achieved in ASM synthesis in last decade and first promising materials are being commercially available. Despite the fact that many of the novel materials are tested in single cells under conditions of the membrane alkaline water electrolysis, the information on the stack behaviour are rare.In our work, we focused on the comparison of the porous diaphragm type and ASM separators in short stack comprising of 3 cells and geometrical area of the separator 78.5 cm2 under different operational conditions in terms of performance achieved. Besides the performance characterisation by mean of the load curve measurement, the complementary information on the gas purity, current efficiency and long-term stability were gathered for ASM separator.Zirfon Perl UTP 500 (Agfa, Belgium) was used as diaphragm type separator, meanwhile chloromethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene backbone functionalized by DABCO groups was used as ASM (TailorMem s.r.o., Czech Republic). Different concentrations of the liquid electrolyte represented by KOH solution were used in the range 1 – 15 wt.%. The temperature effect was verified at the same time by using the temperature of 25 and 40 °C. Load curves were measured galvanostically up to current density of 600 mA cm-2. The current efficiency was calculated based on the Faradays law by measuring the real oxygen production and comparing with theoretical value. The gas purity was measured for oxygen stream as the penetration of hydrogen into the oxygen stream is more important. Gas chromatography was used to measure the oxygen stream composition. The long-term stability was measured in 10 wt.% KOH solution at 40 °C at 240 mA cm-2.The results obtained indicate that at low KOH concentrations the performance of membrane alkaline water electrolysis is significantly better when compare to application of the diaphragm type separator. The differences in performance mitigated at increasing concentration. The characterisation of the membrane alkaline water electrolysis short-stack showed possibility to achieve high purity of the produced gasses and current efficiency even at low current densities. The long-term test showed excellent stability of the used ASM.Acknowledgement:This project has received funding from the European Union‘s Horizon 2020 Research and innovation action under grant agreement No 862509. Financial support by the Technology agency of the Czech Republic within the framework of the project No. TK02030103 is gratefully acknowledged.