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IrO2 is a current state-of-the-art catalyst for polymer electrolyte membrane water electrolyser (PEMWE) applications due to its high stability during the oxygen evolution reaction (OER). However, its activity needs to be significantly improved to justify the use of such a high-cost material. In this study, the activity of the IrO2 catalyst was improved by optimising and comparing two synthesis methods: the modified Adams fusion method (MAFM) and the molten salt method (MSM). Optimum OER performances of the IrO2 catalysts synthesised with the two synthesis methods were obtained at different temperatures. For the MAFM, a synthesis temperature of 350 °C produced the IrO2 catalyst with an overpotential of 279 mV and the highest OER stability of ~ 82 h at 10 mAcm-2. However, for the MSM, the lowest overpotential of 271 mV was observed for IrO2 synthesised at 350 °C, while the highest stability of ~ 75 h was obtained for the IrO2 synthesised at 500 °C. [ABSTRACT FROM AUTHOR]

Zn‐air batteries (ZABs) have drawn an attention due to their high energy density, less complicated chemistry, and relatively low cost. Electrocatalyst plays a significant role in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) that occurs during the operation. Various valance states of manganese make MnO2 as an attractive electrocatalyst. In the present work, MnO2 nanowires with diameter of 80–90 nm are synthesized via hydrothermal route and tested for electrocatalytic performance in a comparative study of ZABs fabricated using liquid and gel polymer electrolytes (GPEs). The open circuit voltage (OCV) of device lies in the range of 1.35–1.4 V. GPEs based devices show higher stability and exhibited 45 cycles (20 min per cycle) having discharge plateau above 1.06 V at a discharge current density of 1.59 mA cm−2 while liquid electrolyte‐based devices show 20 cycles (4 min per cycle at discharge current rate of 3 mA cm−2). [ABSTRACT FROM AUTHOR]

Within the Recycalyse project different research organization and industrial partners worked together to develop catalysts for the oxygen evolution reaction for proton exchange membrane electrolysis that reduce the use of critical raw materials. The catalysts are applied to polymer electrolyte membranes via a direct coating process and tested both in single cells and at stack level. In order to reduce the dependence on material imports in Europe in the long term, recycling is considered for the catalysts, electrodes and the entire system. [ABSTRACT FROM AUTHOR]

Due to the robust oxidation conditions in strong acid oxygen evolution reaction (OER), developing an OER electrocatalyst with high efficiency remains challenging in polymer electrolyte membrane (PEM) water electrolyzer. Recent theoretical research suggested that reducing the coordination number of Ir−O is feasible to reduce the energy barrier of the rate‐determination step, potentially accelerating the OER. Inspired by this, we experimentally verified the Ir−O coordination number's role at model catalysts, then synthesized low‐coordinated IrOx nanoparticles toward a durable PEM water electrolyzer. We first conducted model studies on commercial rutile‐IrO2 using plasma‐based defect engineering. The combined in situ X‐ray absorption spectroscopy (XAS) analysis and computational studies clarify why the decreased coordination numbers increase catalytic activity. Next, under the model studies' guidelines, we explored a low‐coordinated Ir‐based catalyst with a lower overpotential of 231 mV@10 mA cm−2 accompanied by long durability (100 h) in an acidic OER. Finally, the assembled PEM water electrolyzer delivers a low voltage (1.72 V@1 A cm−2) as well as excellent stability exceeding 1200 h (@1 A cm−2) without obvious decay. This work provides a unique insight into the role of coordination numbers, paving the way for designing Ir‐based catalysts for PEM water electrolyzers. [ABSTRACT FROM AUTHOR]

Due to the robust oxidation conditions in strong acid oxygen evolution reaction (OER), developing an OER electrocatalyst with high efficiency remains challenging in polymer electrolyte membrane (PEM) water electrolyzer. Recent theoretical research suggested that reducing the coordination number of Ir−O is feasible to reduce the energy barrier of the rate‐determination step, potentially accelerating the OER. Inspired by this, we experimentally verified the Ir−O coordination number's role at model catalysts, then synthesized low‐coordinated IrOx nanoparticles toward a durable PEM water electrolyzer. We first conducted model studies on commercial rutile‐IrO2 using plasma‐based defect engineering. The combined in situ X‐ray absorption spectroscopy (XAS) analysis and computational studies clarify why the decreased coordination numbers increase catalytic activity. Next, under the model studies' guidelines, we explored a low‐coordinated Ir‐based catalyst with a lower overpotential of 231 mV@10 mA cm−2 accompanied by long durability (100 h) in an acidic OER. Finally, the assembled PEM water electrolyzer delivers a low voltage (1.72 V@1 A cm−2) as well as excellent stability exceeding 1200 h (@1 A cm−2) without obvious decay. This work provides a unique insight into the role of coordination numbers, paving the way for designing Ir‐based catalysts for PEM water electrolyzers. [ABSTRACT FROM AUTHOR]

Polymer electrolyte membrane (PEM) water electrolyzers suffer mainly from slow kinetics regarding the oxygen evolution reaction (OER). Noble metal oxides, like IrO2 and RuO2, are generally more active for OER than metal electrodes, exhibiting low anodic overpotentials and high catalytic activity. However, issues like electrocatalyst stability under continuous operation and cost minimization through a reduction in the catalyst loading are of great importance to the research community. In this study, unsupported IrO2 of various particle sizes (different calcination temperatures) were evaluated for the OER and as anode electrodes for PEM water electrolyzers. The electrocatalysts were synthesized by the modified Adams method, and the effect of calcination temperature on the properties of IrO2 electrocatalysts is investigated. Physicochemical characterization was conducted using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area measurement, high-resolution transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses. For the electrochemical performance of synthesized electrocatalysts in the OER, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted in a typical three-cell electrode configuration, using glassy carbon as the working electrode, which the synthesized electrocatalysts were cast on in a 0.5 M H2SO4 solution. The materials, as anode PEM water electrolysis electrodes, were further evaluated in a typical electrolytic cell using a Nafion®115 membrane as the electrolyte and Pt/C as the cathode electrocatalyst. The IrO2 electrocatalyst calcined at 400 °C shows high crystallinity with a 1.24 nm particle size, a high specific surface area (185 m2 g−1), and a high activity of 177 mA cm−2 at 1.8 V for PEM water electrolysis. [ABSTRACT FROM AUTHOR]

A cell model is developed and validated to analyze the performance of polymer electrolyte membrane water electrolysis (PEMWE) stacks and systems. It is used to characterize the oxygen evolution reaction (OER) activity on a TiO2-supported IrO2 catalyst and an unsupported IrO2 powder catalyst. Electrochemical, stack, and system thermoneutral potentials are defined and determined for isothermal and non-isothermal stack operation. Conditions are determined under which the system thermoneutral potential or flammability of H2 in the O2 anode stream limits the stack turndown and operating temperature. Performance is analyzed of a complete PEMWE system with an electrolyzer stack containing an IrO2/TiO2 anode catalyst (2 mg/cm2 Ir loading) and N117-like membrane mitigated for H2 crossover, anode balance-of-plant (BOP) components, cathode BOP system with temperature swing adsorption for H2 purification, and electrical BOP system with transformer and rectifier. At the rated power condition, defined as 2 A/cm2 at 1.9 V, 80 °C, and 30 bar H2 pressure, the stack/system efficiency is 65.3%/60.3% at beginning of life (BOL), decreasing to 59.3%/53.9% at end of life (EOL). The peak stack/system efficiency is 76.3%/70.2% at BOL, decreasing to 71.2%/65.6% at EOL. Improvements in catalyst activity and membrane are identified for a 50% increase in current to 3 A/cm2 at 1.8 V. [ABSTRACT FROM AUTHOR]

Biomass-derived products, such as lignin, are interesting resources for energetic purposes. Lignin is a natural polymer that, when added to the anode of an alkaline exchange membrane water electrolyser, enhances H2 production rates and efficiencies due to the substitution of the oxygen evolution reaction. Higher efficiencies are reported when different catalytic materials are employed for constructing the lignin anolyte, demonstrating that lower catalytic loadings for the anode improves the H2 production when compared to higher loadings. Furthermore, when a potential of −1.8 V is applied, higher gains are obtained than when −2.3 V is applied. An increase of 200% of H2 flow rates with respect to water electrolysis is reported when commercial lignin is used coupled with Pt-Ru at 0.09 mg cm−2 and E = −1.8 V is applied at the cathode. This article provides deep information about the oxidation process, as well as an optimisation of the method of the lignin electro-oxidation in a flow-reactor as a pre-step for an industrial implementation. [ABSTRACT FROM AUTHOR]

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long‐term stability impose a grand challenge in its large‐scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active‐site over‐oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen‐participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial‐relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE. [ABSTRACT FROM AUTHOR]

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long‐term stability impose a grand challenge in its large‐scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active‐site over‐oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen‐participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial‐relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE. [ABSTRACT FROM AUTHOR]

Acidic water electrolysis is of great importance for boosting the development of renewable energy. However, it severely suffers from the trade-off between high activity and long lifespan for oxygen evolution catalysts on the anode side. This is because the sluggish kinetics of oxygen evolution reaction necessitates the application of a high overpotential to achieve considerable current, which inevitably drives the catalysts far away from their thermodynamic equilibrium states. Here we demonstrate a new oxygen evolution model catalyst-hierarchical palladium (Pd) whose performance even surpasses the benchmark Ir- and Ru-based materials. The Pd catalyst displays an ultralow overpotential (196 mV), excellent durability and mitigated degradation (66 μV h−1) at 10 mA cm−2 in 1 M HClO4. Tensile strain on Pd (111) facets weakens the binding of oxygen species on electrochemical etching-derived hierarchical Pd and thereby leads to two orders of magnitudes of enhancement of mass activity in comparison to the parent Pd bulk materials. Furthermore, the Pd catalyst displays the bifunctional catalytic properties for both oxygen and hydrogen evolutions and can deliver a current density of 2 A cm–2 at a low cell voltage of 1.771 V when fabricated into polymer electrolyte membrane electrolyser. [ABSTRACT FROM AUTHOR]

As a potential candidate for next‐generation energy storage systems, Li–O2 batteries (LOBs) with their attractive theoretical energy density have triggered great interest. However, tough issues of sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics, poor rechargeability, superoxide‐derived side reactions, and Li‐metal corrosion in LOBs limit their practical applications. Herein, a poly(2,2,2‐trifluoroethyl methacrylate) (PTFEMA) additive is introduced into the typical electrolyte, giving superior cycling performance to LOBs. Enabling strong solvation of Li+, PTFEMA regulates a uniform Li+ flow at the cathode and anode sides of LOBs. Induced by homogeneous Li+ flux and favorable adsorption with superoxide species, PTFEMA promotes superoxide transformation in the ORR and inhibits superoxide‐induced parasitic reactions. Uniform Li+ flux gives evenly‐distributed Li2O2 that can be completely decomposed during OER. In addition, PTFEMA protects Li‐metal against corrosion from O2, superoxide, and byproducts shuttle. Hence, accelerated ORR/OER kinetics, facilitated rechargeability, suppressed superoxide‐derived side reactions, and well‐protected Li‐metal can be simultaneously realized with PTFEMA, resulting in significantly enhanced electrochemical performance of LOBs. This electrolyte engineering involving facile multifunctional polymer additives provides a practical alternative to complex componential optimization toward commercial LOBs and gives insight to the understanding of uniform Li+ flux on reaction kinetics and reversibility. [ABSTRACT FROM AUTHOR]

A composite catalyst of nano-grade IrO 2 /TiO 2 powder is synthesized by Adams' fusion method for reducing overvoltage of solid polymer electrolyte (SPE) cell and cost-down of noble metal catalyst, simultaneously. The IrO 2 /TiO 2 catalysts, which has a porous composite nanostructure, are prepared according to molar ratio of Ir and Ti element with a specific surface area of 34.1–55.3 m2 g−1. It is found that crystal structure of TiO 2 is more dominated by the rutile phase than by Anatase. For a SPE system, total catalyst loading of anode which made of TiO 2 and IrO 2 is prepared as low as 0.77 mg cm−2 or less, in which the loading amount of the IrO 2 only is set to 0.6 mg cm−2 or less. The anode catalyst layer of about 10 ㎛ thickness is coated on the membrane (Nafion 212) for the membrane electrode assembly (MEA) by the decal method. The strong adhesion between the catalyst electrode the membrane is observed by Scanning electron microscopy (SEM). Linear sweep voltammetry (LSV) results shows that the nano-composite IrO 2 /TiO 2 catalysts has better oxygen evolution reaction (OER) than that of the synthesis IrO 2 only. Finally, the IrO 2 /TiO 2 catalysts is applied as anode electrode for SPE cells and it is observed that in spite of the lower loading amount of the IrO 2 less than 0.5 mg cm−2, working voltage of 1.68 V is observed at a current density of 1 A cm−2 and operating temperature of 80 °C. • Nano-composite Ir 2 /TiO 2 as anode catalyst is synthesized by Adam's Fusion method. • The loading amount of IrO 2 is reduced less than 0.5 mg cm−2 for SPE electrolysis. • The Ir 2 /TiO 2 shows enhanced durability properties for SPE electrolysis at 1 A cm−2. [ABSTRACT FROM AUTHOR]

Non-precious bifunctional FeCoNiCuMo high entropy alloy catalysts, together with CoNi layered double hydroxide nanowire arrays enhanced gel electrolytes, make high performance rechargeable flexible zinc-air batteries. [Display omitted] • FeCoNiCuMo high entropy alloy exhibits ultrahigh oxygen electrocatalytic efficiency. • Deliver a remarkable peak power density of 298 mW cm−2 at 412 mA cm−2. • Exhibit excellent cycling stability for 1576 cycles (788 h) at 10 mA cm−2. • Develop CoNi layered double hydroxide nanowire array enhanced gel electrolyte. • Flexible ZAB exhibits a peak power density of 102.9 mW cm−2 and lifespan of 58 h. Cost-effective highly efficient and stable bifunctional oxygen catalysts are critical for practical applications of rechargeable zinc-air batteries (ZABs). In this study, a non-precious bifunctional FeCoNiCuMo high entropy alloy (HEA) catalyst was developed, exhibiting outstanding dual-functionality toward catalysis of oxygen reduction and oxygen evolution reactions, with an ultralow voltage gap (ΔE 10) of 0.67 V. The FeCoNiCuMo HEA based ZAB exhibited a high peak power density of 298 mW cm−2 at a high current density of 412 mA cm−2 and an outstanding cycling stability over 1576 cycles (788 h) at 10 mA cm−2 with a minor voltage gap increase of 6.2 %. Incorporation of CoNi layered double hydroxide nanowire arrays to poly(vinyl alcohol) (PVA) gel electrolytes significantly enhanced the aqueous electrolyte adsorption and water retention capabilities and ionic conductivities of the PVA gel, leading to much improved efficiency and stability of the flexible ZAB (FZAB), with an increase in peak power densities from 72 to 102.9 mW cm−2 and a prolonged lifespan from 35 to 58 h at 5 mA cm−2. The FZAB exhibited excellent mechanical stability, with no variations in voltage gaps observed under increasing bending. [ABSTRACT FROM AUTHOR]

The high initial cost of polymer electrolyte membrane water electrolyzers (PEMWEs) has delayed their widespread commercialization. A possible means to reduce cost is by reducing the overvoltage and increasing the current density to reduce the electrode area. This study proposes a novel method in which boiling is superimposed on the oxygen evolution reaction (OER) to decrease electrolysis voltage. The vapor bubbles formed by boiling are expected to decrease the concentration overvoltage. The boiling effect was experimentally analyzed using a three-electrode cell. Although a general catalyst layer (CL) was formed on a working electrode (WE) bar, the structure of the working electrode (WE) bar was special, in which a 10-W heater was embedded and made boiling on the electrode under 1 bar condition. Increasing the electrode temperature under static OER current density slightly decreased the OER potential. However, an abrupt decrease in potential was observed when the temperature was scaled over the boiling temperature. Moreover, this abrupt decrease substantially intensified when, similar to a practical PEMWE, a porous transfer layer (PTL) and flow channel were assembled on the CL. These experimental results suggest that boiling can reduce the concentration overvoltage by reducing the oxygen concentration on the CL, especially when the mass transport resistance caused by the PTL is considerable. Innovatively and simply utilizing boiling, as proposed here, can enhance the oxygen transfer and contribute to reducing the initial cost of PEMWEs. • OER is coupled with boiling. • Three-electrode cell confirms boiling effect on OER. • Theoretical modeling qualitatively predicts the boiling mechanism. • Oxygen activity in catalyst layer is estimated electrochemically. • Boiling reduces concentration overvoltage of OER. [ABSTRACT FROM AUTHOR]

To accelerate the practical application of Li–O 2 batteries. A catalyst with outstanding oxygen reduction reaction and oxygen evolution reaction catalytic activity is required. Here, in-situ growth ZIF-67 within three-dimensional nitrogen-doped reduced graphene oxide (3D-N/rGO) layers was used to investigate a bifunctional catalyst. The inner space is supported by ZIF-67 particles, while 3D-N/rGO extends a suitable three-phase reaction interface and enhances O 2 adsorbability. When assembled battery with normal electrolyte, it delivered a high specific capacity of 12028 mAh g−1, and stabilised for 203 cycles at 200 mA g−1 with a limited capacity of 500 mAh g−1. When LiBr-gel polymer electrolyte (GPE) was used instead of electrolyte the lithium metal anode, the polarisation value of LiBr-GPE in Li/Li batteries was only about 0.0384 V after continuous working for 1600 h. When used as a cathode catalyst in Li–O 2 batteries, the electrode performance was doubled compared with using the normal electrolyte. These findings pave the way for developing of all-encompassing battery materials with high capacity and long cycle life. [ABSTRACT FROM AUTHOR]

For the challenging pursuit of high energy efficiency and mechanical tolerance in flexible solid‐state Zn‐air batteries (FSZABs), a hydrogel electrolyte (HE) consisting of dual‐network crosslinked polyacrylic acid‐Fe3+‐chitosan (PAA‐Fe3+‐CS) polymer host infiltrated with a mixed aqueous electrolyte of NH4Cl and ZnCl2 is developed. The absorbed near‐neutral electrolyte renders the HE high ionic conductivity but low corrosiveness to both electrocatalysts and Zn metal anode (ZMA), ensuring more stable Zn‐OH‐O2 chemistry compared to that in strong alkaline electrolyte and thus endowing the assembled FSZABs with a landmark cycle life up to 120 h (5 mA cm−2). More intriguingly, the CS molecular beams introduced into the PAA hydrogel backbone will precipitate and fold subjecting to the Hofmeister effect when saturated with the near‐neutral electrolyte, which can effectively enhance the interfacial adhesion strength of the HE on both air cathode and ZMA, achieving reliable and robust bonding between them. Thus, the FSZABs simultaneously exhibited a superior tolerance to repeated mechanical deformation during operation, allowing more than 360 continuous bending‐recovery cycles without any decline in voltage efficiency. The ingenious chemistry and interface synergetic engineering on the crucial HEs provides a rational methodology to realize boosted electrochemical and mechanical durability of FSZABs forward for future practical implementation. [ABSTRACT FROM AUTHOR]

[Display omitted] Proton-conducting polymer electrolyte membrane water electrolysis (PEMWE) is a vital clean hydrogen generation technology that can ease the energy crisis resulting from global warming and dependence on fossil fuels. However, the long-term catalytic activity and stability of the extensively studied benchmark RuO 2 catalysts in an acidic environment is insufficient for large-scale renewable energy conversion devices. Thus, significant recent efforts have focused on identifying and exploring acid-stable Ru-based electrocatalysts with low overpotential and high stability for the oxygen evolution reaction (OER). This review offers a comprehensive analysis of recent advances in Ru-based acidic OER catalysts, starting with a detailed understanding of design principles for Ru-based catalysts, encompassing the reaction mechanisms, degradation mechanism, and activity-stability relationships. Subsequently, advanced Ru-based catalysts regulating strategy are into four categories, within each category, a critical assessment of catalyst design and synthesis, electrocatalytic performance, along with typical examples and existing challenges. Representative examples in practical PEMWE are also provided to illustrate these advancements. Finally, the challenges and prospects for future studies on the development of Ru-based acidic OER catalysts towards the ultimate application of PEMWE are also examined. [ABSTRACT FROM AUTHOR]

For safety concerns, polymer-based Li–O2 batteries have received more attention than traditional non-aqueous Li–O2 batteries. However, poor cycling stability, low round trip efficiency, and over charge potential during cycling are the major shortcomings for their future applications. In this work, a soluble redox mediator integrated into a polymer electrolyte provides immediate access to the solid discharged product, lowering the energy barrier for reversible Li2O2 generation and disintegration. Moreover, introducing a redox mediator to the polymer electrolyte boosts the ORR during discharge and the OER during the recharge process. The synergistic redox mediator pBQ (1,4 benzoquinone) dramatically reduces the over-potential. A small proportion of pBQ in the polymer electrolyte allows Li2O2 to develop in a thin film-like morphology on the cathode surface, resulting in a high reversible capacity of ∼12 000 mA h g−1 and an extended cycling stability of 100 cycles at 200 mA g−1 with a cut-off capacity of 1000 mA h g−1. The remarkable cell performance is attributed to the fast kinetics of para benzoquinone for the ORR and OER in Li–O2 batteries. The use of a redox mediator in a polymer electrolyte opens a new avenue for practical Li–O2 battery applications in achieving low charge potential and excellent energy efficiency. [ABSTRACT FROM AUTHOR]

The growth of the hydrogen economy is predicated on advancements in electrochemical energy technologies, with water electrolysis as a key component to the technological portfolio. Much of the focus on anode catalyst development for polymer electrolyte membrane water electrolyzers (PEMWE) is centered on activity as controlled by compositional and morphological impacts on reactant/intermediate/product adsorption. However, the effectiveness of this strategy is found to be limited upon integration of these materials into PEMWE membrane electrode assemblies (MEA). Regardless of catalyst activity, the combination of electrode inhomogeneity, ionomer integration, and high density of oxide–oxide interfaces yields significant performance losses associated with poor catalytic electrode conductivity. Here many of these limitations are addressed through the development of a unique catalyst morphology composed of nanoporous Ir nanosheets (npIrx‐NS) that exhibit high catalytic activity for the anodic oxygen evolution reaction and superior electrode electronic conductivity in comparison to a commercial IrO2 nanoparticle catalyst. The utility of the npIrx‐NS is demonstrated through incorporation into PEMWE MEAs where their performance exceeds that of commercial catalyst coated membranes at loadings as low as 0.06 mgIr cm−2 while exhibiting a negligible loss in performance following 50 000 accelerated stress test cycles. [ABSTRACT FROM AUTHOR]

Efficient water splitting for commercial electrolysis devices is predicated on the development of materials, specifically for the catalytic electrodes, that exhibit an optimal balance between activity and stability. Complicating the development of electrocatalytic materials, particularly for oxygen-evolving anodes in acidic polymer electrolyte membrane electrolyzers, is an inverse relationship between activity and stability. Here the development of a nanostructured oxygen evolution reaction (OER) electrocatalyst for low-pH water splitting is demonstrated. Dealloying of mixed RuIr borides is used to form a high aspect ratio electrocatalytic material that exhibits low OER overpotentials matching that of RuOx and electrolytic stability matching that of IrOx. [ABSTRACT FROM AUTHOR]

Electrochemical scanning tunnelling microscopy (EC-STM) performed under reaction conditions is capable of detecting catalytically active sites. The method is based on the higher noise level of the STM signal at the position of active centres compared to inactive surface areas ('noise' or n-EC-STM). This paper demonstrates for the first time the applicability of n-EC-STM for the oxygen evolution reaction. The investigated system was an amorphous iridium oxide surface grown on Ir(1 1 1). [Display omitted] • Electrochemical scanning tunnelling microscopy under reaction conditions applicable for the oxygen evolution reaction (OER). • The noise level is more pronounced at active than an inactive areas. • Different features on amorphous iridium oxide show comparable noise levels. • OER shows little structure-sensitivity in this case. • The shape of the IrO x nanoparticles should not significantly influence the turnover frequency. Electrocatalytic "green" production of hydrogen from water for sustainable energy provision schemes is currently inefficient due to the sluggish kinetics of the oxygen evolution reaction (OER) at the anodes of the electrolysers. In the case of acidic polymer electrolyte membrane electrolysers, iridium (Ir) oxide catalysts pose a promising compromise between good OER activity and stability. However, the structure–activity relations for these materials remain largely unknown because the surface of a "real" oxide catalyst under reaction conditions becomes amorphous. In order to contribute to the understanding of these systems, we use electrochemical scanning tunnelling microscopy under reaction conditions ('noise' or n-EC-STM). With this technique, active areas can be detected by an increased noise level of the STM signal compared to inactive sites. The n-EC-STM measurements are applied to an amorphous iridium oxide surface, which is formed during electrochemical cycling of Ir(1 1 1). By doing so, we can monitor OER activity in-situ while simultaneously assessing the surface morphology. In order to elucidate the active areas, step and terrace sites were quantitatively compared to each other. The measurements reveal that terraces, step sites and concavities lead to a similar noise level increase in the STM signal. We, thus, conclude that the OER on the amorphous extended iridium oxide surface shows little structure-sensitivity. Subsequently, we suggest that in contrast to, e.g., metallic Pt for the oxygen electro-reduction, the shape of amorphous IrO x nanoparticles in an acidic medium should not significantly influence the OER turnover frequency. [ABSTRACT FROM AUTHOR]

• A Nb 4 N 5 micro-flower was developed to support IrO x as anode catalyst for PEMWE. • The IrO x /Nb 4 N 5 catalyst outperformed commercial IrO 2. • The catalyst-support interaction improved OER performance. • Superior performance and good stability were achieved for practical PEMWE operation. Precious metal Ir is still the best oxygen evolution reaction (OER) catalyst for polymer electrolyte membrane water electrolysis (PEMWE). Developing suitable support materials to improve the utilization rate of Ir is therefore of great significance for the practical application of PEMWE. Herein, we report a unique spherical Nb 4 N 5 µm-flower as an anode catalyst support for PEMWE. Through an organic colloidal method, a supported IrO x /Nb 4 N 5 catalyst was successfully prepared. TEM analysis showed that the IrO x particles were uniformly dispersed on the surface of the Nb 4 N 5 µm-flower, with a particle size of only about 1.8 nm. XPS revealed that Nb 4 N 5 can effectively regulate the d -band center of Ir, thereby reducing the adsorption energy between oxygen molecules and improving the OER efficiency. Half-cell tests and actual PEMWE operation demonstrate that the performance of the IrO x /Nb 4 N 5 catalyst is superior to commercial IrO 2. The overpotential was 247 mV at 10 mA cm−2, and the Tafel slope was only 53.2 mV dec−1 when the IrO 2 content was 40 wt %. Moreover, the mass activity was 857.6 A g –1 IrO2 at 1.7 V, which is 4.7 times that of commercial IrO 2. After an initial 150 h of PEMWE operation, the water decomposition voltage at 1 A cm−2 only increased by 5.5 %. These results suggest that the supported IrO x /Nb 4 N 5 catalyst is a promising candidate for real PEMWE applications. [Display omitted] [ABSTRACT FROM AUTHOR]

In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO 2) has often been utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining step. In general, the performance of PEM water electrolysis is dominantly affected by the specific surface area and the porous structure of the IrO 2 catalyst. Thus, in this study, IrO 2 and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were synthesized through the Adams fusion method. The as-prepared samples showed well-defined porous high-crystalline nanostructures. The ATO nanoparticles as a support were surrounded by IrO 2 nanoparticles as a catalyst without serious agglomeration, indicating that the IrO 2 catalyst was uniformly distributed on the ATO support. Compared to pure IrO 2 , the IrO 2 /ATO mixture electrodes showed superior OER properties because of their increased electrochemical active sites. Image 1 • IrO 2 and ATO nanostructures were synthesized with the Adams fusion method. • The samples showed a well-defined porous high-crystalline nanostructure. • ATO nanoparticles were surrounded by IrO 2 nanoparticles without agglomeration. • IrO 2 /ATO electrodes showed the superior OER properties. • The improved activity results from favorable mass transfer of a reactant of the mesoporous nanostructure. [ABSTRACT FROM AUTHOR]

Abstract Novel IrO 2 /Ti 1−x W x O 2 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 TiO 2 support with various doping amounts (0, 5, 10, and 20 at.%) 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 Ti 0.8 W 0.2 O 2 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 TiO 2 conductivity. The voltammetric charge of 40IrO 2 /Ti 0.9 W 0.1 O 2 (1754 mC (cm2 mg)−1) was higher than that of unsupported IrO 2 , and 40IrO 2 /Ti 0.9 W 0.1 O 2 exhibited enhanced electrochemical activity for the OER process. A single-cell performance of 1.79 V was achieved by using 80IrO 2 /Ti 0.9 W 0.1 O 2 as an anode for the minimum charge-transfer resistance and ohmic resistance. Electrocatalysts with the Ti 0.9 W 0.1 O 2 support exhibited good durability. Thus, Ti 0.9 W 0.1 O 2 is a promising anode electrocatalyst support for SPE water electrolysis. Highlights • The Ti 0.9 W 0.1 O 2 annealing shows adequate conductivity(9.98×10-2 S cm-1) and higher BET surface area(77 m2 g-1). • The 80IrO 2 /Ti 0.9 W 0.1 O 2 shows the lowest OER overpotential (1.79 V). • The durability test of 40IrO 2 /Ti 0.9 W 0.1 O 2 proves that the electrocatalysts possess a desirable stability. [ABSTRACT FROM AUTHOR]

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with submonolayer (1, 2, and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of submonolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts, such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers. [ABSTRACT FROM AUTHOR]

Large-scale deployment of polymer electrolyte membrane water electrolysis, which is a promising technology for green hydrogen production, requires significant reduction of Ir in catalyst layer due to its high material cost. However, as the Ir amount in the catalyst layer decreases, number of electrically-isolated-inactive catalysts increases, resulting in poor performance. Here we show that the nanostructured textiles essentially avoids the formation of such inactive catalysts, enables unprecedented long-term hydrogen evolution with ultralow Ir mass loadings at high rates. We reveal that achieved high performance is derived from the low resistivity of prepared nanostructured textile which have a unique porous, three-dimensional structure. We also show that the nanostructured textiles facilitate simple catalyst-coated membrane production, which does not require liquid-phase processes or produce chemical waste. The method we developed can also be applied to a variety of zero-gap electrochemical conversion cells that require a high-rate catalytic reactions with small amounts of catalyst. [Display omitted] • Conductive IrO 2 nanostructured textile developed for electrolytic H 2 production. • High performance achieved due to low sheet resistance. • Ultralow Ir mass loading will lead to vastly reduced cost. • Liquid-phase processes are not needed, and no chemical waste is produced. • TEM and operando XAFS revealed unique electronic and geometrical structure of the IrO 2 nanostructured textile. [ABSTRACT FROM AUTHOR]

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 IrO 2 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 IrO 2 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 V 2 O 5 precipitate. [ABSTRACT FROM AUTHOR]

Durability of polymer electrolyte fuel cells in automotive applications can be severely affected by hydrogen starvation arising due to transients during the drive-cycle. It causes individual cell voltage reversal, yielding water electrolysis and carbon corrosion reactions at the anode, ultimately leading to catastrophic cell failure. A popular material-based mitigation strategy is to employ a reversal tolerant anode (RTA) that includes oxygen evolution reaction (OER) catalyst (e.g., IrO 2 ) to promote water electrolysis over carbon corrosion. Here we report that RTA performance surprisingly drops under not only water-deficient but also water-excess conditions. This presents a significant technical challenge since the most common triggers for cell reversal involve excess liquid water. Our findings from detailed electrochemical diagnostics and nano-scale X-ray computed tomography provide insight into how automotive fuel cells can overcome critical vulnerabilities using material-based solutions. Our work also highlights the need for improved materials, electrode designs, and operation strategies for robust RTAs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Four different 316 stainless steel mesh structures are investigated. • In pressure drop experiments, M1 sample has the lowest drop by 1.02 kPa. • In Case 3, the current density is increased by %300 at 1.9 V. • Case 3 electrode has the highest charge transfer coefficient by 0.827. • Corrosion potential of SS316 is improved from 218.28 mV to 689.5 mV by Pt coating. Cell performance of the polymer electrolyte membrane water electrolyzers is highly depended on the transferred water amount through the cell. Thus, anode gas diffusion electrodes should be selected carefully due to its effect on the overpotentials like mass transfer limitations and ohmic losses inside the cell. In this study, to reduce the cost of polymer electrolyte membrane water electrolyzers systems, stainless steel mesh electrodes (SS316) were used for a cheaper option as an anodic gas diffusion electrode for the polymer electrolyte membrane water electrolyzers. Pressure change measurement experiments were conducted to select the best mesh electrode pore size. In the pressure change measurement experiments, the M1 sample had the lowest pressure drops around 1.02 kPa. After the pressure change experiments, the M1 sample was coated by Pt noble metal in different molar ratios using electrodeposition method, like 1 mM (Case 1), 2 mM (Case 2), and 3 mM (Case 3). At 1.9 V, the current density was improved around 300% compared to the uncoated electrode in Case 3 sample. According to the results, it was seen that the Pt coated stainless steel electrode could be used as an alternative cost-effective gas diffusion electrode in polymer electrolyte membrane water electrolyzer system. [ABSTRACT FROM AUTHOR]

Rational fabrication of an electrocatalyst with excellent stability, conductivity, and activity is urgently demanded for oxygen evolution reaction (OER). In this work, Ru-regulated hierarchical CoFe/CoFe 2 O 4 nanoparticles with graphitic carbon shells were successfully fabricated via self-assembling and vapor reduction strategies. Our investigation uncovers high concentrations of octahedral Co3+ sites in electrocatalysts can be triumphantly acquired via moderate Ru doping, which could give rise to excellent electrocatalytic activities during OER. In addition, the design of CoFe alloys and graphitic carbon shells can compensate for the weak conductivity of cobalt ferrites. Based on the intrinsic stability, improved conductivity and activity, the as-prepared electrocatalyst yields a low overpotential of 237 mV at 10 mA cm−2 and a small Tafel slope of 47 mV dec−1 in 1.0 M KOH electrolyte, exhibiting outstanding electrocatalytic activities for OER. This work effectually optimized the geometric sites of cations in spinel oxides via Ru introducing, and successfully enhanced conductivities of electrocatalysts by combining with CoFe alloys as well as graphitic carbon shells. [Display omitted] [ABSTRACT FROM AUTHOR]

Designing an efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline media is highly needed but very challenging task. Herein, we used organic polyelectrolytes such as (carboxymethyl cellulose) CMC and polyacrylamide polymers for the growth of Co 3 O 4 nanostructures by aqueous chemical growth method. The morphology and composition studies were performed on scanning electron microscopy (SEM), energy dispersive X-ray (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) techniques. The structural properties and the surface chemistry of the Co 3 O 4 electrocatalysts were correlated to the OER performance, and the enhancement mechanism with respect to pristine Co 3 O 4 was observed to be specifically related to the polyelectrolyte templating role. Co 3 O 4 @CMC composites displayed reduced crystallite size, producing OER overpotential as low as 290 mV at 10 mAcm−2 in 1.0 KOH and Tafel slope of 71 mVdec−1, suggesting fast transfer of intermediates and electrons during water electrolysis. On the other hand, the use of polyacrylamide and its different templating mechanism resulted in similar crystallite size, but preferential exposed faces and larger surface vacancies content, as demonstrated by HR-TEM and XPS, respectively. Consistently, this material displays cutting-edge OER performance, such as overpotential of 260 mV at 10 mAcm−2 and a low Tafel slope of 63 mVdec−1. The proposed strategy for the preparation of Co 3 O 4 nanostructures in the presence of CMC and polyacrylamide is facile, mass production, thus it could equally contributed towards the realization of hydrogen energy. Therefore, these nanostructures of Co 3 O 4 can be regarded as an alternative and promising materials for the different electrochemical applications including fuel cells, metal air batteries, overall water electrolysis and other energy storage devices. [ABSTRACT FROM AUTHOR]

• α-Ni(OH) 2 @Ir core–shell nanoparticles are synthesized at room temperature. • 92 fold better OER mass activity is recorded for α-Ni(OH) 2 @Ir than IrO 2. • α-Ni(OH) 2 @Ir exhibits 5 times higher HER mass activity than Pt/C. The industrialization of solid polymer electrolyte water electrolysis (SPEWE) dominantly relies on electrocatalyst with high stability and efficiency for oxygen evolution reaction (OER) catalysis. In this work, we report a facile method to construct α-Ni(OH) 2 @Ir core–shell nanoparticles as bifunctional electrocatalyst for acidic overall water splitting application. α-Ni(OH) 2 @Ir requires lower overpotentials (238 mV and 20 mV) to achieve 10 mA cm−2 in acidic OER and HER catalysis compared to benchmarked IrO 2 (332 mV) and Pt/C (36 mV); meanwhile, the OER mass activity at 1.55 V vs. RHE was 92 fold higher than IrO 2 ; HER mass activity at overpotential of 80 mV was 5 fold better than Pt/C. The boosted catalytic activity is due to the enhanced utilization efficiency of Ir stemmed from the ultrathin Ir shell (~1 nm). Moreover, 1.49 V is demanded for α-Ni(OH) 2 @Ir to attain 10 mA cm−2 in acidic overall water splitting, 110 mV lower than benchmarked Pt/C-IrO 2 system. Meanwhile, stable catalytic performance is recorded for 20 h. [ABSTRACT FROM AUTHOR]

The main challenge for the anode electrocatalyst in a polymer electrolyte membrane water electrolyzer (PEMWE) is to maintain activity and stability simultaneously under the corrosive environment. We report a highly active and durable iridium electrocatalyst supported on boron carbide for the oxygen evolution reaction. The physical and electrochemical properties of the catalyst are controlled by changing the synthetic reduction temperature from 30 °C to 100 °C. The prepared Ir/B 4 C-100 °C catalyst shows two times higher mass activity than Ir/B 4 C-30 °C, even outperforming two commercial catalysts. The improved activity can be correlated to the high concentration of Ir (III) and OH species on the surface and the well-dispersed iridium nanoparticles on the support. Controlling the reduction temperature is also found to enhance iridium stability by developing the interactions between iridium and B 4 C. These metal-support interactions inhibit the oxidative dissolution of Ir (III) and the aggregation of iridium species. Ir/B 4 C-100 °C also shows better single cell performance than those of two commercial catalysts when tested in a PEMWE. The cell with the synthesized catalyst of Ir/B 4 C-100 °C shows a current density of 1.98 A/cm2 at 1.8 V, whereas those with two commercial catalysts exhibit values of 1.36 and 0.692 A/cm2 at 1.8 V, respectively. [Display omitted] • A series of boron carbide-supported iridium catalysts were synthesized. • The physical and electrochemical properties of the iridium catalyst were controlled. • OER activity and durability of Ir/B 4 C-100 °C outperformed two commercial catalysts. • High amount of Ir(III) and OH species and high dispersion provided better activity. • Metal-support interaction of Ir/B 4 C-100 °C enhanced the durability of Ir. [ABSTRACT FROM AUTHOR]

Abstract: Ir x Ru1−x O2(1 ≥ x ≥ 0) with nanorod structure were successfully synthesized by employed pre-filling the Ir and/or Ru guest species into the peripheral-pore of NH2-modified as-synthesized SBA-15 and explored as electrocatalyst for oxygen evolution reaction (OER) in water electrolyzers. Various physicochemical parameters for zeolite template and/or Ir x Ru1−x O2 were obtained by SEM, TEM, XRD, EDX and N2 gas absorption/desorption measurements. The morphology for prepared Ir x Ru1−x O2 samples with individual and/or cluster nanorods was changed with the component difference. Cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, steady state polarization curves and stability tests were performed to investigate the catalytic activity and stability of these electrocatalysts for OER. The cell with catalyst RuO2 showed best catalytic performance with the lowest onset potential (1.374 V at 10 mA cm−2), which may be ascribed to regular nanoclusters and larger outer active surface area. Meanwhile, the cell stability tests suggested that the addition of IrO2 in Ir x Ru1−x O2 improved the stability of the RuO2 catalyst. [Copyright &y& Elsevier]

Polymer electrolyte membrane water electrolysis (PEMWE) uses intermittent renewable energy and plays a key role in storing energy in the form of hydrogen. However, its widespread application is limited owing to issues associated with oxygen evolution reaction (OER) catalysts, including activity and stability. Thus, it is necessary to develop highly active and durable catalysts in this regard. Herein, an IrO x @IrO 2 catalyst with an ultrahigh surface area (G-450) was synthesized by a simple Adams fusion method and calcined at 450 °C with glycine as an additive. Owing to its micro/mesoporous structure, this catalyst exhibited an ultrahigh specific surface area (SSA) of 403 m2 g-1 and an amorphous structure with average oxidation states of Ir(IV). A trade-off between its OER activity and stability was achieved by controlling its SSA and Ir oxidation states via the optimization of the calcination temperature. Surface-rich Ir(III) species and high SSA enhanced the OER activity of G-450 (309 mV overpotential at 10 mA cm-2) compared with the IrO 2 catalyst prepared without glycine (A-450, 351 mV overpotential at 10 mA cm-2). Further, the G-450 exhibited an Ir dissolution rate that was 2.5-fold lower than that of the IrO 2 catalyst prepared at 350 °C (G-350) after 6 h chronopotentiometry at 10 mA cm-2, implying a higher stability owing to the presence of Ir(IV) species. Additionally, G-450 was introduced into the anode of the polymer electrolyte membrane (PEM) water electrolyzer single cell and demonstrated higher performance than A-450. The balanced activity and stability of IrO x @IrO 2 enhanced the PEM water electrolyzer performance. Moreover, this facile synthetic method is also applicable to the synthesis of binary oxide compounds and other transition metal oxides. [Display omitted] [ABSTRACT FROM AUTHOR]

To improve the efficiency of polymer electrolyte membrane water electrolyzers (PEMWEs), catalysts with high activity and stability in the oxygen evolution reaction (OER) have been extensively investigated. In this study, using a novel solution-reduction process, self-assembled Ir/IrOx OER catalysts are prepared, with a controlled Ir oxidation-state ratios. Spectroscopic and electron microscopic analyses confirm that these catalysts have a gradient oxidation state, which gradually decreases from the shell to the core. In half-cell tests, before and after the accelerated stress test (AST) at 10 mA cm−2, the optimized Ir/IrOx catalyst exhibits an overpotential change of approximately 21 mV RHE , which is 3.3 times lesser than that of the state-of-the-art IrO 2 (approximately 69 mV RHE). Further, quantification of the dissolved Ir content during the AST shows that the Ir/IrOx catalyst has a strong corrosion resistance under acidic OER conditions. Single-cell testing of the as-prepared OER catalyst demonstrates a remarkable cell performance of 1.73 V at 1.0 A cm−2. After 48-h durability testing, the increase in the membrane electrode assembly cell voltage with the as-prepared OER catalyst is approximately 5.3 times lesser than that of the reference at 250 mA cm−2. The proposed novel catalyst with high activity and stability can aid the development of cost-effective PEMWE systems. [Display omitted] • Self-assembled Ir/IrOx catalysts were prepared by a novel solution–reduction method. • Ir/IrO X catalyst has a feature of gradient distribution of oxidation states of Ir. • Investigation of oxygen evolution reaction activity and stability parameters. • Ir/IrO X -[2] catalyst showed excellent OER performance. • The durability of a single-cell with Ir/IrO X exceeds that of commercial IrO 2. [ABSTRACT FROM AUTHOR]

The potential for generating green hydrogen by electrolysis (water splitting) has resulted in a substantial amount of literature focusing on lowering the current production cost of hydrogen. A significant contributor to this high cost is the requirement for precious metals (namely Pt and Ir/Ru (oxides)) to catalyse the two main reactions involved in electrolysis: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein we overview the current literature of non-precious metal HER and OER catalysts capable of efficient water splitting within a polymer electrolyte membrane (PEM) electrolyser, recording the activity and stability of each catalyst and allowing for direct comparison to be made. Additionally, we highlight the inapplicability of catalyst stability testing in many academic studies for commercial electrolyser applications and propose a universal stability-testing regime for HER and OER catalysts that more accurately mimics the conditions within an operating electrolyser. • Stability studies of cathodic catalysts don't translate to commercial electrolysis. • Anodic catalyst stability studies are more comprehensive for electrolysis operation. • Stability of novel catalysts needs to be benchmarked against the optimal catalysts. • Combining active and stable first row transition metals shows promising catalysis. • A stability testing regime for proton exchange membrane electrolysis is proposed. [ABSTRACT FROM AUTHOR]

• TiO 2 -MoO x was prepared as a support for Ir catalyst toward OER. • Interaction between TiO 2 -MoO x and Ir promotes formation of OER-active Ir(III) species. • During the OER, the chemical Interaction between Mo and Ir keeps in Ir(III) species and suppresses dissolution of Ir. • Therefore, Ir/TiO 2 -MoO x exhibited improved OER activity and stability compared Ir black, Ir/C and Ir/TiO 2. • Ir/TiO 2 -MoO x improve performance and durability of the PEMWE single cell than Ir black. To accomplish the goals of cost reduction and durability improvement of polymer electrolyte water electrolysis (PEMWE), we report a titanium oxide and molybdenum oxide composite (TiO 2 -MoO x) as a support for the iridium (Ir) nanoparticle catalyst (Ir/TiO 2 -MoO x) toward oxygen evolution reaction (OER). Incorporation of Mo(V) and Mo(VI) into TiO 2 significantly enhances the electrical conductivity of TiO 2. During the synthesis of Ir nanoparticles on TiO 2 -MoO x , Mo(V) is oxidized to Mo(IV) with suppressing oxidation of Ir(III) to Ir(IV). Consequently, Ir/TiO 2 -MoO x forms higher fraction of the OER-active Ir(III) species on Ir surface enhancing OER activity of Ir catalyst. During OER, the chemical interaction between Mo and Ir keeps in Ir(III) and lessens the dissolution of Ir(III), improving the durability of Ir/TiO 2 -MoO x. Thus, Ir/TiO 2 -MoO x exhibits superior OER activity and durability. The single cell test results demonstrate that, as an anode catalyst, Ir/TiO 2 -MoO x can improve the performance and durability of the PEMWE cell than Ir black. [ABSTRACT FROM AUTHOR]

As state-of-the-art catalysts for polymer electrolyte membrane water electrolysis (PEMWE), IrO 2 and RuO 2 have been extensively developed to accelerate the oxygen evolution reaction (OER) and to achieve high stability under acidic conditions. In this study, RuO 2 catalysts with controlled pore size distributions, particularly with a large number of macropores, are synthesized using a modified Adams method with silica nanosphere particles (SNPs) as a template. The specific surface areas of the prepared catalysts are 240–270 m2 g−1 regardless of the sizes of the SNP template. Also, all RuO 2 catalysts have a rutile structure suggested from X-ray diffraction. The RuO 2 catalysts with large portions of macropores (more than 20% of the total pore volume) exhibited greater OER activity and stability than the catalysts prepared using the conventional Adams method. These enhancements are attributed to fast mass transport during the reaction and the 50% increase in the electrochemically active surface area (ECSA) of the RuO 2 catalysts prepared from the modified method over those prepared by the conventional Adams method. In addition, the enhanced catalytic activity (overpotential of 263 mV at 10 mA cm−2) of RuO 2 with a large portion of macropores is revealed at the single-cell level in PEMWE. Image 1 • RuO 2 catalysts with controlled pore size distribution were synthesized. • The distribution was controlled by silica nanosphere particles as a nano-template. • Macro-RuO 2 was superior to other catalysts in terms of OER activity and stability. • The improvement was ascribed to high ECSA and fast bubble removal during OER reaction. [ABSTRACT FROM AUTHOR]

• Iridium nanoclusters supported on boron carbide are proposed as a catalyst for the oxygen evolution reaction. • A simple chemical reduction method was used to synthesize grape-like iridium nanoclusters supported on B 4 C. • Iridium nanoclusters supported on B 4 C showed better oxygen evolution activity than a commercial iridium oxide catalyst. Reducing the iridium loading for the oxygen evolution reaction (OER) is one of the main challenges of polymer electrolyte membrane water electrolysis (PEMWE). This study introduces grape-like iridium nanoclusters supported on boron carbide (B 4 C) which increase iridium utilization compared to a non-supported iridium catalyst. A simple chemical reduction method with NaBH 4 as a reducing agent was used to synthesize iridium nanoclusters on B 4 C. TEM images indicated that grape-like iridium nanoclusters were successfully dispersed on the B 4 C support. The catalytic performance of Ir/B 4 C was better than that of a commercial catalyst. To reach a current density of 10 mA/cm2, the overpotential of Ir/B 4 C was less than that of the commercial catalyst by 32 mV. Ir/B 4 C also has a mass activity 2.55 times higher than that of the commercial catalyst at 1.55 V. These improvements are attributed to the high electrochemical active surface area, the weak adsorption strength of oxygen, and the presence of Ir(OH) 4 on the surface. The durability of Ir/B 4 C is comparable to that of the commercial catalyst at 1 mA/cm2 for 15 h and higher at 10 mA/cm2 for 3 h. B 4 C may weaken the oxidative dissolution of iridium by transferring electrons even though the high electrochemical surface area of iridium in Ir/B 4 C may reduce its durability. [ABSTRACT FROM AUTHOR]

• In situ growth method to prepare a binder-free IrO x -Ti (IT) anode with a very low IrO x loading amount is described. • The synthesized catalyst shows a high oxygen evolution reaction activity and durability in acidic media. • A flow-type polymer electrolyzer (polymer electrolyte alcohol electrosynthesis cell) that efficiently synthesized glycolic acid from oxalic acid is described. Alcohol is considered as a potential CO 2 -free energy carrier to replace hydrogen because of its high energy density, appropriate chemical stability and low-cost production. In this study, we describe the development of a simple in situ growth method to prepare a binder free IrO x -Ti anode with a very low IrO x loading amount (< 0.25 mg cm−2), and the obtained catalyst shows a high oxygen evolution reaction (OER) activity and durability in acidic media. The performance of a polymer electrolyte alcohol electrosynthesis cell (PEAEC) using this anode catalyst shows a better performance than that of PEAEC using a typical commercial IrO 2 -Ti paper anode with the loading of IrO 2 = 3 mg cm−2. Furthermore, the key factors that affect the performance of the PEAEC are determined by comparing the performances of the PEAEC with different anode catalysts. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Tri-electrode battery configurations using gel polymer electrolytes are studied. • Effects of crosslinking density on OER is evaluated. • Effects of ZnO on ORR and OER are revealed separately. • Cycling performance is shown in different orientations without leaking. To increase the cyclability of rechargeable zinc-air batteries (ZABs) using gel polymer electrolytes (GPEs), battery design and structure need to be optimized. Two different configurations for a tri-electrode battery have been developed in this work; i.e., a planar cell and a sandwich cell. Discharge-recharge cycling tests at 5 mA cm–2 show that the sandwich cell is more suitable for ZABs than the planar cell. Various additives for the GPEs, such as cross linking agents and ZnO, have also been evaluated using the tri-electrode design. Cross linking density affects the physical state of GPEs, which in turn affects the oxygen evolution reaction as there is an adverse effect of excessive GPE/Ni foam electrode contact area because oxygen bubbles may be trapped in the GPE. The addition of ZnO to the oxygen reduction reaction side of the battery enhances the cycling performance of ZABs; i.e., the battery can withstand at least 100 cycles at 5 mA cm–2 with an efficiency of 62% during the first cycle. Image, graphical abstract [ABSTRACT FROM AUTHOR]

For a typical Proton exchange membrane (PEM) water electrolysis system, reducing the amount of Ir loading at the anode in membrane electrode assembly (MEA), while maintaining the performance, is still a big challenge. Here, to improve the performance of catalyst layer for oxygen evolution reaction (OER) in an acidic medium, vertical aligned IrO x nanoarrays (IrO x NAs) were synthesized by electrodeposition with TiO 2 nanotube arrays (TNTA) as template. By tuning the scan rate in the electrodeposition process, IrO x open-end nanotube arrays with tunable length ranged from 500 nm to 1400 nm and hollow nanorod arrays were obtained due to a diffusion-controlled deposition process. By constructing 3-D electrode of 1-D catalyst with enhanced ion transfer, IrO x NAs performed almost the same current density of OER with 1/20 Ir loading amount compared with commercial IrO 2 nanoparticles, which could significantly reduce the cost of catalyst in PEM water electrolyser. Moreover, no apparent current decrease was observed even after 10 h' anodic polarization under 1.55 V, which exhibit excellent OER durability. [ABSTRACT FROM AUTHOR]

• Simultaneous polarization and gas analysis measurements by mixing the gas outlet. • Permeated H 2 is consumed by IrOx during OER at low current densities. • Analysis of the PEMWE anode gaseous effluents aids in E/i corrections. • Tafel region extends by more than three decades of current in an operating PEMWE. • Chemical step as OER rate determining step with IrOx is suggested. It is considered that the oxidation activity of Ir based anodes on H 2 permeating the polymer electrolyte membrane (PEM) in PEM water electrolyzers (WE) is poor, resulting in high H 2 -in-O 2 contents at idle or dynamic operating conditions, especially with asymmetric pressure. In the present work, the dependence of H 2 permeation on cathode pressure was quantified by the pressure drop method. H 2 consumption was detected by chronoamperometry at potentials preceding water splitting. The addition of N 2 in the anode outlet gas allowed the on-line monitoring of the mixture composition during idle PEMWE operation by a thermal conductivity sensor. It is found that H 2 is effectively consumed with IrOx oxyhydroxide anodes at low current densities, with 75% conversion of the zero-current permeation at cathode pressure of 5 bar and 60 °C. Hence, both H 2 -in-O 2 content and lowest current density at which the H 2 -in-O 2 flammability limit is not exceeded decrease by more than 50%. Current density corrections accounting for H 2 consumption and mass transport potential corrections accounting for the enhancement of H 2 permeation with current density were implemented. The resulting Tafel slope values suggest that the rate determining step of the oxygen evolution reaction with IrOx oxyhydroxides is a chemical step. [ABSTRACT FROM AUTHOR]

Membrane–electrode assemblies (MEAs) designed for a polymer electrolyte membrane (PEM) water electrolyser based on a short-side chain (SSC) perfluorosulfonic acid (PFSA) membrane, Aquivion®, and an advanced Ir-Ru oxide anode electro-catalyst, with various cathode and anode noble metal loadings, were investigated. Electrochemical impedance spectroscopy (EIS), in combination with performance and durability tests, provided useful information to identify rate-determining steps and to quantify the impact of the different phenomena on the electrolysis efficiency and stability characteristics as a function of the MEA properties. This technique appears to be a useful diagnostic tool to individuate different phenomena and to quantify their effect on the performance and degradation of PEM electrolysis cells. [ABSTRACT FROM AUTHOR]