Recently, IrV‐based perovskite‐like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance. However, iridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts. In this work, Sr2MIr(V)O6 (M=Fe, Co) and Sr2Fe0.5Ir0.5(V)O4 were explored as OER catalysts in acidic media. Their activities were observed to be roughly equal to those previously reported for La2LiIrO6 or Ba2PrIrO6. Coupling electrochemical measurements with iridium dissolution studies under chemical or electrochemical conditions, we show that the deposition of an IrOx layer on the surface of these perovskites is responsible for their OER activity. Furthermore, we experimentally reconstruct the iridium Pourbaix diagram, which will help guide future research in controlling the dissolution/precipitation equilibrium of iridium species for the design of better Ir‐based OER catalysts.

Increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energy-storage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of the transition metals used in the manufacturing of cathode materials, together with questionable mining practices, are driving development towards more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium, as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different application scenarios, such as stationary energy storage and low-cost vehicles. This potential is reflected by the major investments that are being made by industry in a wide variety of markets and in diverse material combinations. Despite the associated advantages of being a drop-in replacement for LIBs, there are remarkable differences in the physicochemical properties between sodium and lithium that give rise to different behaviours, for example, different coordination preferences in compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt components. This demands a more detailed study of the underlying physical and chemical processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in academia and industry of the current state of the art in 2021 and the different research directions and strategies currently underway to improve the performance of sodium-ion batteries. The aim is to provide an opinion with respect to the current challenges and opportunities, from the fundamental properties to the practical applications of this technology.

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Recently,I r V -based perovskite-like materials were proposed as oxygen evolution reaction (OER) catalysts in acidic media with promising performance.H owever,i ridium dissolution and surface reconstruction were observed, questioning the real active sites on the surface of these catalysts.In this work, Sr 2 MIr (V) O 6 (M = Fe,Co) and Sr 2 Fe 0.5 Ir 0.5 (V) O 4 were explored as OER catalysts in acidic media. Their activities were observed to be roughly equal to those previously reported for La 2 LiIrO 6 or Ba 2 PrIrO 6 .C oupling electrochemical measurements with iridium dissolution studies under chemical or electrochemical conditions,w es howt hat the deposition of an IrO x layer on the surface of these perovskites is responsible for their OER activity.F urthermore,w ee xperimentally reconstruct the iridium Pourbaix diagram, whichw ill help guide future researchi nc ontrolling the dissolution/precipitation equilibrium of iridium species for the design of better Irbased OER catalysts.The electrochemical production of hydrogen fuel by water splitting has long been explored as ap otential way to store clean and renewable energy.T he key challenge for water splitting lies in improving the efficiency of the kinetically slow, rate-limiting oxygen evolution reaction (OER) (2 H 2 O! 4H + + O 2 + 4e À ). [1,2] While alarge variety of transition metal oxides have been studied as promising OER catalysts in alkaline media, [3][4][5][6] thedesign of active and stable catalysts in acidic media has proven challenging. [7][8][9][10][11] Thef ew materials established as suitable OER catalysts in acidic media are mostly Ir-based metal oxides,such as rutile IrO 2 , [12,13] 14] or more recently Ni-substituted IrO x . [15][16][17] Recently,Ir-based perovskites have been reported as promising candidates in acidic media. [18][19][20] Thehigh OER activity of these Ir-based perovskites was ascribed to the formation of electrophilic O (IÀ) surface species favoring the nucleophilic attack of water. [11,20,21] However,dissolution of iridium, alkali and/or rare earth elements in acidic electrolytes has been observed after close examinations of some Ir V -based perovskites,such as La 2 LiIr (V) O 6[20] and Ba 2 PrIr (V) O 6 , [7,19] indicating their drastic structural instabilities in harsh acidic conditions. [12] Furthermore,the presence of IrO 2 nanoparticles was revealed on the surface of La 2 LiIr (V) O 6 after cycling. [8] Given this array of observations,alegitimate question arises over the origin of the OER activity on the surface of these perovskites.Therefore,itisimportant to understand the OER mechanism of these Ir V -based perovskites,aswell as the effect of iridium dissolution on their catalytic behaviors,inorder to improve their performances and/or guide future research in designing new active and stable OER catalysts in acidic media.We herein investigate the catalytic behaviors of three Ir Vbased OER catalysts in acidic media, Sr 2 MIr (V) O 6 (M = Fe, Co) with ad ouble perovskite structure and Sr 2 Fe 0.5 Ir 0.5 (V) O 4 with a...

Fuel cells efficiently convert chemical into electric energy, with promising application for clean transportation. In proton-exchange membrane fuel cells (PEMFCs), rare platinum metal catalyzes today the oxygen reduction reaction (ORR) while iron(cobalt)-nitrogen-carbon materials (Fe(Co)-N-C) are a promising alternative. Their active sites can be classified as atomically dispersed metal-ions coordinated to nitrogen atoms (MeNxCy moieties) or nitrogen functionalities (possibly influenced by sub-surface metallic particles). While their durability is a recognized challenge, its rational improvement is impeded by insufficient understanding of operando degradation mechanisms. Here, we show that FeNxCy moieties in a representative Fe-N-C catalyst are structurally stable but electrochemically unstable when exposed in acidic medium to H2O2, the main ORR byproduct. We reveal that exposure to H2O2 leaves iron-based catalytic sites untouched but decreases their turnover frequency (TOF) via oxidation of the carbon surface, leading to weakened O2 binding on iron-based sites. Their TOF is recovered upon electrochemical reduction of the carbon surface, demonstrating the proposed deactivation mechanism. Our results reveal a hitherto unsuspected deactivation mechanism during ORR in acidic medium. This study identifies the N-doped carbon surface as Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolyte, these insights suggest that durable iron-nitrogen-carbon catalysts are within reach for PEMFCs if rational strategies minimizing the amount of H2O2 or reactive oxygen species (ROS) produced during ORR are developed.

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The electrochemical reduction of CO2 (eCO2RR) using renewable energy is an effective approach to pursue carbon neutrality. The eCO2RR to CO is indispensable in promoting C–C coupling through bifunctional catalysis and achieving cascade conversion from CO2 to C2+. This work investigates a series of M/N–C (M = Mn, Fe, Co, Ni, Cu, and Zn) catalysts, for which the metal precursor interacted with the nitrogen-doped carbon support (N–C) at room temperature, resulting in the metal being present as (sub)­nanosized metal oxide clusters under ex situ conditions, except for Cu/N–C and Zn/N–C. A volcano trend in their activity toward CO as a function of the group of the transition metal is revealed, with Co/N–C exhibiting the highest activity at −0.5 V versus RHE, while Ni/N–C shows both appreciable activity and selectivity. Operando X-ray absorption spectroscopy shows that the majority of Cu atoms in Cu/N–C form Cu0 clusters during eCO2RR, while Mn/, Fe/, Co/, and Ni/N–C catalysts maintain the metal hydroxide structures, with a minor amount of M0 formed in Fe/, Co/, and Ni/N–C. The superior activity of Fe/, Co/, and Ni/N–C is ascribed to the phase contraction and the HCO3 – insertion into the layered structure of metal hydroxides. Our work provides a facile synthetic approach toward highly active and selective electrocatalysts to convert CO2 into CO. Coupled with state-of-the-art NiFe-based anodes in a full-cell device, Ni/N–C exhibits >80% Faradaic efficiency toward CO at 100 mA cm–2.

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