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The oxygen evolution reaction (OER) is the bottleneck reaction of water splitting, which can be used to generate green hydrogen from renewable electricity. Cobalt iron oxyhydroxides (CoFeOxHy) are among the most active OER catalysts in alkaline medium. However, the active sites of these catalysts remain unclear. Here we use operando ultraviolet-visible (UV-Vis), X-ray absorption, and Raman spectroscopy to reveal oxidations of both Fe and Co ions in CoFeOxHy during the OER. By analyzing samples with different Fe contents and thickness, we find that the concentration of Fe4+ species at the surface, but not the concentration of Co4+ in the bulk, scales with the catalytic activity. These results indicate an Fe4+-containing active site in CoFeOxHy.

Hydroxide exchange membrane fuel cell (HEMFC) is a potentially cost-effective energy conversion technology. However, current state of the art HEMFCs require a high loading of platinum-group-metal (PGM) catalysts, especially for the hydrogen oxidation reaction (HOR). Here we develop a porous nitrogen-doped carbon-supported PtRu HOR catalyst (PtRu/pN-C) that has the highest reported intrinsic and mass activity in alkaline condition. Spectroscopic and microscopic data indicate the presence of Pt single atoms (SAs) in addition to PtRu nanoparticles on pN-C. Mechanistic study suggests Ru modulates the electronic structure of Pt for an optimized hydrogen binding energy, while Pt-SAs on pN-C optimize the interfacial water structure. These synergetic interactions are responsible for the high catalytic activity of this catalyst. An HEMFC with a low loading of this catalyst and a commercial Fe-N-C oxygen reduction reaction (ORR) catalyst achieves the highest reported PGM utilization rate. The current density at 0.65 V of this HEMFC reaches 1.5 A/cm2, exceeding the US DOE 2022 target (1 A/cm2) by 50 %.

Hydroxide exchange membrane fuel cell (HEMFC) is a potentially cost-effective energy conversion technology. However, current state of the art HEMFCs require a high loading of platinum-group-metal (PGM) catalysts, especially for the hydrogen oxidation reaction (HOR). Here we develop a porous nitrogen-doped carbon-supported PtRu HOR catalyst (PtRu/pN-C) that has the highest reported intrinsic and mass activity in alkaline condition. An HEMFC with a low loading of this catalyst and a commercial Fe-N-C oxygen reduction reaction (ORR) catalyst achieves the highest reported PGM utilization rate. The current density at 0.65 V of this HEMFC reaches 1.5 A/cm2, exceeding the US DOE 2022 target (1 A/cm2) by 50 %. Spectroscopic and microscopic data indicate the presence of Pt single atoms (SAs) in addition to PtRu nanoparticles on pN-C. Mechanistic study suggests Ru modulates the electronic structure of Pt for an optimized hydrogen binding energy, while Pt-SAs on pN-C optimize the interfacial water structure. These synergetic interactions are responsible for the high catalytic activity of this catalyst.

It has been widely acknowledged that most of the electrocatalysts would undergo a structural reconstruction during the water splitting as well as carbon dioxide (CO2) reduction reactions. Recently ...

In the face of climate change and rising energy consumption, electrocatalysis for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR) have become promising catalytic processes to address these critical issues. By integrating OER and HER/CO2RR with renewable energy, it may potentially provide a promising way for a sustainable human society. Nonetheless, lack of understanding of catalysts under working conditions and reaction mechanisms restrict the development of efficient catalysts. In recent years, in situ techniques that can be used to investigate the structural transformation of catalysts during the reaction have attracted considerable research interest. Herein, recent developments in heterogeneous catalysts for OER, HER, and CO2RR explored by using in situ X-ray diffraction and X-ray absorption spectroscopy techniques have been comprehensively summarized. Finally, challenges and future perspectives in this field are also provided to pave the way for future developments.

The actual chemical environment of metal phosphides or chalcogenides during hydrogen evolution reaction (HER) has been debated. In this work, pyrite-phase NiP2, Se-doped NiP2, and NiSe2 were investigated by means of in situ and ex situ characterizations to dynamically study the transformation during electrocatalysis. We found that doping with selenide would generate more vacancies to facilitate the activation on water molecule, in which hydroxide poisoning on active site would be suppressed by the presence of selenide. Furthermore, the radial distance of Ni–Ni pair can serve as a descriptor to identify the mechanism of HER. For NiP2 and Se-doped NiP2, the radial distance of the metal (Ni–Ni) pair during HER is identical to the as-prepared one, implying that the HER active site is on a single metal site with a Heyrovsky step. A drastic bond contraction of Ni–Ni pair on NiSe2 indicates dual-site activation correlated with a chemical desorption of Tafel step. The role of anion effects and metal sites are com...

RuO2 has been generally considered as the most active catalyst for the oxygen evolution reaction (OER) to date and shows remarkably higher activity in an acidic electrolyte than under alkaline conditions. Nevertheless, the dynamic valence state and local structure of reactive centers (i.e., Ru) in both acidic and alkaline electrolytes have not been systematically investigated yet, especially through in situ approaches. Herein, we employed in situ X-ray absorption spectroscopy to study the dynamic valence state and local structure of RuO2 during the OER in both acidic and alkaline electrolytes. In the acidic electrolyte, the Ru center was reduced near the onset potential prior to launching the OER and was oxidized during the OER process, while the coordination numbers and the bond lengths of the Ru–O path also decreased as revealed by in situ EXAFS analysis. Besides, in the alkaline electrolyte, RuO2 showed a similar behavior as revealed under acidic conditions. These results provide an evident insight into the dynamic change of the RuO2 electrocatalyst during the OER.

Single-atom catalysts (SAs) with the maximum atom utilization and breakthrough activities toward hydrogen evolution reaction (HER) have attracted considerable research interests. Uncovering the nature of single-atom metal centers under operating electrochemical condition is highly significant for improving their catalytic performance, yet is poorly understood in most studies. Herein, Pt single atoms anchoring on the nitrogen-carbon substrate (PtSA /N-C) as a model system are utilized to investigate the dynamic structure of Pt single-atom centers during the HER process. Via in situ/operando synchrotron X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, an intriguing structural reconstruction at atomic level is identified in the PtSA /N-C when it is subjected to the repetitive linear sweep voltammetry and cyclic voltammetry scanning. It demonstrates that the PtN bonding tends to be weakened under cathodic potentials, which induces some Pt single atoms to dynamically aggregate into forming small clusters during the HER reaction. More importantly, experimental evidence and/or indicator is offered to correlate the observed Tafel slope with the dynamic structure of Pt catalysts. This work provides an evident understanding of SAs under electrocatalytic process and offers informative insights into constructing efficient catalysts at atomic level for electrochemical water-splitting system.

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The oxygen evolution reaction (OER) is a key electrochemical reac-tion relevant to energy storage. Ni-containing bimetallic oxyhydrox-ides are among the most active OER catalysts in alkaline medium, but the mechanism of OER on pure Ni oxyhydroxide remains un-clear. Here we combine multiple operando spectroscopic tools including X-ray absorption, ultraviolet visible (UV-Vis), and Raman with electrokinetics to study the mechanism of OER on Ni(OH)2 nanosheets. The spectroscopic data reveal two intermediates. The first one is a Ni(III)-O center dot species formed upon a 2-e oxidation of Ni(OH)2, and the second one is a Ni-OO-Ni species formed upon a 1-e oxidation of Ni(III)-O center dot. The Ni-OO-Ni species is a side-on super-oxide that acts as a site for hole accumulation. The reaction kinetics follows an inner-sphere model. The rate-determining step is OH -attack to Ni(III)-O center dot, chemically driven by the Ni-OO-Ni species. This work provides new experimental fingerprints and mechanistic perspectives for the understanding of Ni-based OER catalysts.