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1. Ultrafine oxygen-defective iridium oxide nanoclusters for efficient and durable water oxidation at high current densities in acidic media

   https://doi.org/10.1039/D0TA07093A  

   Yu, Zhipeng ; Xu, Junyuan ; Li, Yifan ; Wei, Bin ; Zhang, Nan ; Li, Yue ; Bondarchuk, Oleksandr ; Miao, Hongwei ; Araujo, Ana ; Wang, Zhongchang ; et al ( December 2020 , Journal of Materials Chemistry A)

   Iridium oxide (IrO 2 ) is one of the best known electrocatalysts for the oxygen evolution reaction (OER) taking place in a strongly acidic solution. IrO 2 nanocatalysts with high activity as well as long-term catalytic stability, particularly at high current densities, are highly desirable for proton exchange membrane water electrolysis (PEM-WE). Here, we report a simple and cost-effective strategy for depositing ultrafine oxygen-defective IrO x nanoclusters (1–2 nm) on a high-surface-area, acid-stable titanium current collector (H-Ti@IrO x ), through a repeated impregnation–annealing process. The high catalytically active surface area resulting from the small size of IrO x and the preferable electronic structure originating from the presence of oxygen defects enable the outstanding OER performance of H-Ti@IrO x , with low overpotentials of 277 and 336 mV to deliver 10 and 200 mA cm −2 in 0.5 M H 2 SO 4 . Moreover, H-Ti@IrO x also shows an intrinsic specific activity of 0.04 mA cm catalyst −2 and superior mass activity of 1500 A g Ir −1 at an overpotential of 350 mV. Comprehensive experimental studies and density functional theory calculations confirm the important role of oxygen defects in the enhanced OER performance. Remarkably, H-Ti@IrO x can continuously catalyzemore »the OER in 0.5 M H 2 SO 4 at 200 mA cm −2 for 130 hours with minimal degradation, and with a higher IrO x loading, it can sustain at such a high current density for over 500 hours without significant performance decay, holding substantial promise for use in PEM-WE.« less
2. Nanostructured core–shell metal borides–oxides as highly efficient electrocatalysts for photoelectrochemical water oxidation

   https://doi.org/10.1039/c9nr09818f  

   Lu, Can ; Jothi, Palani R. ; Thersleff, Thomas ; Budnyak, Tetyana M. ; Rokicinska, Anna ; Yubuta, Kunio ; Dronskowski, Richard ; Kuśtrowski, Piotr ; Fokwa, Boniface P. ; Slabon, Adam ( February 2020 , Nanoscale)

   Oxygen evolution reaction (OER) catalysts are critical components of photoanodes for photoelectrochemical (PEC) water oxidation. Herein, nanostructured metal boride MB (M = Co, Fe) electrocatalysts, which have been synthesized by a Sn/SnCl 2 redox assisted solid-state method, were integrated with WO 3 thin films to build heterojunction photoanodes. As-obtained MB modified WO 3 photoanodes exhibit enhanced charge carrier transport, amended separation of photogenerated electrons and holes, prolonged hole lifetime and increased charge carrier density. Surface modification of CoB and FeB significantly enhances the photocurrent density of WO 3 photoanodes from 0.53 to 0.83 and 0.85 mA cm −2 , respectively, in transient chronoamperometry (CA) at 1.23 V vs. RHE (V RHE ) under interrupted illumination in 0.1 M Na 2 SO 4 electrolyte (pH 7), corresponding to an increase of 1.6 relative to pristine WO 3 . In contrast, the pristine MB thin film electrodes do not produce noticeable photocurrent during water oxidation. The metal boride catalysts transform in situ to a core–shell structure with a metal boride core and a metal oxide (MO, M = Co, Fe) surface layer. When coupled to WO 3 thin films, the CoB@CoO x nanostructures exhibit a higher catalytic enhancement than corresponding pure cobaltmore »borate (Co-B i ) and cobalt hydroxide (Co(OH) x ) electrocatalysts. Our results emphasize the role of the semiconductor–electrocatalyst interface for photoelectrodes and their high dependency on materials combination.« less
3. Correlation between oxygen evolution reaction activity and surface compositional evolution in epitaxial La _0.5 Sr _0.5 Ni _1-x Fe _x O _3-δ thin films

   https://doi.org/10.1039/D2NR05373J  

   Adiga, Prajwal ; Wang, Le ; Wong, Cindy ; Matthews, Bethany E ; Bowden, Mark E ; Spurgeon, Steven Richard ; Sterbinsky, George E. ; Blum, Monika ; Choi, Min-Ju ; Tao, Jinhui ; et al ( January 2023 , Nanoscale)

   Water electrolysis can use renewable electricity to produce green hydrogen, a portable fuel and sustainable chemical precursor. Improving electrolyzer efficiency hinges on the activity of the oxygen evolution reaction (OER) catalyst. Earth-abundant, ABO3-type perovskite oxides offer great compositional, structural, and electronic tunability, with previous studies showing compositional substitution can increase the OER activity drastically. However, the relationship between the tailored bulk composition and that of the surface, where OER occurs, remains unclear. Here, we study the effects of electrochemical cycling on the OER activity of La 0.5 Sr 0.5 Ni 1-x Fe x O 3-δ (x = 0-0.5) epitaxial films grown by oxide molecular beam epitaxy as a model Sr-containing perovskite oxide. Electrochemical testing and surface-sensitive spectroscopic analyses show Ni segregation, which is affected by electrochemical history, along with surface amorphization, coupled with changes in OER activity. Our findings highlight the importance of surface composition and electrochemical cycling conditions in understanding OER performance on mixed metal oxide catalysts, suggesting common motifs of the active surface with high surface area systems.
4. Hydrous cobalt–iridium oxide two-dimensional nanoframes: insights into activity and stability of bimetallic acidic oxygen evolution electrocatalysts

   https://doi.org/10.1039/D0NA00912A  

   Ying, Yuanfang ; Godínez Salomón, Jose Fernando ; Lartundo-Rojas, Luis ; Moreno, Ashley ; Meyer, Robert ; Damin, Craig A. ; Rhodes, Christopher P. ( January 2021 , Nanoscale Advances)

   Acidic oxygen evolution reaction (OER) electrocatalysts that have high activity, extended durability, and lower costs are needed to further the development and wide-scale adoption of proton-exchange membrane electrolyzers. In this work, we report hydrous cobalt–iridium oxide two-dimensional (2D) nanoframes exhibit higher oxygen evolution activity and similar stability compared with commercial IrO 2 ; however, the bimetallic Co–Ir catalyst undergoes a significantly different degradation process compared with the monometallic IrO 2 catalyst. The bimetallic Co–Ir 2D nanoframes consist of interconnected Co–Ir alloy domains within an unsupported, carbon-free, porous nanostructure that allows three-dimensional molecular access to the catalytically active surface sites. After electrochemical conditioning within the OER potential range, the predominately bimetallic alloy surface transforms to an oxide/hydroxide surface. Oxygen evolution activities determined using a rotating disk electrode configuration show that the hydrous Co–Ir oxide nanoframes provide 17 times higher OER mass activity and 18 times higher specific activity compared to commercial IrO 2 . The higher OER activities of the hydrous Co–Ir nanoframes are attributed to the presence of highly active surface iridium hydroxide groups. The accelerated durability testing of IrO 2 resulted in lowering of the specific activity and partial dissolution of Ir. In contrast, the durability testing of hydrousmore »Co–Ir oxide nanoframes resulted in the combination of a higher Ir dissolution rate, an increase in the relative contribution of surface iridium hydroxide groups and an increase in specific activity. The understanding of the differences in degradation processes between bimetallic and monometallic catalysts furthers our ability to design high activity and stability acidic OER electrocatalysts.« less
5. Regulating the electronic structures of mixed B-site pyrochlore to enhance the turnover frequency in water oxidation

   https://doi.org/10.1186/s40580-022-00311-z  

   Zhang, Cheng ; Wang, Fangfang ; Xiong, Beichen ; Yang, Hong ( December 2022 , Nano Convergence)

   Abstract This paper describes the development of mixed B-site pyrochlore Y 2 MnRuO 7 electrocatalyst for oxygen evolution reaction (OER) in acidic media, a challenge for the development of low-temperature electrolyzer for green hydrogen production. Recently, several theories have been developed to understand the reaction mechanism for OER, though there is an  uncertainty in most of the cases, due to the complex surface structures. Several key factors such as lattice oxygen, defect, electronic structure, oxidation state, hydroxyl group and conductivity were identified and shown to be important to the OER activity. The contribution of each factor to the performance however is often not well understood, limiting their impact in guiding the design of OER electrocatalysts. In this work, we showed mixed B-site pyrochlore Y 2 MnRuO 7 catalyst exhibits 14 times higher turnover frequency (TOF) than RuO 2 while maintaining a low overpotential of ~ 300 mV for the entire testing period of 24 h in acidic electrolyte. X-ray photoelectron spectroscopy (XPS) analysis reveals that this B-site mixed pyrochlore Y 2 MnRuO 7 has a higher oxidation state of Ru than those of Y 2 Ru 2 O 7 , which could be crucial for improving OER performance as the broadened and loweredmore »Ru 4d band resulted from the B-site substitution by Mn is beneficial to the OER kinetics.« less