Rajput Anubha, Adak Mrinal Kanti, Chakraborty Biswarup
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, 110016 New Delhi, India.
Inorg Chem. 2022 Jul 25;61(29):11189-11206. doi: 10.1021/acs.inorgchem.2c01167. Epub 2022 Jul 13.
Nickel-based bimetallic oxides such as NiMoO and NiWO, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO nanorods and NiWO nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO/NF and NiWO/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH) (ED: electrochemically derived) from NiMoO precatalyst, while NiWO remains stable. A controlled study, stirring of NiMoO/NF in 1 M KOH without applied potential, confirms that NiMoO hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH). Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO compared to the Ni-O-W bond in NiWO causes the transformation of NiMoO into NiO(OH). A comparison of the OER performance of electrochemically derived NiO(OH), NiWO, ex-situ-prepared Ni(OH), and NiO(OH) confirmed that in-situ-prepared NiO(OH) remained superior with a substantial potential of 238 (±6) mV at 20 mA cm. The notable electrochemical performance of NiO(OH) can be attributed to its low Tafel slope value (26 mV dec), high double-layer capacitance (, 1.21 mF cm), and a low charge-transfer resistance (, 1.76 Ω). The NiO(OH)/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm. Post-characterization of the anode proves the structural integrity of NiO(OH) even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm. Similar to that on NF, NiMoO deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH) succeeding the NiO phase is intrinsic to its superior activity.
镍基双金属氧化物，如NiMoO和NiWO，沉积在电极基底上时，对电催化析氧反应（OER）表现出显著的活性。然而，这种纳米结构的稳定性仍具有推测性，且对其催化活性物种的研究较少。在此，通过溶剂热法制备了NiMoO纳米棒和NiWO纳米颗粒，并将其沉积在泡沫镍（NF）上（NiMoO/NF和NiWO/NF）。在确保电极上催化剂的化学和结构完整性后，在碱性介质中进行了OER研究。在1.0 - 1.9 V（相对于可逆氢电极（RHE））的电位窗口内进行几个循环伏安（CV）循环后，对电极进行的非原位拉曼分析推断，NiMoO预催化剂形成了NiO(OH)（ED：电化学衍生），而NiWO保持稳定。一项对照研究，即在无外加电位的情况下在1 M KOH中搅拌NiMoO/NF，证实NiMoO水解为可分离的NiO，其在电位偏压下转化为NiO(OH)。也许与NiWO中的Ni - O - W键相比，NiMoO中Ni - O - Mo键的离子性更强，导致NiMoO转化为NiO(OH)。对电化学衍生的NiO(OH)、NiWO、非原位制备的Ni(OH)和NiO(OH)的OER性能进行比较，证实原位制备的NiO(OH)在20 mA cm时具有238（±6）mV的显著过电位，仍然表现优异。NiO(OH)显著的电化学性能可归因于其低塔菲尔斜率值（26 mV dec）、高双层电容（1.21 mF cm）和低电荷转移电阻（1.76 Ω）。NiO(OH)/NF可以进一步制成耐用的OER阳极，以提供25 - 100 mA cm的高电流密度。对阳极进行表征后证明，即使在1.595 V（相对于可逆氢电极（RHE））下进行12 h计时电流法后，NiO(OH)的结构仍然完整。NiO(OH)/NF可以作为一种兼容的阳极来构建一个全水解（OWS）电解槽，该电解槽可以在1.64 V的电池电位下运行，以达到10 mA cm的电流密度。与在NF上的情况类似，沉积在泡沫铁（IF）和碳布（CC）上的NiMoO也会电化学转化为NiO(OH)，以表现出类似的OER活性。可以理解的是，这项工作表明单斜NiMoO是一种本质上不稳定的电（预）催化剂，其从NiO相演变为多晶NiO(OH)是其优异活性的内在原因。
