Structural transformation of metal–organic frameworks and identification of electrocatalytically active species during the oxygen evolution reaction under neutral conditions

Abstract

The electrocatalytic oxygen evolution reaction (OER) under neutral or near-neutral conditions has attracted research interest due to its environmental friendliness and economic sustainability in comparison with currently available acidic and alkaline conditions. However, it is challenging to identify electrocatalytically active species in the OER procedure under neutral environments due to non-crystalline forms of catalysts. Crystalline metal–organic framework (MOF) materials could provide novel insights into electrocatalytically active species because of their well-defined structures. In this study, we synthesized two isostructural two-dimensional (2D) MOFs [Co(HCi)₂(H₂O)₂·2DMF]_n (Co–Ci-2D) and [Ni(HCi)₂(H₂O)₂·2DMF]_n (Ni–Ci-2D) (H₂Ci = 1H-indazole-5-carboxylic acid, DMF = N,N-dimethyl-formamide) to investigate their OER performance in a neutral environment. Our results indicate that Co–Ci-2D holds a current density of 3.93 mA cm⁻² at 1.8 V vs. RHE and an OER durability superior to the benchmark catalyst IrO₂. Utilizing the advantages of the structural transformation of MOF materials which are easier to characterize and analyze compared with ill-defined amorphous materials, we found that a mononuclear coordination compound [Co(HCi)₂(H₂O)₄] (Co–Ci-mono-A) and its isomer (Co–Ci-mono-B) were proved to be active species of Co–Ci-2D in the neutral OER process. For Ni–Ci-2D, mononuclear coordination compounds similar to the structures of the cobalt material (Ni–Ci-mono-A and Ni–Ci-mono-B) together with NiHPO₄ formed by the precipitation were confirmed as active species for neutral OER catalysis. Additionally, the difference in OER activities between Co–Ci-2D and Ni–Ci-2D, approximately one order of magnitude, can be attributed to changes in bond strength resulting from variations in bond length within coordination octahedra after being treated with the PBS solution. These findings contribute to a better comprehension of the OER procedure in neutral media.