Nickel–cobalt oxalate as an efficient non-precious electrocatalyst for an improved alkaline oxygen evolution reaction†

Abstract

The quest for developing next-generation non-precious electrocatalysts has risen in recent times. Herein, we have designed and developed a low cost electrocatalyst by a ligand-assisted synthetic strategy in an aqueous medium. An oxalate ligand-assisted non-oxide electrocatalyst was developed by a simple wet-chemical technique for alkaline water oxidation application. The synthetic parameters for the preparation of nickel–cobalt oxalate (Ni _2.5Co ₅C ₂O ₄) were optimized, such as the metal precursor (Ni/Co) ratio, oxalic acid amount, reaction temperature, and time. Microstructural analysis revealed a mesoporous block-like architecture for nickel–cobalt oxalate (Ni _2.5Co ₅C ₂O ₄). The required overpotential of Ni _2.5Co ₅C ₂O ₄ for the alkaline oxygen evolution reaction (OER) was found to be 330 mV for achieving 10 mA cm _geo ⁻², which is superior to that of NiC ₂O ₄, CoC ₂O ₄, NiCo ₂O ₄ and the state-of-the-art RuO ₂. The splendid performance of Ni _2.5Co ₅C ₂O ₄ was further verified by its low charge transfer resistance, impressive stability performance, and 87% faradaic efficiency in alkaline medium (pH = 14). The improved electrochemical activity was further attributed to double layer capacitance ( C _dl), which indefinitely divulged the inferiority of NiCo ₂O ₄ compared to Ni _2.5Co ₅C ₂O ₄ for the alkaline oxygen evolution reaction (OER). The obtained proton reaction order ( ρ _RHE) was about 0.80, thus indicating the proton decoupled electron transfer (PDET) mechanism for OER in alkaline medium. Post-catalytic investigation revealed the formation of a flake-like porous nanostructure, indicating distinct transformation in morphology during the alkaline OER process. Further, XPS analysis demonstrated complete oxidation of Ni ²⁺ and Co ²⁺ centres into Ni ³⁺ and Co ³⁺, respectively under high oxidation potential, thereby indicating active site formation throughout the microstructural network. Additionally, from BET-normalised LSV investigation, the intrinsic activity of Ni _2.5Co ₅C ₂O ₄ was also found to be higher than that of NiCo ₂O ₄. Finally, Ni _2.5Co ₅C ₂O ₄ delivered a TOF value of around 3.28 × 10 ⁻³ s ⁻¹, which is 5.56 fold that of NiCo ₂O ₄ for the alkaline OER process. This report highlights the unique benefit of Ni _2.5Co ₅C ₂O ₄ over NiCo ₂O ₄ for the alkaline OER. The structure–catalytic property relationship was further elucidated using density functional theory (DFT) study. To the best of our knowledge, nickel–cobalt oxalate (Ni _2.5Co ₅C ₂O ₄) was introduced for the first time as a non-precious non-oxide electrocatalyst for alkaline OER application.

Abstract

Nickel–cobalt oxalate (Ni _2.5Co ₅C ₂O ₄– nH ₂O) based block-like nanostructure has been introduced as superior electrocatalyst compared to nickel–cobalt oxide (NiCo ₂O ₄) for alkaline water oxidation.