Ultrasmall surface functionalized nanoclusters (Ni, Cu and Co) for high performance oxygen evolution catalysis

Water electrolysis is an appealing approach for the production of molecular H₂ as a sustainable and clean energy carrier. However, the intrinsic sluggish kinetics of the oxygen evolution reaction (OER) restrict its practical application. In this context, various approaches, particularly engineering of materials at the nanoscale length, offer high surface area and tunable electronic and redox properties, significantly accelerating the challenging OER process. This research primarily focuses on the development of cost-effective, thiol-stabilized transition metal ultrasmall nanoclusters (MNCs) as electrode materials, with precise control over size and surface chemistry. Herein, an optimized protocol is presented for the synthesis of transition metal nanoclusters (MNCs, M = Ni, Co and Cu) using 1-dodecanethiol and 2-phenylethanethiol as stabilizing agents. The synthesized MNCs were initially characterized using UV-visible and FT-IR spectroscopic techniques, indicating nanoclusters composed of metal-based nanoclusters stabilized by thiol ligands. Elemental composition was confirmed by EDX and XPS analysis. STEM and HRTEM revealed the uniform distribution (size ≤ 2 nm) of MNCs. The electrochemical evaluation showed that NiNCs emerged as the best electrocatalyst, exhibiting low onset potential of ∼1.46 V vs. RHE (η ≈ 230 mV), with a maximum current density of ∼125 mA cm^-2 at 1.7 V vs. RHE, and showing a low Tafel slope value of 99 mV dec^-1. The activity of Ni NCs is superior to the benchmark electrocatalysts, namely RuO₂ (η ₁₀ ≈ 300-350 mV) and IrO₂ (η ₁₀ ≈ 300-400 mV). Moreover, the Ni NCs also exhibit maximum exchange current density (I _o ≈ 0.9 mA cm^-2), efficient mass activity (∼400 A g^-1@1.7 V) and prolonged OER performance, which are indeed worth considering as attributes for further advancements in catalysis.