Decrypting the hydrogen evolution in alkaline water with novel magnetoactive cobalt(ii) complex-driven cobalt oxide electrocatalysts

Abstract

Under the gravity of future socio-economic development, the viability of water electrolysis still hinges on the accessibility of stable earth-abundant electrocatalysts and net energy efficiency. This work emphasizes the design and synthesis of two newly developed cobalt(II) complexes, [Co(HL)₂(NCS)₂] (Co^mono) and [Co₂(L)₃(CH₃OH)]ClO₄ (Co^di), with a (N,O)-donor ligand, HL (2-methoxy-6-(((2-methoxyphenyl)imino)methyl)phenol). The study delves into understanding their structural, morphological, magnetic, and charge transport characteristics. Moreover, the study explores the potential of these complexes in catalyzing hydrogen production through heterogeneous electrocatalysis. The X-ray crystal structure of Co^mono reveals the octahedral geometry of the Co(II) ion, adopting two HL units and two NCS⁻ units. The Co^di complex exhibits a doubly-phenoxo-O-bridged (μ_1,1) dinuclear complex, forming a typical octahedral geometry for both the Co(II) centres in coupling with three units of L⁻. Temperature-dependent magnetic susceptibility measurements showed that all of the Co(II) ion in Co^mono shows a typical paramagnetic behaviour for high spin octahedral Co(II) ions while the Co(II) centres in Co^di are coupled with doubly-phenoxo-bridges bearing weak ferromagnetic characteristics at low temperature. Electron transport properties of the Co(II) complex-mediated Schottky device address the superior carrier mobility (μ) for Co^di (9.21 × 10⁻⁵) over Co^mono (2.02 × 10⁻⁵ m² v⁻¹ s⁻¹) with respective transit times of 1.70 × 10⁻⁹ and 7.77 × 10⁻⁹ s. Additionally, electron impedance spectral analysis supports the lower electrical transport resistance of Co^di relative to Co^mono. The heterogeneous electrocatalytic HER activity of Co^di and Co^mono in 0.1 M KOH shows excellent electrocatalytic efficiency in terms of the various electrochemical parameters. Constant potential electrolysis, multi-cycle CVs, and post-HER analysis reveal the pre-catalytic nature of the complexes, which in turn delivers Co₃O₄ nanoparticles as the active catalysts for efficient hydrogen evolution.