Cobalt Bis(Pyridinethiolate N‐Oxide) as a Precursor for HER Active Co Nanoparticles and Particle Size‐Dependent Electrocatalytic Properties

Abstract The hydrogen evolution reaction (HER) produces (di)hydrogen (H₂), a clean energy carrier, through the cathodic side of the water splitting reaction. Specifically, this method of producing hydrogen is applicable to converting clean electricity and/or solar energy into a chemical fuel. Herein, a cobalt(pyridinethiolate N‐oxide)₂ complex was synthesized through the reaction of cobalt sulfate with the aforementioned ligand and shown to be a four coordinate paramagnetic cobalt complex using paramagnetic nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and mass spectrometry. This complex was then tested for HER activity in homogeneous phase and embedded into reduced graphene oxide thin films and physisorbed onto a graphite rod electrode. Despite its similarity to other highly active molecular catalysts for HER, surprisingly, this complex did not show any reliable HER activity. Instead, in acidic DMF, HER active nanoparticles were reductively deposited onto a glassy carbon electrode. This is the first example, to the best of our knowledge, of a molecular cobalt thiolate complex that decomposes to make nanoparticles upon electrolysis rather than acting as a molecular catalyst for HER. The ellipsoidal Co nanoparticles were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), SEM energy‐dispersive X‐ray spectroscopy (EDS), X‐ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP‐MS). The amount of deposited material, and the size and number of nanoparticles, was shown to increase with the number of deposition scans. Cyclic voltammetry scans showed that the onset potential for HER decreases and the catalytic current increases with the diameter of the nanoparticles. A drop‐cast Nafion thin film improved the durability of the nanoparticle‐covered electrodes, allowing for HER for at least 8 hrs. These electrodes have a Faradaic efficiency of 100±3 %, and produce 14.1 mmol H₂ per gram Co per second, at pH 1. The complex cobalt bis(mpo) is thus identified as an ideal precursor for the controlled electrodeposition of metallic Co nanoparticles with a defined size and shape.