The electrochemistry and performance of cobalt-based redox couples for thermoelectrochemical cells

Thermoelectrochemical cells are a promising technology for sustainably generating electricity from waste heat. These electrochemical devices directly convert heat into electricity, with a performance governed by the properties of the redox couple, electrolyte and electrode. In this work the influence of the nature of the redox couple on fundamental properties such as the Seebeck coefficient, diffusion coefficient and charge transfer resistance was investigated. Four different cobalt complexes containing the ligands 2-(1H-pyrazol-1-yl)pyridine) (Co^2+/3+(py-pz)₃), 2-(1H-pyrazol-1-yl)-4-tert-butylpyridine (Co^2+/3+(bupy-pz)₃), 2,6-di(1H-pyrazol-1-yl)pyridine (Co^2+/3+(pz-py-pz)₂) and 1,10-phenanthroline (Co^2+/3+(phen)₃) were examined in a 3:1 dimethyl sulfoxide: 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate mixture. The performance of each redox couple was governed by the ligand properties. The highest Seebeck coefficient was measured for Co^2+/3+(py-pz)₃ (2.36 mV K⁻¹) which was attributed to a combination of its small radius, bi-denticity and lower degree of aromaticity. This is higher than the previously reported Co(bpy)₃ couple. The highest power output was achieved with the Co^2+/3+(py-pz)₃ redox electrolyte, using platinum electrodes coated with a carbon layer, which gave 36 mW m⁻² from a ΔT of 30 ^○C. The power outputs achieved using the different redox couples was highest for those with a high Seebeck coefficient, good electrochemical reversibility and fast ion diffusion. The electrochemical reversibility depends significantly on the nature of the electrode substrate and it is demonstrated that carbon-coated platinum electrodes can be used to improve the electrochemical reversibility of these redox couples.