3. Cobalt

Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0–14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity

“…Electrolyses performed under conditions that are compatible with a stable CoO x cf result in near-quantitative Faradaic yields for O 2 evolution (e.g., see Figure ), as has been observed previously. ,, In contrast, electrolysis of 1 mM of Co in 0.1 M tetrafluoroborate electrolyte with η = 680 mV at pH 2.1 (1.79 V vs NHE) showed initial Faradaic efficiencies far from unity followed by an increase in the O 2 production rate, eventually reaching a plateau at the expected value (Figure ). This observation is consistent with studies of Co-mediated water oxidation under acidic conditions using chemical oxidants, which support the formation of hydrogen peroxide as the water oxidation product . Treatment of aliquots of the anolyte with iodide showed signs of peroxide through the formation of the brown-colored I 3 – ion .…”

“…Instead, the data can be rationalized by a mechanism in which oxidation of 3 takes place to afford a Co IV species 7 that dissolves from the surface, promoted by the accumulation of positive charge. The oxygen evolution data implicate the formation of H 2 O 2 , which subsequently undergoes disproportionation (Scheme , step 8) . Co IV intermediates similar to 7 have been invoked to explain homogeneous oxidation of water to H 2 O 2 , catalyzed by cobalt in the presence of chemical oxidants. , The formation of H 2 O 2 from such species suggests that hydrolysis of an O–O-bonded species proceeds more rapidly than the additional oxidation steps needed to form O 2 .…”

“…In contrast to the studies noted above, cobalt electrocatalysts for water oxidation under acidic conditions have received little attention . The aqueous chemistry of cobalt solutions is complex between pH 0–5, where hydrolysis and anation of Co aq III is significant. , Chemical oxidants have been employed in combination with dissolved aquocobalt ions to achieve water oxidation, giving hydrogen peroxide as the initial product. In the presence of Co III , hydrogen peroxide undergoes disproportionation to O 2 and water; however, the kinetics and electrochemical potential for this process differ substantially from oxygen evolution mediated by CoO x cf s …”