The electrochemical characteristics of air fuel cell electrodes used in an electrolytic system for spent chromium plating solution regeneration

The electrochemical characteristics of lab prepared Pt|Nafion and commercial Pt-C|Nafion air fuel cell electrodes in sulfuric and chromic acid electrolytes were examined to help in the design of an electrochemical system that might potentially be used in the regeneration of spent chromium plating solutions to save energy. In both solutions, the Pt-C|Nafion electrode obtained higher cathodic currents from oxygen reduction than the Pt|Nafion electrode, mainly due to an order of magnitude greater active area in the Pt-C|Nafion than in the Pt|Nafion electrode. The currents significantly increased after the cathodic sweeps passed ∼0.7-0.8 V versus a standard hydrogen electrode (SHE) in both systems. The currents for the oxygen reduction reactions were higher in the sulfuric acid system than in the chromic acid system, which were associated with less available electroactive sites on the electrodes and/or the inhibition of oxygen reduction in the chromic acid. In addition, impurities (i.e., Cu) were more difficult to deposit on Pt in the chromic acid than in the sulfuric acid. In sulfuric acid, the Tafel slopes were similar for the two electrodes, but in chromic acid, the Pt-C|Nafion electrode had a Tafel slope close to −120 mV decade−1 while the Pt|Nafion did not have a clear Tafel region in ∼600-400 mV versus Ag/AgCl. The polarization results suggest that, in practical applications, flooding, if it occurs, will be more responsible than slow oxygen reduction kinetics for the cathodic potential/cell voltage increase in the fuel cell cathode system using chromic acid.