Cobalt Hexacyanoferrate as a Selective and High Current Density Formate Oxidation Electrocatalyst

Herein we report the selectivity, stability, and electrochemical characterization of cobalt hexacyanoferrate, the Co–Fe Prussian Blue derivative (CoFePB), as a formate/formic acid oxidation electrocatalyst in aqueous media. CoFePB is able to quantitatively catalyze (100% Faradaic efficiency within less than 8% standard error at pH 5) the electrochemical oxidation of formate to CO2 over a pH range of 1–13. This quantitative formate elecrooxidation is possible due to the exclusive selectivity of the catalyst in a wide potential window (from ca. 1.2 to 1.7 V vs RHE), where no other substrate in aqueous conditions is activated: neither other organic molecules, such as alcohols or acids, nor water itself. CoFePB is one of the first heterogeneous noble-metal-free catalysts reported for the electrooxidation of small hydrocarbon molecules. Importantly, the catalyst showed a very high tolerance against surface poisoning during the reaction, as supported by the cyclic voltammetry and electrochemical impedance spectroscopy data, thereby allowing CoFePB to operate at greater current density than state-of-the-art noble metal catalysts. For example, we observed that CoFePB is able to achieve a formate oxidation current ∼10 mA cm–2 at pH 5, 0.4 M formate at 1.4 V vs RHE, whereas a Pt disk and Pd(5%)/C electrodes had currents of 0.4 and 1.4 mA cm–2, respectively, under identical conditions. The remarkable selectivity, stability, and high current density of CoFePB, in contrast to state-of-the-art catalysts based on platinum-group metals, is an important step in the search for inexpensive earth-abundant materials for oxidation of organic molecules for use in liquid fuel cells or for selective organic molecule sensors. Furthermore, because CoFePB is not poisoned by intermediates and can achieve higher current density than Pt or Pd, improvement of the catalyst onset potential can lead to higher power density formate oxidation fuel cells using earth-abundant metals than with Pt or Pd.