Nano-modulation of palladium surfaces with iridium and iron oxide for boosted catalysis of formic acid electro-oxidation

In an attempt to boost the workability and durability of the direct formic acid fuel cells (DFAFCs), a highly efficient Pd-based electrocatalyst for formic acid electrooxidation (FAO) was developed. This was achieved by adopting the layer-by-layer "sequential" approach to electrodeposit palladium (nano-Pd, semi-spherical of average particle size of ca. 90 nm), iron oxide (nano-FeOx, nanowires of ca. 177 nm in average length and 33 nm in average diameter) and iridium (nano-Ir, spherical of ca. 80 nm in average diameter) nanostructures onto a glassy carbon electrode (GCE). The layers' hierarchy of the catalyst was varied, and the Ir/FeOx/Pd/GCE sequence was optimized for FAO. This catalyst exhibited remarkable enhancement in the electrochemical activity (up to 2.35-fold higher), stability (10-times lower in poisoning rate), turnover frequency (2.5 times higher) and charge transfer resistance (36-times lower) than the Pd/GCE catalyst toward FAO. According to XPS analysis, nano-Pd existed in mixed metallic (Pd⁰) and oxide (Pd²⁺) forms, nano-FeOx retained the Fe³⁺ state, and nano-Ir appeared in the metallic (Ir⁰) form. Deep analyses of the catalyst with electrochemical impedance spectroscopy, stripping voltammetry and Tafel measurements were sought to evaluate the reaction mechanism and kinetics, where all together confirmed both the geometrical and electronic influences in the catalytic enhancement. This catalyst competed perfectly with commercial catalysts for FAO, demonstrating its efficacy and robustness under continuous operation.