Palladium containing nanomaterials for electrocatalytic formic acid oxidation

Abstract

The development of a hydrogen-based economy requires an efficient and cost-effective method for hydrogen production, with a focus on fuel cells, particularly low-temperature direct formic acid fuel cells. Direct formic acid fuel cells have the advantages of lower toxicity and reduced membrane crossover of formic acid compared to methanol and ethanol. Platinum-based electrocatalysts are considered the benchmark for many fuel cell reactions which hinders cost-effective commercialization. Palladium has emerged as an alternative of Pt for formic acid oxidation at the anode. In our study, we synthesized reduced graphene oxide (rGO) supported palladium-based electrocatalysts promoted with various metal oxides, including samarium oxide (Sm2O3), yttrium oxide (Y2O3), and gallium oxide (Ga2O3). The total metal content in each catalyst was 20 wt. %, and we varied the proportions of palladium and metal oxide promoters. All the synthesized catalysts are characterized via a range of analytical techniques, specifically powder X-ray diffraction (pXRD), transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The structural and morphological characterization of electrocatalysts suggests that Pd is deposited as nanoparticles whilst metal oxides as an amorphous phase. The electrochemical results show that amongst the series of palladium promoted with samarium oxide, Pd6Sm4/rGO exhibited exceptional performance with a high electrochemical surface area of 52.709 m2/g and mass activity 1088 mA/mg-Pd surpassing 1.6 and 2.1 times than commercial palladium in formic acid oxidation. Pd6Sm4/rGO demonstrated remarkable stability, maintaining 56 % of the oxidation current after 5500 s. Similarly, in the series of palladium catalysts promoted with yttrium oxide, Pd6Y4/rGO shows an electrochemical surface area of 119.4 m2/g and an electrocatalytic activity of 1377 mA/mg-Pd in formic acid oxidation. These values were 3.7 and 4.8 times higher than commercial palladium black, and the catalyst maintains 70 % of the oxidation current after 5500 s. Likewise, in the series of palladium promoted with gallium oxide, Pd6Ga4/rGO exhibited significant activity with an electrochemical surface area of 102.4 m2/g and a mass activity of 764 mA/mg-Pd, surpassing commercial palladium black by 2.1 and 2.6 times. Pd6Ga4/rGO also demonstrated excellent stability, maintaining 62% of the oxidation current after 5500 s. In addition, palladium and PdFe alloy nanomaterials are supported over micro-flower shaped manganese oxide to fabricate novel electrocatalysts for formic acid oxidation reaction. Pd/Mn2O3 exhibited an electrochemical surface area of 125 m2/g and a mass activity of 536 mA/mg-Pd. Similarly, PdFe/Mn2O3, displays an electrochemical surface area of 161 m2/g and an electrocatalytic ability of 630 mA/mg-Pd which also maintains 45% of the oxidation current after 7200 s. Overall, based on the electrochemical surface area, mass activity, stability parameters, Pd6Y4/rGO and PdFe/Mn2O3 emerge as best optimized materials to catalyze formic acid oxidation