Influence of Nanostructured Carbon Supports on Nanocatalysts Towards Electrooxidation of Formic Acid for Direct Formic Acid Fuel Cells

Direct formic acid fuel cells (DFAFCs) have been reported as a prominent source of alternative green energy and solution to imminent energy crisis for the last two decades. The challenge to commercialize DFAFCs is mainly the utilization of cost effective, high performance and durable anodic catalyst for electrooxidation of formic acid (HCOOH). Low metal loading, high surface area, good electrical conductivity, excellent stability in acidic and alkaline media, and easy availability make nanostructured carbon supports attractive for fuel cell applications. We report here the comparative studies of three different carbon based support materials and their influence on catalytic activities towards the electrochemical oxidation of HCOOH using mono (20 wt% Pd) and ternary (10 wt% Pd-Ni-Co) composite nano-catalysts with commercial 20 wt% Pd/C (activated carbon, ~500 m 2 g -1 ). The nano-catalysts were synthesized using Pd 2+ , Ni 2+ and Co 2+ precursors using Vulcan-XC72 (250 m 2 g -1 , <50 nm), Ketjen Black (1400 m 2 g -1 ) and graphite nano-particles (GNP) (~10 nm) support materials and NaBH 4 as reducing agent. The electrochemical oxidation of HCOOH was conducted at a glassy carbon disk electrode (GCE, 3 mm diameter) pre-casted with 9 µL of each catalyst ink covered with 3 µL Nafion ® polymer (5 wt%) in 0.50 M HCOOH-0.10 M H 2 SO 4 . The catalytic oxidation behavior of HCOOH of all the catalysts was compared using cyclic voltammetry. Multi-potential step chronoamperometry (CA) was utilized for stability tests. The catalytic behavior of all the ternary nano-catalysts is believed to be ascribed to the direct oxidation pathway of HCOOH as indicated by the oxidation peak potential of ~ -0.2 V vs Hg/Hg 2 SO 4 (satd. K 2 SO 4 ) reference electrode. In contrast, the synthesized mono catalysts and commercial Pd/C showed both direct and indirect oxidation pathways. The use of Ni and Co also showed synergistic effect as the HCOOH oxidation peak potential was shifted negatively by ~200 mV with respect to that of the commercial catalyst containing even doubled amounts of Pd. Furthermore, CA data showed that GNP and Ketjen Black supported catalysts were almost as stable as the commercial Pd/C, although the electrooxidation current at the Vulcan-XC72 based catalysts decreased by ~50% within about 20 min. On the other hand, Vulcan-XC72 supported catalysts showed the highest HCOOH oxidation current and the rest were in close proximity to one another. Finally, morphology and elemental analyses employing scanning electron microscope and energy dispersive X-ray spectroscopy revealed the different catalytic activity on various support materials were concordant with the electrochemical data as the turbostratic substrates contributed better metal dispersion and uniform size distribution. Financial support from the CAREER Award (CHE-0955878) is gratefully acknowledged.