Electrochemical Analysis and Density Functional Theory (DFT) Simulation to Investigate the Phosphoric Acid Adsorption on Pt3m (M = Fe, Co, Ni) Nanoparticles

High temperature-polymer electrolyte membrane fuel cell (HT-PEMFC) utilizing phosphoric acid doped polybenzimidazol membrane is a promising power supply for a number of stationary applications. At high operating temperature, significant enhancement in oxygen reduction reaction (ORR) kinetics is expected. However, typically the cell performance of HT-PEMFC is lower than that of conventional Nafion-based PEMFCs. One of important reason is the strong adsorption of phosphoric acid or anions, so called ‘phosphoric acid poisoning’, which blocks the active sites on Pt/C.

Alloying with transition metals have been suggested for an efficient way to improve the performance of HT-PEMFC as the alloying may provide enhanced intrinsic ORR activity and improved tolerance against the phosphoric acid poisoning. Experimental observation and computational simulations revealed that lowered adsorption energy of OH on alloy of Pt and transition metal nanoparticle catalysts (PtM/C) provided the enhanced ORR activity¹.

However, there were few reports investigating effect of transition metal alloying on the phosphoric acid tolerance. Recently, He et al. demonstrated improved tolerance against the phosphoric acid poisoning in bulk PtSn² and PtNi nanoparticle³ alloy surfaces. They postulated that ligand effect or geometric effect may be responsible for the tolerance of alloy surfaces. As a theoretical investigation of the alloying effect, Savizi and Janik calculated potential dependence of phosphoric acid adsorption free energy⁴ on Pt(111) surfaces, using density functional theory (DFT) simulations. However, there has been no further study, including PtM/C electrocatalysts.

In this research, we investigated the effect of transition metal alloying on the phosphoric acid poisoning in Pt₃M (where, M = Fe, Co, Ni) nanoparticle catalyst (Pt₃M/C) surfaces. Using the cyclic voltammetry, adsorption strength of phosphoric acid on Pt₃M/C was analyzed. Also, the effect of transition metal alloying on the adsorption strength of phosphoric acid in Pt₃M/C was analyzed based on the DFT simulations. Combination of electrochemical analysis and DFT simulations allowed investigating systematically the effect of alloying on the phosphoric acid adsorption in Pt₃M/C surfaces.

References
1. I.E.L. Stephens, A.S. Bondarenko, U. Gronbjerg, J. Rossmeisl, I. Chorkendorff, Energy Environ. Sci., 5, 6744 (2012).
2. Q. He, X. Yang, W. Chen, S. Mukerjee, B. Koel, S. Chen, Phys. chem. Chem. Phys., 12, 12544 (2010).
3. Q. He, B. Shyam, M. Nishijima, D. Ramaker, S. Mukerjee, J. Phys. Chem. C, 117, 4877 (2013).
4. I. S. P. Savizi, M. J. Janik, Electrochim. Acta 56, 3996 (2011).