Effect of Substituent in Phosphonic Acid on Anion Adsorption and Oxygen Reduction Reaction Activity
Authors: Lin Honghong , Wang Siwen , Wang Liang , Zhu Gaohua , Ling Chen , Jia Hongfei 
Published: December 2024

Acid anions affect Pt’s oxygen reduction reaction (ORR) activity. For weakly solvated anions like SO4
2- and H2PO4
-, they could adsorb onto Pt and block active sites for O2 adsorption, leading to lower ORR activity than that of nonspecifically or weakly adsorbed F- and ClO4
- anions 1. In low temperature proton exchange membrane fuel cells (PEMFCs) (60-80 °C), where the acid is perfluorinated sulfonic acid (PFSA) polymer, the adsorption of the terminal sulfonate anions (-SO3
-) suppresses ORR activity, and it was also found that the functional group adjacent to the sulfonate affects the anion adsorption and its effect on ORR activity 2,3. In high temperature PEMFCs (120-200 °C), where the acid could be phosphoric acid or phosphonated polymer 4–7, it is unclear yet how the adjacent organic group in those phosphonated polymers affect the terminal phosphonate group’s interaction with Pt and ORR activity. In this work, we select several low-molecular-weight organic phosphonic acids (R-PAs) as model molecules to study their interaction with Pt and the effect on ORR. With density function theory (DFT) simulation, we compare the binding strength of these organic phosphonic acids and their effect on ORR activity to H3PO4. At the same time, we use Pt disk electrode to study the adsorption/desorption behavior of the H, O, and anion species, with diluted acid solutions. By comparing the simulation and experiment results, we found that the electronic and steric effect contribute together to the reaction site number and the site’s reactivity, which provides useful information to address the acid poisoning issue in HT-PEMFCs.

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