A Structured Hybrid Catalyst to Improve ORR Activity and Phosphoric Acid in the High-Temperature Exchange Membrane Fuel Cell

Abstract

The high-temperature proton exchange membrane fuel cell (HT-PEMFC) based on a phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane electrolyte has been attracted attention because of its higher kinetics of oxygen reduction reaction, no flooding issues and excellent carbon monoxide tolerance [1, 2]. However, PA leaching from PBI membrane to electrode is one of the biggest problems as the loss of phosphoric acid causing worse proton conductivity of PBI membrane and preventing gas transfer in the electrode [3]. Furthermore, HT-PEMFC requires higher amount of platinum (Pt) (normally between 0.7 and 1.2 mg cm-2) to provide high electrochemical surface area, which accounts for a higher proportion in the cost of a total fuel cell stack [4]. Therefore, the reduction of phosphoric acid permeation and catalyst dosage are two challenges in HT-PEMFC.Many studies and technologies have been conducted to prevent the permeation of PA. Currently, the modification of PBI membrane structure is the main method by introducing materials with more -NH2 functional groups to form stable chemical bonds with phosphoric acid molecules and prevent their movement [5-7]. However, the composite membrane materials are not uniform and the intercalation of materials (SiO2, Graphene et al) will cause crossover issues to influence the durability of HT-PEMFC [7].Based on our previous research, platinum supported on nitrogen doped reduced electrochemically exfoliated graphene oxide/carbon black hybrid catalysts (Pt/NrEGOx-CBy) have higher oxygen reduction reaction (ORR) activity. This work aims to optimize the ratio of NrEGO and CB in HT-PEMFC and explore the effect of NrEGO flakes on PA permeation to improve the durability of HT-PEMFC. The preliminary results showed that Pt/NrEGO2-CB3 catalyst in a membrane electrode assembly (MEA) had the mass power density of 1.652 W mgPt -1 with a decay rate of 0.01 mV h-1 within 100 hours, which is much lower than that of 1.125 mV h-1 in the commercial Pt/C as shown in Figure 1.Reference[1] J. Lobato, H. Zamora, J. Plaza, P. Cañizares, M.A. Rodrigo. Enhancement of high temperature PEMFC stability using catalysts based on Pt supported on SiC based materials. Applied Catalysis B: Environmental. 2016. 198: 516-524.[2] Y. Hu, Y. Jiang, J.O. Jensen, L.N. Cleemann, Q. Li. Catalyst Evaluation for Oxygen Reduction Reaction in Concentrated Phosphoric Acid at Elevated Temperatures. Journal of Power Sources. 2018. 375: 77-78.[3] G. Liu, H. Zhang, J. Hu, Y. Zhai, D. Xu, Z.-g. Shao. Studies of performance degradation of a high temperature PEMFC based on H3PO4-doped PBI. Journal of Power Sources. 2006. 162 (1): 547-552.[4] H. Su, T.-C. Jao, O. Barron, B.G. Pollet, S. Pasupathi. Low platinum loading for high temperature proton exchange membrane fuel cell developed by ultrasonic spray coating technique. Journal of Power Sources. 2014. 267: 155-159.[5] F. Liu, S. Wang, J. Li, X. Tian, X. Wang, H. Chen, Z. Wang. Polybenzimidazole/ionic-liquid-functional silica composite membranes with improved proton conductivity for high temperature proton exchange membrane fuel cells. Journal of Membrane Science. 2017. 541: 492-499.[6] K.-J. Peng, J.-Y. Lai, Y.-L. Liu. Nanohybrids of graphene oxide chemically-bonded with Nafion: Preparation and application for proton exchange membrane fuel cells, Journal of Membrane Science. 2016. 514: 86-94.[7] C.-L. Lu, C.-P. Chang, Y.-H. Guo, T.-K. Yeh, Y.-C. Su, P.-C. Wang, K.-L. Hsueh, F.-G. Tseng. High-performance and low-leakage phosphoric acid fuel cell with synergic composite membrane stacking of micro glass microfiber and nano PTFE. Renewable Energy. 2019. 134: 982-988. Figure 1