PEM-Type Water Electrolysis/Fuel Cell Reversible Cell with Low PGM Catalyst Loadings

To utilize fluctuating renewable energy stably and effectively, large-scale energy storage systems such as Li-ion, redox-flow and Na-S batteries have been extensively studied.  Battery storage system have advantages of high efficiency in energy utilization and rapid response for energy demand, however initial system cost tend to be expensive.  Electrochemical hydrogen production and storage is recently attracting attentions as another practical technology because of high energy efficiency of electrochemical water splitting, relatively low technological barrier for large-scale storage, and ease of long-term energy storage and long-range transport without self-discharge. 

Regenerative fuel cell is an energy storage system using hydrogen as an energy medium.  In particular, reversible (unitized) regenerative fuel cells (RFCs) can be functioned both as water electrolyzer and fuel cell in the same electrochemical cell by switching the operation mode, so that much more compact and cheaper system could be realized.  In the PEM-type RFC, unsupported platinum/iridium-based catalysts are usually selected in terms of activity and stability for both operation modes.  Loading amount of platinum group metals (PGMs) of RFC is relatively high (typically >1 mg cm^-2) compared to the fuel cell dedicated system due to stability issue at high potentials during OER.  Therefore, reducing PGM loadings without expense of performance and durability is high priority to meet the capital cost of the systems. 

To reduce the PGM loadings of oxygen electrode of RFC, we examined electro-conductive Magneli phase titanium oxides (in particular Ti₄O₇) as a catalyst support material [1,2].  High surface area oxide support was prepared by UV laser technique; TiO₂ nano particles dispersed in an appropriate solvent (typically acetonitrile) are reduced by pulsed UV laser irradiation [3].  Pt, Ir, and Pt-Ir alloy nano particles were deposited on the oxide support, and their electrocatalytic activities were examined by rotating disk electrode and MEA.  Ti₄O₇-supported Pt catalyst showed 3 times larger electrochemical active area (ECA) than that of commercially available Pt black, and 2.9 and 2.4-fold mass activity was obtained for ORR@0.9V and OER@1.7V, respectively.  Initial ORR/OER cycle durability test was conducted for Pt/Ti₄O₇ catalyst MEA at 80^oC between >1.8V for OER and <0.8V for ORR, and almost no performance loss was observed.  These results show titanium oxide based materials can be applied as RFC catalyst support, which leads to effective use and lowering PGM loading in the oxygen electrode.

References

1. G. Chen, S. Bare, T. Mallouk, J. Electrochem. Soc., 149, A1092 (2002).

2. T. Ioroi, T. Akita, M. Asahi, S. Yamazaki, Z. Siroma, N. Fujiwara, K. Yasuda, J. Power Sources, 223, 183 (2013).

3. T. Ioroi, H. Kageyama, T. Akita, K. Yasuda, Phys. Chem. Chem. Phys., 12, 7529 (2010).