Ho-Young Jung

Development of High Durability Bi-functional Oxygen Electrode for Unitized Regenerative Fuel Cell (URFC)

Performance, durability and cost are the most important considerations for the successful application of unitized regenerative fuel cell (URFC). Among them, the durability of the URFC system can be improved by developing structurally stable bi-functional oxygen electrode (BOE), gas diffusion layer (GDL), and bipolar plate which are capable of withstanding the high potentials when the URFC is operated in the water electrolyzer mode. In one of our previous studies [1] we optimized the electrochemical performance of Pt black with varying amounts of Ir black as BOE catalysts. Rotating ring disc electrode (RRDE) and URFC studies indicated that 85 wt.% Pt black with 15 wt.% Ir black is an optimum catalyst composition and can be used as a BOE catalyst. The unsupported Pt85Ir15 black catalyst showed the highest round-trip energy conversion efficiency (e RT = 49%), as shown in Table 1. The conventional carbon based bipolar plates have poor mechanical strength, poor manufacturability, high cost and high corrosion rate in the high potential region [2, 3] and are not suitable for URFC application. Specifically, carbon corrosion decreases the thickness of the bipolar plate leading to poor electrical contact with the gas diffusion media and subsequent increase in the ohmic resistance. In addition, oxidation of the carbon surface decreases the hydrophobic property and affects water management, leading to increased mass transport losses. This effect causes the stability of URFC performance to degrade quickly. In order to solve these problems, noble metal coated titanium metal bipolar plate was developed and used in this study. Fig. 1 shows the surface morphology of titanium based bipolar plate after noble metal coating. Fig. 2 shows the performance of URFC performance obtained using titanium bipolar plates before and after noble metal coating. The performance of the unit cell is significantly increased with the noble metal coating on titanium bipolar plate due to the decreased ohmic resistance. Thus it is found that the application of nanolayers of noble metal on the titanium bipolar plates can be effective for the improvement of cell performance by inhibiting the formation of passive oxide layer on the surface of titanium based bipolar plates. The overall objective of this research is to develop high durability Pt85Ir15 black based BOE catalyst layer which will have no or minimum performance degradation during cycling test (continuous operation in both fuel cell and water electrolyzer modes). For these studies, new electrode with high performance and durability, high sustainable GDL, and high corrosion resistant bipolar plate are essential. In order to make high durability BOE, the structural stability and loss of hydrophobicity in the BOE will be improved by introducing the hydrophobic PTFE into the catalyst layer [4]. This study will involve: (i) optimization of the hydrophobic/hydrophilic character by selecting appropriate amounts of Teflon® and Nafion® (ii) decrease of the catalyst loading and the catalyst layer thickness. For the preparation of high sustainable GDL, graphitized carbon, which is less corrosive in the high potential region, will be used in the microporous layer. The performance of the developed catalyst layer will be evaluated in an URFC. Cycling test will be carried out according to the conditions exist at the low earth orbit (LEO). The performance and the cycling test results of the developed catalyst layer will be presented at the meeting.