Effect of Anode Microporous Layer on the Performance of Direct Methanol Fuel Cell Using Carbon Nanocoil-Supported PtRu Catalyst

Abstract

Direct methanol fuel cell (DMFC) is a promising energy source for portable and automotive applications, mainly due to their low operating temperature, direct use of liquid fuel, and simple structure without the stringent need for a reformer. Nevertheless, issues such as water management and methanol crossover still limit the widespread commercial application of DMFC. In our previous study, we analyzed the use of carbon nanocoils (CNCs) as a catalyst support in DMFC [1]. Due to their three dimensional structure, CNC is considered to be a unique support material for electrocatalyst materials. However, when CNC-supported PtRu catalyst was used in the anode of DMFC, it resulted in poor DMFC performance. Therefore, to utilize the advantages of CNC as an anode catalyst support, we applied the anode microporous layer (MPL) to DMFC for improving the efficiency of utilization of the CNC-supported PtRu catalyst [2]. The anode MPL is expected to play a crucial role in preventing the permeation of methanol across CNCs in the anode catalyst layer (CL). Carbon nanoballoon (CNB) and Vulcan were used as the anode MPL materials. CNB is a unique material because of its hollow structure and high electrical conductivity, while Vulcan has a high surface area and high electrical conductivity. CNCs were synthesized using an automatic chemical vapor deposition system with a consecutive substrate transfer mechanism [1]. As seen in the transmission electron micrograph, the fiber diameter of the CNCs is ~300 nm, the coil diameter is ~1000 nm, and the coil length is ~10 μm. Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O) and ruthenium trichloride (RuCl3) were used as the Pt and Ru precursors, respectively. The molar ratio of Pt and Ru was set at 1:1. Each of the carbon nanomaterials (200 mg) was dispersed in 500 mL of deionized water by sonication for 20 min. H2PtCl6·6H2O and RuCl3 were stirred in 50 mL of deionized water at 60 rpm for 10 min. The solutions were mixed and stirred at 600 rpm for 10 min. Next, a 30-fold molar excess of sodium borohydride (NaBH4) with respect to the metal precursors was added to 400 mL deionized water. This NaBH4 solution was added to the metal precursor and carbon nanomaterial mixture and stirred. The solution was then filtered, washed and dried to obtain the supported catalyst. A Nafion®115 membranes (Du Pont, K.K., Tokyo, Japan) were cleaned several times with deionized water, hydrogen peroxide solution, and sulfuric acid. The anode and cathode inks were prepared by mixing PtRu/CNCs as the anode catalyst or Pt/C as the cathode catalyst with 5 wt% Nafion® solution in 1-propanol and isopropyl alcohols. Subsequently, the inks were uniformly sprayed onto the PTFE substrate. The substrates were then hot-pressed on both sides of the electrolyte membrane at a pressure of 15 MPa at 130 °C for 10 min. The amount of Pt in the anode and cathode CLs was maintained at 0.3 mg cm−2. The MEA thus obtained was mounted in the DMFC (Japan Automobile Research Institute, Tsukuba, Japan), and the performance of the DMFC was tested by supplying 0.5 M methanol to the anode at a flow rate of 0.1 mL s−1 and dry air to the cathode at a flow rate of 5 mL s−1. We investigated the effect of using an anode MPL on the performance of DMFCs. According to the polarization experiments, the anode MPL with CNB and Vulcan loadings of 1.5 mg cm−2 resulted in the best DMFC performance among the different types of anode MPLs investigated for a methanol concentration of 0.5 M. The EIS results indicated that the MEA with an anode MPL exhibited improved conduction of electrons between the anode MPL and the anode CL, and lower high-frequency resistance and charge transfer resistance.