Performance of Membrane-Electrode-Assembly Using Anode Catalyst Layers with Carbon Nanomaterials of Particle and Fiber Geometries in Direct Methanol Fuel Cell

Abstract

Direct methanol fuel cell (DMFC) is 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 [1-3]. Carbon black, nanometer-size carbon particles, is commercially used as the catalyst support in fuel cell owing to its high surface area, porosity, electric conductivity, low density, and low cost. In the previous work, we have used various carbon nanomaterials as a catalyst support for DMFC [4]. In this study, we measured the powder conductivity of carbon nanomaterials including carbon nanocoil (CNC), carbon nanoballoon (CNB), Vulcan XC-72R (Vulcan), and vapor-grown carbon fiber (VGCF-H). Under compression of these materials, it is shown that the electrical conductivity of carbon nanomaterials did not only depend on its intrinsic morphological properties, which determine the degree of packing of the material and hence the change in density, but also on such extrinsic factors as the applied pressure and the ambient humidity. In addition, this study investigates the effects of the conductivity and structure of the carbon nanomaterials used in the anode catalyst layer (CL) on the performance of DMFC using transmission and scanning electron microscopies, polarization technique, and electrochemical impedance spectroscopy (EIS). CNCs were synthesized using an automatic chemical vapor deposition system with a consecutive substrate transfer mechanism. The fiber diameter of the CNCs is ~300 nm, the coil diameter is ~1000 nm, and the coil length is ~10 μm. Arc black (AcB) was synthesized using the twin-torch arc discharge apparatus developed in our laboratory. CNB was obtained by heating AcB in a Tammann oven in Ar gas at 2600 ºC for 2 h. Commercially available Vulcan (Cabot Corp., Boston, MA, USA) and VGCF-H (SHOWA DENKO K. K., Tokyo, Japan) were used as the Vulcan and VGCF-H samples, respectively. CNB and Vulcan were composed of spherical with a particle diameter of ~50 nm. The fiber diameters of the VGCF-H were ~15 nm, and their length was ~3 μm. Powder conductivity of each sample was measured by a source meter, with an applied voltage of 0.1 V at room temperature. 300 mg of the sample was set in acrylic pipe. Subsequently the sample was compressed between the brass pistons. The compressive force was varied from 0.01 to 1.0 MPa. Nafion®115 membrane (Dupont) was used as electrolyte membrane. The anode and cathode catalysts used were 30-wt.% PtRu/CNC, /CNB, /Vulcan, and /VGCF-H and 50-wt.% Pt/C (Tanaka Kikinzoku International K.K), respectively. The membrane electrode assembly (MEA) was mounted into the DMFC cell (Japan Automobile Research Institute). In this performance testing, 0.5 M (M = moldm-3) methanol solution were supplied to the anode at a flow rate of 0.1 mLs-1, and dry air was supplied to the cathode at a flow rate of 5 mLs-1. The DMFC was operated 60 ºC and its polarization characteristics and EIS were measured using a fuel cell impedance meter (Kikusui Electronics Corp., KFM2030). The carbon nanomaterials and the surface morphologies of the anode catalyst layer were examined by TEM (JEM-2100F, JEOL, Tokyo, Japan) and SEM (S-4500 II and SU8000, Hitachi, Tokyo, Japan), respectively. The density of the carbon nanomaterials by compressive forces depends on the rearrangement and fragmentation of agglomerates [5]. In addition, the powder conductivity during compaction is mainly governed by the increase of particle contact area. From the measurement results of the powder compression electrical conductivity, CNB showed comparable powder conductivity to Vulcan. Moreover, VGCF-H showed the highest conductivity. The figure shows the cell polarization and power density of the DMFCs with different carbon nanomaterials in the anode CL. The DMFC performance exhibited the highest power density (15.3 mW cm-2) when CNC was used as the catalyst support in the anode CL, while that using CNB showed the lowest power density (8.1 mW cm-2). Therefore, the DMFC performance was not correlated with the results obtained by the measurement of the powder compression electrical conductivity. From the results of the EIS measurement and SEM images, it is suggested that the interface state of the catalyst supports was also an important factor in the DMFC performance.