Anode backing layer

C. Xu, T. S. Zhao, and Q. Ye. Effect of anode backing layer on the cell performance of a direct methanol fuel cell. Electrochimica Acta 51 (2006) 5524—5531. 

Induce strong lowering of the crossover rate with increase in cell current through proper choice of the anode backing layer... 

Fig. 51 Lowering the rate of methanol crossover by operation at cell current close to the limiting current defined by the anode backing layer. Solid - MeOH concentration profile at open circuit dashed - MeOH concentration profile nearJ iman.

In DMFCs, the methanol flux through the membrane is large and it cannot be ignored. The flux of methanol in the anode backing layer of DMFC obeys an equation... 

Condition 7 = 1 is equivalent to = j, or 6FD c °// = AFDld fP/ll- In other words, at the inlet the methanol flux in the anode backing layer equals the oxygen flux in the cathode backing layer. Both fluxes are maximal since they provide limiting current density. The concentrations of oxygen and methanol in the respective catalyst layer thus tend to zero. 

MEAs used in this study were prepared in the following procedure [5]. The diffusion backing layers for anode and cathode were a Teflon-treated (20 wt. %) carbon paper (Toray 090, E-Tek) of 0.29 mm thickness. A thin diffusion layer was formed on top of the backing layer by spreading Vulcan XC-72 (85 wt. %) with PTFE (15 wt. %) for both anode and cathode. After the diffusion layers were sintered at a temperature of 360 C for 15 min., the catalyst layer was then formed with Pl/Ru (4 mg/cm ) and Nafion (1 mg/cm ) for anode and with Pt (4 mg/cm ) and Nafion (1 mg/cm ) for cathode. The prepared electrodes were placed either side of a pretreated Nafion 115 membrane and the assembly was hot-pressed at 85 kg/cm for 3 min. at 135 C. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

In electrochemical systems, metal meshes have been widely used as the backing layers for catalyst layers (or electrodes) [26-29] and as separators [30]. In fuel cells where an aqueous electrolyte is employed, metal screens or sheets have been used as the diffusion layers with catalyst layers coated on them [31]. In direct liquid fuel cells, such as the direct methanol fuel cell (DMFC), there has been research with metal meshes as DLs in order to replace the typical CFPs and CCs because they are considered unsuitable for the transport and release of carbon dioxide gas from the anode side of the cell [32]. 

After testing a number of DLs with and without MPLs, Lin and Nguyen [108] postulated that the MPL seemed to push more liquid water back to the anode through the membrane. Basically, the small hydrophobic pores in the MPL result in low liquid water permeability and reduce the water transport from the CL toward the DL. Therefore, more liquid water accumulated in the CL is forced toward the anode (back diffusion). This reduces the amount of water removed through the cathode DL, decreases the number of blocked pores within the cathode diffusion layer, and improves the overall gas transport from the DL toward the active zones. 

DMFC modeling thus aims to provide a useful tool for the basic understanding of transport and electrochemical phenomena in DMFC and for the optimization of cell design and operating conditions. This modeling is challenging in that it entails the two-phase treatment for both anode and cathode and that both the exact role of the surface treatment in backing layers and the physical processes which control liquid-phase transport are unknown. 

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

Figure 39. Images of bubble dynamics in the DMFC anode with carbon paper backing layer for 2 M MeOH feed and nonhumidified air at 100 mA/cm and 85...

Figure 2.1 Schematic diagram of a DMFC, its electrode reactions and material transport involved, where (b) is the anode backing, (f) the cathode backing, (c) the Pt-Ru anode catalyst layer, (d) the Nafion 117 membrane and (e) the Pt cathode catalyst layer.

The electromembrane reactor used in the study was a flow-through undivided electrocatalytic cell. The principal feature of the cell is the ceramic-based sheet, which was coated with the carbon-supported catalyst on one side. The coated side was used as the anode and the cathodic side was not coated with any electroconductive substance or catalyst. Current was supphed to the anode and cathode by means of backing layers, which are connected, to the external power source by means of a conducting wire. The backing layers that were used in this study are carbon cloths 6100-200 purchased from Lydall, United States. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

The electrode-electrolyte assembly was investigated in a single cell test station. After installing the MEA in the fuel cell housing, water was supplied to the anode and cathode backing layers and the cell was warmed-up step-wise from room temperature to 145°C. The polarization curves obtained for the fuel cells equipped with the Nafion-silica and Nafion-silica-PWA membranes, under same conditions in presence of oxygen feed at cathode and 2M methanol solution at anode, are reported in Fig. 6. 

Physically, r is proportional to the ratio of mass transfer coefficient of liquid water in membrane to mass transfer coefficient of water vapour in the backing layer. The parameter r thus describes the competition of two opposite water fluxes back diffusion, which wets the anode side of the membrane and leakage through the backing layer to the channel, which facilitates membrane drying. Physically, r controls the local water-limiting current density (see below). 

Fig. 7.16 Effect of PTEE content on the anode and cathode backing layer in the performance of DMFC [82] (Reprinted by permission of the publisher)...

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