Anode catalyst layer

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

Efficient transport of protons from the anode catalyst layer to the cathode catalyst layer ... 

Influence of PTFE content in the anode DL of a DMFC. Operating conditions 90°C cell temperature anode at ambient pressure cathode at 2 bar pressure methanol concentration of 2 mol dm methanol flow rate of 0.84 cm min. The air flow rate was not specified there was a parallel flow field for both sides. The anode catalyst layer had 13 wt% PTFE, Pt 20 wt%, Ru 10 wt% on Vulcan XC-73R carbon TGP-H-090 with 10 wt% PTFE as cathode DL. The cathode catalyst layer had 13 wt% PTFE, Pt 10 wt% on carbon catalyst with a loading 1 mg cm Pt black with 10 wt% Nafion. The membrane was a Nafion 117. (Reprinted from K. Scott et al. Journal of Applied Electrochemistry 28 (1998) 1389-1397. With permission from Springer.)... 

As mentioned, the reaction distribution is the main effect on the catalyst-layer scale. Because of the facile kinetics (i.e., low charge-transfer resistance) compared to the ionic resistance of proton movement for the HOR, the reaction distribution in the anode is a relatively sharp front next to the membrane. This can be seen in analyzing Figure 10, and it means that the catalyst layer should be relatively thin in order to utilize the most catalyst and increase the efficiency of the electrode. It also means that treating the anode catalyst layer as an interface is valid. On the other hand, the charge-transfer resistance for the ORR is relatively high, and thus, the reaction distribution is basically uniform across the cathode. This means... 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

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 electric circuit of membrane electrode assemblies is a combination of anode and cathode catalyst layers plus the membrane. In general, the anode catalyst layer is considered an electric circuit, the cathode catalyst layer is considered another electric circuit similar to that of the anode but with different RC values, and the membrane is treated as a resistance. These three electric circuits are connected in series to construct a whole-cell equivalent circuit. A typical impedance spectrum is shown in Chapter 1 as Figure 1.16. Since the anode reaction is significantly faster than the cathode, the RC electric circuit of the anode can be disregarded. 

When the impedance versus the reciprocal of the square root of the frequency is plotted, the slope gives (Rf/Cdi)U2, where Rp is the ionic resistance of the anode catalyst layer. The double-layer capacitance can be obtained from the cyclic voltammogram. According to the definition of capacitance, the capacitance of the double layer equals the current divided by the scan rate (Cdt = TV-1). The ionic resistance can be obtained from the capacitance of the double layer. 

Havranek A, Wippermann K (2004) Determination of proton conductivity in anode catalyst layers of the direct methanol fuel cell (DMFC). J Electroanal Chem 567(2) 305-15... 

Zhao X, Fan X, Wang S, Yang S, Yi B, Xin Q, Sun G (2005) Determination of ionic resistance and optimal composition in the anodic catalyst layers of DMFC using ac impedance. Int J Hydrogen Energy 30(9) 1003-10... 

Figure 33 shows the profiles of reaction rates across the catalyst layers in the inlet element (along the white line in Fig. 32). In this example, the reaction rate on the cathode side is almost constant along x, whereas on the anode side it rapidly grows with x. Most of the anode catalyst layer thickness is not used for reaction, except for the thin sublayer near the membrane, with the thickness of the order of 2-3 pm.

At high anodic overpotentials, methanol oxidation reaction exhibits strongly non-Tafel behavior owing to finite and potential-independent rate of methanol adsorption on catalyst surface [244]. The equations of Section 8.2.3 can be modified to take into account the non-Tafel kinetics of methanol oxidation. The results reveal an interesting regime of the anode catalyst layer operation featuring a variable thickness of the current-generating domain [245]. The experimental verification of this effect, however, has not yet been performed. 

Fig. 20 Distributions at current density of 1 A cm 2, of electrode potential (top), reactant concentration (middle), and current generation (bottom) in a PEFC anode catalyst layer 5 pm thick, as result of limited transport rate of the hydrogen gas reactant and/or the limited transport rate of protons. Two cases of reactant concentration, 100% hydrogen and 10% hydrogen in the dry gas and two cases of effective protonic conductivity in the catalyst layer, 0.1 and 0.01 S cm-1, are considered in these calculations. A value of 2 x 10-4 cm2 sec-1 was used as estimate for effective Dh2 in the catalyst layer.

Completed study of the effect of platinum-to-ruthenium (Pt-to-Ru) ratio on anode performance, demonstrating optimum performance with 55 5 atomic percent of Ru in the anode catalyst layer. 

While the membrane represents the heart of the fuel cell, determining the type of cell and feasible operating conditions, the two catalyst layers are its pacemakers. They fix the rates of electrochemical conversion of reactants. The anode catalyst layer (ACL) separates hydrogen or hydrocarbon fuels into protons and electrons and directs them onto distinct pathways. The cathode catalyst layer (CCL) rejoins them with oxygen to form liquid water. This spatial separation of reduction and oxidation reactions enables the electrons to do work in external electrical appliances, making the Gibbs free energy of the net reaction, —AG, available to them. 

In planar fuel eells, the membrane is part of a layered sandwich structure (the membrane-eleetrode assembly) eonsisting of a thin eatalyst layer and a porous eleetrode (gas dilfusion layer) on either side of the membrane. The oxygen reduction and hydrogen oxidation reaetions take plaee at the eathode and anode catalyst layers, and the reaetants and produets are transported through the porous electrodes. A fuel eell model thus requires appropriate coupling of the membrane sub-model to the adjacent transport and electrochemical reactions. Detailed strategies for implementing complete fuel cell models have been discussed elsewhere [11-15]. 

Strictly speaking, variation of ip is negligible if t 95. This condition is not fulfilled in the anode catalyst layer, where tj can be on the order of p. In that case rj feels the variation of p, which leads to two-dimensional effects [8]. 

From there hydrogen and water vapor are transported by diffusion through the porous structure of the electrode to the anode catalyst layer as the electrochemical interface where molecular hydrogen is oxidized giving off two protons and two electrons per molecule. 

Fig. 1.5 Scheme of a PEM direct alcohol fuel monocell showing (1) proton conducting membrane (2) anodic catalyst layer, (3) cathodic catalyst layer, (4) gas diffusion layers (5) current collector with flow channels (6) seals (7) electric load... 

There are different kinds of DAFC operation conditions depending of the way the fuel and the oxidant (oxygen/air) are fed into the cell. In complete active fuel cells the liquid fuel (neat alcohol or aqueous solution) is pumped and gas is compressed, using auxiliary pumps and blowers, in order to improve mass transport and reduce concentration polarization losses in the system. On the other hand, in complete passive DAFC the alcohol reaches the anode catalyst layer by natural convection and the cathode breathes oxygen directly from the air. A number of intermediate options have been also studied and tested. 

Witham CK, Oran W, Valdez n, Narayanan SR (2000) Performance of direct methanol fuel cells with sputter-deposited anode catalyst layers. Electrochem Solid State Lett 3 497—500... 

Use of a high methanol concentration but maintaining an adequate concentration in the anode catalyst layer at a given current density to maximize the system specific energy and cell performance. For achieving this aim it is fundamental an optimum design of the fuel supply system (that allows the orientation-independent operation of the fuel cell), the anode current-collector, and the anode diffusion layer. 

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