Methanol oxidation kinetics

Fig. 2 shows the dynamic response of stack voltage to the step changes of various applied current densities. Like the former case of applied current pulses, the response exhibits the overshooting and relaxation which is caused by the methanol oxidation kinetics on the catalyst surface. The steady state stack voltage was found to be the same for both pulse and step loads with the same current density. 

In addition to the slow methanol oxidation kinetics, methanol that crosses over from the anode to the cathode side through the membrane can react with 02 at the cathode catalyst, leading to a mixed potential at the cathode side and thereby reducing cell performance. To solve this problem, methanol-tolerant catalysts as well as membranes with low methanol permeability have been investigated. However, these materials are still in the research stages and commercial applications have not been developed. 

However, DMFCs do suffer some drawbacks such as lower electrical efficiency and higher catalyst loadings as compared to H2 fuel cells. Efforts should be continued on developing anode catalysts with improved methanol oxidation kinetics, cathode catalysts with a high tolerance to methanol, membranes with lower methanol permeation rates, and strategies to reduce the methanol crossover rate. Attention should also be given to other direct-feed fuel cells using other liquid fuels (such as formic acid). 

Tripkovic et al. presented a eomparison of the mass-speeifie steady-state current densities for methanol oxidation in 0.5 M H2SO4 and 0.1 M NaOH, respeetively, as a funetion of temperature (Figure 4.19) [115]. On both Pt and Pt2Ru3 the methanol oxidation kinetics was much faster in alkaline media. The mass speeifie eurrent density at a constant potential (e.g., 0.5 V vs. RHE) was typieally over one order of magnitude higher for 0.1 M NaOH compared to 0.5 M H2SO4. Furthermore, in alkaline media there was virtually no difference in the aetivities of Pt and Pt2Rus eatalysts. 

When liquid methanol is employed as the fuel, the sluggish methanol oxidation kinetics on the anode catalyst may be the main hindrance, dramatically reducing the overall MEA performance. Much work has focused on exploring anode catalysts that can effectively enhance the methanol electrooxidation kinetics. Currently, only Pt-based electrocatalysts display the necessary activity and stability in the acidic environment of the DMFC. Figure 22.3 shows the generally accepted... 

Krewer, U., Christov, M., Vidakovic, T., and Sundmacher, K. 2006. Impedance spectroscopic analysis of the electrochemical methanol oxidation kinetics. 

Cell voltage of 1.21 V in Eq. (4.6) is not achieved in practical DMFCs due to (1) a slow ORR kinetics, (2) a slow methanol oxidation kinetics, and (3) methanol crossover through the membrane that causes parasitic methanol oxidation current at the cathode. 

Goussis, D.A., Lam, S.H. A study of homogeneous methanol oxidation kinetics using CSP. Proc. Combust. Inst. 24, 113-120 (1992)... 

Methanol oxidation on Pt has been investigated at temperatures 350° to 650°C, CH3OH partial pressures, pM, between 5-10"2 and 1 kPa and oxygen partial pressures, po2, between 1 and 20 kPa.50 Formaldehyde and C02 were the only products detected in measurable concentrations. The open-circuit selectivity to H2CO is of the order of 0.5 and is practically unaffected by gas residence time over the above conditions for methanol conversions below 30%. Consequently the reactions of H2CO and C02 formation can be considered kinetically as two parallel reactions. 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

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. 

This results from the slow kinetics of methanol oxidation and oxygen reduction. An additional loss is due to the cell resistance (arising mainly... 

III. ELECTRODE KINETICS AND ELECTROCATALY SIS OF METHANOL OXIDATION—ELECTROCHEMICAL AND SPECTROSCOPIC INVESTIGATIONS... 

Kauranen, R, E. Skou, and J. Munk, Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts, J. Electroanal. Chem., 404, 1 (1996). 

Ab initio methods allow the nature of active sites to be elucidated and the influence of supports or solvents on the catalytic kinetics to be predicted. Neurock and coworkers have successfully coupled theory with atomic-scale simulations and have tracked the molecular transformations that occur over different surfaces to assess their catalytic activity and selectivity [95-98]. Relevant examples are the Pt-catalyzed NO decomposition and methanol oxidation. In case of NO decomposition, density functional theory calculations and kinetic Monte Carlo simulations substantially helped to optimize the composition of the nanocatalyst by alloying Pt with Au and creating a specific structure of the PtgAu7 particles. In catalytic methanol decomposition the elementary pathways were identified... 

Jusys Z, Behm RJ. 2001. Methanol oxidation on a carbon-supported Pt fuel cell catalyst—A kinetic and mechanistic study by differential electrochemical mass spectrometry. J Phys ChemB 105 10874-10883. 

For potentials higher than 0.5 V vs. RHE, the formation of adsorbed oxygen species at Ru as well as at Pt will block the catalytic surface, leading to a decrease in the methanol adsorption kinetics. Therefore, in a potential range higher than 0.5 V vs. RHE, the kinetics of methanol oxidation is optimized at a Ru-poor catalyst, because methanol adsorption is not blocked and because the presence of Ru provides the extra oxygen atom needed to complete the oxidation of adsorbed CO to CO2. 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

Reaction Kinetics of the Steady-State Methanol Oxidation.242... 

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