Direct methanol fuel cell reaction kinetics

The construction of a cell permitting both FTIR measurements and electrochemical impedance measurements at buried polymer/metal interfaces has been described [266]. Ingress of water and electrolyte, oxidation (corrosion) of the aluminum metal layer, swelling of the polymer and delamination of the polymer were observed. A cell suitable for ATR measurements up to 80°C has been described [267]. The combination of a cell for ATR measurements with DBMS (see Sect. 5.8.1) has been developed [268]. It permits simultaneous detection of stable adsorbed species and relatively stable adsorbed reaction intermediates (via FTIR spectroscopy), quantitative determination of volatile species with DBMS and elucidation of overall reaction kinetics. An arrangement with a gas-fed electrode attached to the ATR element and operated at T = 60°C has been reported [269]. In this study, the establishment of mixed potentials at an oxygen consuming direct methanol fuel cell in the presence of methanol at the cathode was investigated. With infrared spec-... 

The generated potential via exergonic reactions in direct fuel cells is partly used to promote electrode reactions (kinetic overpotential), while in indirect fuel cells, the fuel is first processed into simpler fuels (to reduce the kinetic overpotential) via conventional catalytic reactors, or CMRs, in which temperature is used as the key operating parameter to accomplish the desired kinetics and equilibrium conversions. Among the direct fuel cells, e.g., PEMFCs use hydrogen, direct methanol fuel cell (DMFC) uses methanol, while the SOFC can operate directly on natoral gas (Figiue 15.3). However, in indirect fuel cells, a complex fuel must be suitably reformed into simpler molecules such as H2 and CO before it can be used in a fuel ceU. 

There is another fuel cell working under the ambient condition, that is, direct methanol fuel cells (DMFCs). Difference in the PEFCs and DMFCs is their anode fuels (the cathode fuel is oxygen in both cases). In the DMFCs, methanol (CH3OH) is supplied to the anode instead of the hydrogen for the PEFCs and this difference is crucial for their ceU performances. Although the Pt is known to be an active catalyst for both HOR and methanol oxidation reaction (MOR), kinetics of the MOR is much slower than that of the HOR and ORR on the Pt catalyst, which increases anode overpotential and gives an inferior cell performance in the DMFCs as demonstrated in Fig. 1. Therefore, an important research topic is... 

Normally, the kinetics of ORR and OER occurring at the cathode of fuel cells, including direct methanol fuel cells (DMFCs) is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, ORR catalyst is needed at the air cathode. Platinum (Pt)-based materials are the most practical catalysts used in PEM technology. These Pt-based catalysts are too expensive to make fuel cells commercially viable, and hence extensive research over the past several decades has been focused on development of alternative catalysts. These alternative electrocatalysts include noble metals and allo37S, carbon materials, quinone and its derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, transition metal carbides and transition metal oxides. In this chapter, we focus on both noble and nonnoble electrocatalysts being used in air cathodes and the kinetics and mechanisms O2 reduction/oxidation reaction (both ORR and OER), catal37zed by them. 

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

All fuel cells exhibit kinetic losses that cause the electrode reactions to deviate from their theoretical ideal. This is particularly true for a direct methanol PEFC. Eliminating the need for a fuel reformer, however, makes methanol and air PEFCs an attractive alternative to PEFCs that require pure hydrogen as a fuel. The minimum performance goal for direct methanol PEFC commercialization is approximately 200 mW/cm at 0.5 to 0.6 V. 

Abstract The faster kinetics of the alcohol oxidation reaction in alkaline direct alcohol fuel cells (ADAFCs), opening up the possibility of using less expensive metal catalysts, as silver, nickel, and palladium, makes the alkaline direct alcohol fuel cell a potentially low-cost technology compared to acid direct alcohol fuel cell technology, which employs platinum catalysts. In this work an overview of catalysts for ADAFCs, and of testing of ADAFCs, fuelled with methanol, ethanol, and ethylene glycol, formed by these materials, is presented. 

In direct alcohol fuels cells (DAFCs), some simple organic molecules such as methanol, ethanol, formic acid, and ethylene glycol are used as alternative fuels. Besides the slow kinetics of ORR in the cathode, the slow alcohol oxidation reaction on Pt is another major contribution to low DAFC performance. 

Fthanol is a nontoxic renewable energy source that can be produced from agricultural products its theoretical energy density (8.0 kWh/kg) is higher than that of methanol (6.1 kWh/kg). However, the commercialization of direct ethanol fuel cells (DFFCs) is considered more difficult than that of DMFCs, because the kinetics of ethanol oxidation reaction is slower than that of methanol oxidation even on the best available catalysts. 

Considering (Mily the thermodynamics of the DMFC (used here as a representative of direct alcohol fuel cells), methanol should be oxidized spraitaneously when the potential of the anode is above 0.05 V/SHE. Similarly, oxygen should be reduced spontaneously when the cathode potential is below 1.23 V/SHE, identical to a H2-O2 fuel cell. However, kinetic losses due to side reactions cause a deviation of ideal thermodynamic values and decrease the efficiency of the DMFC. This is presented in Fig. lb, which includes various limiting effects as kinetics, ohmic resistance, alcohol crossover, and mass transport. The anode and cathode overpotentials for alcohol oxidation and oxygen reduction reduce the cell potential and together are responsible for the decay in efficiency of approximately 50 % in DMFCs [13, 27]. 

Chapter 1 discusses the current status of electrocatalysts development for methanol and ethanol oxidation. Chapter 2 presents a systematic study of electrocatalysis of methanol oxidation on pure and Pt or Pd overlayer-modified tungsten carbide, which has similar catalytic behavior to Pt. Chapters 3 and 4 outline the understanding of formic acid oxidation mechanisms on Pt and non-Pt catalysts and recent development of advanced electrocatalysts for this reaction. The faster kinetics of the alcohol oxidation reaction in alkaline compared to acidic medium opens up the possibility of using less expensive metal catalysts. Chapters 5 and 6 discuss the applications of Pt and non-Pt-based catalysts for direct alcohol alkaline fuel cells. 

The left half of Figure 15.3, thus, depicts the catalytic processes that different fuels imdergo, such as reforming, high-temperature shift and low-temperatore shift, and preferential oxidation (PrOx), which are individually conducted at different temperatures so as to make the most of their thermodynamic limitations and kinetics. In direct fuel ceUs with relatively complex fuels, such as methanol, in fact, many of these reaction steps occur electrocatalylicaUy within the fuel cell, thus accounting for the large oveipotentials, e.g., in a DMFC. 

Other similar cases exist, such as direct electro-oxidation of alcohols such as methanol and formic acid in low-temperature fuel cells. In these cases, an alternative to the BV formulation that accounts for the limiting adsorption and charge transfer steps is appropriate. Two common models for a surface adsorption limited reaction are the Langmuir and Temkin kinetics. In the simpler Langmuir model, the surface adsorption rate constant is independent of surface coverage. In the Temkin model, the adsorption rate constant is modeled as a function of the surface coverage of adsorbed species. In both models, a two-step reaction mechanism is assumed [6] ... 

---