Overall DMFC Reaction

We have already noted in Section 2.2 that the change in molar Gibbs energy for this reaction is -698.2 kJmol As we shall see below, six electrons are transferred for each molecule of methanol, and so, from equation 2.2, the reversible no-loss cell voltage is 

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In the case of the direct alcohol fuel cells (DAFCs), e.g., direct methanol fuel cell (DMFC), the anode of a liquid-feed DMFC is supplied with a diluted methanol aqueous solution, while the cathode is fed with air or pure oxygen, which can be either forced by an external blower or driven by natural convection [19, 20], Due to the combined effect of both convection and diffusion, the methanol and the oxygen reach the anode and cathode CLs, respectively, where they undergo the overall electrochemical reactions ... 

Current DMFC research and development is centred on and around PEM electrolytes. In this case, the overall anode reaction is... 

Water Management in DMFC In DAFCs, some water is needed at the anode to provide additional oxygen for the oxidation reaction. As a resnlt, flooding at the cathode can be more severe becanse the diffusion gradient will favor flow toward the cathode and electro-osmotic drug is enhanced. From Eq. (6.42), 1 mol of water is needed per mole of methanol for the overall oxidation reaction. One can define an anode methanol and an anode water stoichiometry in this case ... 

FIGURE 13.1 Basic reaction and structure of direct methanol fuel cell (DMFC). Overall,... 

When methanol is oxidized directly at the anode as the fuel, the fuel cell is called a direct methanol fuel cell (DMFC). The anode, the cathode, and the overall reactions are shown in Reactions 1.4, 1.5, and 1.6, respectively. It is important to note that methanol oxidation needs the presence of water. In other words, if there is no water at the anode, methanol will not be oxidized. So, supplying enough water to the anode is a necessity for a DMFC. 

In a DMFC, methanol is directly oxidized at the anode, as shown by Reaction 7.3. Each methanol molecule requires one water molecule for the reaction to proceed. Without water, the reaction cannot proceed, and this must be remembered when designing a DMFC system. The product is gaseous CO2, and it must be vented out so it does not impede the diffusion of methanol to the anode catalyst layer. At the cathode, oxygen combines with protons and electrons to form water, as shown by Reaction 7.4. The overall reaction is shown by Reaction 7.5, where each methanol molecule reacts with 1.5 O2 molecules to produce 1 CO2 molecule and 2 H2O molecules. 

The maximum reversible cell voltage is determined by the thermodynamics of the overall reaction. For both fuel-cell types, PFMFC and DMFC, an open-circuit... 

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. 

The basic setup of polymer electrolyte membrane fuel cells (PEMFC) as well as direct methanol fuel cells (DMFC) is in line with the standard fuel cell setup the two electrodes are separated by means of an electrically insulating but ionically conductive membrane (polymer electrolyte). In a PEM fuel cell, the anodic reactant is hydrogen, which is oxidized to form protons on the cathode, oxygen is reduced and forms water with protons that are transported through the proton conductive membrane. The overall reaction is... 

The electrode reactions taking place at the electrodes of DMFCs, the overall current-producing reactions and the corresponding thermodynamic values of equilibrium electrode potentials and EMF of the fuel cell are as follows ... 

Thus, the overall rate of methanol electroadsorption is determined by the potential-independent chemisorption (4.222). It is, therefore, reasonable to approximate the MOR kinetics in a DMFC anode by a two-step reaction mechanism ... 

A direct methanol fuel cell (DMFC) uses liquid methanol (CH3OH) as fuel. The electrochemical half and overall reactions in a DMFC is summarized in Table 4.1. The overall reaction in a DMFC is given as... 

The effectiveness of EIS can be greatly enhanced with the use of a reference electrode, which has a stable potential at the time of measurement [3]. A suitable reference electrode allows discernment of the different electrode losses from the overall cell response, resulting in a more appropriate equivalent circuit. Ideally, the collective responses of the anode and cathode will add to the full cell resistance. Because the use of a stable reference electrode in many fuel cell systems is difficult, one common way to examine fuel cell behavior is the use of a dynamic hydrogen electrode (DHE). In this case, one of the electrodes is used as the DHE, with hydrogen flow at this location. It is assumed that the losses associated with the DHE are minor, and all polarizations measured can be attributed to the other electrode. This approach can be dubious and is not appropriate when there are phenomena at the DHE that can affect losses, such as anode dryout in a PEFC. Note that the DHE does not have to be the actual anode in the fuel cell but can be used at either electrode to examine the polarization of the opposing electrode. For example, a DHE can be used at the cathode of a DMFC to examine the polarization behavior of the anode in the DMFC. In this case, of course, the reaction does not galvanically proceed in the desired direction, and external power from a galvanostal/potentiostat system must be applied to drive the reaction in the desired direction. 

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