Fuel DMFC anodes

The Pt/Ru catalyst is the material of choice for the direct methanol fuel cell (DMFC) (and hydrogen reformate) fuel cell anodes, and its catalytic function needs to be completely understood. In the hrst approximation, as is now widely acknowledged, methanol decomposes on Pt sites of the Pt/Ru surface, producing chemisorbed CO that is transferred via surface motions to the active Pt/Ru sites to become oxidized to CO2... 

Finally, we have discussed the effect of incomplete Cj oxidation product formation for fuel cell applications and the implications of these processes for reaction modeling. While for standard DMFC applications, formaldehyde and formic acid formation will be negligible, they may become important for low temperature applications and for microstructured cells with high space velocities. For reaction modeling, we have particularly stressed the need for an improved kinetic data base, including kinetic data under defined reaction and transport conditions and kinetic measurements on the oxidation of Ci mixtures with defined amounts of formaldehyde and formic acid, for a better understanding of cross effects between the different reactants at an operating fuel cell anode. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

New Ternary Fuel Cell Catalysts for DMFC Anodes... 

Strasser, P., Fan, Q., Devenney, M., Weinberg, W. H., Combinatorial Exploration of ternary fuel cell electrocatalysts for DMFC anodes — a comparative study of PtRuCo, PtRuNi and PtRuW systems, AIChE fall meeting, 2003, San Francisco. 

Figure 5.38. Nyquist plot of a typical DMFC anode impedance spectrum [43], (Reprinted from Journal of Power Sources, 75, Muller JT, Urban PM. Characterization of direct methanol fuel cells by ac impedance spectroscopy, 139M3, 1998, with permission from Elsevier and the authors.)...

A special case of Eq. (62) arises when the DMFC anode is (methanol) dead-ended, that is, all methanol fed into the anode at a current equivalent rate of /feed. either reacts at the anode or recombines with oxygen at the cathode. These operation conditions apply in 1-3 W, passive DMFC platforms developed for consumer electronics applications described later in this section. Under these conditions, rjf = /cell//fuel feed... 

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. 

Much worse is the situation with a direct methanol fuel cell (DMFC) anode. For this electrode, jcrit is very low, about 20mAcm (Table 23.1). This means that at a working current of 100 mAcm , a typical DMFC anode operates in the double-Tafel regime, which dramatically increases the anode polarization voltage. 

In cells of various t3rpes, the flow in the channels can be purely gaseous (SOFC and anode of PEFC), two-phase (PEFC and DMFC cathodes, and DMFC anode) or purely liquid (DMFC anode at low current). A typical inlet flow velocity of liquid methanol-water solution in the anode channel of the DMFC varies between 0.1 and 1 cm s. The velocity of gaseous flow in fuel cells is between 10 and 10 cm s. With the typical channel diameter in the order of 0.1 cm, the Reynolds number varies in the range of 100-1000 and hence the flows are laminar. 

Regardless of fuel cell type, in a certain range of polarization voltages this rate is well approximated by the Butler-Volmer or Tafel equations. A better (non-Tafel) approximation of the reaction rate for DMFC anode is considered in Section 2.7. 

Scheiba F, Scholz M, Cao L, Schafranek R, Roth C, Cremers C, et al. On the suitability of hydrous ruthenium oxide supports to enhance intrinsic proton conductivity in DMFC anodes. Fuel Cells 2006 6 439-46. 

W.D. King, J.D. Com, O.J. Murphy, D.L. BoxaU, E.A. Kenik et ai, R-Ru and Pt-Ru-P/Carbon nanocomposites Synthesis, characterization, and imexpected performance as direct methanol fuel cell (DMFC) anode catalysts , J. Phys. Chem. B 107 (2003) 5467. 

The simplest approach to a liquid-fed DMFC is to supply an aqueous solution of methanol directly to the fuel cell anode and use it in combination with an air-breezing cathode without any auxiliary equipment in a so-called passive design. 

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