Proton-exchange membrane fuel cells reaction mechanisms

Camara, G.A. et al.. The CO poisoning mechanism of the hydrogen oxidation reaction in proton exchange membrane fuel cells, J. Electrochem. Soc., 149, A748, 2002. 

Chemical reactions are temperature sensitive, and indeed, chemical rate constants and reactions mechanism are expected to vary considerably with temperature. Most investigations on the electrocatalysis of the ORR are usually performed at ambient conditions, which do not necessarily represent the behavior of the materials and the reaction at the conditions of practical interest. For example, in proton exchange membrane fuel cells, the temperature of operation is between 80 and 100 °C. Significant discrepancy in behavior may arise if reactions and materials are tested at ambient conditions and their behavior at high temperatures is merely deduced firom extrapolation. Schafer et al. introduced variable temperature SECM, with an operational range of 0-100 °C, by integrating a temperature control unit (Peltier element) into an SECM setup, as shown in the schematic of Fig. 23 [66]. At the heart of the temperature control unit is the Peltier element, which is housed in a stainless steel block. 

Reaction mechanism graphs are particularly useful in comparing several different reaction mechanisms that yield the same overall reaction, and also in considering the relative locations of various steps. This is illustrated through several examples from molten carbonate and proton-exchange membrane (PEM) fuel cell. 

Fundamental Research that Underlay Development of this Cell. Three U.S. universities were involved in the work that culminated in manufacture of the proton-exchange membrane by Ballard Power Systems. First, Case-Western Reserve University must be recognized because of the sustained investigations there (Yeager et al., 1961-1983) on the mechanism and catalysis of the reduction of02, the reaction that causes most of the energy losses in the fuel cell. The Electrochemistry of... 

HOR on Pt-Ru/C anodes has also been studied in a symmetrical H2IH2 polymer electrolyte membrane fuel cell, because the polarisations involved in this reaction are small [70], This cell contained a MEA in which a proton exchange membrane is sandwiched between a Pt-Ru catalysed anode and a Pt catalysed cathode. The anode (working electrode) was then fed with H2, as well as the cathode, which was used as counter and reference electrode. The analysis of the anodic polarisation scans together with the dependence of the exchange current density on the partial pressme of hydrogen allowed concluding that the Pt-Ru catalysed anode follows the Tafel-Volmer mechanism. 

---