Electronic/protonic conductivity

Acceptor-doped SrCe03 is, without doubt, the mixed electron-proton conducting oxide system most studied, both with respect to actual measurements of... 

Rolison DR, Hagans PL, Swider KE, Long JW (1999) Role of hydrous rathenium oxide in Pt — Ru direct methanol fuel cell anode electrocatalysts the importance of mixed electron/ proton conductivity. Langmuir 15(3) 774—779... 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Inzelt, G., M. Pineri, 1. W. Schultze, and M. A. Vorotyntsev, Electron and proton conducting polymers recent developments and prospects, Electrochim. Acta, 45, 2403 (2000). 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

For a fast catalytic reaction, free access of gas, electrons, protons and water is needed. This leads to a best compromise of the volume fractions of protonconducting polymer, electron-conducting carbon, active sites and void space. 

The fabrication of catalyst layers for PEM fuel cells involves maintaining a delicate balance between gas and water transport, and electron and proton conduction. The process of CL fabrication should be guided by both fuel cell performance and cost reduction. 

An effective catalyst layer must serve multiple functions simultaneously electron and proton conduction, oxygen or hydrogen supply, and water management. The composition and structure of a CL can affecf all fhese functions... 

Proton conduction at high temperatures occurs in certain perovskites such as doped strontium cerate, Sr Ceo.95Ybo.o503 t. In air, this material is primarily an electronic conductor due to the mixed valence of Ce. In the presence of moisture, water is absorbed by the reaction with positive holes to generate protons ... 

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