Phosphoric acid fuel cells electrode/electrolyte system

The earliest models of fuel-cell catalyst layers are microscopic, single-pore models, because these models are amenable to analytic solutions. The original models were done for phosphoric-acid fuel cells. In these systems, the catalyst layer contains Teflon-coated pores for gas diffusion, with the rest of the electrode being flooded with the liquid electrolyte. The single-pore models, like all microscopic models, require a somewhat detailed microstructure of the layers. Hence, effective values for such parameters as diffusivity and conductivity are not used, since they involve averaging over the microstructure. 

The beginning of modeling of polymer-electrolyte fuel cells can actually be traced back to phosphoric-acid fuel cells. These systems are very similar in terms of their porous-electrode nature, with only the electrolyte being different, namely, a liquid. Giner and Hunter and Cutlip and co-workers proposed the first such models. These models account for diffusion and reaction in the gas-diffusion electrodes. These processes were also examined later with porous-electrode theory. While the phosphoric-acid fuel-cell models became more refined, polymer-electrolyte-membrane fuel cells began getting much more attention, especially experimentally. 

UTC) has been using SiC for 50 years as an electrolyte matrix in PAFC because of its extreme stability in hot phosphoric acid. The system could not be used in fuel cell electrodes due to its poor catalytic activity and electrical conductivity. However, SiC has been evaluated as a catalyst support with addition of carbon black to enhance conductivity in the catalyst layer. The approach included the deposition of Pt particles on SiC by chemical route followed by mixing with carbon to formulate catalyst. Authors claimed that a higher Pt loading has led to improved electrode performance even with large particles of Pt. This indicated that electrode performance depends not only on surface area of Pt but also on the interaction nature between support and metal catalyst [19]. 

As long as fuel cells are using liquid electrolytes like phosphoric acid or concentrated caustic potash, the catalyst utilization is usually not limited by incomplete wetting of the catalyst. Provided the amount of electrolyte is sufficiently high, the hydrophilic porous particles are not only completely flooded but due to their expressed hydrophilicity are wetted externally by an electrolyte film that together with the whole electrolyte-filled hydrophilic pore system establishes the ionic contact of an electrode to the respective counterelectrode. 

PAFCs were the first fuel-cell technology to be commercialised and represented almost 40% of the installed fuel cell units in 2004 (Sammes et al., 2004). Most of the demonstration units are in the range of 50-200 kW, but larger plants (1-10 MW) or smaller systems (1-10 kW) have also been built (Ghouse et al., 2000 Yang et al., 2002). Lifetimes of 5 years with > 95% durability have been demonstrated. Phosphoric acid electrode/electrolyte technology has reached maturity. However, fiirther increases in power density and reduced cost are needed to achieve economic competitiveness (US DOE, 2002 Larminie et al., 2003 Haile, 2003). 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

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