Phosphoric acid-based electrolytes

PAFCs run at temperatures around 200 °C. Unlike PEM fuel cells, the electrolyte in PAFCs does not rely on water for proton conduction therefore water management is not a concern. PAFC systems usually are used in coupled heat and power (CHP) applications for improved efficiency. Due to corrosion caused by phosphoric acid-based electrolyte and dissolution/corrosion of Pt-based catalyst in PAFCs, the operating voltage conditions of PAFCs are limited to less than 0.8 V per ceU. The dissolution/corrosion problems also eliminate the option of hot idling of PAFCs. Borohydride energy density at 60 °C would allow a smaller, more adaptive system. 

Phosphoric acid-based electrolytes are used mainly to generate oxides that provide enhanced adhesion with paints and adhesives (Anowsmith and Clifford 1985 Arrowsmith et al., 1992 Johnsen et al, 2004 Zhang et al, 2008). The baths generally contain between 10% and 30% wt. phosphoric acid, with a temperature between 15 °C and 30 °C. The films produced are generally unsuited to hydrothermal sealing (Konno et al, 1982). 

Polishing, electropolish (surface modification) Polishing a surface that is the anode of an electrolysis cell using a suitable electrolyte. Example Electropolishing stainless steel in a phosphoric acid-based electrolyte. 

In addition to its use in phosphoric acid fuel cells (PAFC) and more recently in phosphoric acid-based high temperature polymer electrolyte membrane fuel cells (HT-PEMFC), phosphoric acid is a widely used compound in the chemical industry. Phosphoric acid is the second most important mineral acid in terms of volume and value, being exceeded only by sulfuric acid. It is mainly used for the production of fertilizers and industrial phosphates, metal surface treatment, and the acidulation of beverages [1]. 

Park CO, Akbar SA, Weppner W (2003) Ceramic electrolytes and electrochemical sensors. J Mater Sci 38 4639-4660 Park CO, Fergus JW, Miura N, Park J, Choi A (2009) Solid-state electrochemical gas sensors. Ionics 15 261-284 Phair JW, Badwal SPS (2006) Review of proton conductors for hydrogen separation. Ionics 12 103-115 Ponomareva VG, Lavrova GV, Hairetdinov EF (1997) Hydrogen sensor based on antimonium pentoxide-phosphoric acid sohd electrolyte. Sens Actuators B 40 95-98... 

As is well known, and as discussed by Neyerlin and others, there is an increase in polarization losses with phosphoric acid-based fuel cells, in comparison to low-temperature PFSA-based fuel cells, and this effect is thought to be due to the presence of phosphoric acid and/or its anions that adsorb onto the surface of the catalyst [47]. Because of this, high-temperature stacks based upon phosphoric acid-doped polymer electrolyte membranes are larger in order to get the same power output. The overall question is whether the benefit in system simplification overcomes the need for larger stacks, so that there is an overall net system benefit. Reducing the effect of the adsorbed anion species would, of course, have significant benefit at the stack and system levels. 

Inorganic Methods. Before the development of electrolytic processes, hydrogen peroxide was manufactured solely from metal peroxides. Eady methods based on barium peroxide, obtained by air-roasting barium oxide, used dilute sulfuric or phosphoric acid to form hydrogen peroxide in 3—8% concentration and the corresponding insoluble barium salt. Mote recent patents propose acidification with carbon dioxide and calcination of the by-product barium carbonate to the oxide for recycle. 

These three approaches to reject heat and exhaust fuel recovery with power generation apply primarily to the higher temperature, solid oxide (1800 F) and molten carbonate (1200 F), fuel cell systems operating on CH4 fuel. The lower operating temperatures of the phosphoric acid (400 F) and polymer electrolyte (175 F) fuel cells severely limit the effectiveness of thermal cycle based power generation as a practical means of heat recovery. 

Figure 19.18. Data of electrochemical fuel cells, (a) Processes in a fuel cell based on the reaction between hydrogen and oxygen, (b) Voltage-current characteristic of a hydrogen-air fuel cell operating at 125°C with phosphoric acid electrolyte [Adlharl, in Energy Technology Handbook (Considine, Ed.), 1977, p. 4.61). (c) Theoretical voltages of fuel cell reactions over a range of temperatures, (d) Major electrochemical systems for fuel cells (Adlharl, in Considine, loc. cit., 1977, p. 4.62).

Based on the fact that aromatic sulfonic and carboxylic acids were successfully separated by reversed-phase chromatography in the presence of organic electrolytes, Chaytor and Heal (158) developed a method for the separation of 15 synthetic colors using a mobile phase containing o-phosphoric acid (Table 7). The presence of the electrolyte provided lower variation in response and retention over a period of time. Furthermore, eluted peaks were sharper than those seen in ion-pair chromatography. 

The phosphoric acid cell has been under research for a longer time than that of any other kind of fuel cell. Alloys of Pt with Cr, V, and Ti and other non-noble metals are better than Pt (Appleby, 1986). The particle size of the catalyst has been reduced to that of tens of atoms (Stonehart, 1993).10 Much attention has been given to the search for non-noble (hence cheaper) catalysts that are stable in hot acids. The best are the porphyrins, the formulas for which are shown in Fig. 13.20. They are applied to a base of graphite. These electrocatalysts are more effective in alkaline fuel cells than in those with acid electrolytes. Curiously, these substances are more stable and give better catalysis after pyrolysis in He at 800 °C, a process that would decompose the organic part of the structure. Perhaps the only active part of the porphyrin catalyst is the central... 

The PAFC is based on an immobilized phosphoric acid electrolyte. The matrix universally used to retain the acid is silicon carbide, and the catalyst for both the anode and cathode is platinum [8], The active layer of platinum catalyst on a carbon-black support and a polymer binder is backed by a carbon paper with 90% porosity, which is reduced to some extent by a Teflon binder [6,9]. 

Asensio, J.A., Borros, S., and Gomez-Romero, R, Polymer electrolyte fuel cells based on phosphoric acid-impregnated poly(2,5-benzimidazole) membranes, J. Electrochem. Soc., 151, A304, 2004. 

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