Phosphoric acid fuel cells introduced

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

The 200 kW phosphoric acid fuel cell (PAFC) was introduced into the market in 1991 by International Fuel Cells/ ONSI, now called UTC Fuel Cells. It is the only commercialized fuel cell technology. PAFC units have been installed in various applications—commercial, small industrial, landfill, and military—and some are used for cooling, heating, and power. To date there have been 250 units sold, at roughly 4500/kW. The U.S. Department of Defense (DOD) has cost-shared the purchase of three-quarters of the units sold to date. The units have performed well they have operated at 95 to 98 percent availability and 99.99 to 99.9999 percent reliability and have served 4 million customers and accumulated 4 million hours of operation. The cost of PAFC units has not decreased and in fact has increased from 3500/... 

In the early 1960s, concentrated solutions of phosphoric acid, with which the working temperature could be raised to 150 C, were introduced. Even at that temperature, and even in large quantities, the platinum catalysts proved not to be sufficiently active for a practical fuel cell working with natural fuels (hydrocarbons and other fuels). The experience gathered in these attempts later led to successful hydrogen-oxygen phosphoric acid fuel cells. 

By alloying Pt with transition metals M (M = V, Cr, Co, Ni, Fe, Ti, etc.), the ORR activity can be enhanced remarkably in both phosphoric acid fuel cell (PAFC) and PEMFC [20-22]. The activity enhancement mechanisms have been an open question for more than three decades and ascribed to decreased Pt-ft bond distance [23], enhanced surface roughness [24], increased Pt d-band vacancy [25-27], weakened OH adsorption [28, 29], and downshifted d-band center [30-35]. Nprskov and Mavrikakis et al. combined the stmctural and electronic effects by introducing a d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. 

PAFCs are the first fuel cells to be commercially available. The major manufacturers of these fuel cells are UTC Power, Toshiba Corporation, HydroGen Corporation, Fuji Electric Corporation and Mitsubishi Electric Corporation. UTC Power introduced for sale a 200 kW PAFC system in 1991, and over 260 units were delivered to various customers worldwide. The design operational lifetime for these units was 40,000 h and most of the fielded units have met or exceeded this requirement A number of these units are still operational today with fleet leader at Mohegun Sun in Uncasville, Connecticut, USA, accumulating more than 76,(X)0 h [48]. Fuji s phosphoric acid fuel cell power plants, launched in 1998 have also demonstrated 40,000 h of life in field and some units after overhaul have exceeded 77,000 h of operational lifetime [1]. 

Polybenzimidazole (PBI) (initially manufactured by Hoechst-Celanese, now PE ME A) is one of the few polymers under consideration for high-temperature operation. The application of PBI [206, 207] and the noncommercial AB-PBI [208] in fuel cells was introduced by Savinell and coworkers. For that, the membrane was immersed in concentrated phosphoric acid to reach the needed proton conductivity. Operation up to 200 °C is reported [209]. A disadvantage of this class of membranes is the acid leaching out during operation, particularly problematic for cells directly fed with liquid fuels. Additionally, the phosphoric acid may adsorb on the platinum surface. A review on membranes for fuel cells operating above 100 °C has been recently published [209]. 

The positive effect of water on the fuel cell performance was further proven by introducing several H2O/H2 mixtures into the anode compartment. I-V curves were recorded for each water vapor partial pressure as shown in Fig. 22. I-V curves were recorded fast, within a time interval less than 30 s, so that the water produced at the cathode would not equilibrate with the membrane. It is evident in Fig. 22 that by increasing the water partial pressure a threefold increase of the cell performance is observ ed, thus proving the vital importance of steam for the efficient operation of the phosphoric acid imbibed high temperature MEA. 

Other groups have investigated additional inorganic additives, such as zir-conivun tricarboxybutylphosphonate (ZrPBTC), for use in a direct methanol fuel cell [35-37]. They also applied a post-sulfonation thermal treatment to the m-PBI/ZrPBTC membrane to increase the conductivity. ZrPBTC was introduced into the polymer by dispersing ZrPBTC powder in a DMAc solution of m-PBI [37]. The solvent was evaporated, leaving behind a 50 wt% ZrPBTC/m-PBI composite membrane. The membrane was then soaked in hydrochloric acid to introduce protons. Further immersion in either phosphoric acid or sulfuric acid produced a doped membrane. The sulfuric acid (SA)-doped membrane was then thermally treated at 480 °C for 60 s. 

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