Alkaline fuel cells electrolyte system

Alkaline fuel cells (AFCs). The electrolyte is 40 to 70% KOH, the working temperatures are 60 to 240°C. Such systems were used in the spacecraft of the Apollo program and in the U.S. space shuttle. 

Aqueous, alkaline fuel cells, as used by NASA for supplemental power in spacecraft, are intolerant to C02 in the oxidant. The strongly alkaline electrolyte acts as an efficient scrubber for any C02, even down to the ppm level, but the resultant carbonate alters the performance unacceptably. This behavior was recognized as early as the mid 1960 s as a way to control space cabin C02 levels and recover and recycle the chemically bound oxygen. While these devices had been built and operated at bench scale before 1970, the first comprehensive analysis of their electrochemistry was put forth in a series of papers in 1974 [27]. The system comprises a bipolar array of fuel cells through whose cathode chamber COz-containing air is passed. The electrolyte, aqueous Cs2C03, is immobilized in a thin (0.25 0.75 mm) membrane. The electrodes are nickel-based fuel cell electrodes, designed to be hydrophobic with PTFE. 

Jacques [5] was the first person to succeed in making large fuel cell systems. In 1895, he built a 1.5 kW battery consisting of about 100 small single fuel cells. Later, he also designed a stack with 30 kW of power. This was the first alkaline fuel cell, and it used molten KOH as the electrolyte and coke as the fuel. This AFC could provide 100 mA/cm2 at 1.0 V. 

Whereas the hot systems can consume CO, the cool systems suffer CO-poisoned platinum catalysts, and must have a shift reactor to consume the CO. Platinum poisoning is an irreversibility. The alkaline fuel cell (AFC), although without platinum, is especially incompatible with CO because of its KOH electrolyte. It needs a pure hydrogen fuel, and air with CO removed. The latter two purifications carry their own irreversibilities. 

In the past two decades, fuel cells and in particular imi-exchange membranes have become a top priority topic in material research. Fuel cells are seen as promising alternative energy conversion systems replacing the combustion-based techniques. Among the various types of fuel cells, the low-temperature fuel cells like the polymer electrolyte membrane fuel cell (PEMFQ, DMFC, or alkaline fuel cell (AFC) are the most flexible ones concerning range of appUcations e.g. portable, automotive, and stationary. 

The use of an alkaline fuel cell also involves an ammonia cracker, but since the electrolyte of the AEC is not sensitive to ammonia traces, the system is slightly simpler because of the elimination of the purification step. In addition, alkaline cells can run at slightly elevated temperatures (like the high-temperature PEM or higher), and they do not contain expensive platinum. AFCs could be operated with the same choice of metal ammine complex as the PEM. 

Metal/air cells are a semi-fuel cell, a hybrid between a battery and a fuel cell. The system combine in situ a metal anode, which dissolve in an alkaline or neutral electrolyte, and a porous diffusion cathode, in which air is continuously admitted. 

The Alkaline Fuel Cell, AFC, is the most efficient low-temperature fuel cell presently available with a very high power density, therefore ideally appropriate for mobile applications. The AFC shows a similar performance as the PEFC, but with a much more demanding process control which is complicated because of the requirements of fuel purity (no CO2) and of the corrosive liquid electrolyte. Efficiencies of more than 60 % have been achieved with clean hydrogen and oxygen and noble electrode materials. The AFC was demonstrated to also work with a hydrogen-air system, in a 1 kW stack in Japan and in a 6 kW Russian system with more than 5(XX) h lifetime [34]. PEFC and AFC may play a... 

Alkaline fuel cells (AFC) using aqueous KOH as electrolyte were the first type of fuel cells with practical applications at the beginning of the last century [9] but the formation of carbonate in the liquid electrolyte due to the CO2 contamination in the oxidant gas stream has limited its application to systems miming with pure oxygen, such as the fuel cells used by the NASA in the I960 Apollo space program, and currently used in the shuttle missions [10]. 

Electrochemical Impedance Studies of AFC Cathodes. A review of the state of the art of alkaline fuel cell is given by McLean et al. [2002]. The electrolyte in AFC is a concentrated KOH solution, 25-50 wt% when operated at 120°C or below and in older AFC systems, operated at 250°C a 85 w/o KOH solution was used. The AFC was first fuel cell developed to technical maturity, based on the fundamental research of Bacon [1952] and has been employed extensively in the NASA space programs Apollo program (1960-1965) and space shuttle program (since 1981). 

In Chapter 10, the authors will demonstrate the preparation techniques for ASPEM and the characterization results. The relationship between structure and properties will be discussed and compared. The double-layer carbon air cathodes were also prepared for solid-state alkaline metal fuel cell fabrication. The alkaline solid state electrochemical systems, sueh as Ni-MH, Zn-air fuel cells, Al-air fuel cells, Zn-Mn02 and Al-Mn02 cells, were assembled with anodes, cathodes and alkaline solid polymer electrolyte membranes. The electrochemical cells showed excellent cell power density and high electrode utilization. Therefore, these PVA-based solid polymer electrolyte membranes have great advantages in the applications for all-solid-state alkaline fuel cells. Some other potential applieations include small electrochemical devices, sueh as supercapacitors and 3C electronic products. 

Venkatesan, S., Gradinarova, L., Menjak, A., and Wang, H. (2004) Alkaline fuel cell pack with gravity fed electrolyte circulation and water management system. US Patent Application 2004-0161652. 

Although for a long period the focus of research and development in low-temperature fuel cells was on polymer electrolyte membrane (PEM)-based systems working with a solid acid electrolyte (at least for the intended daily-life applications such as cars and portable electronics), recently there has been a clear tendency towards the development of alkaline fuel cells (AFCs) [2-4], which was... 

The alkaline fuel cell (AFC) uses an alkaline electrolyte such as potassium hydroxide (usually in a solution of water) in order to operate. AFC systems are classified as low-temperature fuel cells and usually operate between 60°C and 90°C. AFCs use a variety of metals to speed up the reactions at the anode and cathode, with Nickel being the most commonly used catalyst. 

Priestnall et al. (2002) proposed a new concept of MR fuel cells called compact mixed-reactant (CMR) fuel cells, based on the use of a structure containing porous flow-though electrodes. Such a structure can be a single cell or stacks of cells connected in series or parallel. In their initial proof-of-principle experiments, the authors used an alkaline fuel cell system with an electrolyte 0.008 M NBH4 in 10 M KOH saturated with air. 

Figure 5.3 Diagram of an alkaline fuel cell with mobile electrolyte. The electrolyte also serves as the fuel cell coolant. Most terrestrial systems are of this type. 

This mobile electrolyte system was used by Bacon in his historic alkaline fuel cells of the 1950s and in the Apollo mission fuel cells. It is almost universally used in terrestrial systems, but the Shuttle Orbiter vehicles use a static electrolyte, as described in the next section. 

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