Hydrogen fuels, electrochemical oxidation

Several activities, if successful, would strongly boost the prospects for fuel ceU technology. These include the development of (/) an active electrocatalyst for the direct electrochemical oxidation of methanol (2) improved electrocatalysts for oxygen reduction and (2) a more CO-tolerant electrocatalyst for hydrogen. A comprehensive assessment of the research needs for advancing fuel ceU technologies, conducted in the 1980s, is available (22). 

Grove recognized that electrodes above the surface of an electrolyte, (e.g., sulfuric acid) would be wetted by capillary action and so allow the platinum electrodes to catalyze the electrochemical reactions of a fuel and oxidant stich as hydrogen and oxygen. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

The principle of the fuel cell was first demonstrated by Grove in 1839 [W. R. Grove, Phil. Mag. 14 (1839) 137]. Today, different schemes exist for utilizing hydrogen in electrochemical cells. We explain the two most important, namely the Polymer Electrolyte Membrane Fuel Cell (PEMFC) and the Solid Oxide Fuel Cell (SOFC). 

Fuel cells are electrochemical devices transforming the heat of combustion of a fuel (hydrogen, natural gas, methanol, ethanol, hydrocarbons, etc.) directly into electricity. The fuel is electrochemically oxidized at the anode, whereas the oxidant (oxygen from the air) is reduced at the cathode. This process does not follow Carnot s theorem, so that higher energy efficiencies are expected up to 40-50% in electrical energy and 80-85% in total energy (heat production in addition to electricity). 

One of the fundamental requirements for a high performance anode is to have excellent catalytic activity toward the electrochemical oxidation of the fuel (e.g., hydrogen). This is reflected as low anode polarization or interfacial resistance. This area has seen intensive research for quite some time and is covered very well by the reviews of McEvoy [2], Zhu and Deevi [3], and Jiang and Chan [4], In this section, the focus will still be on revealing the influences of processing and testing parameters on the obtained anode electrochemical performance. 

Yates C and Winnick J. Anode materials for a hydrogen sulfide solid oxide fuel cell. J Electrochem Soc 1999 146 2841-2844. 

It is widely agreed that carbon monoxide (CO) is a m u or adsorbate when fuels such as methane, methanol, fi>rmaideh3rde, formic add, and higher molecular wei t hydrocarbons are electrochemically oxidized. CO is also found in a gas reformed from hydrocarbon for hydrogen- oxj en fuel cells. In both cases, it is certain that CO has inhibiting effects on the oxidation of the foels. For this reason, extensive works have been done. ... 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

As shown in Figure 18, the potential is almost proportional to the logarithm of H2 concentration diluted in air. When H2 is diluted in N2, the observed potential corresponds to the electromotive force of a H2-02 fuel cell, and in fact the EMF was as large as about 1.0 V with a theoretical slope of 30 mV/decade, as shown in the same figure. It has been shown that in the case of H2 diluted in air, the following electrode reaction, i.e., electrochemical oxidation of hydrogen (2) and electrochemical reduction of oxygen (3), are important. 

Figure 3.3.2 Principle of a fuel cell as an electrochemical energy conversion device. Inside a fuel cell, fuel, e.g. hydrogen, and an oxidant, typically oxygen, combine electrochemically to form products, e.g. water, and electricity and some excess heat (not shown). Figure adapted from ref. [4]...

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