Fuel cell, incomplete

Role of Incomplete Oxidation Products for Fuel Cell Applications... 

The hnding of very substantial amounts of incomplete oxidation products for methanol and formaldehyde oxidation can have considerable consequences for technical applications, such as in DMFCs. In that case, the release of formaldehyde at the fuel cell exhaust has to be avoided not only from efficiency and energetic reasons, but in particular because of the toxicity of formaldehyde. While in standard DMFC applications the catalyst loading is sufficiently high that this is not a problem, i.e., only CO2 is detected [Arico et al., 1998], the trend to reducing the catalyst loading or applications in micro fuel cells may lead to situations where the formation of incomplete oxidation products could indeed become problematic (see also Wasmus et al. [1995]). For such purposes, one could dehne a maximum space velocity above which formation of incomplete oxidation products may become critical. 

Finally, we have discussed the effect of incomplete Cj oxidation product formation for fuel cell applications and the implications of these processes for reaction modeling. While for standard DMFC applications, formaldehyde and formic acid formation will be negligible, they may become important for low temperature applications and for microstructured cells with high space velocities. For reaction modeling, we have particularly stressed the need for an improved kinetic data base, including kinetic data under defined reaction and transport conditions and kinetic measurements on the oxidation of Ci mixtures with defined amounts of formaldehyde and formic acid, for a better understanding of cross effects between the different reactants at an operating fuel cell anode. 

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. 

All the other gaseous reactant fuel cell systems are incomplete in that they do not have circulators, and move reactants and products by irreversible diffusion. Systems such as the proton exchange fuel cell and its companion the direct methanol fuel cell are doubly incomplete, since they lack a hydrogen mine which can produce cheap hydrogen and cheap methanol from natural gas (see Section A.1.4 and Eigure A.4, and Barclay, 2002). 

Currently, the stationary power market penetration of the fuel cell is based on reduced local pollution rather than superior performance. Indeed the internationally demonstrated Ballard fuel cell bus generates more pollution from the power plant stack which generates its hydrogen supply from an inefficient incomplete electrolyser, than it saves by emitting steam from its exhaust. The industry has to rescue itself from this untenable position. 

Systems to be described. One main objective of the book has evolved to become the presentation of a limited, rather than comprehensive, list of incomplete fuel cells, as available at the time of writing. There is small value in an omnivorous approach, because no present-day fuel cell without circulators has long-term... 

The placing into administration of ZEVCO, the Zero Emission Vehicle Company, in the latter part of 2001, encouraged the author not to write a chapter on the incomplete alkaline fuel cell. However, there is a transatlantic revival of this incomplete system at present (end of 2004) which the author has ignored as ephemeral. 

The USA, Germany and Japan are large players in the business of the incomplete MCFC. Fuel Cell Energy, USA, its licensee MTU Friedrichshafn, and the Mitsubishi Materials Company all have to face... 

The most famous blues singer of them all had a void to fill. In the fuel cell industry the author finds all fuel cells to be incomplete and imperfect, with only the power storage system of the last chapter being an approach to completeness. This chapter goes into detail on the shortfalls of present-day fuel cell systems. 

Sulzer Hexis went into bankruptcy in late 2005, so the following account is history. The Sulzer Hexis fuel cell was exclusively aimed at the domestic market (Batawi 1996 1999 2001 Schuler, 2001 Ballhausen, 2001). After a long development history the project matured as the Sulzer Hexis Premiere, an incomplete fuel cell, calculated using calorific value theory. 

The work reported by Ralph etal. (2003) is a well-rounded, self-contained essay on the DMFC. (See DMFC flow sheet in Figure 6.6.) Moreover, because Ballard/Johnson Matthey did not contribute on fuel cells at the Palm Springs Fuel Cell Seminar in 2002 (see below), Ralph etal. (2003) is the current information source, additional to the patents in the list of references. Note that the methanol-water mixture presents to the fuel electrode its associated methanol vapour pressure. The DMFC does not have an incompressible fuel. The cell needs circulators. It is incomplete. 

Complete fuel cells are engineered for isothermal oxidation by the addition of perm-selective membranes and isothermal concentration cells. They would, if developed, generate much higher Nernst potential difference than an existing incomplete fuel cell. That would give a chance for fuel cells to draw level, a chance the industry does not seem to understand. 

In Donelson etal. (1998) Ceramic Fuel Cells Ltd, of Australia, gave itself a target of A 1500/kW for SOFC stationary power installations of 200 kW. A rough translation is 600/kW, or US 960/kW of incomplete cell. 

The Ricardo conclusion is not out of line with a US paper (Williams, 2002) by TIAX LLC of Cupertino, CA, which omits to give its full name, but is a successor to the Technology and Innovation Business of A D Little. The paper concludes that (incomplete) hydrogen fuel cell vehicles will have a 1-2000 per annum operating cost penalty (from high capital cost), relative to engine-driven vehicles. 

Predictions by several fuel cell organisations for incomplete systems are not in unison, but all see economic improvements coming from mass production, notably ITM. That will be very necessary to meet the intense competition as improved vehicles with new engine schemes enter the market. The industry needs complete fuel cells to achieve competitive performance and any kind of mature economics. The difficulty of the situation is highlighted by the fuel cell bus which saves local pollution on the road, but generates at the power plant stack more pollution... 

Figure 3.4. Fuel cell negative electrode potential 0, and positive electrode potential 0., as a function of current. The main cause of the diminishing potential difference A0., for increasing current is at first incomplete electrocatalysis at the electrodes, for larger currents also ohmic losses in the electrolyte solution, and finally a lack of ion transport (cf. Bockris and Shrinivasan, 1969). From B. Sorensen, Renewable Energy, 2004, used by permission from Elsevier.

Other hand, aim at the formation of useful chemicals, with possible energy recovery or conservation as a bonus (16, 17, 33, 52-54. Thus, complete oxidation of hydrocarbons to COj would yield maximum energy output and is considered to be a fuel cell process. Incomplete, selective oxidation of an alkene to an aldehyde, however, would qualify as an electrogenerative process, since energy is sacrificed for the sake of production of an intermediate oxidation product. 

Oxidation of hydrocarbons and alcohols If reasonably effective oxidation catalysts can be identified for aqueous electrolytes, hydrocarbon and alcohol oxidation processes would make possible promising fuel cells operating directly on quite practical fuels at moderate temperatures. The currently used platinum and platinum-family metals and alloys have substantial activity, but it is not sufficient for practical fuel cells with aqueous electrolytes. With the many electrons involved in the complete oxidation, the detailed mechanisms for the oxidation are likely to be quite complex. To avoid incomplete oxidation it is probably necessary to have the reactants remain adsorbed on the electrode surface through the complete oxidation to C02 and H20. Here again, new promising catalysts and new experimental approaches are needed. 

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