Fuel cell performance Carbonate

Virdis, B., Rabaey, K., Yuan, Z.G., Rozendal, R.A., and Keller, J. (2009) Electron fluxes in a microbial fuel cell performing carbon and nitrogen removal. Environ. Sci. Technol, 43 (13), 5144-5149. 

Fuel Cell Energy presented a computer model for predicting carbonate fuel cell performance at different operating conditions. The model was described in detail at the Fourth International Symposium on Carbonate Fuel Cell Technology, Montreal, Canada, 1997 (93). The model equations are listed as follows ... 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

As discussed previously, a number of different materials have been considered as potential candidates to be used as diffusion layers in PEMFCs and direct liquid fuel cells (DLFCs). The two materials used the most so far in fuel cell research and products are carbon fiber papers and carbon cloths, also known as carbon woven fabrics. Both materials are made from carbon fibers. Although these materials have been quite popular for fuel cells, they have a number of drawbacks—particularly with respect to their design and model complexity—that have led to the study of other possible materials. The following sections discuss in detail the main materials that have been used as diffusion layers, providing an insight into how these materials are fabricated and how they affect fuel cell performance. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Campbell, Chisham, and Wilkinson [121] found that the catalyst utilization in the electrode and fuel cell performance could be improved by making the carbon-supported catalyst hydrophilic. This was done by treating the carbon-supported catalyst with a suitable acid such as nitric acid in order to introduce the surface oxide group on the carbon. In principle, this same approach could be applied to the carbon components of the DL and MPL. 

In addition to the way in which the MPL is manufactured, other MPL parameters directly affect fuel cell performance. These include fhickness of fhe MPL, carbon loading, PTFE content, type of carbon parficles, efc. The following subsection will briefly discuss them. 

DL thicknesses on overall fuel cell performance and concluded fhaf fhe performance of DLs wifh MPL increased considerably when the MPL thickness was increased from 7.5 to 17.5 pm. Their explanation was that very thin MPLs provide poor electrical contact between the CL and the current-collecting FF plate because the electrical resistance is increased due to the roughness of fhe carbon clofh DL. If fhe MPL is too fhin, fhe amount of carbon/PTFE is insufficienf to provide good elecfrical confacf for the collection of fhe currenf generated in fhe fhree-phase reaction zone of fhe catalyst layer. 

Figure 4.19 shows results by Park, Lee, and Popov [155] for the effect of carbon loading in the MPL on fuel cell performance. It can also be seen that a fuel cell without an MPL performs very poorly compared to the others. Jian-hua et al. [137] recently presented very similar results. 

Anfolini et al. [161] also compared SAB and Vulcan XC-72 as possible candidates in MPLs, but in this case they used carbon cloth DLs with two MPLs. From their results, it was concluded that the Vulcan XC-72 gave slightly higher electrocatalytic activity for fhe ORR. On the other hand, MPLs near the FF that used SAB had better performance. Thus, it was suggested that for improved fuel cell performance af high pressures (around 3 atm), the ideal cathode MPL compositions would use the Vulcan XC-72 in the MPL next to the CL and SAB in the MPL next to the flow fields. 

S. Park, J. W. Lee, and B. N. Popov. Effect of carbon loading in microporous layer on PEM fuel cell performance. Journal of Power Sources 163 (2006) 357-363. 

Figure 20. Fuel cell performance demonstration for the Battelle methanol processor and the carbon monoxide removal reactor.

Recent kinetic studies indicate that carbon corrosion can be significant under normal transient operation.56,57,60-62 The rate of voltage change, common in the automotive application, enhances cathode carbon-support corrosion.16 Hence, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling. Currently, the relationship between fuel cell performance decay and accumulated carbon-support loss is only empirical.22 More effort has to be made to incorporate mechanisms that can accurately quantify voltage decay with carbon-support loss.31,32... 

In recent decades, research has intensified to develop commercially viable fuel cells as a cleaner, more efficient source of energy, due to the global shortage of fossil fuels. The challenge is to achieve a cell lifetime suitable for transportation and stationary applications. Among the possible fuel cell types, it is generally believed that PEM fuel cells hold the most promise for these uses [10, 11], In order to improve fuel cell performance and lifetime, a suitable technique is needed to examine PEM fuel cell operation. EIS has also proven to be a powerful technique for studying the fundamental components and processes in fuel cells [12], and is now widely applied to the study of PEM fuel cells as well as direct methanol fuel cells (DMFCs), solid oxide fuel cell (SOFCs), and molten carbonate fuel cells (MCFCs). 

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

Figure 3.51. Single PEM fuel cell performance. Current-voltage relationship (open symbols) and implied power density (filled symbols), are shown for different operational temperatures in the range of 45-90°C, for a cell with a catalyst layer with incorporated PTFE (polytetrafluoroethene) to reduce water flooding, a low loading of a carbon-supported Pt catalyst layer (120 pg Pt cm ) and finally Nation intrusions. (From Z. Qi and A. Kaufman (2003). Low Pt loading high performance cathodes for PEM fuel cells. /. Power Sources 113,37-43. Used with permission from Elsevier.)...

The performance of a catalyst is determined by its intrinsic activity and dispersion. This latter is defined as the ratio between the number of surface and bulk atoms. Metal nanoparticles supported on a high-surface-area carbon are thermodynamically unstable and give rise to sintering phenomena upon thermal treatment. In this regard, the influence of the nature of the support on the properties of dispersed catalysts has been continuously investigated [2]. For fuel cell catalysts, carbon support provides a framework that allows electron conduction and enhances the dispersion of the active phase. 

Designing alloy electrocatalysts by the so-called ad-atom method, and by alloy sputtering for oxidation of CH3OH and CO, and for CO tolerance in H2 oxidation, respectively, as well as for O2 reduction are discussed. Many years of experience are summarized and collaborations with other groups are highlighted. The particle size effect in electrocatalysis by small particle electrodes, and the effect of corrosion of carbon-black supported nanoparticles on the electrocatalytic activity are also discussed. All these factors, as well as catalyst lifetimes, are very important in fuel cell performance and in the final cost estimates for the practical fuel cell applications. 

To prolong the life of MCFC, the amount of electrolyte in the matrix must be maintained at an appropriate level over long-term operation. The growth of particles of L1A102 as an electrolyte retention material in molten carbonates leads to a decrease in the electrolyte retention ability. These phenomena result in a decrease of the fuel cell performance. It was found that zirconia powder added to lithium aluminate keeps the electrolyte retention ability constant for over 7000 hr in Li/Na carbonates and pCOx = 0.1. ... 

Subramanian, N. Haran, B.S. Ganesan, P. White, R.E. Popov, B.N. Analysis of molten carbonate fuel cell performance using a three-phase homogeneous model. J. Electrochem. Soc. 2003, 150 (1), A46-A56. 

Iwase, Y. Okada, H. Kuroe, S. Mitsuishima, S. Takeuchi, M. Enhancement and stabilization of the molten carbonate fuel cell performance by optimization of electrode pore distributions. Denki Kagaku 1994, 62 (2), 152-157. 

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