Half-cell PEMFC

Ref. Year Precursor materials Half cell RRDE PEMFC ESR IR Mossbr NMR Raman SIMS UV XAS XPS XRD SEM TEM Elemental Thermal... 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

If the results from the half-cell measurements are matched in the actual fuel cell operation, the DOE goals to reduce the Pt loading of the fuel cell by lOX by 2008 will be met. The decreased amount of Pt in the fuel cell will also result in a significant decrease in the cost of the PEMFCs. 

From the second half of twentieth century, acidic (proton exchange) Polymer Electrolyte Membrane Fuel Cells (PEMFC) have attracted much attention due to their potential as a clean power source for portable applications (alcohol feed). 

At the anode of a PEMFC, hydrogen is oxidized, creating protons and electrons. The polymer membrane provides proton-conducting pathways, whereas the electrons are forced through an external circuit by a potential difference between the anode and cathode. Within the CCL, oxygen is reduced to water in the presence of protons and electrons. The respective half-cell reactions, typically catalyzed by cost-intensive platinum, are... 

Fuel cell electrodes can be examined for their electrocatalytic behavior by ex situ or in situ voltammetry tests. In the case of ex situ tests, also known as half-cell tests, the properties of the electrode are evaluated using a standard three-electrode cell where an aqueous solution (e.g., perchloric acid) simulates the proton-conducting electrolyte in a PEMFC. Half-cell tests are a convenient and relatively fast method of screening electrocatalysts however. 

Fuel cells are electrochemical cells where the chemical energy of the fuel was converted into electricity for power generation with high efficiency [1,2]. Industrial purified hydrogen and air are often used in fuel cells to eliminate any pollution or emission, which is known as proton exchange membrane fuel cells (PEMFCs). In a typical PEMFC, a steam of hydrogen is deUvered to the anode side of the membrane electrode assembly (MEA) [3,4], At the anode, it is catalyzed by platinum (Pt) and split into protons and electrons. This oxidation half-cell reaction is represented as follows ... 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

Table 4 and Fig. 18 illustrate the performance levels achieved by the active players in DMFC R D. The main goal in DMFC research in the U.S. and European programs is to achieve a stable performance level of 200 mW/cm at a cell potential of 0.5 to 0.6 V. It is because of the relatively low activity of the electrocatalyst for methanol electrooxidation that this power level is less than half that of a PEMFC with Hj as a fuel. A higher power level of the DMFC is essential for a transportation application, but the present power level goal is quite adequate for small portable power sources. 

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. 

Today, almost half of all efforts worldwide to develop fuel cells for portable equipment come from North America, which has to do with the relatively large resources made available by the U.S. military (see Crawley s review, 2006). Japan carries a share of about 20%, the support coming from the many large companies making portables Sony, Casio, Fujitsu, Hitachi, NEC, Toshiba, and others. Considerable contributions come from other countries, such as South Korea (Samsung), France, Italy, Germany, and some others. In almost 90% of all such developments, the fuel cells involved are those of the PEMFC and DMFC type (at approximately equal shares). Mini-fuel cells based on SOFC constitute a minor part they are of interest primarily for military purposes. 

The fuel cell stacks (5) are arranged as two units connected in parallel. Each half consists of 10 stacks, each of power 13kW. The maximum electrical power is thus 260 kW. The cells are operated at 90°C, and both the fliel and the air are humidified, as explained in Sections 4.4.5 and 4.4.6 of Chapter 4. The operating pressure of the stacks is increased when higher power is needed, up to a maximum of 207 kPa above ambient pressure. Each half unit probably consists of about 750 cells in series. They are constructed in the mainstream fashion for PEMFC, that is, with graphite, or graphite/polymer mixture, bipolar plates. They are water-cooled, as we would expect from Section 4.5.3. 

---