PEFC Irreversibility

Some types of PEFC employ bipolar steel plates with carefully machined gas grooves to convey a uniform (or desired) gas flow per unit area to the reaction zone. The current of electrons from the tops of the upstands or groove walls into the same area of the reaction zone is influenced by surface films. The same comments apply to titanium flow plates. The optimisation of this situation is, at the same time, the minimisation of a local irreversibility, namely electrical resistance. 

at ambient temperature, the slow growth of a crystal is attempted, then, against a background of thermal agitation, the systematic assembly of a perfect crystal turns out to be impossible. Dislocations and interstitial atoms are unavoidable. The second law is obeyed, in that entropy growth is unavoidable. Hence fuel cell materials are real and imperfect, and will need careful optimisation. 

A perfectly pure material would corrode. It would have a voracious, concentration-difference-driven hunger for impurities, and would be difficult to employ in practice. 

Precisely symmetrical models of aqueous electrode interfaces are often shown for educational purposes. Polar water molecules and positive ions are assembled in a repetitive array. In reality the array would be interrupted by impurities and irregularities, all thermally agitated. The precise achievement of a potential difference V , corresponding to Nernst s equation (4.1), would require microscopically uniform conditions at the interface between electrode and electrolyte, so as to avoid local variations of potential difference and circulating currents in the electrode. 

Such perfection is aided by assuming, in equilibrium calculations, a pure water electrolyte. The small self-ionisation of pure water is sufficient at zero-current equilibrium. Practical features such as strong electrolytes can be added as perturbations. 

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Under ideal operation of PEFCs, the membrane would retain a uniformly saturated level of hydration, providing the highest proton conductivity, (7p The PEM would therefore perform like a linear ohmic resistance, with irreversible voltage losses ... 

Obviously, the CCL not only determines the rate of currenf conversion and the major portion of irreversible voltage losses in a PEFC, but also plays a key role for the water balance of the whole cell. Indeed, due to a benign porous structure with a large portion of pores in the nanometer range, the CCL emerges as favorite water exchanger for PEFCs. Once liquid wafer arrives in gas diffusion layers or flow fields, PEFCs are unable to handle if. 

This is close to the simulated value of 2.57 W, including the irreversible, entropic, and Joule heat. The energy-conversion efficiency of this cell, defined as the ratio of the electric power (Jjvg VreiiA) to the total energy consumption IsvgVceuA + eat), is then calculated to be 40%. These simple calculations help demonstrate the validity and accuracy of a noniso-thermal PEFC model. 

The PEFC system itself will have to face the consequences of irreversibility due to lack of circulators. Crucially, the system must have a hydrogen mine . Perhaps Ballard will need to join with Methanex and Air Products to solve this major development and capital investment problem. There are no signs of that happening. 

The performance of the apparatus in the isothermal enclosure of the equilibrium diagram, Figure A. 1, which may be a fuel cell or an electrolyser depending on which way the equilibrium is tilted, is 1.23Vn, 237.1 AG. That equal and opposite performance could not be achieved by a PEFC with irreversible chemistry at its cathode as discussed below. 

Finally, it should be realized that CO is not the only fuel (or fuel-derived) contaminant expected to affect anode performance in the PEFC. In a test of other possible contaminants that could result, in principle, from methanol reforming, Seymour et al. [27] reported strong and irreversible effects of formic acid at a PEFC platinum (high-loading) anode, whereas methanol, formaldehyde, and methyl formate were found to have much smaller and reversible effects. The fuel impurity aspects of coupling between natural gas (or gasoline) reformers of various types and a PEFC stack are even wider, and make it essential to probe and address, either by removal upstream or by use of modified catalysts, the possible detrimental effects of low levels of sulfur, H2S, COS, and NH3 [28]. 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

Nowadays, efficiencies for automotive PEFC stacks, reported by car manufacturers, are in the range of 60% or above. In order to determine a precise value of the voltage efficiency, the irreversible heat losses Qi must be quantified. Knowing these values, we obtain the terms r]i and the corresponding values of Wout and Cceii. Therefore,... 

In spite of the progress, the ORR in the cathode still incurs about 40% of all irreversible energy losses in the cell, as well as a proportional fraction of voltage losses. Moreover, at the current mass loadings required for high cell performance, Pt is responsible for 30-70% of the total cost of a fuel cell stack, although it only amounts to about 0.1 % of the stack volume. The foremost challenge in PEFC research remains to maximize performance with a minimal amount of Pt. 

COad monolayer (ML) electrooxidation on Pt nanoparticles has a long history as a prototype electrochemical reaction. Furthermore, irreversibly adsorbed COad is a catalyst poison in PEFCs. Numerous studies have revealed strong effects of particle size and morphology on the electrocatalytic activity of COad electro-oxidation (Arenz et al., 2005 Cherstiouk et al., 2003 Friedrich et al., 2000 Maillard et al., 2004 Mayrhofer et al., 2005 Solla-Gulldn et al., 2006). 

Figure 4.40 shows the total heat flux from the CCL of a PEFC and its components. As illustrated, for a typical current density of 1 A cm", the heat flux is about 1W cm , which is close to the useful electric power density generated by the fuel cell at this current density. Further, the dominating contributions give the thermodynamic (the first term in the square brackets in Equation 4.299) and the irreversible (the second term) heating. The Joule term contribution is marginal. 

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