Equilibrium fuel cells

Isothermal chemistry in fuel cells. Barclay (2002) wrote a paper which is seminal to this book, and may be downloaded from the author s listed web site. The text and calculations of this paper are reiterated, and paraphrased, extensively in this introduction. Its equations are used in Appendix A. The paper, via an equilibrium diagram, draws attention to isothermal oxidation. The single equilibrium diagram brings out the fact that a fuel cell and an electrolyser which are the thermodynamic inverse of each other need, relative to existing devices, additional components (concentration cells and semi-permeable membranes), so as to operate at reversible equilibrium, and avoid irreversible diffusion as a gas transport mechanism. The equilibrium fuel cell then turns out to be much more efficient than a normal fuel cell. It has a greatly increased Nernst potential difference. In addition the basis of calculation of efficiency obviously cannot be the calorific value of the... 

The reader, even when reassured by the foreword, may still be reluctant, after reading Chapter 1, to forsake the ingrained notion that irreversible combustion is somehow relevant to equilibrium fuel cells. But here in ESS-RGN are fuel cells, with liquid reactants and products, and without oxygen Combustion cannot be relevant to the Regenesys system, as described below, and in the abb.com reference (Power Conversion Section, 2004) Electricity from the Store . 

Appendix A shows that for equilibrium fuel cells, high-temperature operation offers no advantage, but indeed a theoretical disadvantage. However, in a practical non-equilibrium set-up, where unused fuel and unconsumed oxygen are features and these have to be combusted in a gas turbine, the high-temperature fuel cell is likely to be at an advantage. Further discussion appears on this point in Chapters 4 and 5, in respect of the SOFC and the MCFC. The PEFC (Chapter 6) must, for... 

In Appendix A, calculations show a status, for fuel cell isothermal Faradaic oxidation, of a high vacuum of reactants relative to a high concentration of product. That calculated status cannot even be approached in the laboratory, for lack of adequate semi-permeable membranes and circulators (concentration cells). The equilibrium fuel cell of Figure A.l is dead-ended, whereas the air-breathing open-ended design must have both of its electrodes swept by a parallel flow, with an inlet and an... 

Repeating from earlier, an equilibrium fuel cell directly generates an electrical potential difference, V, proportional to the work function, the... 

A. 1.2 Steady Flow Equilibrium Fuel Cell Equilibrated via Concentration Cells with the Environment and both Internally and Externally Reversible... 

The to and fro, equilibrium homogeneous dissociation reactions, and the heterogeneous Faradaic electrode reaction, which co-exist in an equilibrium fuel cell, have the equation... 

By raising the operating pressure and temperature, the output of equilibrium fuel cells is shown to be reduced. The output includes electrical power from the fuel cell, power from the isen-tropic and isothermal circulators, and power from the Carnot cycle. No artificial credit is given for producing combined heat and power by the popular but erroneous addition of power and heat, which have different units, to produce mythical high efficiencies. The addition of similar units (e.g. power plus power, or heat plus heat) is the valid procedure, in line with Joule s irreversible experiment, from which 1W s 1J. The 3> sign can never mean =. In particular the stirred liquid of the experiment cannot be unstirred. 

To a good approximation, the temperature dependence of the equilibrium fuel cell potential is linear (Kulikovsky, 2010a) ... 

When no net current is flowing, the equilibrium potential of the fuel cell is given by the Nernst equation ... 

To be ionicaUy conducting, the fluorocarbon ionomer must be wet under equilibrium conditions, it will contain about 20 percent water. The operating temperature of the fuel cell must be less than 373 K (212°F), therefore, to prevent the membrane from drying out. 

Reactions (3.9) to (3.11) proceed rapidly to equilibrium in most anodic solid oxide fuel cell (SOFC) environments and thus H2 (Eq. 3.8) rather than CH4 is oxidized electrochemically resulting in low polarization losses. Upon doubling the stoichiometric coefficients of equation (3.8), summing equations (3.8) to (3.11) and dividing the resulting coefficients by two one obtains ... 

This is considerably higher than that of an H2-O2 fuel cell (i.e., 83%). However, under normal operating conditions, at a current density j, the electrode potentials deviate from their equilibrium values as a result of large overpotentials, r, at both electrodes (Fig. 5) ... 

Kreutz, T., Steinbugler, M., and Ogden, J., Onboard fuel reformers for fuel cell vehicles Equilibrium, kinetic and system modeling, Proc. 1996 Fuel Cell Seminar, Orlando, FL, 714, 1996. 

Gary Jacobs and Burt Davis (University of Kentucky) review catalysts used for low-temperature water gas shift, one of the key steps in fuel processors designed to convert liquid fuels into hydrogen-rich gas streams for fuel cells. These catalysts must closely approach the favorable equilibrium associated with low temperatures, but be active enough to minimize reactor volume. The authors discuss both heterogeneous and homogeneous catalysts for this reaction, with the latter including bases and metal carbonyls. 

Initially, both electrodes are at equilibrium. Since the anode has accumulated electrons and the cathode has depleted electrons, electrons begin to flow from electrode from the anode to the cathode. The thermodynamic driving force for the electron flow is the electrode potential difference, which for the fuel cell reaction is 1.23 Y at standard conditions. In addition to electron flow, H + ions produced at the anode diffuse through the bulk solution and react at the cathode. The reaction is able to continue as long as H2 is fed at the anode and 02 at the cathode. Hence, the cell is not at equilibrium. The shift in electrode potential from equilibrium is called the overpotential (>/). 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

The advantage of handling H2 in solution. In thermodynamic terms, the most efficient processes are those that are close to equilibrium. Hydrogenases catalyse the oxidation of H2 reversibly, in contrast to flames or explosions. Fuel cells can achieve higher efficiencies than heat engines. 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur (3). Carbon deposition not only represents a loss of carbon for the reaction but more importantly also results in deactivation of catalysts in the processor and the fuel cell, due to deposition at the active sites. Thermodynamic equilibrium calculations provide a first approximation of the potential for coke formation. The governing equations are ... 

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