Efficiency of Galvanic Cells

The efficiency of galvanic cells, the permissible current and operating voltage, as well as the power of the cell, can all be calculated from current potential curves of the two electrode reactions. Figure 103 shows typical... 

The main attractiveness of fuel cells follows from the definitions given above. It comprises the high theoretical efficiency associated with direct conversion of chemical energy into electrical energy by means of galvanic cells [10] the selectivity of the electrochemical process and the advantage of a continuous metabolism by using the ambient air to oxidize the steadily supplied fuel. 

The heat produced on merely burning 1 mole H2, with no work obtained directly, is — AH = 68.32 kcal. This is the <7h in the expression for efficiency the heat that is put into the working substance in a heat engine when the fuel bums. If the reaction can be made to take place in a galvanic cell, the maximum electrical work available (when the cell operates reversibly) per mole Hj is —AG = 56.69 kcal. This is the w in the expression for efficiency the work done by the device. The maximum efficiency of the cell then is... 

In 1800 Volta described the first voltaic pile or galvanic cell , i.e. a source of direct electric current at low potential. The first cell was made from discs of copper and zinc separated from each other by a cloth saturated with salt solution. Ever since then, the search for better cells to supply electricity has continued. This section will start by classifying the various kinds of batteries and fuel cells, proceed to describe some of the principles of operation and define the efficiency of the cells, and then describe the general features of each kind of cell. 

This can be accomplished by applying an electrical potential in the external circuit in such a manner that an emf occurs in opposition to that of the galvanic cell. The opposing emf is varied by means of a potentiometer until the current flow from the cell is essentially zero. Under these conditions, the cell may very well approach reversibility. This is readily tested by changing the direction of the current and allowing an infinitesimally small current flow in the opposite direction. If the cell is reversible, the cell reaction will proceed in the reverse direction with the same efficiency as in the forward direction. For a reversible reaction... 

Obviously, plasmas can be used very efficiently within the synthetic approach (i), and all examples given in this paper are assigned to the synthetic approach. It is much less obvious whether plasmas can be used also in the counter-direction. In order to measure a stable and reproducible electromotive force (EMF) the corresponding electrochemical (galvanic) cell must be in (local) thermodynamic equilibrium. Low-temperature plasmas represent non-equilibrium states and are highly inhomogeneous systems from a thermodynamic point of view, often not... 

Although we can never actually realize the maximum work from a cell reaction, its value is still useful for evaluating the efficiency of a real process based on the cell reaction. For example, suppose a certain galvanic cell has a maximum potential of 2.50 V. In a particular experiment 1.33 moles of electrons passes through this cell at an average actual potential of 2.10 V. The actual work done is... 

Figure 1.5 describes basic galvanic elements discriminated according to energy density (per mass) and temperature. All those galvanic cells that directly convert chemical into electrical energy, without thermal detours, are hence not bound by Carnot s efficiency, and offer high theoretical efficiencies. The application of solid... 

Solid electrolytes can withstand higher temperatures than liquids which is important for the Carnot efficiency of a thermo-galvanic cell. In the case of devices featuring a liquid electrolyte and a redox couple, the electroactive species diffuse from one electrode to the other. To have high steady state current, diffusion gradient should be as steep as possible which means bringing the electrodes close to each other. There results an increase in thermal loss by conduction. On the other hand, with a solid electrolyte such as 3-A 20s the electroactive species migrate in the electrolyte where it is the only possible current carrier. Consequently the current will not be limited by mass diffusion but by heat diffusion in metallic electrodes or by the electrical resistance of the solid electrolyte. 

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