Energy converter, electrochemical efficiency

The intrinsic performance advantages of hydrocarbon-based energy conversion systems are formidable. Currently, electrochemically based energy converters have found application in limited, niche markets. Electrochemical processes are frequently advantageous in terms of intrinsic efficiency, process control, and pollution reduction. Many systems await advances in electrocatalysis and materials. 

It is in this sense it is said that in an electrochemical energy converter, the ideal maximum efficiency is 100% for, as in the above idealized situation, if one could carry out reactions in such a way that the electrode potentials were infinitely near the equilibrium values, the electrical energy one could draw2 from the reaction would be nFVe and this is all of the free-energy change AG, which is the maximum amount of useful work one can obtain from a chemical reaction. 

Fig. 13.9. Efficiency vs. current density relations for an electrochemical energy converter (calculated for curve 1 of Fig. 13.5).

The distinction between the situation in which one needs predominantly high efficiency and one in which one wants high power becomes clear when one compares the P vs. I and e vs. I curves (Fig. 13.11). When its efficiency is at a maximum, the electrochemical energy converter is a less good power source. As the current density is increased, the power output increases, but the efficiency decreases. Of course, at the highest current drains, both the power and efficiency fall toward zero. 

However, there are electrochemical energy converters (fuel cells), such as the one shown in Fig. 1.6. An electrochemical energy converter is not restrained by the Carnot efficiency of 25% and can have efficiencies up to AH/AG for the heat content and free energy changes in the oxidation reactions involved in digesting food. This ratio is often as much as 90% (cf. Chapter 13). 

The need to convert chemical energy to electricity, efficiently and at low temperature, has increased the development of materials with electrocatalytic activity toward multi-electron charge transfer. Reactions of technical relevance are, for example, cathodic processes, such as the oxygen reduction reaction (ORR),3 13 and anodic processes, such as small organics (R-OH, where R = CH3-3-12 or CH3CH-13-17) and sugars.18-20 These complex electrochemical processes are useful in low-temperature systems such as the direct methanol FC (DMFC) or biofuel cell systems. 

For a fuel cell the maximum thermodynamic efficiency can be calculated from the Gibbs energy (AG) and the enthalpy change (AH) of the electrochemical reaction. Ideally, the free energy of the reaction can be completely converted into electrical energy and the efficiency e is given by ... 

Fuel cells are power generation devices converting chemical energy into electric energy by electrochemical reactions. A typical fuel cell is comprised of two electrodes separated by an electrolyte, with a provision of reactant supply and product removal. Among various types of fuel cells, Ha-O -based polymer electrolyte membrane (PEM) fuel cells (PEMFC) have attracted special attention due to their high efficiency, low temperature operation and suitability for low to medium power generation. Basic components of a PEMFC are PEM, catalyst layer, gas diffusion layer and... 

Fuel cells are efficient energy converters, based on electrochemical principles. They convert the chemical energy (heating value) of a fuel directly into electricity, circumventing the various steps of thermal conversion and electricity generation. 

Fuel Cell Catalysts. Euel cells (qv) are electrochemical devices that convert the chemical energy of a fuel direcdy into electrical and thermal energy. The fuel cell, an environmentally clean method of power generation (qv), is more efficient than most other energy conversion systems. The main by-product is pure water. 

Because batteries direcdy convert chemical energy to electrical energy ia an isothermal process, they are not limited by the Carnot efficiency. The thermodynamic efficiency S for electrochemical processes is given by ... 

Electrochemical power sources differ from others, such as thermal power plants, by the fact that the energy conversion occurs without any intermediate steps for example, in the case of thermal power plants fuel is first converted in thermal energy, and finally electric power is produced using generators. In the case of electrochemical power sources this otherwise multistep process is achieved directly in only one step. As a consequence, electrochemical systems show some advantages, such as energy efficiency. 

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