Fuel Cell Systems Explained - from and for SET students

Fuel cells can be used successfully in CHP plants or in mobile power systems, like vehicles orelectronic equipment.2. Efficiency and open circuit voltgage2.1 Energy and the EMF of the hydrogen fuel cellThe chemical energy in and output of a fuel cell is not easily derived. Parameters as enthalpy,Hemholtz function, Gibbs free energy or exergy are used. Gibbs free energyenergy available to do external work, neglecting any work done by changes in pressureand/or volume. In case of a fuel cell are the moving electrons in the electric circuit meantwith external work. Exergyis all external work that can be extracted, including that due to pressure and volume changes EnthalpyGibbs free energy plus the energy connected to the entropy.The chemical energy has similarities to mechanical potential energy.1. Reference energy level can be defined as almost anywhere. Working with chemical reactionsthe reference level with zero energy is defined as pure elements, in the normal state, atstandard conditions (25°C or 298.15K, 0.1MPa).Using this convention, the terms 'Gibbs free energy of formation', G f , and 'enthalpy offormation' is used. In case of a hydrogen fuel cell operating at standard temperature and pressurethe Gibbs free energy of formation at the input is zero (useful simplification).2. Energy difference between in and output is the important value. In a fuel cell, the change ofthe Gibbs free energy of formation, ΔG f , is the energy released. Given by the equationΔG f = G f of products - G f of reactantsIt is convenient to use the molar specific Gibbs free energy of formation, written g̅ f.