Measures of Reactant Utilization Efficiency

There are various metrics utilized to quantify the efficiency of different aspects of an electrochemical reaction. One type of efficiency for a purely electrochemical reaction is based on species consumption. For a galvanic process, there will be a minimum amount of reactant required for a given reaction, as calculated by Faraday s law, Eq. [2.33]. In practice, we are not constrained to provide exactly the minimum amount of reactant. For a given current, there is a calculated minimum amount of reactant, but there is no maximum. The actual flow rate of reactants is a function of the pumps and blowers that are used for reactant dehvery. Obviously, the more flow delivered, the higher the parasitic power required, so we generally seek to dehver something close to the minimum requirement. The Faradic efficiency is a measure of the percent utilization of reactant in a galvanic process  

Faradic efficiency is often called the fuel utilization efficiency (Hf) when applied to the fuel in a galvanic redox reaction  

For an electrolytic process, some side reactions and inefficiencies may occur and result in less than complete conversion. The current efficiency is defined as  

Stoichiometric Ratio In fuel cell parlance, the term stoichiometry is defined as the inverse of the Faradic efficiency. Smdents may be confused with this terminology, since the stoichiometric condition typically describes a balanced chemical reaction equation with no excess oxidizer. Here, the term stoichiometry is used slightly differently, and its meaning is similar to the definition of equivalence ratio used in combustion. Unlike chemical reactions, the reduction and oxidation reactions are separated by electrolyte, so each electrode can have a discrete stoichiometry  

To avoid confusion the reader should be aware that other symbols for stoichiometty, besides A, are commonly used in the literature, including f and The theoretical rate of reactant required is calculated by Faraday s law, and the actual rate of reactant dehvered is a funchon of the fuel or oxidizer delivery system. One important point is worth mentioning Fuel cells must always have an anode and cathode stoichiometry greater than 1. For a value less than unity, the current specified could not be produced. For reasons explained in Chapter 4, a stoichiometry of exactly 1 is not possible either, so that a Faradic efficiency of 100% is not possible on the anode or cathode for a single pass of reactant.  

---