Reactant utilization efficiency, measures

In PEM fuel cells, uniformity of the current density across the entire active area is critical for optimizing the fuel cell performance. A non-uniform current density in the fuel cell can drastically affect different parameters of the fuel cell, such as reduced reactant and catalyst utilization along the active area, decrease in total efficiency and lifetime, and durability failure modes. Thus, determination of the current density distribution information is vital for designing PEM fuel cells that achieve higher performance and longer life.130 A number of methods for measuring current distribution in PEM fuel cells have been demonstrated the following sections discuss some of these methods in further detail. 

It is instructive to compare the atom economies of the two pathways. Atom economy is a measure of the efficiency of a chemical process, defined in percentage terms as x (formula wt. of atoms utilized)/(formula wt. of all reactants). For the old six-step ibuprofen synthesis the atom economy was only 40% (with MeC02H, EtOH, NaCl, Et0C02H, 2H2O and NH3 as waste). This is dramatically improved to 77% for the new three-step route with only MeC02H as a by-product from the first step. Recovery and use of this increases the atom economy to 99%. Additionally, the catalytic amounts of HF and Pd complex used in the BHC process are recovered and reused, whereas stoichiometric quantities of AICI3 hydrate were produced as waste by the old route. 

A central issue in making chemistry greener is the efficiency of incorporating the reactant atoms into the product compound. Any atoms that are used in the reaction but are not part of the product are wasted, and this waste must subsequently be disposed of. If chemists can design reactions that do not have wasted atoms, there are no disposal issues and no potential for environmental impact. This type of efficiency can be measured in terms of percentage atom utilization ... 

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 ... 

The Faradic efficiency is a measure of the percent utilization of reactant in a galvanic process... 

From an environmental and economic point of view, it would be useful to know how many atoms present in the reactants end up in the desired products and how many end up as waste. This concept was developed by Barry Trost from Stanford University and published in Science in 1991. The concept of atom economy or atom utilization allowed Barry Trost to begin to change the way in which chemists measure the efficiency of the reactions they design (award ceremony by Paul Anderson 1997 ACS President). Professor Trost received the Presidential Green Chemistry Challenge Award in 1998. 

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