The voltage at which a fuel cell functions is known as the operating voltage. The operating voltage can range from 0 V up to a theoretical maximum known as the Nernst Potential. The Nernst Potential is, in essence, the electrical potential difference (voltage) that would result from a given chemical reaction, such as the combination of hydrogen and oxygen, at very specific conditions. In the case of the formation of water from hydrogen and oxygen at standard temperature and pressure, the Nernst Potential is approximately 1.2 V. The voltage efficiency of a single cell can therefore be calculated by dividing the operating voltage by this voltage. For an multi-cell stack, voltage efficiency is calculated by taking the stack voltage and dividing it by the Nernst Potential multiplied by the number of cells in the stack.

Similarly, the current load on a stack is also an important parameter used to measure the performance of a fuel cell. The maximum current, or ideal current, is dictated by the amount of fuel and air supply. The ideal current can be calculated using Faraday’s current law, which states that the ideal current is proportional to the reactant flow rate multiplied by the number of electrons being transferred in the reaction. Detailed calculations will not be carried out here, but using Faraday’s current law, it can be shown that a 1 A current is equivalent to 7.5 mL/min H₂ at room temperature and standard pressure.

Based on the total current produced by the stack and fuel flow going into the stack, the fuel utilization can be calculated. Fuel utilization is defined as the ratio of actual current produced to the ideal current that could be produced from a given fuel flow rate. For instance, if the fuel flow rate is set at 15 mL/min and the current load produced by the stack is 1.5 A, the fuel utilization is 75% (1.5 A actual current / 2 A ideal current. The inverse of utilization is, mathematically, the ratio of ideal current to actual current; this is referred to as NOS or “stoics.” Air/oxygen supply is commonly considered in terms of stoics, which effectively describe the excess reactant supplied. For example, if the air is supplied at 2 stoics, this means that there is exactly twice as much oxygen flowing into the stack as is required for a given current load. For air breathing stacks, the number of stoics is not reported.