A fuel cell is an electrochemical device used to create electricity through a reaction between a fuel (such as hydrogen) and an oxidant (such as oxygen) in the presence of an electrolyte. In addition to producing electricity, the reaction generates by- products, which typically are only water and heat. Therefore, using fuel cells is an environmentally friendly way to produce electricity. If you are involved in any of the various stages of developing fuel cells, you need to draw current from your fuel cell or fuel cell stack. In addition, measuring the AC impedance of your fuel cell can help you identify the kinetic and ohmic resistances, which can uncover effi ciency issues by revealing internal cell losses. Measuring impedance across a range of frequencies (electrochemical impedance spectroscopy or EIS) can help you identify problems with the fuel cell components or deviations in the fuel cell assembly process.