Principle of Low‐temperature Fuel Cells Using an Ionic Membrane

Abstract

Low-temperature fuel cells (LTFC), such as a proton exchange membrane fuel cell (PEMFC) or an alkaline fuel cell (AFC), fed either with pure hydrogen or with hydrogen contained in a reformate gas, which can be produced for example, by steam methane reforming, will greatly improve the air quality of our environment, since these devices do not producegreenhouse gases (CO2) or polluting gases (NOx,SOx, etc.), unlike internal combustion engines that still use fossil fuels. In particular, hydrogen/airPEMFCs, working at relatively low temperatures (ranging from ambient to 70–80 °C), are becoming a mature technology for powering electricalvehicles with an autonomy range approaching 600 km without hydrogen refueling or for stationary power plants with relatively good energy efficiencies: 40–55% in electrical energy to 80–95% in total energy (electricity + heat production in combined heat and power systems), depending on the applications and the working conditions. On the other hand, the direct alcohol fuel cell (DAFC), the architecture of which is similar to that of aPEMFC, but which is fed either with methanol or with ethanol, directly converts the chemical energy of alcohol combustion with oxygen into electrical energy. These fuel cells are particularly suitable for portable electronics either to recharge their lithium battery or to power them directly. The development and commercialization ofLTFCs still need great improvement of their electric characteristics that depend mainly on the properties (activity, stability, and availability) of the electrocatalysts used to activate the electrochemical reactions involved: electrooxidation of the fuel (hydrogen, alcohol, etc.) and electroreduction of oxygen. In order to evaluate the energy efficiencies and the role of electrocatalysts, the thermodynamic data of the combustion reaction of these fuels with oxygen will be first presented. Then the electrical characteristicsEcell(j) of the fuel cell, that is, the cell voltageEcell versus the current densityj, will be established using the Butler–Volmer law for charge transfer overpotential and the Fick's laws for concentration overpotential. The effects of the properties of the fuel cell components on their electrical characteristics will be discussed, particularly the key role of the catalytic behavior of electrodes. Finally, two typical examples ofLTFCs under development and commercialization will be given, the first one concerning the direct methanol fuel cell (DMFC) for portable electronics and the second one theH2/airPEMFC for theelectrical vehicle.