Although there is plenty of potential for fuel cells to produce electricity from chemical energy, there are still many processes occurring within a fuel cell stack that we do not fully understand and cannot adequately predict. The need for fuel cell diagnostic techniques is critical in ensuring high performance is achieved.
There are a variety of techniques that are used to check fuel cell performance while the stack is running normally or offline. These techniques range from current interruption which shows the magnitude of ohmic and charge transfer resistances to neutron imaging which can indicate water concentration in flow channels. The most common diagnostic tool is Electrochemical Impedance Spectroscopy (EIS) which is an electrical method which infers various performance losses based on voltage and current variations.
Performance is dependent on many parameters both in design and operation of the fuel cell. A common way of showing performance losses is with a plot of the potential against the current during operation (known as a V/i curve). The three main performance loss regions on the V/i curve are the activation, ohmic and mass transport regions. Outlined below are some of the factors which affect the losses in each region:
Activation Region (I)
- Catalyst Type, Age, and Morphology
- Impurities in Gases or Catalyst
Ohmic Region (II)
- Material Resistances
- Stack Compression
- Membrane water content
Mass Transport Region (III)
- Reactant Concentration
- Gas diffusion
- Water accumulation in Gas Diffusion Layer
- Surface blockages
- Inert Gas Build up