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Fuel Cell Research
A fuel cell is an electrochemical device which converts chemical energy stored in the hydrogen directly into electrical energy with only by-product as water. Hydrogen is used as a fuel and oxygen (air) serves as an oxidant. The basic fuel cell consists of a electrolyte with an electrode on each side. Fuel cells are classified based on the type of electrolyte used. Besides the electrolyte type, they also differ in operating temperature, application and efficiency.
In practical situations, the actual fuel cell voltage is less than the thermodynamically predicted voltage (1.23 V at 25 °C and 1 atm) due to irreversible kinetic losses. If more current is drawn from fuel cell, the cell voltage declines due to an increase in losses. In general, there are four fuel cell losses which define the current voltage characteristics of a fuel cell.
- Fuel crossover
- Activation losses
- Ohmic losses
- Concentration or mass transport losses
PEM Fuel Cell
The proton exchange membrane (PEM) fuel cell uses an ion conducting solid electrolyte membrane. The PEM fuel cell is also termed as polymer electrolyte membrane fuel cell. A solid polymer electrolyte has advantages such as high power density and less corrosion over liquid electrolytes. PEM fuel cells operate at lower temperature which allows faster start up and immediate response to changes in the load. The wide power range and versatility allows their use in large number of applications. The PEM fuel cells are seen as promising choice for transportation applications due to their power ranging up to several kilowatts with temperature only varying from room temperature to 100 °C.
PEM fuel cells use hydrogen as a fuel and oxygen, generally from air, as an oxidant to produce electricity and water. The PEM fuel cell primarily consists of a positively charged electrode, i.e. anode, a polymer electrolyte membrane and a negatively charged electrode, cathode. Hydrogen gas introduced at the anode oxidizes to protons (H+). Protons migrate through polymer electrolyte membrane from anode to cathode and electrons flow to the cathode through an external circuit. Oxygen introduced at the cathode reacts with protons and electrons to form the product water and heat. A catalyst is essential for the electrochemical conversion of hydrogen and oxygen in the fuel cell. Therefore, in PEM fuel cells both the anode and the cathode contain catalyst, usually platinum (Pt), to boost the electrochemical reaction.
The PEM fuel cell electrode reactions are shown below,
The basic PEM fuel cell operation is shown in following figure.
UND Fuel cell Research Test Facility
UND's renewable hydrogen test facility has two 1.2 kW Ballard's Nexa™ and a 600 W PEM fuel cell stacks. The Ballard's Nexa™ PEM fuel cell system has 47 membrane electrode assemblies (MEA). It is a fully integrated system that produces 1200 W unregulated DC power using hydrogen and air. An integrated control board, onboard sensors and a microprocessor monitor the system performance and allow a fully automated fuel cell operation. This PEM fuel cell stack is capable of supplying 44 A. Its output voltage varies ranging from 43 V at no load and 26 V at full load. The Nexa™ power module operates on pure and dry hydrogen and room air. The air is humidified before reaching the fuel cell to maintain the membrane saturation. The stack is air cooled. Hydrogen inlet, oxidant air and cooling air flow rates are controlled using an embedded control board. The compressor's speed that supply oxidant air and the cooling air fan speed are adjusted by the control board to match the current requirement.
The following researches have been conducting at UND's research facility:
- Characterization of a PEM fuel cell at cell and stack level using electrochemical impedance spectroscopy
- Equivalent circuit modeling of PEM fuel cell stacks
- Dynamic simulation and verification of a 1.2 kW PEM fuel cell stack