Introduction

Conclusions

Fuel Cells

Hydrogen can produce electricity directly via the following reactions in a fuel cell:

Fuel cells are electrochemical devices that convert chemical energy directly to electrical energy, resulting in high-efficiency power generation with low environmental impact. Most fuel cell power systems are comprised of a number of components:

• Unit cells, where the reactions take place
• Stacks, where individual cells are modularly combined by electrically connecting the cells
• Balance of plant, which may include a fuel processor, thermal management, and electric power conditioning, among other auxiliary functions.

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Source: www.fctec.com

The efficiency is not limited by the heat engine formula and is, theoretically, 80%. However, build up of electrical resistance, resistance to the reactants and products moving to and from the electrodes, respectively (mass transport), and reaction kinetics decrease the efficiency.

Typically, fuel is fed to the anode (negative electrode) and an oxidant (often oxygen from air) is fed to the cathode (positive electrode).

Hydrogen is the most popular fuel because it has a high reactivity for most anode reactions and can be produced chemically from a variety of fuels. Hydrogen is oxidized at the anode and electrons are removed and passed through the circuit to the cathode where oxygen is released. The Hydrogen and Oxygen then combine to form water.

Many developments to optimize fuel cell performance have been made, such as reducing electrolyte thickness, and developing improved electrode and electrolyte materials to broaden the operational temperature ranges. Catalysts to speed up reaction kinetics, increasing the surface area of the electrodes by making them porous, reacting at high temperatures, and using high conductance wires to connect the electrodes can minimize these barriers but cannot eliminate them.

The electrolyte of a fuel cell transports dissolved reactants to the electrode, bars against fuel and oxidant gas mixing, and conducts ionic charge between the electrodes, completing the fuel cell’s circuit. The electrodes conduct electrons, provide current collection and connections with other cells, ensure that reactant gasses are evenly distributed over the cell, and ensure that reaction products are led away from the reaction sites.

For an area to be active, it must be exposed to the reactant, in electrical contact with the electrode, and in ionic contact with the electrolyte – a three phase interface. In addition, it must contain sufficient electro-catalyst for the reaction to occur at an effective rate. In liquid electrolyte fuel cells, the reactant gasses diffuse through a thin electrolyte film that wets portions of the porous electrode and react. If there is excessive electrolyte, the electrode can flood and restrict mass transport, reducing performance. Therefore, a balance must be maintained for optimized function. A solid electrolyte fuel cell has a different challenge, engineering catalyst sites into the interface that are electrically and ionically connected to the electrode and electrolyte, respectively, and is exposed to the reactant gasses.

Research is focused on developing low temperature fuel cells for transportation and distributed energy systems. Key barriers to fuel cell development are:

• Cost and reliability
• High efficiency
• Durability
• Heat Utilization
• Startup time
• Power and Load requirements
• Performance of components such as:
  • Proton exchange membranes
  • Oxygen reduction electrodes
  • Advanced catalyst

Sources:

1. Bellona Foundation. www.bellona.no

2. EG&G Technical Services Under Contract No. DE-AM26-99FT40575 for U.S. Department of Energy.  Fuel Cell Handbook 7th Edition.  November 2004.

3. Department of Defense. www.dodfuelcell.com

4. Fuel Cell Test and Evaluation Center. www.fctec.com

5. Fuel Cell Today. www.fuelcelltoday.com