Fuel cells

Fuel Cells: The Energy of the future

At the beginning of the 20th century, houses lit their nights by burning fuels: gas lighting in privileged urban areas, oil or paraffin lamps or burners in the rest, even though in 1879 Thomas A. Edison had patented the electric light bulb. Edison had patented the electric light bulb. But twenty or thirty years later, these modern inventions were still curiosities within the reach of very few. When incandescent electric lamps replaced fuel lamps and banished their smell and soot, houses became brighter, cleaner and safer.

At the beginning of the 21st century, this struggle between combustion and clean technologies has moved to the street. Ninety years of combustion cars have led to a heavy reliance on individual rapid transport, but at the cost of polluting our air. A possible alternative lies in so-called fuel cells.

A bit of chemistry

Fuel cells are based on electrochemical reactors where chemical energy is converted into electricity without any “combustion” process. The device is very simple: it consists of two electrodes separated by an electrolyte (a specially treated material that allows the passage of ions – positively or negatively charged atoms – but not electrons). At the negative electrode (anode), the oxidation of the fuel (usually hydrogen but can be methanol or others) takes place, and at the positive electrode (cathode), the reduction of the oxygen in the air.

Thus, one of the simplest reactions that can occur in a cell is for oxygen and hydrogen to combine to form water and produce electrical energy and heat.

Reacción en el ánodo 2H2 ———- 4H+ + 4e

Reacción en el cátodo O2 + 4H+ + 4e ——— 2 H2O

The cell consists of an anode into which the fuel – hydrogen, ammonia or hydrazine – is injected, and a cathode into which an oxidant – air or oxygen – is introduced. The two electrodes of a fuel cell are separated by a conductive ionic electrolyte.

Hydrogen enters at the anode and in the presence of a catalyst dissociates into positive ions and electrons. Oxygen from the air enters at the cathode and dissociates into negative ions, also in the presence of the catalyst. The hydrogen ions migrate through the electrolyte while the electrons circulate through the external circuit (such as the car’s electric motor). In this way, they recombine again at the cathode, producing water and a significant amount of thermal energy.

A fuel cell

A single cell generates about one volt in an open circuit; if higher voltage and high power are required, the required number of cells are connected in series to form a stack called a fuel cell.

In this stack, systems must be added to dissipate heat, to allow gases to circulate easily, and to provide electrical terminals so that the electrical energy produced can be used.

Types of fuel cells

Fuel cells are generally classified according to the type of electrolyte they use. The main differences between them are the temperatures at which they operate, the purity of the fuel they use, the efficiencies and the applications in which they can be used.

Low temperature fuel cells

In the case of a hydrogen-oxygen fuel cell with an alkali metal hydroxide electrolyte, the anode reaction is 2H2 + 4OH ——- 4H2O + 4e and the cathode reaction O2 + 2H2O + 4e ——- 4OH.

The electrons generated at the anode move through an external circuit containing the charge and pass to the cathode. The OH- ions generated at the cathode are conducted by the electrolyte to the anode, where they combine with hydrogen and form water. The voltage of the fuel cell in this case is about 0.8 V but decreases as the charge increases. The water produced at the anode must be continuously removed to prevent it from flooding the cell.

They use high purity hydrogen as fuel without any carbon monoxide or carbon dioxide. Efficiency is around 55%. Their main advantage is that they operate at low temperatures.

Hydrogen-oxygen fuel cells using ion exchange membranes or phosphoric acid electrolytes were used in the Gemini and Apollo space programmes respectively. Phosphoric acid fuel cells have limited use in electrical power generating facilities.

Fuel cells operating at high temperature

Fuel cells with molten carbonate electrolytes are currently being developed. The electrolyte is solid at room temperature, but at operating temperature (650 to 800 °C) it is a liquid. This system uses carbon monoxide as fuel, so mixtures of carbon monoxide and hydrogen such as those produced in a coal gasifier can be used as fuel.

Fuel cells are also being developed that use solid zirconium dioxide as an electrolyte. These are called solid oxide fuel cells. Zirconium dioxide is converted into an ionic conductor at about 1,000 °C. The most suitable fuels are hydrogen, carbon monoxide and methane, and the cathode is supplied with air or oxygen. The high operating temperature of solid oxide fuel cells allows the direct use of methane, a fuel that does not require expensive platinum catalysts.

Origin

As is often the case, the basic scientific principles behind fuel cells were discovered long before their applications could be intuited.

In 1839, Sir William Robert Grove (1811-1896), a lawyer by profession and physicist by vocation, published an experiment demonstrating the possibility of generating electric current from the electrochemical reaction between hydrogen and oxygen. His experiment consisted of joining four electrochemical cells in series, each consisting of an electrode containing hydrogen and an electrode containing oxygen, separated by an electrolyte. Grove found that the reaction of hydrogen at the negative electrode combined with oxygen at the positive electrode generated an electric current that could in turn be used to generate hydrogen and oxygen.

Some fuel cell applications

What do we do at INMA?

At INMA, as the Materials Institute that we are, we do research on materials for fuel cells. In particular, we study electrolytes and anodes for solid oxide fuel cells, and we address the processing and study of their physical properties (conductivity, structure, microstructure, etc.).

The conditions to which these materials are subjected in use are severe (high temperature, thermal cycling, oxidising and reducing conditions, etc.), so there is scope for research into the search for and optimisation of the most suitable materials. They will be those that better withstand the cycles and high temperatures or which, with better conductivity, allow the working temperature to be reduced.

We have an experimental facility to measure the I-V curves of the single cells that are manufactured. The figure shows the detail of a conventional zirconia cell with NiO?zirconia anode and LSM cathode ready for testing.

The conventional anodes of these cells consist of mixtures of nickel oxide and zirconia prepared by ceramic methods in which the layer in contact with the electrolyte is specially treated to have a very fine microstructure (Functional Layer), and therefore many reaction points for the fuel. At INMA we are investigating methods of modifying this functional layer using directional laser solidification treatments.

We are also investigating methods of localised sintering of the electrolyte on a metal support to process intermediate temperature SOFCs (ITSOFC) supported on metal.

More information

New materials for fuel cells
www.aragoninvestiga.com

Asociación Española de Pilas de Combustible (APPICE)
www.appice.es

Tecnociencia: Especial Pilas de Combustible de Hidrógeno
www.tecnociencia.es/especiales/hidrogeno

Website of the Spanish Hydrogen Association
www.aeh2.org

Websites with a full description of fuel cells
www.fuelcells.org

www.fuelcellonline.com

Instituto de Nanociencia y Materiales de Aragón