Fuel cells

Basic operation principles

A fuel cell is an electrochemical device, which directly converts the chemical energy of the fuel into electricity, without intermediate stages of combustion and production of mechanical work.

In its basic form a fuel cell operates as follows: hydrogen reacts with oxygen in the presence of an electrolyte and produces water, while at the same time an electrochemical potential is developed which causes the flow of an electric current in the external circuit (load). The following electrochemical reactions take place on the two electrodes:

                        Anode:            

                        Cathode:          

Thus, the total reaction is:         

At the anode, ions and free electrons are produced. Ions move towards the cathode through the electrolyte. Electrons move towards the cathode through the external circuit, which includes the load (external resistance). The reaction is exothermic. The heat released can be used in thermal processes.

The hydrogen required is usually produced from hydrocarbons, most frequently natural gas, by a process known as reforming, which can be either external or internal to the fuel cell unit, depending on the type of fuel cell.  It can also be produced by electrolysis of water. In certain types of fuel cells, carbon monoxide can be used as fuel, instead of hydrogen.

A single cell develops an electric voltage slightly lower than 1 Volt. The proper number of cells connected in series produces the required voltage, while with parallel connection the required power is produced. Thus, a stack of cells is created and a direct current is produced. An inverter - usually static - is used to transform the direct current to alternating current of the appropriate voltage and frequency.

Types of Fuel Cells

Several classifications of fuel cells have appeared in the literature throughout the years. The prevailing one is based on the type of electrolyte.

Direct methanol fuel cells (DMFC)

Sulphonic acid incorporated into a solid polymer membrane or sulphuric acid solution is the electrolyte. DMFC is expected to operate at a temperature of 50-110°C, but it is still at the R&D stage, requiring fundamental research.

Alkaline fuel cells (AFC)

Potassium hydroxide (KOH), which is the most conductive of all alkaline hydroxides, is the electrolyte, at a concentration of around 30%. Pure hydrogen is the fuel and pure oxygen or air is the oxidizer. Alkaline fuel cells operate at a temperature of 60-80C. This is why they are characterized as low temperature fuel cells. The operating pressure in some cases is a few atmospheres, but most often this coincides with atmospheric pressure.

Alkaline fuel cells have been used in NASA’s Apollo mission. Today they are still used in space applications. Also they are one of the most attractive systems for transportation applications. Units with a power up to 100 kW have been constructed.

Polymer electrolyte fuel cells (PEFC)

They are also known with the initials PEM (polymer electrolyte membranes). The electrolyte consists of a solid polymeric membrane, which is sandwiched between two platinum-catalyzed porous electrodes. The operating temperature is around 80ºC and the operating pressure 1-8 atm. PEFC units with a power output up to 100 kW have been constructed.

Phosphoric acid fuel cells (PAFC)

PAFC are at the moment the most advanced fuel cell technology for terrestrial applications. Packaged units of 200-250 kWe are already commercially available for electricity generation or cogeneration, while demonstration systems of 25 kW–11 MW have been constructed in Europe, USA and Japan.

Phosphoric acid (H₃PO₄) is the electrolyte. Hydrogen is produced by an external reformer from fuels such as natural gas or methanol. Air is the oxidizer. The operating temperature is around 200ºC, which makes PAFCs attractive for cogeneration applications, in particular in the tertiary sector.

Molten carbonate fuel cells (MCFC)

Molten alkali carbonate mixture, retained in a porous lithium aluminate matrix, is used as the electrolyte. The eutectic mixture consists of 68% Li₂CO₃ and 32% K₂CO₃, which at the operating temperature of 600-700ºC is in a liquid phase. The fuel consists of a gaseous mixture of H₂, CO and CO₂, which is obtained with reforming of hydrocarbons such as natural gas, or with coal gasification. The high operating temperature makes internal reforming possible. For this purpose, the heat released by the fuel cell itself is used.

MCFC’s have good prospects for utility and industrial applications of medium to large size (at the order of MW). Efficiencies higher than 50% are expected. The available high temperature heat can be used either for thermal processes (cogeneration) or in a bottoming cycle for additional power production. Experimental units have been constructed, but the MCFC technology is still in the development phase.

Solid oxide fuel cells (SOFC)

The solid oxide fuel cell is an all-solid-state power system, which uses yttrium -stabilized zirconium (Y₂O₃-ZrO₂), a ceramic material, as the electrolyte layer. It operates at temperatures of 950-1000ºC. Pure hydrogen or a mixture of H₂ and CO is used as fuel, which is produced with internal reforming of hydrocarbons or with coal gasification.

Intermediate temperature SOFC (IT-SOFC)

The electrolyte is ceria-gadolinia, a ceramic, solid oxide. IT-SOFC operates at a temperature of 650-750ºC. It has good prospects for commercial and residential CHP, power generation, ship propulsion, for fuelling trains, but it still requires fundamental research.