The FC - known since 1839 by the Englishman Sir William Robert Grove (1811-1896) - is one of the electrochemical energy sources (Table 1). In contrast to a battery, it supplies uninterrupted power without recharging, provided it is continuously supplied with chemical "fuel". Their useful life is unlimited. Due to their high achievable output, energy efficiency (no Carnot process) and environmental friendliness, FCs play a key role in supplying stationary and mobile consumers with electrical energy. Decentralized, environmentally friendly provision of electricity and heat from a single system (CHP) and electromobility are becoming economically viable.

Functional principle
Energy generation with the FC as a reversal of electrolysis converts gaseous or gasifiable fuels in cold combustion directly into direct current. Instead of water being_{broken down} into its gaseous components hydrogen H2_and oxygen O2, the FC processes both substances into water. In the process, electrical energy is extracted from the fuels (e.g. H2_{and O2}) with the simultaneous exothermic release of heat. In the redox process, H2 is oxidized_to water _H2Oor cold-burned.

2 H2_(gas) + O2_(gas) = 2 _{H2O(liquid/vapor)}

Efficiencies in comparison (principle curves). © Krause

To avoid normal combustion or an oxyhydrogen reaction, H2_and O2 must_not come into direct contact with each other. For this reason, the porous electrodes are separated from each other by an electrolyte (e.g. potassium or sodium alkali). H2 is_oxidized at the anode and positive H2 _ions are formed. The cathode ensures _O2 reduction, negative _O2 ions are formed. Both processes require catalysts (metals of the platinum-iron group) with which the gas-permeable electrodes are coated. The different polarity between them leads to the DC voltage DC. It can be used as soon as both poles are connected by a circuit.

The cell/open-circuit voltage U0_≈ 1.2 V decreases under load (shunt characteristic). A voltage between approx. 0.6 and 0.7 V has proven to be the optimum operating point for the individual cell. However, in order to achieve higher voltages and outputs, cells are connected in series and parallel. The resulting stack (up to > 800 cells) is stored in a container.

Theoretically, the FC releases the amount of electrical energy that was required for the electrolysis process and_stored in the H2. This provides a store of electrical energy with considerable capacity. O2_manages without buffering, as the majority of FCs work with air.

Table 1: Electrochemical energy sources

FC vs. conventional energy generation
In FC, the conventional, lossy multiple energy conversion is eliminated. Therefore, efficiencies η > 65 % can be achieved (for comparison: steam power plants ≤ 50 %, CCGT plants > 60 %. Gas turbines and diesel-gas engines approx. 35 %). In addition, conventional generators are bound to minimum output values in the MW range.

Another advantage is the use of their waste heat (utilization rate of primary energy up to 90 %). In addition, the thermodynamic efficiency is highest at low operating temperatures and remains roughly constant in the partial load range.

Fuels and FC types
In addition to the energy sources H2_{and O2}, which allow the highest efficiency in their purest form, methanol, natural gas, coal, biogas and, as an oxidant, air are also suitable. The process always requires different catalysts depending on the FC type.

Table 2: FC types KW power plant

In addition to the electrolyte used, the various types differ in terms of operating temperature and fuel. There are three operating temperature ranges: low (60-90 °C AFC, PEMFC), medium (160-220 °C PAFC), high (600-1000 °C MCFC, SOFC) (Table 2).

Realizations
The maturity of FCs now allows commercially interesting projects (Table 2). They are still mainly used for testing and demonstration purposes. The core is the provision of the energy storage medium hydrogen _H2. As it is not freely available in nature, it has to be produced. H2 _surpluses from industry can be used, as can the electrical energy available during off-peak periods.

Economic and ecological outlook
FC technology promotes decentralized energy generation with smaller units close to the consumer. The elimination of energy transportation from large power plants also results in economic benefits. At the same time, FC favors e-mobility and the development of the necessary infrastructure, which is currently in deficit. It also supports the trend towards DC. Its use leads to a significant reduction in carbon dioxide emissions. Thanks to CHP and higher efficiency, the overall ecological balance is more favorable. FCs are superior to all conventional generators in terms of their environmental impact. Their emissions achieve significantly lower (reference) values compared to established technologies (reference value 100 %): NOx₇ %, COx₂₀ %, CHx₀ %. Only FCs_using ultra-pure H2 are zero-emission systems. That is why an important part of the future is _H2.

Combined heat and power generation CHP
Simultaneous generation of electricity and heat in one system. A compact small system is called a combined heat and power unit (CHP).

DC technology
Renewable generators (wind, solar with battery storage, etc.) are increasing, as are DC consumers. The conventional AC grid structure forces multiple, lossy conversion of the electricity. This reduces the overall efficiency of the energy supply to less than 60 %. In contrast, a complete DC grid (without capacitive losses, with HVDC) would alternatively achieve approx. 90 %.