4 T HE NATURE OF FUEL CELLS
4.1 I NTRODUCTION
In most countries, the energy supply consists of a small percentage of intermittent re‐
sources as well as combustion technologies in vehicles, power plants and CHP plants. The perspective in replacing conventional technologies with more efficient fuel cells is depend‐
ent on the characteristics of the different fuel cell types available.
Fuel cells generally consist of the cell, in which an electrochemical reaction takes place; the stacks, in which the cells are combined to the desired power capacity; and the balance of the plant, which comprises systems for handling fuel, heat, electric power conditioning, and other systems required around the cell.
Fuel cells are comparable to batteries, except from the fact that they are not limited by the amount of energy stored in the cell itself. In these cells, chemical energy is converted di‐
rectly into electricity. This provides higher efficiencies than in traditional technologies, in which the energy content in fuels is converted into thermal energy, then mechanical energy and finally electricity. The higher efficiencies also imply a significant reduction of emissions.
Although certain types of fuel cells are mainly considered for mobile and others for station‐
ary use, this is not determined yet. The characteristics of the fuel cell types, however, make certain potential applications more probable than others. Fuel cell types are named after their electrolyte, which also determines their operating temperature. In Table 1, the main characteristics of the five main types of fuel cells are listed.
Please note the fact that such comparisons are subject to the different preconditions and characteristics of the different fuel cells. Thus, these preconditions should be taken into account when comparing e.g. efficiencies. In Mathiesen and Nielsen (2008) [1], the data sheets for different fuel cell systems are presented.
Fuel cells AFC PEMFC PAFC MCFC SOFC
Name (electrolyte) Alkaline
Polymer exchange membrane
Immobilised phosphoric acid
Immobilised molten carbon‐
ate
Solid oxide conducting ceramic
Catalyst Platinum Platinum2 Platinum Nickel Perovskite3
Operating temp. 40‐100 ºC 60‐200 ºC 180‐220 ºC 550‐700 ºC 500‐1000 ºC
Fuel(s) Perfectly pure
H2
Pure H2 or
CH3OH Pure H2
H2, CO, NH3, hydrocarbons,
alcohols
H2, CO, NH3, hydrocarbons,
alcohols
Intolerant to CO, CO2 CO, S, NH3 CO, S, NH3 S S
Potential electric eff. %4 60 40‐55 45 60 60
Potential applications Mobile units space, military
Mobile units, micro‐CHP
Smaller CHP units
Larger CHP units
From large to micro‐CHP Table 1, Characteristics of the five main types of fuel cells and potential areas of use. [24‐37].
In all fuel cell types, the core consists of a cell with an electrolyte and two electrodes; the anode and the cathode. In Fig. 7, the reactions in different fuel cells are illustrated. Hydro‐
gen and oxygen are converted into water producing electricity and heat. The conversion of fuels takes places in a chemical process, in which the catalytic active electrodes convert the fuel into positive ions and oxygen into negative ions. The precise reactions depend on the type of fuel cell. The ions cross the electrolyte and form water and possibly CO2, depending on the fuel and the fuel cell. Only protons can cross the electrolyte while creating a voltage difference between the anode and the cathode in the cell; thus, the electrons cross to the anode section in an external circuit. The output is DC electricity from the flow of electrons from one side of the cell to another. [25]
The advantages of lower temperature fuel cells are mainly related to the fact that they are compact, lightweight and have a quick start‐up and shut‐down potential. This, combined with the fact that the efficiency of the fuel cells cannot compete with other larger power‐
producing technologies, makes transport and mobile applications most promising. In these cases, fuel cells can compete with the efficiencies of existing technologies. They may poten‐
tially contribute to the supply as small‐scale micro‐CHP plants. For larger stationary applica‐
tions, other technologies have already today proven to have better efficiencies.
Alkaline fuel cells (AFCs) are highly reliable, rather compact, and have low material costs;
but no widespread commercial use is expected, because of the costs related to the exten‐
sive gas purification needs [25;26]. AFCs have been used for extraterrestrial applications,
2 May also consist of platinum in combination with ruthenium and molybdenum depending on the CO contents in the fuel. This is espe‐
cially the case of DMFC. In HT‐PEM, the catalyst is often pure platinum.
3 May contain nickel if the fuel is hydrocarbons, e.g. natural gas or methanol.
4 Potential efficiencies depend on the stack load. Total efficiency may be more than 90 per cent, but is dependent on the cooling system
and the operation temperature. For AFC, the efficiency is dependent on the existence of perfectly pure hydrogen at the anode and pure
e.g. the manned Apollo missions, which has no price issue, availability of pure oxygen prob‐
lems, and in which the excess water is useful for astronauts. The lifetime of AFCs is rather short and is not expected to increase with further research; thus, mainly mobile applica‐
tions should be considered. In recent years, research has shown that the purification needs may be lower than expected. Micro‐CHP based on AFC is also still being investigated [27].
Phosphoric acid fuel cells (PAFCs) are widely used today as emergency power and stand‐
alone units in hospitals, schools and hotels. They have been commercially available since 1992, but the costs of PAFCs are still about three times higher than those of other compa‐
rable alternatives. The main problems related to PAFCs are based on the fact that they are dependent on noble metals for the electrodes and the fact that their reported efficiencies are not considerably better than those of other technologies. [24‐26;38]
The AFCs and the PAFCs are often considered the most developed fuel cells of the five types mentioned [28]. However, although variants of both types are still developed, it will hardly be possible to improve the two main challenges, namely the lifetime of AFCs and the cost level of PAFCs, respectively [25;27].
Fig. 7, Schemas of different fuel cell types
PEMFCs are characterised by a rather simple design and fast start‐up. Different variants of PEMFCs are available, including low temperature fuel cells operating at 60‐80 ºC; high tem‐
perature fuel cells (HT‐PEMFC) operating at 140‐200 ºC, and direct methanol fuel cells (DMFCs) ) typically operated at temperatures somewhat below 60 °C due to issues related to the system water balance. PEMFCs and HT‐PEMFCs can be utilised in almost all applica‐
tions in which high temperature heat is not required, such as in micro‐CHP as household heating systems, transport or smaller devices. DMFCs are mainly considered for small port‐
able devices, such as mobile phones, computers, etc. [25;29;39]
The main advantages of high temperature fuel cells are the higher efficiencies and the fuel flexibility which they offer. Other advantages include high operating temperatures, which allow internal reforming or direct conversion and thus enable a rather simple system design as well as the option of integrating these systems with heat engine based bottoming cycles enabling even better net system efficiencies. Moreover, they are constructed from rather cheap materials and do not contain noble metals.
While molten carbonate fuel cells (MCFCs) have high efficiencies, they require the input of
CO2 with ambient air on the cathode side. Also the electrolyte of the MCFC is heavily corro‐
sive, which is the main problem in these cells today. Research is still being conducted in order to improve the cells, mainly for applications to larger CHP and power plants, though the efforts have decreased. [25;34]
SOFCs may be more promising in the future. They have already proven to have rather long lifetimes when not thermally cycled, and theoretically, high efficiencies may be achieved in SOFC systems. However, SOFCs may have problems with thermal stresses and degradation.
Third generation metal‐supported cells, which are currently being developed, are expected to reduce these problems and increase the power density of the cells [40].
All fuel cells can operate on hydrogen. While some fuel cells require higher hydrogen puri‐
ties than others, high temperature SOFCs and MCFCs can operate directly on methane rich fuels, such as natural gas. Electrolyses as well as biomass‐derived fuels can be combined with a synthesis process, thus enabling the production of other fuels than hydrogen for fuel cells.
In the MCFC and the SOFC, the electrolyte conducts ions from the cathode side to the an‐
ode side. In the PEMFC and PAFC, hydrogen passes from anode to cathode. For the two high temperature fuel cells, this means that a wide range of fuels, including natural gas, biogas, ethanol, diesel, LPG, methanol, etc., can be used without reforming the fuel com‐
pletely into hydrogen and CO2 [41;42].
Some fuel cells are very versatile in terms of their ability to utilise different fuels. Others
paths can be divided into two categories; one involving the fuels that can be converted di‐
rectly, i.e. meets the required characteristics, and the second involving the fuels that can be procured to meet these standards.
PEMFCs and SOFCs are described in further detail below, as are the balance of plant equipment and the start‐up, operation and regulation abilities of grid‐connected fuel cells.
Further details about AFCs, PAFCs and MCFCs are included in Mathiesen and Nielsen (2008) [1].
4.2 The characteristics and applications of proton exchange membrane fuel cell