In an internal combustion engine in your car, the fuel – petrol, diesel, LPG – reacts directly with oxygen. The larger volume of combustion gases pushes a piston back and forth, and a crankshaft converts this back-and-forth movement into a rotating movement for the car wheels. Chemical energy from the fuel is converted into useful mechanical energy and (useless) heat.
In a fuel cell, combustion takes place indirectly. A fuel cell performs a controlled combustion reaction by supplying the two reactants – for example methanol, methane or hydrogen on the one hand and oxygen on the other – to either side of the cell. Inside the cell they react with each other where electrons are part of the electrochemical reaction, and outside the cell these
electrons can be used meaningfully to power an electrical device such as an electric motor. So, just like a battery, a fuel cell converts chemical energy into electrical energy – and also heat. Fuel cells do not contain any moving parts, and this has the advantage that they can operate quietly and that there is no mechanical wear.
Solid oxide fuel cell (SOFC)
Fuel cells come in different shapes and sizes, but their structure is similar: a sandwich of two electrodes – an anode and a cathode – with an electrolyte in between. This electrolyte gives the fuel cells their name. Solid oxide fuel cells, or SOFC for short, are ceramic fuel cells which, because of their materials, can operate at high temperatures (600-1000 °C) and achieve high efficiency. Yttria-stabilised zirconia or YSZ is the ideal electrolyte material because of its high oxygen ionic conductivity at high temperature. An electrolyte must allow ions to pass, but electrons are not allowed to move through it. The material should also be gas-tight, so that the reactants do not have the opportunity to react directly with each other. The porous anode consists of a mixture of nickel and YSZ (Ni/YSZ), and the porous cathode of lanthanum-strontium-manganate (La1-xSrxMnO3). The electrodes are porous because at the electrode/electrolyte/gas triple phase boundary oxygen is built into/out of the electrolyte. Ni/YSZ has sufficiently high electrical conductivity and is chemically and thermally stable enough at the side of the fuel cell where the fuel such as hydrogen is supplied, and lanthanum-strontium-manganate has these properties at the oxygen side.
When the fuel cell is operating, an oxygen-containing reactant (such as pure oxygen or air) is fed to the cathode where oxygen is reduced, thereby forming oxygen ions (O2-). These oxygen ions move through the solid-state electrolyte layer to the anode where they are oxidised with fuel (e.g. hydrogen or natural gas) to form a reaction product (water). The cathode and anode are externally connected, and transport electrons that are released at the anode to the cathode where they are needed for the reaction. This electron flow is meaningfully used to drive an electrical device.
Operation of the solid oxide fuel cell or SOFC
Solid oxide fuel cells are well suited for large-scale stationary applications, for example to supply power for houses or factories. Because they operate at high temperatures, and therefore cannot be switched on and off quickly, they are particularly suitable for providing a constant base load for a long period of time. Another type of fuel cell, the polymer exchange membrane fuel cell or PEMFC, has a higher energy density and is less heavy, and is therefore good for use in mobile applications such as buses. The bus has the necessary hydrogen on board in high-pressure tanks (about 700 bars), and the electricity that is supplied by the fuel cell drives an electric motor that runs the wheels of the bus. This already indicates: a hydrogen-powered bus (or car) is in fact an electric bus (or car). However, the propulsion is different. Electric vehicles – like the battery electric vehicles (BEV) you see more and more on the road – get their energy from charged lithium ion batteries. In hydrogen-powered vehicles (cars and buses, but also trains and trucks), the ‘fuel’ comes from the high-pressure tank on board, and the oxygen from the ambient air, for example through large intake openings at the front of the vehicle.
Electrolyser for hydrogen production
You can also make a fuel cell work the other way around, just like an LED lamp does the opposite as a solar cell in converting electrical energy into light. Such an inverted fuel cell – a device that splits water into hydrogen and oxygen by passing an electric current through it – is called an electrolyser. Electrolysis is an effective way of producing hydrogen, where the electrical energy is transformed into hydrogen as an energy carrier – ‘power to gas‘. This hydrogen gas can then be easily stored or transported through gas pipelines or in cooled liquid form.
Elemental hydrogen is abundant on earth, for example as one of the building blocks chemically bound inside the water molecule. However, hydrogen gas (H2) in its free molecular form is not abundant at all, and needs to be synthesized artificially in order to occur in adequate quantities. Currently, the major way to synthesize hydrogen gas is by steam reforming of natural gas. Therefore, it has a fossil origin with the greenhouse gas carbon dioxide as an unwanted by-product. More than half of this hydrogen is being used in ammonia synthesis, which in turn is a major raw material for fertilizer manufacturing. An alternative way to generate hydrogen gas is by electrolysis of water.
In electrolysis with a solid oxide electrolyser cell or SOEC – which does the opposite as a SOFC – steam is supplied to the porous cathode which at the cathode/electrolyte boundary is converted to hydrogen gas and oxygen ions. The applied voltage difference across the electrolyser causes diffusion of the oxygen ions to the anode side, where they are converted into molecular oxygen. A SOEC operates at the same high temperatures as a SOFC, with the steam supplied already providing part of the heat required.