Regenerative fuel cell
A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.
A hydrogen fueled Proton exchange membrane fuel cell, for example, uses hydrogen gas (H2) and oxygen (O2) to produce electricity and water (H2O); a regenerative hydrogen fuel cell uses electricity and water to produce hydrogen and oxygen.
When the fuel cell is operated in regenerative mode, the anode for the electricity production mode(fuel cell mode) becomes the cathode in the hydrogen generation mode(reverse fuel cell mode), and vice versa. When an external voltage is applied, water at the cathode side will undergo electrolysis to form hydrogen and oxide ions; oxide ions will be transported through the electrolyte to anode where it can be oxidized to form oxygen. In this reverse mode, the polarity of the cell is opposite to that for the fuel cell mode. The following reactions describe the chemical process in the hydrogen generation mode:
At cathode: H2O + 2e− → H2 + O2−
At anode: O2− → 1/2O2 + 2e−
Overall: H2O → 1/2O2 + H2
One example of RFC is solid oxide regenerative fuel cell. Solid oxide fuel cell operates at high temperatures with high fuel-to-electricity conversion ratios and it is a good candidate for high temperature electrolysis. Less electricity is required for electrolysis process in SORFC due to high temperature.
The electrolyte can be O2− conducting and/or proton(H+) conducting. The state of the art for O2− conducting yttria stabilized zirconia(YSZ) based SORFC using Ni–YSZ as the hydrogen electrode and LSM (or LSM–YSZ) as the oxygen electrode has been actively studied. Dönitz and Erdle reported on the operation of YSZ electrolyte cells with current densities of 0.3 A cm−2 and 100% Faraday efficiency at only 1.07 V. The recent study by researchers from Sweden shows that ceria-based composite electrolytes, where both proton and oxide ion conductions exist, produce high current output for fuel cell operation and high hydrogen output for electrolysis operation. Zirconia doped with scandia and ceria (10Sc1CeSZ) is also investigated as potential electrolyte in SORFC for hydrogen production at intermediate temperatures(500-750°C).It is reported that 10Sc1CeSZ shows good behavior and produces high current densities, with suitable electrodes.
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