Electrolysis is a promising option for Green hydrogen production from renewable resources. Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production.
Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in slightly different ways, mainly due to the different type of electrolyte material involved.
There are three different water electrolysis technologies:
- Alkaline Electrolysers
- Proton Electron Membrane Electrolysers
- Solid Oxide Electrolysers
Alkaline water electrolysis is one of the easiest methods for hydrogen production although it is relatively expensive technology. This technique is very clean and produces more than 99.989% purity of hydrogen gas. In addition, electrolysis can be linked to renewable electricity-producing technologies and hence could become even more important in the future. Usually an alkaline medium is employed (25–30% KOH). The electrolytic reactions that occur on each electrode are given by the following equations:
- Cathode: 2H2O + 2e− → 2OH− + H2
- Anode: 2OH− → ½O2 + H+ + 2e−
- Overall: H2O → H2 + ½O2
Proton-Electron Membrane Electrolyser(PEMEC)
PEMEC systems are based on the solid polymer electrolyte (SPE) concept for water electrolysis that was first introduced in the 1960s by General Electric to overcome the drawbacks of AECs . These electrolyzers have been used for years at scales up to 100’s of kW. Newer materials and manufacturing methods are emerging to support higher efficiency and lower cost system development. With increasing demand for electrolyzers at the MW-scale and up, the industry is currently significantly scaling up its manufacturing capacity. Commercial products in this range have recently been introduced to the market by several suppliers like ITM Power.
In a (PEM) electrolyzer, the electrolyte is a solid specialty plastic material.
- Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons): 2H2O → O2 + 4H+ + 4e-
- The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.
- At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas: 4H+ + 4e- → 2H2
Solid Oxide Electrolysis Cell (SOEC)
SOEC is the least developed electrolysis technology. It is not yet widely commercialised, but systems have been developed and demonstrated on laboratory scale and individual companies are currently aiming to bring this technology to market. SOECs use solid ion-conducting ceramics as the electrolyte, enabling operation at significantly higher temperatures. Potential advantages include high electrical efficiency, low material cost and the options to operate in reverse mode as a fuel cell or in co-electrolysis mode producing syngas (CO2 + H2) from water steam (H2O).
Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions; and the oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit. SOEC operates at temperatures high enough for the solid oxide membranes to function properly (~700°–800°C, compared to 70°–90°C for PEM).With the ability to effectively use heat available at these elevated temperatures (from various sources, including nuclear energy), SOEC electrolyzers can maintain high H2 production rates with high electrical efficiencies.
Why are Electrolysers being considered?
Hydrogen produced via electrolysis can result in zero greenhouse gas emissions, depending on the source of the electricity used. The source of the required electricity—including its cost and efficiency, as well as emissions resulting from electricity generation—must be considered when evaluating the benefits and economic viability of hydrogen production via electrolysis. In many regions of the country, today’s power grid is not ideal for providing the electricity required for electrolysis because of the greenhouse gases released and the amount of fuel required due to the low efficiency of the electricity generation process. Hydrogen production via electrolysis is being pursued for renewable (wind) and nuclear energy options. These pathways result in virtually zero greenhouse gas and criteria pollutant emissions.
Source:@Solar_Edition @ScienceDirect @IEA_Hydrogen
Fig 1: @Solar_Edition @ScienceDirect
Fig 2: @Solar_Edition @ScienceDirect
Fig 3: @Solar_Edition @ScienceDirect
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Reference: Schmidt, O. & Gambhir, A. & Staffell, Iain & Hawkes, A. & Nelson, J. & Few, Sheridan. (2017). Future cost and performance of water electrolysis: An expert elicitation study. International Journal of Hydrogen Energy. 42. 30470-30492. 10.1016/j.ijhydene.2017.10.045.