At present the most commercially developed technology available to produce hydrogen is from electrolysis. Electrolysis is an optimistic course of action for carbon-free hydrogen production from renewable and nuclear resources. Water electrolysis is the decomposition of water (H2O) into its basic components, hydrogen (H2) and oxygen (O2), through passing electric current. Water is a complete source for producing hydrogen and the only by-product released during process is oxygen. This process uses electrical energy that can then be stored as a chemical energy in the form of hydrogen.
What is the Process?
To produce Hydrogen, Electrolysis converts electrical energy into chemical energy by storing electrons in stable chemical bonds. Like fuel cells, electrolysers are composed of an anode and a cathode separated by an aqueous electrolyte according to the type of electrolyte material involved and the ionic species it conducts. The electrolyte is an obligatory part as pure water does not have the ability to carry enough charge as it lacks ions. At the anode, water is oxidised into oxygen gas and hydrogen ions. While the cathode, water is reduced to hydrogen gas and hydroxide ions. At present there are three leading electrolysis technologies.
Alkaline Electrolysers (AEL)
This technology has been used on an industrial scale for over 100 years. Alkaline electrolysers operate via transport of hydroxide ions (OH-) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Operating at 100°–150°C, Electrolysers use a liquid alkaline solution of sodium or potassium hydroxide (KOH) as the electrolyte. In this process the anode and cathode are separated using a diaphragm that prevents remixing. At the cathode, water is split to form H2 and releases hydroxide anions that pass through the diaphragm to recombine at the anode where oxygen is produced. As this is a well-established technology it is relatively low in cost of production as well as it provides a long-time stability. However, it does have a crossover in gases possibly compromising its degree of purity and requires the use of a corrosive liquid electrolyte.
Polymer Electrolyte Membrane Electrolysers (PEM)
Polymer Electrolyte Membrane is the latest technology to be commercially used to produce hydrogen. In a PEM electrolyser, the electrolyte is a solid specialty plastic material. PEM electrolysers operate at 70°–90°C. In this the process the water reacts at the anode to form oxygen and positively charged hydrogen ions (protons). The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode. At the cathode, the hydrogen ions combine with electrons from the external circuit to form hydrogen gas. Compared to AEL there are several advantages: the product gas purity is high in a partial load operation, the system design is compact and has a rapid system response. However, component cost is high and durability is low.
Solid Oxide Electrolysers (SOE)
AEL and PEM electrolysers are known as Low-Temperature Electrolysers (LTE). However, Solid oxide Electrolyser (SOE) is known as High-Temperature Electrolyser (HTE). This technology is still at development stage. In SOE, solid ceramic material is used as the electrolyte which conducts negatively charged oxygen ions (O2-) at elevated temperatures, generates hydrogen in a slightly different way. At a temperature about 700°–800°C steam at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. 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. Advantages of this technology is that it combines high heat and power efficiency as well as it producing low emissions at a relatively low cost. Although, due to the high heat and power required, start-up time takes longer.
Why is Hydrogen being considered as an alternative fuel?
Hydrogen is considered an alternative fuel under the Energy Policy Act of 1992. Hydrogen produced via electrolysis can contribute zero greenhouse gas emissions, depending on the source of the electricity used. This technology is being pursued to work with renewable (wind, solar, hydro, geothermal) and nuclear energy options to allow virtually zero greenhouse gas and other pollutant emissions. Although, this type of production will require the cost to be decreased significantly to be competitive with more mature carbon-based pathways such as natural gas reforming. There is potential for synergy with renewable energy power generation. Hydrogen fuel and electric power generation could be distributed and sited at wind farms, thereby allowing flexibility to shift production to best match resource availability with system operational needs and market factors.