Hydrogen Production by Electrolysis of Water

Polymer Electrolyte Membrane (PEM) and Alkaline (AEL) electrolysers are both mature and reliable technologies. Their respective characteristics and suitability according to the utilisation are compared on this radar graph. Alkaline technology is better in terms of cost and track record while footprint and reactivity talk for PEM. Electrolyser reactivity is of interest when associated with solar or wind electricity sources (the capacity factor slider in the calculator allows to consider the typical load factor of renewables).

- AEL is the most mature electrolyser technology. It operates at temperatures between 60°C and 90°C and reaches up to 35 bar of pressure. Electrodes are submerged in a liquid electrolyte/ionic conductor, typically a 25-30% aqueous KOH-solution, which is a conductive corrosive solution. The solution is circulated either by a pump or by the natural circulation occurring due to the temperature gradients and/or movement of gas bubbles. The electrolyte is stored in two different chambers for each product gas (O2 and H2) separated by a solid barrier called a diaphragm.

- PEM is a built up around a proton exchange membrane in the middle of the cell and consists of an anode for oxygen production and a cathode for hydrogen production. The design of the PEM is compact, it operates at temperatures between 50°C and 85°C and due to the solid assembly, its operation supports high-pressure conditions and are typically operating around 70 bars. Typically, the electrodes are in direct contact with the proton exchange membrane. At the anode, water is oxidized to produce oxygen, electrons, and protons that circulate across the membrane to the cathode where they are reduced, closing the circuit and producing hydrogen that bubbles towards the cathodic gas outlet. In most cases a highly conductive Nafion proton membrane will separate the two chambers and the electrodes are typically applied directly to the membrane, which makes up the membrane electrode assembly. The corrosive acidic solution in the PEM requires noble metal catalysts like iridium for the anode and platinum for the cathode. The membrane features a very low cross-permeation of the gases, producing hydrogen with a high purity after drying.

- SOE is a technology that enables water, or more specific steam electrolysis, to operate at high temperatures ranging from 600°C to 1000°C with a pressure between 15 and 25 bars. It is a highly efficient technology in which an electric current is passed through two electrodes, which can be made up of relatively cheap nickel, separated by an electrolyte. Electrolysis of steam convert electric energy and heat into hydrogen and oxygen. In the overall process for steam electrolysis gaseous water is fed to the negative electrode where it is split into hydrogen and oxide ions. The oxide ions are conducted through the solid oxide electrolyte from the negative electrode to the positive by the applied electric field. At the positive oxygen electrode, the oxide ions recombine to produce gaseous oxygen.

At The Heart Of The Challenge : The Electrolyser Efficiency

The global efficiency of the electrolyser is the result of several sub level efficiencies. More precisely, it is a function of (a) the DC efficiency representative of the stack efficiency, the very core of the process (which can be divided in faraday and voltage efficiencies) and (b) the efficiency of all the auxiliaries necessary to run an electrolyser system, it can include the cooling, H2 purification, water treatment and AC conversion among other.

At HyJack we use a global system efficiency for our calculator. The definition of the system boundaries vary from a manufacturer to another and deserve to be specified when comparing quotes. For now, in the tool, the efficiency follows a curve specific to each given technology. Note that the efficiency varies because of different specifications used by each manufacturer. It is also sensitive to the outlet pressure and the specific needs of the underling project (such as cooling and gas treatment). In particular, small unit efficiency is typically lower due to a higher proportion of electricity needs related to auxiliaries.