Stack life Stack e ciency Capex (system) Opex Stack life Stack e ciency Capex (system) Opex Stack life Stack e ciency Capex (stack) Opex
= 90,000 h = 78% (LHV) (3.8 kWh/Nm ) = 700–900 $/kW for 20 MW size = 3% of Capex = 65,000 h = 66% (LHV) (4.5 kWh/Nm ) = 900–1200 $/kW for 20 MW size = 2% of Capex = <20,000 h = 75% (LHV) (3.7 kWh/Nm ) = >2000 $/kW for 5 MW size = 2% of Capex
Alkaline water electrolysis (AWE)
Durability
E ciency
Proton exchange membrane (PEM)
Capex
Solid oxide electrolysis (SOE)
Opex
Figure 2 Key factors for technology selection
reaction (OER) (Sakthivel, S, 2021a). When a direct current (DC) is applied, electrons flow from the negative terminal of the DC source to the cathode, at which the electrons are consumed by hydrogen ions (protons) to form hydrogen. Hydroxide ions (anions) transfer through the electrolyte solution to the anode, at which the hydroxide ions release electrons that return to the positive terminal of the DC source, thereby maintaining a balance. When a DC source is applied at the two electrodes, the water is reduced at the cathode surface, and concurrently water oxidation occurs at the anode surface. The typical reactions are given below: 𝐶𝑎𝑡ℎ𝑜𝑑𝑒 : 2𝐻 2 𝑂 (𝑙) + 2𝑒 - → 𝐻 2 (𝑔) + 2 𝑂𝐻 - (𝑎𝑞) 𝐴𝑛𝑜𝑑𝑒: 2𝑂𝐻 - (𝑎𝑞) → 0.5 𝑂 2 (𝑔) +𝑒 - + 𝐻 2 𝑂 (𝑙) Overall reaction 𝐻 2 𝑂 (𝑙) → 𝐻 2(𝑔) + 0.5 𝑂 2 (𝑔) In the water electrolysis reaction, for every 1 mol of H2 produced, ½ mol of O2 is also generated. As per first principles, demineralised water (8.9 kg) produces 1 kg H2 and 7.9 kg O2 . The purity levels of H2 and O 2 produced can be 99.9 vol% and 99.7 vol%, respectively. The water electrolyser consists of three major components: the cell, stack, and system. The cell is the core of the electrolyser, where the electrochemical process occurs. The stack comprises multiple cells connected in series, together with spacers, seals, frames (mechanical support), and end plates (to avoid leaks and collect fluids). The system includes equipment for cooling, processing the hydrogen (for purity and compression), converting the electricity input (transformer
and rectifier), and treating the water supply (demineralisation) and gas output (of oxygen). Technology selection Various types of electrolysers include alkaline water electrolyser (AWE), proton exchange membrane electrolyser (PEM), solid oxide electrolyser (SOE), and anion exchange membrane electrolyser (AEM). These are classified by the type of electrolyte used, ionic agent as ion carrier (OH - , H + , O 2 - ), and operating conditions (Sakthivel, S., 2021b), (Sakthivel, S., 2021c). At present, AWE and PEM are used for the commercial production of hydrogen. These two technologies dominate the current market for electrolysers. AEM water electrolysis uses less expensive transition metals in place of noble metal electrocatalysts, but compared to AWE and PEM, it is less technologically advanced. At present, each technology has its own challenges, from critical materials to performance, durability, and maturity level. Generally, the technology selection is categorised by the durability of cell and stack component(s), efficiency, and operating parameters, including operating expenditure (Opex) and capital expenditure (Capex) and their returns. Figure 2 depicts the key factors for technology selection. At present, original equipment manufacturers (OEMs) claim stack lifetimes of 90,000 hours, 65,000 hours, and <20,000 hours for AWE, PEM, and SOE, respectively. The stack lifetime is dependent on parameters such as the nature of electrolyte used, applied current density, voltage range, selection of material for electrodes, nature of diaphragm or membrane composition, type
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