Mar-2023

Stability and durability of water electrolysers

A review of developments in water electrolysis for hydrogen production and the potential to increase efficiency and hydrogen production rates over longer lifetimes.

Dr Sakthivel. S
TATA Consulting Engineers Limited

Article Summary

Currently, hydrogen production is around 70 Mtpa (million metric ton per annum) worldwide. Global hydrogen demand is projected to increase to more than 800 Mtpa by 2050, driven by actions to mitigate climate change via energy transition and the decarbonisation of industrial processes. India’s total hydrogen demand is likely to grow from 6.7 Mtpa in 2021 to 11.7 Mtpa by 2030 and 28 Mtpa by 2050 (India Government, 2021). Many countries have plans to increase electrolyser capacity to meet the growing demand for low-carbon-intensity hydrogen.

The global electrolyser capacity is expected to increase from 0.3 GW in 2020 to about 16.7 GW by 2026 (IEA, 2021). In comparison with the global picture, India’s electrolyser manufacturing ecosystem is at a nascent stage. However, the levelised cost of green hydrogen remains high ($6-7 per kg) relative to fossil-based grey hydrogen ($1-2 per kg). A number of challenges need to be overcome to raise the technology maturity level for water electrolysis. These include a high capital requirement, relatively high specific energy consumption, and other technical challenges impacting electrolyser stability and durability. This article reviews the current understanding of cell and stack construction, electrolyser technology selection, and the effect of current density, electrodes, electrolyte, anode-cathode separator, and dynamic/variable load on the durability and stability of electrolyser cell and stack.

Chemistry of water electrolysis
The electrochemical process takes place between electrodes and electrolyte in a cell when an electrical current is applied. A basic water electrolysis unit or cell consists of an anode, a cathode, a power supply, and an electrolyte, as shown in Figure 1. The cell is composed of two electrodes (anode and cathode) immersed in a liquid electrolyte or adjacent to a solid electrolyte membrane, two porous transport layers (which facilitate the transport of reactants and removal of products), and the bipolar plates that provide mechanical support and distribute the flow.

Typically, hydrogen and oxygen are produced from the decomposition of water by two half-reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution 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:

???ℎ???:              2?2?(?) + 2?- → ?2(?) + 2??-(??)

?????:                     2??-(??)→ 0.5 ?2(?) + 2 ?- + ?2?(?)

Overall reaction:  ?2?(?) → ?2(?) + 0.5 ?2 (?)

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 O2 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+, O2-), 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 of catalyst, number of operating hours, and load variation, which are discussed in this article.

The electrolyser efficiency depends on the specific energy consumption with respect to the hydrogen production rate (Sakthivel, S., 2022). Measures for electrolyser efficiency include voltage efficiency, faradaic efficiency, thermal efficiency, electrical efficiency, and net efficiency, based on energy loss and so on.