Decarbonisation Technology - February 2023

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. Electrolysers incur losses due to inefficiencies in the electrochemical reactions. These losses are related to the selection of electrolyte, the materials used in the electrode, and the electrical and ionic resistance as the current flows. At present, OEMs claim the electrolyser efficiency with respect to low heating value (LHV), and for AWE, PEM and SOE, it is 65%, 60%, and 75%, respectively. Figure 3 depicts typical electrolyser efficiency with specific energy consumption for an electrolyser with a capacity of 1 MW. AWE and PEM electrolysers are considered production from fossil fuels. The Capex of a PEM water electrolyser is 50-60% more expensive than AWE due to the use of rare earth metals like iridium, platinum, and gold and membranes, representing an additional barrier to market penetration. Both have the potential for cost reduction related to economies of scale (>20 MW), optimisation of modular size, and balance of plant (BOP) with automation, improvements, and optimisation of the basic cell components of current density, membrane thickness, rare earth metal, catalyst usage, and an increase in the availability of components (IRENA, 2020). In this context, OEMs should focus on further improvement of electrolyser efficiency and capital cost reduction. expensive from both Capex and Opex perspectives compared with hydrogen Cell stability and durability The durability, reliability, and safety of the cell and stack are important criteria for selecting an

350

Eciency, % H production rate

80 60 70

300

250

200

150

100

6.0 5.5 5.0 4.5 4.0 3.5 Speci f ic energy consumption, kWh/Nm 0

electrolyser. The following criteria affect the cell and stack performance: • Characteristic of current density vs voltage (IV-curve) • Material selection for the electrodes • Types of electrolytes used • Water impurities • Dynamic electrical load as intermittent energy input • Electrode separator - membrane/diaphragm Current density vs voltage: Increasing the current density (the amount of charge per unit time that flows through a unit area of electrode in 1 ampere/cm²) is necessary to achieve a higher hydrogen production rate (or other performance indicators for gas evolution) for a given voltage. Table 1 shows ranges in current density and voltage for different technologies. AWE is a well-established technology with relatively low costs. However, AWE can only operate at low current densities of <0.4-0.8 A/ cm 2 (at moderate temperatures of 70-90°C with a cell voltage in the range of 1.4-3V), which limits the potential of this technology. A recent study has achieved current densities for AWE of Figure 3 Typical electrolyser efficiency with specific energy consumption for an electrolyser with a capacity of 1 MW

Parameters

AWE

PEM

SOE

AEM

Nominal current density, A/cm 2 Voltage range (limits), V Operating temperature, °C

0.4-0.8 1.4-3 70-90

1-2.3

0.3-1

0.2-2

1.4-2.5 50-80

1.0-1.5

1.4-2.0 40-60

700-850

Table 1 Current density with voltage