Decarbonisation Technology - February 2023

1.0 A/cm 2 at 1.55-1.65V at high temperatures and as high as 1.2 A/cm 2 at a higher voltage of 2V (Jensen et al , 2019). In comparison, PEM electrolysers can operate at higher current densities of about 1-2.3 A/cm 2 . Furthermore, current developments suggest that the range of current densities for PEM is likely to increase to 2-3 A/cm 2 in the near future. This would reduce the capital investment for PEM. As current density increases, efficiency decreases, which leads to a higher power (voltage) requirement. The current density and efficiency can be improved by increasing the operating temperature of the cell, which can speed up activated electrochemical reactions and use of stable hydrogen evolution reaction catalyst. However, there are some generic challenges, independent of the technology used, including:  When increasing current density, ohmic losses in the electrolyser increase, which leads to increasing power costs.  High temperature in the electrolyser causes extreme corrosion due to the temperature and nature of the electrolyte.  Higher current density will create a lot of gas bubbles, which will prompt ohmic, activation, and mass transport losses. At high current density, massive hydrogen gas bubbles are rapidly formed on the electrode surface. Usually, these gas bubbles gather on the electrode contact surface of the catalyst and the electrolyte, which delays the mass transfer of the liquid and causes ohmic losses. It slows down the electron transfer and reduces the exposed active sites, resulting in decreased electrocatalytic activity and stability (Zhang L, 2022). OEMs should focus on the use of super- hydrophilic or super-aerophobicity surface electrodes. Super-hydrophilic electrodes can increase the wettability between the catalyst

surface and aqueous electrolyte. Then, the super-aerophobicity surface will accelerate the separation of gas bubbles from the electrode surface, which could repeal the bubbles underwater. Generally, the gas bubble diameter depends on the adhesion force of the catalyst film, and the adhesion force originates from the solid-liquid-gas phase as three-phase. Therefore, OEMs should concentrate on the best way of gas bubble generation and separation, which optimises the current density to increase the hydrogen production rate. Electrode : General criteria for selection of suitable anode materials include (a) stable at oxygen evolution potential or oxidation reaction, (b) resistance to open-circuit (c) good electrocatalyst, (d) does not react with other cell materials, and (e) economically viable. At present, electrolyser developers are looking to improve the anode efficiency, durability, and scaling-up cost. Approaches to improve electrode efficiency include:  Optimising the temperature for increased efficiency while avoiding increasing the temperature to the point it causes anode corrosion.  Increasing the electrochemically active surface area of the electrode.  New electrode (anode and cathode) electrocatalysts based on mixed metal oxides.  Low internal electrode resistance, which depends on both the anode materials and structure. Once the desired efficiency is achieved, the electrode must be evaluated for its stability or durability with respect to physical and chemical degradation. At present, OEMs are commonly using electrodes made from high surface area nickel (Ni) coated perforated stainless steel for

Parameters

AWE

PEM

SOE

AEM

Electrode (anode) for

Nickel-coated

Iridium

Perovskite-type (e.g., LSCF, LSM)

Nickel or

O 2 evolution reaction (OER)

perforated

oxide

Ni-FeCo alloys

stainless steel Nickel-coated

Electrode (cathode) for H 2 evolution reaction (HER)

Platinum

Ni/YSZ

Nickel

perforated

nanoparticles on

stainless steel

carbon black

Table 2 Type of anode and cathode for different technologies