Decarbonisation Technology - February 2023

AWE, iridium oxide/platinum nanoparticle on carbon black for PEM, Ni or nickel, iron, cobalt, (Ni, Fe, Co) for AEM and perovskites, Ni/ Yttria stabilised zirconia (YSZ) for SOE. Nickel has traditionally been used for both anode and cathode material. It is relatively high activity in alkaline electrolytes, highly corrosion-resistant at positive potentials, and has good availability at low cost. Recently, nickel/iron (Ni, Fe)-based materials have demonstrated good catalytic activity towards OER and stability. However, the introduction of these conductive components in the electrolyte environment could also bring about some side effects. These side effects will reduce efficiency and durability. Table 2 shows the type of anode and cathode used as electrodes for different water electrolysis technologies. In PEM electrolysers, a harsh oxidative environment is created on the anode surface area in an acidic environment (by PFSA membrane) due to high voltage. Typically, noble metal electrodes are used to mitigate this. The noble metal provides long-term stability to cell components and optimal electron conductivity. This is one of the reasons that PEM is more expensive than AWE. Electrolyte : The electrolyte is the medium in which the generated chemical charges (anions (-) or cations (+)) migrate from cathode to anode and vice versa. AWE and AEM use potassium hydroxide (KOH) and sodium hydroxide (NaOH) as electrolytes. Figure 4 depicts the effect of electrolyte concentration with different temperatures and different types of electrolytes on specific conductivity (Brauns & Turek, 2020). The most common electrolyte for alkaline water electrolysis is an aqueous solution of KOH with 25 to 30 wt% KOH, as the specific conductivity is optimal within the typical temperature range from 50 to 80°C. The high temperatures improve

120

100

KOH, 50˚C

NaOH, 50˚C

KOH, 25˚C

NaOH, 25˚C

Electrolyte concentration , wt%

the electrode kinetics as well as the conductivity of the electrolyte. However, the challenge of exploiting the benefits of high temperatures lies primarily in the design and material selection for constructing an efficient and durable electrolyser (perfluoro sulphonic acid, PFSA or Nafion or solid polymer electrolyte) for the conduction of protons, separation of product gases, and electrical insulation of the electrodes is also in development. The PFSA membrane is chemically and mechanically robust, which allows for high pressure differentials. Thus, the PEM cells can operate at up to 70 bar with the oxygen side at atmospheric pressure. Table 3 depicts the type of electrolyte used for different electrolyser technologies. Increasing the operating temperatures in an electrolyser is not a trivial task. PEM electrolysis is unsuitable for high operating temperatures, as under acidic conditions, high temperatures lead to corrosion issues on polymer electrolytes and cause membrane stability problems. for these harsh operating conditions. In PEMs, the use of solid electrolyte Figure 4 Typical electrolyte concentration vs specific conductivity

Parameters

AWE

PEM

SOE

AEM

Electrolyte

KOH/NaOH

PFSA, Nafion, solid polymer electrolyte

Yttria-stabilised zirconia

NVF, DVB supported polymer matrix (PTFE) or NaHCO 3 or ceramic metal

(YSZ)/polysulfone

Carrier

OH -

H +

OH -

Table 3 Different type electrolytes