Anion Exchange Membrane Electrolysis Technology

What Is AEM Electrolysis Technology?

Anion exchange membrane (AEM) electrolysis technology combines the advantages of traditional alkaline electrolysis technology and proton exchange membrane (PEM) electrolysis technology. In AEM electrolysis, on the one hand, low-cost catalytic materials of alkaline electrolysis are used, and on the other hand, the solid polymer electrolyte architecture of PEM electrolysis technology is borrowed.

Comparison of AEM electrolysis, traditional alkaline electrolysis and PEM electrolysis technology. [1]

How AEM Electrolysis Technology Works

AEM water electrolysis cells consist of a hydrocarbon anion exchange membrane and two electrodes based on transition metal catalysts. Distilled water or low-concentration alkaline solutions can be used as electrolytes in AEM instead of concentrated KOH solutions (in alkaline electrolysis systems). AEM water electrolysis technology consists of two simultaneous half-cell reactions, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).

Schematic diagram of AEM electrolysis. [2]

The alkaline HER reaction steps:

The Volmer step: H₂O + e⁻ + catalyst = catalyst-Had + OH⁻ (1)

Heyrovsky step: H₂O + catalyst-Had + e⁻ = catalyst+ H₂+ OH⁻ (2)

The combined Tafel step: 2catalyst-Had = catalyst + H₂ (3)

The alkaline OER process:

M + OH⁻ = MOH + e⁻ (4)

MOH + OH⁻ = MO + H₂O (l) + e⁻ (5)

MO + OH⁻ = MOOH + e⁻ (6)

MOOH + OH⁻ = M + O₂ (g) + H₂O (l) + e⁻ (7)

Overall: 4 OH⁻ = O₂ (g) + 2 H₂O (l) + 4 e⁻ (8)

(Had=hydrogen intermediates; M indicates the active catalyst.)

Key Factors Influencing AEM Electrolysis

Electrocatalysts: Compared with traditional noble metal electrocatalysts, such as Ir, Pt, Ru and their oxides, catalysts such as Ni, Fe and Co alloys, and graphene are potential low-cost electrocatalyst alternatives.
Selection of AEM: The stability of AEM, including thermal, mechanical and chemical stability, is another critical aspect of the success of AEM electrolyzers.
Ionomers: Ionomers often act as bridges, acting as reliable conductors between the AEM and the surface active sites on the catalyst layer. Increasing the interaction between the ionomer and the polymer matrix increases the mechanical strength of the AEM while also decreasing the chain mobility and total pore volume of the AEM.

Membrane Electrode Assembly

A membrane electrode assembly (MEA) is a collective system containing an AEM, ionomer, anode, and cathode. Common methods for manufacturing MEA are the catalyst coated substrate (CCS) and catalyst coated membrane (CCM).

CCS Method

The electrocatalyst and binder are mixed to form a uniform ink, and then the ink is coated on both sides of the AEM by spraying. After placing the AEM between gas diffusion layers (GDL), it is processed using mechanical or thermal pressure.

CCM Method

Catalyst ink is deposited directly on GDL and then sintered to obtain a gas diffusion electrode (GDE). The sandwich GDL (or GDE with AEM as the inner layer) is then heat pressed to produce the final MEA.

Comparison of CCS and CCM methods for preparing MEA. [1]