Analysis of Catalyst for PEM Electrolyzer

1. Introduction to PEM water electrolysis for hydrogen production

PEM Electrolyzer for hydrogen production, also known as proton exchange membrane water electrolysis for hydrogen production, refers to a hydrogen production process that uses a proton exchange membrane as a solid electrolyte and uses pure water as the raw material for electrolysis of water to produce hydrogen.

Compared with alkaline water electrolysis hydrogen production technology, PEM water electrolysis hydrogen production technology has the advantages of large current density, high hydrogen purity, and fast response speed. PEM water electrolysis hydrogen production technology has higher working efficiency.

However, since PEM electrolyzers need to operate in a highly acidic and highly oxidizing working environment, the equipment is more dependent on expensive metal materials such as iridium, platinum, titanium, etc., resulting in high costs.

PEM electrolyzer

2.PEM water electrolysis hydrogen production principle

PEM hydrogen production is mainly divided into the following four steps.

1. Water electrolysis and oxygen evolution

Water (2H2O) undergoes a hydrolysis reaction on the positive electrode and splits into protons (4H+), electrons (4e-) and gaseous oxygen (O2) under the action of the electric field and catalyst, as shown in equation (1).

2H2O=4H++4e- +O2 (1)

2. Proton exchange

4H+ passes through the solid PEM containing sulfonic acid functional groups and reaches the negative electrode under the action of the electric field.

3. Electronic conduction

4e-Electrons pass from the positive electrode to the negative electrode through the external circuit.

4. Hydrogen gas evolution

The 4H+ that reaches the negative electrode gets 4e- to generate 2H2, as shown in equation (2).

4H++4e-=2H2 (2)

3. PEM water electrolysis hydrogen production catalyst

The common commercial product of proton exchange membrane is perfluorosulfonic acid polymer membranes. Therefore, the working environment of PEM hydrogen production membrane electrode is highly acidic. The materials of each component need to consider corrosion resistance, and the catalyst is no exception. Generally, precious metals such as Platinum, iridium, ruthenium, etc.

The catalysts of the cathode and the anode of PEM electrolyzer for hydrogen production are different. The cathode is a platinum carbon catalyst, and the anode is generally an iridium-based catalyst such as iridium dioxide and iridium black. Low load capacity is one of the future technological development directions. In addition, catalytic structure optimization and waste precious metal recycling are also hot topics in the industry.

1. Cathode hydrogen evolution: platinum carbon catalyst

As a good catalyst, Pt can adsorb hydrogen molecules and promote dissociation, and is currently the first choice for commercial use. Platinum-on-carbon catalyst, referred to as Pt/C, also known as platinum-on-carbon catalyst, refers to a carrier catalyst that loads platinum onto activated carbon and is one of the subcategories of precious metal catalysts. Pt loading is generally 0.4-0.6 mg/cm2.

The chemical reduction method is currently the most commonly used platinum carbon catalyst production method. It refers to a method that uses activated carbon, distilled water, hexachloroplatinic acid solution, etc. as raw materials, and produces a platinum carbon catalyst through mixing and dissolution, ultrasonic vibration, chemical reduction treatment and other steps.

2. Anode oxygen evolution: iridium-based catalyst

Since the anode side is a high-oxygen environment, the anode electrochemical catalyst can only choose a few precious metal elements and their oxides such as Ir, Ru, which are highly resistant to oxidation and corrosion.

RuO2 and IrO2 have the best catalytic activity for oxygen evolution electrochemical reactions, and IrO2 has better stability, so IrO2 is the main material of oxygen evolution catalyst.

The preparation methods of iridium oxide mainly include thermal oxidation method, chemical precipitation method, Adams (Admas) melting method, sol-gel method, etc. For example, the chemical precipitation method usually adds an alkali (such as sodium hydroxide) to an iridium aqueous solution, and after the reaction, a hydroxide (Ir(OH)3(H2O)3 or IrOx·nH2O) precipitation is obtained, and then calcination is performed to obtain iridium oxide.