Anode Catalyst Materials for AEM Water Electrolysis

Anode Catalyst Materials for AEM Water Electrolysis – High Volume Electrolyzer Coatings – Cheersonic

The anodic oxygen evolution reaction (OER) has been the focus of many research papers since the early twentieth century, due to its relatively weak electrochemical activity compared to, for example, the acidic hydrogen evolution reaction (HER). This is best described by comparing their exchange current densities (ECDs), i.e., their intrinsic activity at zero net current density. It has been estimated that the exchange current densities for OER and HER under acidic conditions are 1×10-4mA/cm2 and 1mA/cm2, respectively. The advent of PEM technology in the 1960s brought more attention to this topic, as these ECDs represent severe kinetic boundaries that limit the efficiency of PEM electrolyzers and fuel cells.

Under alkaline conditions, the situation is reversed, as OER catalysts that tend to corrode and dissolve rapidly under acidic conditions show both activity and stability in alkaline electrolytes. Furthermore, the difference in ECDs (exchange current densities) between alkaline OER and HER is smaller compared to acidic conditions, which means that research efforts are more balanced when studying both reactions in alkaline electrolytes.

New alkaline OER catalysts are almost completely free of precious metals (PGMs), and are becoming less dependent on critical raw materials (CRMs) as elements such as cobalt and vanadium become increasingly scarce due to their increased use in battery/consumer electronics production. The most studied alkaline OER catalyst materials include iron, nickel, cobalt, manganese and chromium, usually in bimetallic or ternary combinations. One or more of the above elements are usually combined in the form of spinels, perovskites and general oxides, showing activity and stability under alkaline conditions.

Nickel-iron catalyst materials are one of the most common OER materials, for example, 3D-NiFe-LDH films generated on nickel foam/nickel mesh by hydrothermal method (hydrothermal method, in simple terms, refers to a method of preparing materials by dissolving and recrystallizing powders in a sealed pressure vessel with water as solvent.) show good activity. This is because LDHs are converted in situ to NiOOH, where NiOOH is the OER active phase. Its stability was tested by time-varying potentiometry under ambient conditions, and it maintained 97.8% activity (-3.30 mV/h) over a 10-hour test. This is attributed to the strong binding force between the catalyst and the substrate, and even ultrasonic treatment could not cause the catalyst film to fall off. Although NiOOH may be the active phase of NiFe catalyst materials during OER, it was found that iron incorporation is crucial for OER activity.

The activity of chemically reduced Ni100-xFex/50wt.% CeO2 (x = 0, 5, 10, 20, 40 at.%) peaked at Ni80Fe20/50wt.% CeO2, which is due to the fusion of iron with NiOOH species. Further increasing the iron content produces a less active iron-rich separated phase. The activity of NiFe catalysts was also studied in detail, and nanoscale NiFe LDHs in nitrogen-doped graphene frameworks (NGFs) showed potential for alkaline OER.

NiOOH was also identified as the source of OER activity by comparing the oxidation peaks corresponding to the Ni(II)/Ni(III) redox and the corresponding LSV curves. The stability was determined by a degradation rate of 3.57 mV/h based on the LSV curve data measured before and after a 3.3-hour constant current potential polarization test at around 15mA/cm2. Although the activity peak was determined in nickel-iron layer double hydroxides (LDHs) with an iron content of 20~25 at.%, the associated stability was questioned. The higher the iron content, the higher the stability, showing good stability even under strict square wave potential cycling conditions. Another different Ni-Fe LDH material showed excellent activity and durability, with an iron content close to 50at.%. This Ni-Fe LDH electrode was produced by etching an iron substrate in a nickel salt solution, and the resulting Ni-Fe LDH film maintained activity and durability for more than 5000 hours at 1.0 mA/cm2. The excellent durability is attributed to the stable LDH, which acts as both a protective film for the iron substrate and an active catalytic layer. Similar to the NiFe catalyst, the iron content was varied in several WxCoyFe1-x-y/NF catalysts to produce W0.5Co0.4Fe0.1/NF, which showed activity and stability in terms of OER. The addition of iron reduced the degree of cobalt oxidation, thereby optimizing the binding energy of OER intermediates and thus improving the kinetics of OER. The stability of 504 hours at 20 mA/cm2 by electrochemical impedance spectroscopy is likely due to the presence of Fe, which suppresses the typical stability issues associated with Co oxidation. In recent years, the use of heteroatoms (such as B, S, N, and P) as dopants to change the electronic structure of transition metals has also shown good potential, such as Ni2Fe8-Ni2S3/NF catalysts.

The Ni2Fe8-Ni2S3/NF catalyst performs well in maintaining relevant current density (j≥1.0mA/cm2) and has little degradation over 300 hours of operation. XPS and XRD after stabilization show that S seeps out of the surface oxide layer but remains stable inside the material, allowing the material to remain both active and stable. Other transition metals can also be used in combination with LDHs, among which CoFeCe0.5 has been developed to good effect. The interface between CoFe LDHs and CeO2 improves OER activity by reducing the OH- adsorption energy to change the rate-determining step (RDS), making the OER kinetics more facile. Using multi-interface materials to reduce the energy barriers associated with RDS is a common method for making composite reaction catalyst materials.

To this end, V-doped CoCOx were synthesized on nickel foam (NF) and showed activity for both oxygen evolution (OER) and hydrogen evolution (HER) in alkaline electrolytes. V doping changes the electronic structure of CoCO, bringing the d-band closer to the Fermi level, which significantly improves the OER activity. Another example involving V and Co is the addition of P, which results in a catalyst material that is most effective for both OER and HER. A series of V-CoP catalysts were developed by tuning the current density employed during the electrodeposition process. All catalysts showed the same intrinsic OER activity (η@20 mA/cm2), suggesting that the electrodeposition current density helps tune the mass transport properties. The highly porous structure resulting from the high current density electrodeposition process enables the electrode to effectively handle large currents during the oxygen evolution process, resulting in rapid detachment of gas bubbles from many active sites. The relatively high degradation rates found in some studies may be related to the short time for which stability was measured. Similar catalyst materials based on NiFe LDHs showed comparable performance.

Basic overview of alkaline OER catalyst materials, mainly composed of various nickel-iron combinations. Stability has been primarily tested in alkaline electrolytes, but there is growing interest in improving the activity of the catalysts in deionized water. This coincides with the need to use deionized water as the electrolyte for alkaline water electrolysis, where PEM-based water electrolysis may become obsolete if acceptable performance (E ≤ 2 V, j ≥ 10 A/cm², T ≥ 50 °C) is achieved using non-PGM materials.

Creating a material that is catalytically active in an initially ion-free analyte is quite difficult, which in many cases has led to a redoubling of efforts to develop CRM-free catalyst materials for use in KOH electrolyte environments. In this regard, stainless steel is inexpensive, ubiquitous, and widely used, and has recently shown good OER activity in alkaline electrolytes. Therefore, the current trend is to use stainless steel GDLs as both GDLs and OER catalysts.

Hydrogen production by electrolysis of water is the most advantageous method for producing hydrogen. Utrasonic coating systems are ideal for spraying carbon-based catalyst inks onto electrolyte membranes used for hydrogen generation. This technology can improve the stability and conversion efficiency of the diaphragm in the electrolytic water hydrogen production device. Cheersonic has extensive expertise coating proton exchange membrane electrolyzers, creating uniform, effective coatings possible for electrolysis applications.

Cheersonic ultrasonic coating systems are used in a number of electrolysis coating applications. The high uniformity of catalyst layers and even dispersion of suspended particles results in very high efficiency electrolyzer coatings, either single or double sided.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

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