Enhanced Oxygen Evolution Reaction Performance in Anion Exchange Membrane Water Electrolysis by Ru-Doped Ni-Fe Catalysts
Authors: Ahn SunBeom , Han Gyeong Ho , Ahn Sang Hyun 
Published: December 2024

Research on future energy is underway due to the problems caused by global warming and fossil fuel depletion. Of particular interest among these is hydrogen energy, which is garnering significant attention due to its high energy density per unit mass and its abundance in space. Additionally, it has the advantage of being eco-friendly since it can be produced by electrolyzing water and does not produce carbon dioxide as a by-product when used.

The methods for electrolyzing water include alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE), and anion exchange membrane water electrolysis (AEMWE). In alkaline water electrolysis (AWE), achieving high current density is challenging due to significant ohmic losses arising from the considerable distance between the anode and cathode. For proton exchange membrane water electrolysis (PEMWE), the electrodes are closely situated within the membrane electrode assembly (MEA) structure, allowing for high current density. However, it suffers from the drawback of requiring precious metals due to the harsh corrosion conditions. AEMWE combines the advantages of PEMWE's compact structure and AWE's utilization of transition metals as catalysts. However, further research is needed on AEMWE as it has not yet reached the performance level of PEMWE, which uses noble metal catalysts known for their high reactivity in electrolysis.

In water electrolysis, the hydrogen evolution reaction (HER) occurs at the cathode, while the oxygen evolution reaction (OER) takes place at the anode. In the HER, two electrons are involved in the reaction, whereas in the OER, four electrons are involved, resulting in a slower reaction rate for OER. Therefore, since the overall reaction rate of water electrolysis is determined by OER, research on the OER catalyst is essential. Among transition metals, Ni is known to be a good catalyst for OER, and research is underway on Ni-based catalysts enhanced in activity and durability through synergy effects with other transition metals such as Fe, Co, and Mo. However, as the performance still falls short compared to precious metals, incorporating a small amount of precious metals alongside can enhance the catalyst's performance. Precious metals such as Ir and Ru are known for their good activity in the OER. Nonetheless, Ir is considerably more expensive than Ru, and since Ru exhibits slightly superior activity, we intend to use Ru.

There are several methods for synthesizing catalysts, including chemical precipitation, chemical vapor deposition, hydrothermal synthesis, sol-gel method, and electrodeposition. Among them, electrodeposition allows for the variation of catalyst composition, morphology, thickness by changing the composition of the solution, applied voltage, and time. Moreover, it occurs at room temperature and pressure and offers rapid fabrication times, making it advantageous for scaling up from lab scale to industrial scale in the future.

Galvanic replacement is a spontaneous substitution reaction that occurs due to the standard reduction potential difference between two metals. The metal with a relatively higher (more negative) standard reduction potential undergoes oxidation to form ions, while the metal with a lower standard reduction potential is reduced from ions to metal. Through this reaction, a small amount of precious metals can be doped onto the surface in nanoparticle form. Since this reaction occurs spontaneously, no additional devices are required, and it can occur at room temperature and pressure, similar to electrodeposition, making it suitable for scaling up to industrial scale.

Here in, we synthesized NiFe catalysts on SUS paper via the electrodeposition method. Moreover, we incorporated a small amount of Ru through galvanic replacement. We evaluated the performance of the catalysts under 1 M KOH conditions. In the half-cell test, NiFe/SUS exhibited an overpotential of 256 mV at 10 mA/cm2, which was reduced to 229 mV with Ru doping. In the single-cell test, Ru-NiFe/SUS showed excellent performance, achieving 8.3 A at 80°C and 2 V.