Simple and Scalable Manufacturing Process of Raney-Ni Electrode Via Dip-Coating Method for Alkaline Water Electrolysis
Authors: Lee Hae In , Kim Suhyun , Kim Hee Soo , Cho Jaehoon , Lim Dong-Ha 
Published: December 2024

Green hydrogen from water electrolysis coupled with renewable energy is important to overcome the climate change crisis and drives the energy paradigm shift from fossil fuels to eco-friendly energy. Water electrolysis technology was first discovered in the 19th century, and hydrogen for ammonia synthesis was produced in a 165 MW water electrolysis system linked to the Aswan Dam in Egypt in the early 20th century. Currently, research is being actively conducted to implement a water electrolysis system using wind and solar power generation. Among water electrolysis technologies, alkaline water electrolysis has a high technological maturity based on its long history and has the advantage of being highly economical as it does not use precious metal catalysts. However renewable energy has intermittent and irregular power production characteristics, the water electrolysis system must include load-following technology that can follow load changes when connected to renewable energy.

Most electrode materials used in alkaline water electrolysis are Ni-based catalysts, and Raney-Ni is an advanced catalyst through the enlarged specific surface area by forming an intermetallic compound layer such as combined with Ni, Al, and Zn on the surface of the Ni electrode. Complex structural electrodes including many pores are generally used in order to realize a zero-gap design in alkaline water electrolysis. VPS (vacuum plasma spraying), PVD (physical vapor deposition), and electroplating are generally used to form Raney-Ni on the surface of a complex structural substrate. However, above mentioned methods have a technical issue for scaling up the area of the electrode because certain conditions must be established in the vacuum chamber or plating bath.

In this study, high-performance Raney-Ni electrode was manufactured using a dip-coating method which is to soak the Ni substrate in the slurry consisted of Al powder and polymeric binder. Heat treatment condition was adjusted to find the optimal temperature range for forming suitable Ni-Al intermetallic compounds on the Ni foam (NF) surface. As a result, the appropriate heat treatment temperature was 700 oC, and the prepared electrode was evaluated by linear sweep voltammetry (LSV) to confirm the hydrogen generation reaction performance. The ratio of polymer binder and Al was adjusted to confirm the optimal slurry viscosity to determine the optimal Al loading conditions on the surface of the complex structural NF. LSV, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were conducted to measure HER performance of prepared Raney-Ni electrodes. Fig. 1a. shows a comparison of HER performance between conventional NF and prepared electrode, and the Raney Ni/NF is achieved for low overpotential of more than 170 mV at the current density of -0.3 A/cm2. In addition, the outstanding kinetics of Raney-Ni were confirmed on the Tafel slope (Fig. 1b). Performance improvement was investigated through CV analysis by improving the electrochemically active area due to the Ni-Al intermetallic compound layer formed on the Ni surface. As shown in 1c, the improvement in surface roughness was confirmed through SEM analysis (Fig. 1c).

It is important to respond to load fluctuations from renewable energy and the durability of components in electrolytic stack because the value of alkaline water electrolysis lies in high economy for utilizing surplus energy from renewable energy. Durability and responsiveness were confirmed by repeatedly applying currents from -0.03 to -0.3 mA/cm2 of electrolytic cell equipped with the manufactured Raney-Ni electrode for 100 cycles. The excellent HER performance of the Raney-Ni electrode, which was already confirmed through the previous electrochemical analysis, was also confirmed in repeated current application experiments, and excellent responsiveness and durability were also confirmed (Fig. 1d). It was confirmed that the prepared Raney-Ni electrode showed high responsiveness, with voltage appearing immediately according to the applied current (Fig. 1e). Additionally, there was small or no changes of performance despite repeated load fluctuations during 100 cycles (Fig. 1f).

In the previous experiment, HER performance, responsiveness, and durability were confirmed through a half-cell experiment. The water electrolysis efficiency of the prepared electrode was confirmed through in-situ analysis, and durability was confirmed for over 100 hours in a constant current condition. In addition, the electrode corrosion resistance caused by shunt current, which is important in the alkaline water electrolysis stack, was confirmed through the alkaline water electrolysis stack equipped with the prepared electrode and stack was performed on/off test over 100 cycles. As a result, the electrode showed only low performance degradation. Finally, an m2 electrode was manufactured by dip-coating method, and to conduct performance evaluation through the sample which is at a random location with a size of 25 cm2. Samples achieved uniform performance, although efficiency difference was observed.

Figure 1