Stability Investigation on NiFeOx Electrocatalysts during On and Off Cycles in Harsh Alkaline Conditions
Authors: Suzuki Reona , Obata Keisuke , Takanabe Kazuhiro 
Published: December 2024

Water electrolysis is the most practical and mature technique to produce green hydrogen. In addition to its efficiency, its durability also influences the production cost of hydrogen. The major concern is the durability of anodes for oxygen evolution reaction (OER), which are exposed to highly oxidative and acidic/alkaline environments at elevated temperatures. Additionally, intermittent operation is expected when coupled with renewable electricity, which leads to severe potential shifts on the electrodes during turning on and off. In this study, the stability of benchmark electrocatalysts, NiFeOx, was evaluated during on/off cycles in various electrolytes including commercially used highly alkaline solution (7 M KOH) at 80 °C. The NiFeOx showed a significant degradation particularly during on/off operation in 7 M KOH for 80 h.

A protocol for on/off test was constructed based on the reported protocol,[1] which is intended to reproduce power-on and power-off conditions repeated in a short period of time. At the beginning of a unit cycle, chronopotentiometry (CP) was conducted at 600 mA cm−2 for 30 s to perform OER followed by a potential sweep to 0.3 V vs. reversible hydrogen electrode (RHE) to reproduce the sudden voltage drop. The power-off condition is reproduced by chronoamperometry (CA) at 0.3 VRHE for 30 s. The degradation of the electrocatalyst was evaluated by steady state CP and cyclic voltammetry (CV) every 100 cycles. These operations are defined as one set, and continued till the catalytic performance obviously changes. The electrolyte was refreshed every 24 or 48 h before the electrolyte concentration changed due to evaporation. A benchmark NiFeOx electrocatalyst was deposited on a Ni foam by reported hydrothermal method.[2] Using the three-electrode system, NiFeOx durability tests were conducted in 1 M KOH at 298 K and 7 M KOH at 353 K. In 1 M KOH at 298 K, the potential to reach 600 mA cm−2 remained relatively constant even after 3000 on/off cycling (Figure 1). Although the initial potential in 7 M KOH was superior to that in 1 M KOH, the potential in 7 M KOH continuously increased with cycles and reached the OER potential using Ni foam substrate. We note that, during constant current operation at 600 mA cm−
2, the potential remained stable at 1.47 VRHE for 80 h indicating that a redox reaction of material itself during the potential sweep causes the degradation of NiFeOx. The volume change between Ni(OH)2 and NiOOH may cause a mechanical stress and promote the loss of catalyst into the electrolyte. Indeed, the Ni redox peak in CV and double layer capacitance sharply dropped within 5 h. A severe increase of the potential was observed when the electrolyte was replaced after 24 h. Fe in NiFeOx is considered to be the active site for OER.[3] The degradation in 7 M KOH may come from the formation of soluble Fe complex ions between the hydroxide and iron,[4] which is favored in the highly alkaline solution leading to the dissolution in the electrolyte. Inductively coupled plasma optical emission spectra (ICP-OES) measurements performed on the spent electrolytes showed that the Fe continued to dissolve in the electrolyte (purple diamond symbol in Figure 1). At the end of the test, 1.2 mg of the Fe was lost, which corresponds to 80 % of that in the original catalyst (1.4 mg).

To further reveal the degradation behavior, the electrolyte was saturated with Fe(NO3)3, and durability tests were conducted at 80 °C. It was found that the potential was maintained at around 1.53 VRHE in the solution saturated with Fe(NO3)3, even after replacing the electrolyte (closed symbol), indicating that severe deactivation was due to the loss of Fe species. However, the initial degradation to 1.53 VRHE still occurred even in the presence of the extra ferric ions, which may come from the mechanical stress due to the potential sweeping. The present results demonstrate the importance to investigate the stability at industrially relevant harsh conditions rather than the standard laboratory condition. Step-by-step clarification of the degradation mechanism would help to reveal the limitation and to develop highly durable electrocatalysts.

Figure 1