• Splitting of water through use of the oxygen evolution reaction (OER) is difficult because the environment close to the electrode surface becomes very acidic leading to catalyst degradation. 
• Mixed nickel-iron hydroxide (MNF) has emerged as a potential catalyst, but the harsh environment during the OER has led to a phase segregation hindering performance. 
• The MNF catalyst can be regenerated through intermittent electrochemical reduction that reduces the acidic environment and reverses phase segregation.

One source of renewable energy involves the use of hydrogen as a fuel that can be produced through the splitting of water. Such a concept has the initial appearance of being relatively easy, but researchers have found that the half-reaction driving the oxidation of a hydroxyl ion to oxygen to be very challenging.

The main process used for splitting or the electrolysis of water has involved the use of a polymer electrolyte membrane that utilizes expensive platinum catalysts.

Feng Lin, assistant professor of chemistry at Virginia Tech in Blacksburg, Va. (see Figure 3), says, “Electrocatalysts seem to perform very well at the beginning of the reaction. The challenge is that the reaction governing splitting of water wants to operate in a thermodynamic state that is unfavorable to a catalyst that is trying to speed up the process kinetically. The oxygen evolution reaction (OER) becomes very difficult to run over the long term because, as the hydroxyl ions are consumed, the environment becomes very acidic close to the electrode surface, leading to catalyst degradation. It is hard to design a catalyst that is stable not only under acidic conditions but also over a wide pH range.”


Figure 3. Assistant professor Feng Lin led a research program that showed how a catalyst that is used in water splitting can be regenerated. Figure courtesy of Virginia Tech.

A catalyst that has shown promise in facilitating the OER is based on mixed nickel-iron hydroxide (MNF). Lin says, “MNF exhibits a crystal structure based on a nickel hydroxide hexagonal structure. The first clue that iron could be integrated into this structure came when Edison developed a battery with a nickel hydroxide electrode placed in a casing that contained iron. Iron was found to dissolve in the electrolyte surrounding the electrode and then eventually was found in the nickel hydroxide matrix.”

One of the degradation mechanisms that hinders catalyst performance is now known as phase segregation. Lin explains, “In the MNF, iron is evenly distributed throughout the nickel hydroxide lattice leading to the formation of a solid solution. Once the electrocatalyst is charged to a high voltage, both iron and nickel will be oxidized. Iron moves from the +2 to the +3 state, while nickel also can be oxidized from +2 to +3 and in some cases to the +4 state.”

Each of these metal’s oxidation states has a different bond length with oxygen in the MNF crystal lattice. Lin continues, “The change in bond length leads to lattice distortions and an increase in the mechanical stress of the MNF. This bond cleavage is facilitated by the acidic environment close to the surface of the catalyst during the OER reaction. The result of the lattice distortion and the acidity is the promotion of metal dissolution leading to a phase segregation where a secondary phase forms that is partitioned from the primary phase of the catalyst.”

Phase segregation causes a significant drop in catalyst performance. Lin and his colleagues found that in a chronoamperometry study conducted at 1.63 Volts versus the reversible hydrogen electrode, fading of the current density occurred rapidly at the beginning of the experiment and dropped by 25% after only 15 hours.

The researchers used synchrotron X-ray fluorescence microscopy to evaluate the nickel:iron ratio in the catalyst during the chronoamperometry study. As time progressed, phase segregation was detected in the MNF catalyst.

The researchers found that the ratio of nickel to nickel + iron gradually increases also providing a strong indication of iron depletion in the catalyst. Nickel also left the catalyst further reducing its effectiveness. Once the experiment is stopped and the catalyst is allowed to rest for four hours, the ratio of nickel to nickel + iron recovered to its original state as iron and nickel migrated back into the nickel hydroxide lattice.

Developing an approach to reverse phase segregation (beyond allowing the catalyst to rest, which is not feasible) is required to extend the operating life of MNF. A new approach has now been developed to achieve this goal.

Intermittent electrochemical reduction
Lin and his colleagues determined that a small energy difference exists between homogeneously distributed and segregated MNF. He says, “We found that applying a low reductive voltage will temporarily stop the OER and facilitate the transport of hydroxyl groups to the MNF so they can neutralize protons and reduce the acidic environment surrounding the catalyst’s surface. This process favors deposition of iron in the nickel hydroxide matrix and is called an intermittent electrochemical reduction. By using this approach, we have demonstrated that phase segregation can be reversed regenerating the MNF catalyst.”

Another aspect that governs this process is the physical size of the catalyst. The researchers evaluated the performance of thin MNF versus a bulk MNF. The thin MNF contains two atomic layers that enables all the metal sites to be exposed to the surface. In contrast, bulk MNF is physically a larger-sized catalyst that also contains some inert materials.

Lin says, “We determined that thin MNF could be fully recovered through intermittent electrochemical reduction in a short period of time while only a partial performance increase could be realized with bulk MNF under similar experimental conditions. By downsizing the catalyst, we can show better performance over a longer operating time.”

The researchers will be trying to better understand the fundamental aspects of how MNF catalyzes the OER reaction and how it can be regenerated.

As part of this work, Lin is looking to find the optimum pH for MNF to operate in and to identify a method that can be used to maintain the desired pH. Additional information on this work can be found in a recent article¹ or by contacting Lin at fenglin@vt.edu.

REFERENCE
1. Kuai, C., Xu, Z., Xi, C., Hu, A., Yang, Z., Zhang, Y., Sun, C., Li, L., Sokaras, D., Dong, C., Qiao, S., Du, X. and Lin, F. (2020), “Phase segregation reversibility in mixed-metal hydroxide water oxidation catalysts,” Nature Catalysis, 3 (9), pp. 743-753.