In the face of climate change, our need to switch to clean energy sources is more pressing than ever before. However, many of these energy sources, such as solar and wind energy, face a key issue with intermittency, as the energy they supply is dependent on the time of day and weather. A potential solution to offset intermittency is electrochemical water splitting, which generates hydrogen fuel and oxygen.
However, electrochemical water splitting also comes with its challenges. Besides being slow, the process is also costly due to the use of noble metals such as ruthenium and iridium as electrocatalysts. Switching to cheaper metals such as nickel would help to reduce costs associated with electrochemical water splitting. While scientists have been working on nickel hydroxide as an alternative for more than two decades, its performance as a catalyst has not been up to par with industry standards thus far.
Seeking to boost the effectiveness of nickel hydroxide catalysts, researchers at A*STAR’s Institute of High Performance Computing (IHPC), joined by others from the Institute of Chemical and Engineering Sciences (ICES) and the National University of Singapore, have employed a combination of theoretical and experimental methods to optimize catalyst design.
"The oxygen evolution reaction (OER) is the bottleneck in electrochemical water splitting due to the sluggish kinetics arising from the oxygen double bond formation and multiple electron transfer reactions,” explained Zhigen Yu, a Senior Scientist at IHPC and one of the study’s authors. Hence, nickel hydroxide’s catalytic action in OER is crucial for water splitting efficiency.
“Based on our density functional theory (DFT) simulations, we found that reducing nickel coordination to four from six dramatically reduces the OER overpotential to 0.36 V,” explained Yu. The reduction in overpotential indicates that less energy is required to kickstart OER, thereby increasing reaction rate and catalytic efficiency.
To achieve the chemical configuration needed for reduced overpotential, the group designed a nickel hydroxide catalyst in the form of nanoribbons. Using X-ray absorption spectroscopy and scanning transmission electron microscopy, they confirmed that the nickel hydroxide nanoribbons were able to provide stable four-coordinated nickel sites, which resulted in improved OER and water splitting efficiency. The group also found that the overpotential was a mere 162 mV, one of the best performances among nickel hydroxide catalysts to date.
“For the first time, we present a novel alternating nickel 4-/6-coordinated nickel hydroxide that is successfully stabilized by introducing tensile strain,” Yu said. Apart from replacing noble metals in OER catalysts, the group’s methodology could reveal novel ways to reduce OER overpotential, which will make the switch to clean energy easier than before.
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing (IHPC) and the Institute of Chemical and Engineering Sciences (ICES).
