Electrochemical/photoelectrochemical water splitting is widely considered to be a critical step for efficient renewable energy production, storage and usage such as sustainable hydrogen production, rechargeable metal-air batteries and fuel cells. Currently, the state-of-the-art catalysts to split water are IrO₂ and Pt for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, with ~1.5 V to reach 10 mA cm⁻² current (for integrated solar water splitting). However, the price and scarcity of these noble metals present barriers for their scale-up deployment.

A great deal of effort and progress have been made towards efficient OER and HER catalysts with earth-abundant materials, such as cobalt phosphate, perovskite oxides and transition metal oxides (TMOs)/layer-double-hydroxides for OER and transition metal dichalcogenides and nickel molybdenum alloy for HER. However, pairing the two electrode reactions together in an integrated electrolyzer for practical use is difficult due to the mismatch of pH ranges in which these catalysts are stable and remain the most active. In addition, producing different catalysts for OER and HER requires different equipment and processes, which could increase the cost. Therefore, developing a bifunctional electrocatalyst with high activity towards both OER and HER in the same electrolyte becomes important yet challenging.

… Here we demonstrate a novel bifunctional catalyst of lithium-induced ultra-small NiFeO_x nanoparticles (NPs), with a remarkable performance of only 1.51 V (280 mV overpotential) to achieve 10 mA cm⁻² current in 1 M KOH solution for long-term operation.

—Wang et al.

A conventional water-splitting device consists of two electrodes submerged in a water-based electrolyte. A low-voltage current applied to the electrodes drives a catalytic reaction that separates molecules of H₂O, releasing bubbles of hydrogen on one electrode and oxygen on the other. Each electrode is embedded with a different catalyst, typically platinum and iridium.

In 2014, Stanford chemist Hongjie Dai developed a water splitter made of inexpensive nickel and iron that runs on an ordinary 1.5-volt battery. (Earlier post.)In the new study, Profesor Yi Cui and his colleagues advanced that technology further.

In conventional water splitters, the hydrogen and oxygen catalysts often require different electrolytes with different pH—one acidic, one alkaline—to remain stable and active. An expensive barrier is needed to separate the two electrolytes, adding to the cost of the device, said graduate student Haotian Wang, lead author of the study.

Our water splitter is unique, because we only use one catalyst, nickel-iron oxide, for both electrodes. Our single-catalyst water splitter operates efficiently in one electrolyte with a uniform pH.

—Haotian Wang

To find catalytic material suitable for both electrodes, the Stanford team borrowed a technique used in battery research called lithium-induced electrochemical tuning. The idea is to use lithium ions to chemically break the metal oxide catalyst into smaller and smaller pieces.

Breaking down metal oxide into tiny particles increases its surface area and exposes lots of ultra-small, interconnected grain boundaries that become active sites for the water-splitting catalytic reaction. This process creates tiny particles that are strongly connected, so the catalyst has very good electrical conductivity and stability.

—Yi Cui

Haotian discovered that nickel-iron oxide is a world-record performing material that can catalyze both the hydrogen and the oxygen reaction, Cui said. “No other catalyst can do this with such great performance.”

By improving both OER and HER activities, the galvanostatic cycling method successfully elevates the efficiency of water-splitting electrolyzer at 10 mA cm⁻² current to 81.5% using only one material, making good preparations for the scale-up of water photolysis/electrolysis with high efficiency and low cost. Synthesizing catalysts on conducting substrates can maximally reduce the use of carbon additives and also get rid of polymer binders, which enables high-current operations (circumvent bubble-releasing problems introduced by the hydrophobic nature of carbon) and also performs superior stabilities. In addition, the successful demonstration of the Li conversion reaction method in improving water-splitting catalysis may help to inspire the improvements of other important TMOs applications including oxygen reduction reactions, supercapacitors, carbon dioxide reductions and so on.

—Wang et al.

Resources

• Haotian Wang, Hyun-Wook Lee, Yong Deng, Zhiyi Lu, Po-Chun Hsu, Yayuan Liu, Dingchang Lin & Yi Cui (2015) “Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting” Nature Communications 6, Article number: 7261 doi: 10.1038/ncomms8261