The robust, flexible electrodes recorded 98% water-splitting efficiency

Using paper coated with nickel nanoparticles and model catalysts as electrodes, researchers at Indian Institute of Science Education and Research (IISER) Kolkata have been able to split water and generate oxygen and hydrogen gas with very low overpotentials (voltage applied over and above the theoretical voltage to split water). The flexible electrodes recorded 98% water-splitting efficiency and maintained robustness and durability even after more than 10 continuous days of operation.

The current produced by the nickel-coated paper electrodes remained constant even when the material was subjected to extreme deformation — bending up to 180 degree. Similarly, the electrodes remained stable even when the electrolyte (potassium hydroxide) was made highly corrosive — more than 10 days of continuous operation at 1M (pH 14) and at least 12 hours at 10 M concentration.

The researchers led by Prof. Sayan Bhattacharyya from the Institute’s Department of Chemical Sciences showed that their method could be extended to other common substrates such as a cotton cloth which has similar structural characteristics like paper.

“Besides using it as paper-based electrodes, this is a platform technology for multiple applications. We are currently testing the nickel-coated paper as a glucose biosensor for detection of diabetes. It can be used in resource-poor settings,” says Prof. Bhattacharyya. The team has used their electrodes to fabricate wearable paper-based batteries.

Porous surface

“The paper-electrode has highly porous catalytic surface and this increases the kinetics of the reaction — ability of water molecules to reach the active sites in the electrode. The high porosity is responsible for the high efficiency of the paper-electrode,” says Atharva Sahasradudhe from the Department of Chemical Sciences, IISER Kolkata and first author of a paper published in Nature Communications.

The porous nature of the paper and abundance of functional groups on cellulose microfibres help in strongly binding different metal ions and finally nickel nanoparticles in a three-step immersion process. Coating the paper with nickel makes it electrically conductive. The nickel-coated paper is then coated with two different catalysts (nickel-iron oxyhydroxide and nickel-molybdenum alloy) to serve as an anode and a cathode.

Splitting water to generate oxygen and hydrogen gas requires cost-effective and stable catalysts that have high activity — generate higher current at lower applied voltage. The more current produced the more will be amount of water split and hydrogen gas produced.

The researchers were able to split water and generate oxygen when the voltage applied (on top of the theoretical voltage to split water) was as low as 240 millivolt to get a current density of 50 milliamperes per sq.cm. In the case of hydrogen, the applied voltage was just 32 millivolt to generate 10 milliamperes per sq.cm current density. “The porosity increases the ability of the water molecules to reach the catalytic sites where the molecules get split into hydrogen and oxygen gas,” says Sahasradudhe.

The researchers also used nickel-coated paper electrodes that had no catalysts. According to Prof. Bhattacharyya, the paper electrodes coated with only nickel, which can be used both as anode and cathode for splitting water, had far better performance than other such catalysts reported till date.

The team achieved “excellent” water splitting ability when nickel-paper electrodes coated with catalysts were used in electrolysis cells.

The total cell voltage required was merely 1.51 V at 10 milliamperes per sq.cm current density. “The lower cell voltage required to split water reflects the performance of the electrodes. This is one of the lowest reported values for water splitting in basic medium,” says Prof. Bhattacharyya.