A microscopic view of the nickel felt material that had the optimum combination of surface area and bubble release for producing hydrogen via electrolysis. Image: Wiley Lab, Duke University.
A microscopic view of the nickel felt material that had the optimum combination of surface area and bubble release for producing hydrogen via electrolysis. Image: Wiley Lab, Duke University.

Electrolysis – passing a current through water to break it into gaseous hydrogen and oxygen – could offer a handy way to store excess energy from wind or solar power. The hydrogen can be stored and used as fuel later, when the sun is down or the winds are calm. Without some kind of affordable energy storage like this, billions of watts of renewable energy are wasted each year.

For hydrogen to be the solution to the storage problem, however, water-splitting electrolysis would have to be much more affordable and efficient, said Ben Wiley, a professor of chemistry at Duke University. And he and his team have some ideas about how to accomplish that.

They recently tested three new materials that might be used as a porous, flow-through electrodes to improve the efficiency of electrolysis. Their goal was to increase the surface area of the electrode for reactions, while avoiding trapping the gas bubbles that are produced.

"The maximum rate at which hydrogen is produced is limited by the bubbles blocking the electrode – literally blocking the water from getting to the surface and splitting," explained Wiley.

In a paper in Advanced Energy Materials, Wiley and his team report comparing three different configurations of a porous electrode through which the alkaline water can flow as the reaction occurs. This involved fabricating three kinds of flow-through electrodes, each a 4mm square of sponge-like material, just 1mm thick. One was made of a nickel foam, one was made of a 'felt' of nickel microfibers and the third was made of a felt of nickel-copper nanowires.

Pulsing current through the electrodes for five minutes on, five minutes off, they found that the felt made of nickel-copper nanowires initially produced hydrogen most efficiently, because it had a greater surface area than the other two materials. But within 30 seconds, its efficiency plunged because the material became clogged with bubbles.

The nickel foam electrode was best at letting the bubbles escape, but it had a significantly lower surface area than the other two electrodes, making it less productive. The sweet spot turned out to be the felt of nickel microfiber, which produced more hydrogen than the felt of nickel-copper nanowires despite having 25% less surface area for the reaction.

Over the course of a 100-hour test, the microfiber felt produced hydrogen at a current density of 25,000 milliamps per square centimeter. At that rate, it would be 50 times more productive than the conventional alkaline electrolyzers currently used for water electrolysis, the researchers calculated.

The cheapest way to make industrial quantities of hydrogen right now isn't by splitting water, but by breaking natural gas (methane) apart with very hot steam. This is an energy-intensive approach that creates 9–12 tons of carbon dioxide for every ton of hydrogen it yields, not including the energy needed to create 1000°C steam.

Wiley said that commercial producers of water electrolyzers may be able to make improvements in the structure of their electrodes based on what his team has learned. If they could greatly increase the hydrogen production rate, the cost of hydrogen produced from splitting water could go down, perhaps even enough to make it an affordable storage solution for renewable energy.

He is also working with a group of students in Duke's Bass Connections program who are exploring whether flow-through electrolysis might be scaled up to make hydrogen from India's abundant solar power.

This story is adapted from material from Duke University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.