Cheaper green hydrogen catalyst nearly matches costly ruthenium

A team of researchers has developed a cost-effective catalyst for green hydrogen production that performs nearly as well as a ruthenium-based benchmark, achieving results within 50 millivolts during oxygen evolution. This development offers a potential alternative to catalysts made from rare and expensive precious metals.

The researchers engineered catalysts using nickel and iron oxides anchored to carbon nanofibers, which were grown on a conductive two-dimensional material known as MXene. This approach could help reduce the cost of water electrolysis, a carbon-free hydrogen production process.

Green hydrogen is produced by splitting water into hydrogen and oxygen using electricity, a process that emits no carbon dioxide when powered by renewable energy. In contrast, conventional hydrogen production relies heavily on fossil fuels and requires electrocatalysts made from costly metals like platinum, ruthenium, and iridium.

The researchers highlighted that approximately 96 percent of global hydrogen production currently depends on fossil fuels. Producing one kilogram of conventional gray hydrogen results in about 22 pounds (10 kilograms) of carbon dioxide emissions, while blue hydrogen produces around 13 pounds (6 kilograms).

To reduce dependence on precious metals, the team used Ti₃C₂Tₓ MXene as a conductive scaffold and grew carbon nanofibers directly on its surface via chemical vapor deposition. Iron or nickel oxide particles formed at the fiber tips, resulting in two catalyst materials: FeO-CNF@MX and NiO-CNF@MX.

Microscopy and spectroscopy analyses confirmed the successful formation of carbon nanofibers across the MXene sheets, maintaining the material's layered structure. The nickel-based composite yielded a more graphitic and less defective carbon structure compared to the iron-derived version.

The team tested both materials in an alkaline electrolyte for water splitting. For the hydrogen evolution reaction, the nickel-based catalyst required an overpotential of 490 millivolts (mEV) to achieve a current density of 10 milliamps per square centimeter, compared to 520 mEV for the iron version. Both outperformed similar catalysts without MXene support.

The most significant improvement was observed during the oxygen evolution reaction, where the nickel catalyst required only 300 mEV, compared to 360 mEV for the iron catalyst. Ruthenium oxide, a benchmark catalyst, required 250 mEV, leaving the nickel material just 50 mEV behind despite using less expensive metals.

The nickel composite also demonstrated slightly lower charge-transfer resistance, indicating more efficient electron movement during the reaction. "Growing the fibers directly on the MXene sheets provided a conductive backbone and numerous accessible active sites," the researchers explained.

In their study, scientists assembled complete water-splitting cells using the same catalyst on both electrodes. They found that the iron-based material delivered stronger initial performance, reaching target current density at 2.2 volts, while the nickel catalyst required 2.7 volts. Iron's ability to stabilize reaction intermediates during oxygen production contributed to enhanced overall cell performance.

Both prototype electrolyzers remained stable during a 24-hour test, with cell voltage increasing by only 0.2 to 0.3 volts. The nickel catalyst exhibited slightly less performance drift over time.

Although these new materials do not yet match platinum catalysts, which require only about 70 millivolts for hydrogen production, the researchers suggest this work demonstrates a simple and scalable method to create efficient electrocatalysts from abundant, low-cost materials.

The findings were published in ACS Applied Nano Materials.