A team led by researchers at the University of Tsukuba in Japan has fabricated non-noble-metal electrodes for the hydrogen evolution reaction (HER) for electrochemical water splitting using a bicontinuous and open porous NiMo alloy covered by nitrogen-doped (N-doped) graphene with nanometer-sized holes.

This noble-metal-free HER catalyst exhibits performance almost identical with that of a Pt/C electrode, while its original catalytic activity is preserved even in acidic electrolytes. A paper on the work is published in the journal ACS Catalysis.

Schematic illustration of the fabrication of a porous NiMo composite covered with holey graphene layers for hydrogen evolution in acidic electrolytes. Credit: ACS, Ito et al. Click to enlarge.

The electrolysis of water to hydrogen is vital for energy storage in a green economy. One of the major obstacles, however, is the high cost of noble-metal electrodes. Cheaper non-nobles do work, but mainly in alkaline conditions, where the reaction is electricity-hungry; the more efficient acid-phase reaction requires scarce commodity metals such as platinum. Worse still, the acid electrolytes are corrosive and eat away at the core metal.

The researchers found that “holey” graphene offers a way round this problem. They used nitrogen-doped graphene sheets to encapsulate a nickel–molybdenum (NiMo) electrode alloy; the graphene was punched full of nanometer-size holes, like a colander. In acid conditions, the new HER system significantly outperforms an electrode using regular non-holey graphene.

Density functional theory (DFT) calculations suggest that the fringes (hole region) between the graphene nanoholes and the NiMo surface enhances hydrogen adsorption and desorption, while the non-holey graphene layer (non-hole region) prevents excess impregnation and corrosion of the non0-noble metal by the acidic electrolyte. Our results reveal that the combination of a non-noble metal with holey graphene is a promising design route for noble-metal-free catalysts that are chemically stable in acidic electrolytes.

—Ito et al.

The use of graphene in HER electrodes is not new—this flexible, conductive carbon sheet is ideal for wrapping around the core metal. However, while protecting the metal against corrosion, graphene also suppresses its chemical activity. In the new Tsukuba system, the all-important holes promote the reaction in two ways, while the intact graphene part protects the metal.

We created holes by decorating the NiMo surface with silica nanoparticles. Then, when we deposited the graphene layer, gaps were left behind at the nanoparticle positions—like a relief artwork. In fact, the holes are more than just gaps—they are ringed by chemically active ridges called ‘fringes’. Technically, these fringes are structural defects, but they drive the chemistry of the electrode.

—co-author Kailong Hu

Compared with normal graphene, the fringes are more hydrophilic. This attracts hydronium (H₃O⁺) in the acid solution, which plays a crucial role in one of the two HER mechanisms. The fringes are also excellent at adsorbing single H atoms, which provides extra surface area for the other important HER process. As a result, H₂ is produced as efficiently as on a conventional (but expensive) Pt/C electrode. Meanwhile, the non-holey part of graphene delays the metal catalyst from dissolving in the acid.

This is a versatile new concept for hydrogen evolution electrodes. The goal is to minimize the overpotential needed for the reaction. Therefore, it’s not limited to one particular catalyst. We tuned our holey graphene layer specifically to NiMo by optimizing the size and number of holes. What’s impressive is that the catalyst was still stable in acid, despite the holes. In the future, holey graphene could be customized to a range of metals, pushing the efficiency of hydrogen production toward full-scale adoption.

—lead author Yoshikazu Ito

Resources

• Yoshikazu Ito, Tatsuhiko Ohto, Daisuke Hojo, Mitsuru Wakisaka, Yuki Nagata, Linghan Chen, Kailong Hu, Masahiko Izumi, Jun-ichi Fujita, and Tadafumi Adschiri (2018) “Cooperation between holey graphene and NiMo alloy for hydrogen evolution in an acidic electrolyte” ACS Catalysis 8 (4), 3579-3586 doi: 10.1021/acscatal.7b04091