Punching holes in a sheet of the carbon material graphene can make an electrode for hydrogen generation far more effective in acidic conditions, according to work by researchers at the University of Tsukuba. [Y. Ito et al., ACS Catal, 2018; DOI: 10.1021/acscatal.7b04091]
The electrolysis of water to hydrogen is one of several options for "green" and sustainable energy supply. Unfortunately, the usual electrode materials are expensive noble metals. Alternative, less costly materials only work in alkaline conditions, which makes the hydrogen evolution reaction far less efficient and so commercially untenable. Acid conditions work well with pricey platinum but also dissolve the electrode.
Now, researchers led by a team at Tsukuba have developed "holey" graphene, which seems to offer a way to circumvent all of these problems. The team experimented with nitrogen-doped graphene sheets to encapsulate a nickel-molybdenum (NiMo) electrode alloy. The graphene, which was punched with nanometer-sized holes is key to their success. They found that it allowed the HER to carried out under the more efficient acid conditions, while protecting the core metal from dissolution, and precluding the need for a costly noble metal. The HER system dramatically outperforms the equivalent non-holey graphene electrode system.
Graphene has been tried before as a protective layer for the core metal in HER electrodes. But the protective layer suppresses chemical activity. The Tsukuba system shows that holes in the graphene can promote HER in two ways.
"We created holes by decorating the NiMo surface with silica nanoparticles," explains team member Kailong Hu. "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," he adds.
These hydrophilic fringes attract hydronium ions in the acid solution and this, the team explains, could be crucial to the first of two HER mechanisms. The fringes can also adsorb protons and provide extra surface area for the second. "This is a versatile new concept for hydrogen evolution electrodes," adds Yoshikazu Ito. "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."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".