Electrochemical water splitting: Mathematically designed graphene on edge

08/26/2019

Holey sheets of chemically doped graphene have great potential for generating clean hydrogen fuel

Introducing defects and chemical dopants can boost the electrocatalytic activity of graphene for the hydrogen evolution reaction.
Introducing defects and chemical dopants can boost the electrocatalytic activity of graphene for the hydrogen evolution reaction.

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Mathematically optimized graphene-based structures containing the right combination of nitrogen and phosphorus atoms could match — or even outperform — expensive platinum catalysts for producing hydrogen, an AIMR co-led team has shown1. This could make hydrogen production cheaper and thereby facilitate the widespread adoption of hydrogen in place of fossil fuels.

Platinum is currently the material of choice for catalyzing the hydrogen evolution reaction (HER). In this process, renewable electricity from solar panels or wind turbines is used to electrolyze water, releasing clean hydrogen, which can be stored, transported and used as a fuel. When hydrogen is burned, water vapor is the only emission.

However, because platinum is rare, electrocatalysts made from cheap, abundant carbon — in the form of single-atom-thick sheets called graphene — are being investigated. Graphene’s HER activity is affected by various factors, including chemical dopants and atomic defect structures such as those around holes in the graphene sheet, says Akichika Kumatani of the AIMR at Tohoku University. “However, there’s no direct evidence as to which factors — atomic structures, chemical dopants or both — give the highest performance,” he explains.

To answer that question, the researchers designed atomic structures of graphene mathematically and then produced the structures using chemical vapor deposition with silicon dioxide nanoparticles. They also developed scanning electrochemical cell microscopy (SECCM) for characterizing the graphene structures. It essentially enabled them to recreate an electrochemical cell inside a scanning microscope, so that they could measure the HER at different points across the graphene surface with a high spatial resolution.

Using SECCM, the team compared the HER performance of graphene with and without holes, and with and without nitrogen and phosphorus dopant atoms. They found that the most catalytically active sites were areas of doped graphene around the edges of holes. The graphene around the holes has the greatest concentration of defects in the carbon lattice, which can accommodate many dopant atoms in close proximity.

Computational studies gave further insights. The most catalytically active sites were nitrogen atoms in a ‘pyridinic’ bonding arrangement with surrounding carbon atoms, especially if they were co-located with phosphorus atoms, which enhanced the charge on the pyridinic nitrogen, further boosting catalytic activity.

"Remarkably, our theoretical results suggested that the optimal structure beats platinum’s performance for water electrolysis," Kumatani says. "Importantly, the atom structure was inspired by mathematical analysis. We believe this carbon-based structure for HER can be essential for developing a sustainable hydrogen economy."

The team plans to optimize methods for creating graphene with more holey edges with chemical dopants, as well as to use SECCM to study other important electrochemical reactions such as carbon dioxide reduction.

References

  1. Kumatani, A., Miura, C., Kuramochi, H., Ohto, T., Wakisaka, M., Nagata, Y., Ida, H., Takahashi, Y., Hu, K., Jeong, S. et al. Chemical dopants on edge of holey graphene accelerate electrochemical hydrogen evolution reaction. Advanced Science 6, 1900119 (2019). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.