Increasing the Hot‐Electron Driven Hydrogen Evolution Reaction Rate on a Metal‐Free Graphene Electrode

Abstract

Recently, it has been shown that a semiconductor–insulator–graphene device can drive the hydrogen evolution reaction (HER) at the graphene surface with a reduced onset potential by injecting hot electrons into graphene. However, the catalytic properties of graphene are limited by the large hydrogen adsorption energy and lack of electrochemically active sites. To address these limitations, a n‐silicon/insulator/plasma etched graphene device is investigated, where a dry etch process is used to increase the number of active sites on the graphene by creating a greater number of active edge sites, increasing hydrogen adsorption at a given potential. This has been shown to improve the properties of devices with cold electrons. However, here it is shown that this approach can improve the HER rate with hot electrons. The electrons injected into the graphene from the silicon shift the onset potential of HER by as high as ≈0.8 V reaching a current density of 90 mA cm−2 at an overpotential of ‐0.5 V versus RHE. Furthermore, the comparison between device with pristine graphene shows a ≈2X improvement in current density at high overpotentials. This result shows that hot‐electron devices can be improved by modifying the catalytically active sites without metal catalysts.

A review of defect engineering in two-dimensional materials for electrocatalytic hydrogen evolution reaction

“…Compared with other materials, for instance nanopore materials, nanowires and nanotubes, 2D materials are likely to expose more active centers. Moreover, the uniform structure of 2D materials makes them easier to A,B) Reproduced with permission [22]. Copyright 2021, Wiley-VCH.…”

“…The curve fluctuation in stability tests was due to gas leakage and water evaporation, as well as electrolyte loss in HER process [114]. Using dry etching, Kapadia et al [22] injected hot electrons into graphene, resulting in many pores on the substrate and many edge defect sites, which favored hydrogen adsorption/desorption properties.…”

A study of highly activated hydrogen evolution reaction performance in acidic media by 2D heterostructure of N and S doped graphene on MoOx

“…From this perspective, many studies have focused on the use of graphene‐based heterostructures as catalysts for electrochemical water splitting. However, the practical application of these heterostructures is limited because of their low chemical stability and highly activated Fermi level resulting from their highly unstable intrinsic bonding energy state 16–18 . To enhance their chemical activity and stability, methods that entail doping with heteroatoms such as N, S, B, and/or F are widely used 19,20 .…”

“…However, the practical application of these heterostructures is limited because of their low chemical stability and highly activated Fermi level resulting from their highly unstable intrinsic bonding energy state. [16][17][18] To enhance their chemical activity and stability, methods that entail doping with heteroatoms such as N, S, B, and/or F are widely used. 19,20 Owing to their electronegativity, these heteroatom dopants increase the electrical activity for the adsorption of water molecules, which is critical for the catalytic activity and stability of the modified energy state.…”

Electrostatically Generated Air-Stable Negative Electron Affinity Silicon Photocathode

“…When a positive voltage is applied between graphene and silicon, photogenerated electrons in silicon can tunnel through a thin insulating barrier via the Fowler-Nordheim tunneling process , and reach the insulator conduction band where it gains enough energy from the electric field to be emitted over the workfunction barrier of graphene. This hot electron laser-assisted cathode (HELAC) can operate under vacuum conditions as low as 10 –3 Torr compared to 10 –9 and 10 –11 Torr vacuum requirements of typical metallic and semiconductor photocathodes. , Interestingly, observation of hot electron emission from silicon/oxide-based devices dates back to the early studies on gate leakage behavior of MOSFETs where scattering processes at the gate metal suppressed the emission yield. − However, a 2D semimetal such as graphene serves as an electronically and optically transparent conductive electrode that allows more efficient extraction of the electrons thanks to its ultrathin nature and favorable scattering rates compared to the metals. − …”