Hot Electron Metal-Free Electrode for Hydrogen Evolution Reaction

Abstract

Abstract Body: Recently, carbon-based electrode, especially graphene, has been introduced to replace the noble metals as a low-cost alternative. In this work, we show that the turn on voltage of the electrochemical reaction can be tuned in a semiconductor-insulator-plasma etched graphene (SIEG) device, especially for hydrogen evolution reaction (HER). By using this structured device, the relative energy level can be shifted to modify the onset potential. Besides, O2 plasma etched graphene was introduced to increase the number of active sites for HER on the surface of the graphene. Figure 1a shows a schematic of the SIEG device. A plasma-etched graphene on an insulator–semiconductor is introduced here. Reactive ion etching (RIE) by oxygen flow method was used to dry etch the graphene to introduce the active reaction sites. Compared to pristine graphene (PG) in Raman Spectroscopy, the ID/IG ratio was increased from ~ 0 to 0.58 from PG to EG shown in Figure 1b, suggesting the disruption of the hexagonal configuration by oxygen plasma etching. The result implies the existence of a higher amount of defect sites due to the introduction of plasma-induced edge defects. Figure 2a shows the HER measurement result of SIEG device in both biased and unbiased conditions. It is noteworthy that the onset potential of HER can shift up to ~0.8 V and 90mA/cm2 current density could be achieved at -0.5V vs RHE at VGr-Si = 1.5V. Figure 2b shows the schematic of the mechanism of catalysis due to hot electrons. The high performance of the device comes from the hot electrons possessing high energy which requires a significantly smaller activation energy barrier to overcome compared to the thermal electrons. Figure 3 shows the mechanism of how hot electrons are introduced into the graphene from the silicon. The applied bias between graphene and silicon shifts the conduction band of silicon relative to the reduction potential of H+/H2 redox couple, injecting carriers from silicon through graphene, while driving the reaction before any scattering happens in the graphene. To clarify this effect of the applied bias, we analyzed the different current components from the system individually. Here we find out that the silicon current is the major current component for chemical reaction (Redox current) when graphene-silicon voltage is applied as shown in Figure 3b. This result shows that the hot electrons from silicon current directly drive the electrochemical reaction, without affecting the graphene current. Figure 4 shows the overlapping of HER current between SIPG and SIEG device. The result shows that the current density of SIEG device at same experimental condition is ~ 2x higher than that of SIPG device. This indicates that the newly introduced active site for the hydrogen ions breaking through the limiting factor of graphene surface area. Electrochemical surface area (ECSA) achieved by cyclic voltammetry measurement also indicates the enhanced electrochemical area of the plasma etched graphene. In conclusion, we demonstrate that a plasma etched graphene based SIEG device can be a promising electrocatalysts candidate. SIEG device is not confined to this certain structure, we could possibly get better results by replacing each component with other materials. Future experiments could explore other redox reactions for carbon dioxide reduction to useful materials, and other 2-D materials with a high density of active sites that transport electrons more efficiently and adsorb more protons on top of the surface to see if high efficiencies can be achieved at lower voltages.