# Grain Boundary--A Route to Enhance Electrocatalytic Activity for Hydrogen Evolution Reaction
## 3 Results and Discussion
### Experimental Verification of GB-Assisted HER Activity
#### 3.2.2 Electrochemical HER Characterization  
The effect of GBs on the HER activity of the Au NPs was examined in acid aqueous solution (0.5 M H\({}_{2}\)SO\({}_{4}\)). Figure 6a shows representative LSV curves for the CFP substrate, pristine Au/CFP and annealed Au/CFP electrodes (Au/CFP\({}_{200}\), Au  
Au/CFP\({}_{400}\)). The CFP substrate has negligible HER activity and, thus, is expected to exert no influence on the current density contribution by the GBs. The pristine as-deposited Au/CFP has the highest HER activity and shows an overpotential of 330 mV at a current density of 10 mA/cm\({}^{2}\). By contrast, the overpotentials required to achieve 10 mA/cm\({}^{2}\) HER current density for the annealed Au/CFP electrodes are 470 mV (Au/CFP\({}_{200}\)), 530 mV (Au/CFP\({}_{300}\)) and 550 mV (Au/CFP\({}_{400}\)). In combination with the above TEM analysis, the HER activity significantly decreases with the decrease in the density of the GBs in the Au NPs. Au/CFP\({}_{300}\) has only a 20 mV lower overpotential than Au/CFP\({}_{400}\), as the GBs in the Au NPs almost diminish after annealing at 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, which is consistent with the above TEM observation (Figure 5b,c). Obviously, the experimental LSV curve is in good agreement with our simulated current density curve (Figure 3b). Although it is quite difficult to measure the GB density accurately due to the irregular shape of Au NPs, the overpotential difference (220 mV) still indeed suggests that the GBs in Au NPs could significantly enhance HER activity. A comparison of Tafel plots for various Au/CFP electrodes is shown in Figure 6b. It can be seen that the introduction of GBs in Au NPs leads to a smaller Tafel slope compared with the GB-free counterparts, resulting in values of 145 and 185 mV dec\({}^{-1}\) for Au/CFP and Au/CFP\({}_{400}\), respectively. As widely accepted, Tafel slopes of 120, 40 and 30 mV dec\({}^{-1}\) have been observed for the Volmer, Heyrovsky and Tafel determining rate steps, respectively [31]. Regarding the HER mechanism, the rate-determining step for this Au catalyst is the Volmer reaction, i.e., the initial adsorption of protons from the acid solution to form adsorbed H [10]. Of note is that GBs may change the rate-determining step (RDS); that is, RDS may be changed from binding H* to desorbing hydrogen at Au atoms around the GBs by increasing the capability of binding H (Figure 2a, Table S2), which is in agreement with the theoretical \(\Delta\)G\({}_{\text{H}^{*}}\) for the GB sites and normal sites (Figure 2a). To exclude the improvement in HER activity caused by particle size and surface areas, we calibrated the electrochemically active (EA) surface area (Figure S4) of the as  
Figure 6: Effect of GBs on Au surface on the electrochemical HER activity of Au/CFP electrodes. (**a,b**) Electrochemically active (EA) surface area\(-\)calibrated linear sweep voltammetry (LSV) curves and Tafel plots for four Au/CFP electrodes with a size of 0.5 \(\times\) 1 cm\({}^{2}\) (electrolyte: 0.5 M H\({}_{2}\)SO\({}_{4}\), scan rate: 5 mVs\({}^{-1}\)). (**c**) Turnover frequency per surface Au atom of various Au/CFP samples, and the inset is the TOF value obtained at an overpotential of 300 mV. prepared Au/CFP electrodes. Based on the EA surface area, we then calculated the turnover frequency (TOF) per surface Au atom to compare the intrinsic activity of various Au/CFP electrodes. Figure 6c shows the experimental TOF as a function of applied overpotential. The TOF value increases with the overpotential following the Tafel behavior for all Au/CFP electrodes. The TOF value of the as-deposited Au/CFP is even higher than those of the other annealed Au/CFP electrodes, indicating that, after annealing treatment, the highly effective sites for the HER significantly decrease. Additionally, the TOF values of Au/CFP\({}_{300}\) and Au/CFP\({}_{400}\) are almost the same over the whole overpotential range.