# Grain Boundary--A Route to Enhance Electrocatalytic Activity for Hydrogen Evolution Reaction
## 1 Introduction  
In recent decades, human beings have relied on fossil fuels for over 80% of their total energy needs. Hydrogen, as a clean, economic and renewable energy carrier, is considered to be an attractive alternative to traditional fossil fuels, and it can greatly alleviate the global greenhouse effect and energy crisis at present [1]. Amongst various hydrogen production techniques, the electrocatalytic hydrogen evolution reaction (HER) via electrochemical water splitting, an important energy recovery technique, has received great attention as a hot research topic [2; 3; 4]. Consequently, ever-increasing efforts have been devoted to developing a variety of new catalysts with the aim of improving HER performance [5; 6]. However, noble metal-based materials such as platinum (Pt) (e.g., Pt/C) are still the most efficient catalysts widely used to catalyze hydrogen evolution, owing to their outstanding thermodynamical and kinetic features for the HER [3]. Unfortunately, Pt is an extremely scarce and precious metal, which results in a high cost for the HER. Therefore, designing a new catalyst or boosting the activity of existing non-noble materials other than Pt has become urgent.  
As an important descriptor, hydrogen bonding ability is often taken into account in the screening or designing of HER catalysts [7; 8; 9]. According to the Sabatier principle, either a too weak or too strong adsorption ability results in a drop in HER performance in that a too weak adsorption of H* leads to a pronounced decline in intermediate stabilization, while a too strong adsorption inhibits the desorption of gaseous hydrogen [10]. The superior HER of Pt is due to its optimal H* binding energy, or Gibbs free energy, which is close to zero. Todate, many non-noble materials suffer from a weak H* binding ability. Identifying how to enhance their H* binding activity remains an enormous challenge. Therefore, various routes have been proposed to control the microstructure of catalysts in order to expose more active facets or sites [11; 12; 13]. However, further improving HER activity is still a great challenge due to the inherent electronic structure of the catalysts themselves, leading to a certain capacity for H* binding, which almost cannot be tuned. Inspired by a previous study on the relationship between grain boundaries (GBs) and catalytic activity in various reactions [14; 15; 16], GBs rich in the atomic arrangement disorder were found to alter the local electronic structure around GBs [17; 18]. In addition, GBs feature under-coordinated sites and microstrains [19; 16], which favor H binding. [PERSON] [17], using extensive density functional theory (DFT) calculations to model the atomistic structure of GBs on a Au(111) surface, concluded that the grain boundary of Au is conducive to the adsorption of COOH by the active site, thus enhancing electrochemical CO\({}_{2}\) reduction. [PERSON] [18] believes that the GB sites on a Au(110) surface lead to a high selectivity toward CH\({}_{3}\)OH. Thus, GBs are expected to have a stronger H* binding capability so as to compensate for the unsaturated coordination of GB atoms resulting from such a disorder. However, these studies lack an investigation of the atomic and electronic structures of grain boundaries and an explanation of the enhanced catalytic performance of grain boundaries.  
Herein, we first predict the impact of grain boundaries on a Au surface on the HER occurring at GBs via density functional theory (DFT) calculations, in light of the much weaker H* binding capacity of Au [20]. Then, a Au NP catalyst with GBs is prepared for the verification of GB-assisted HER activity. HER activity greatly increases with an increase in the density of GBs in Au NPs. Additionally, the atomic and electronic structures are analyzed to determine the role of the GBs. On the basis of the theoretical and experimental studies, it is found that the GBs can prodigiously promote the chemical bonding ability of H* intermediate, resulting in enhanced catalytic activity in hydrogen evolution.