# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 1 Introduction  
Electrochemical reactions are the redox reactions which take place solely at the interface of an electrode and the electrolyte. Due to their attractive advantages, such as the simple control of reaction rate via changing the applied bias and the easy separation of reaction products from both half reactions, electrochemical systems have been utilized in various practical applications, including electrochemical sensors [1], supercapacitors [2], electrolysis [3; 4; 5], fuel cells [6; 7], batteries [8], and electrochromic devices [9]. Frequently, a layer of electrochemically active materials is coated on the surface of the electrode as a thin film; the thin film deposited on the electrode can be served as either the active material for the corresponding application or the material that can further enhance the performance of the corresponding application (e.g., electrocatalysts for electrolysis, fuel cells, and some electrochemical sensors, capacitive materials for supercapacitors, materials enabling ion intercalation for batteries, and electrochromic materials for electrochromic devices). For electrocatalytic applications, the design of thin-film electrocatalysts thus becomes crucial to achieve a faster reaction rate at a relatively lower overpotential. The reaction rate of the thin-film electrochemical process can be affected by the following factors [10; 11; 12]: (1) the total number of electrochemically active sites presented in the entire thin film, (2) the charge-transport rate between those active sites and from the active sites to the underlying electrode, (3) the rate of physical diffusion of the active sites within the film (if there is any), and (4) the diffusivity of counter ions within the thin film for maintaining electroneutrality. The total number of electrochemically active sites is directly related to the accessible surface area of the thin film deposited on the electrode. Therefore, to achieve a high reaction rate for the electrochemical reaction occurring at the active thin film, which is beneficial for electrocatalytic applications, electrocatalytically activethin films composed of nanoporous materials with high surface area, facile charge-transport rate, and rational porosity for ion diffusion are highly desirable. Metal-organic frameworks (MOFs) consist of a class of porous materials constructed from metal-rich nodes and organic linkers [13; 14]. Compared to other categories of nanoporous materials such as zeolites, nanoporous carbons, and inorganic oxides, MOFs exhibit several attractive characteristics, including regular, interconnected and permanent porosity, high structure tunability, and ultrahigh specific surface area. The experimental Brunauer-Emmett-Teller (BET) specific surface area of MOFs has achieved more than 7000 m\({}^{2}\)/g [15; 16], which is much higher than those of conventional nanoporous materials. Besides, due to their crystallinity, MOF-based materials are more structurally characterizable compared to amorphous materials. Owing to these unique features, MOFs have attracted great attention in several applications such as gas storage [17], separation [18; 19], and chemical sensors [20] over the past two decades. Moreover, the periodic intra-framework functionality of MOFs, either provided by the metal-based nodes or organic linkers, may be further utilized to install well-separated and accessible active sites within the entire framework with a high active-site density [21; 22], which renders MOFs attractive porous supports for a range of catalytic applications [23; 24; 25]. Although the majority of MOFs still show poorer chemical and thermal stability compared to other nanoporous materials, several MOFs possessing water stability and moderately thermal stability have been designed during the past decade [26; 27; 28], which further extends the feasibility of utilizing MOFs in aqueous systems and high-temperature catalysis.