# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 3 The Stability of Metal-Organic Frameworks (MOFs) under Electrochemical Environments  
Besides the efficient charge transport and facile diffusion of counter ions within the framework, the stability of MOFs during electrochemical operations is another crucial factor for utilizing MOFs in electrocatalytic applications. If the MOF is not chemically stable in the electrolyte, the framework may collapse during or before the electrochemical operation; dissolution of the MOF or formation of the MOF-derived metal oxides tethered with organic ligands may occur [70]. The resulting modified electrode may exhibit a stable electrochemical performance with long-term stability, but in fact the active electrode material that shows the electrochemical activity is no longer the crystalline MOF with regular porosity. For example, the early findings suggest that a cobalt-based MOF constructed from 1,4-benzenedicarboxylic acid linkers shows exceptional pseudocapacitive performance in alkaline electrolytes [71, 72], but the active material achieving the observed capacitive behavior is indeed cobalt hydroxide derived from the MOF in the electrolyte [73]. Another example is Cu-BTC, i.e., HKUST-1, a common copper-based MOF that has been found to be unstable in water [26], but got reported sometime for the electrocatalytic applications in aqueous electrolytes. In some circumstances, the collapse of MOF may solely occur on the surface of MOF crystals, which generates a layer of electrochemically active MOF-derived metal hydroxide covering the inactive MOF [74]. Thus, to avoid recognizing the electrocatalytic activity originated from MOF-derived materials as the property of the crystalline MOFs, it is crucial to examine the chemical stability of the targeted MOF in the electrolyte before utilizing the MOF for corresponding electrocatalytic applications. As mentioned previously that more than 90% of existed MOFs are not stable in water, for the electrocatalytic applications that need to be operated in aqueous electrolytes such as water splitting, oxygen reduction, and most of the electrochemical sensing systems, the use of water-stable MOFs is necessary. Among MOFs constructed from carboxylate-based linkers, zirconium-based MOFs and their hafnium-based analogs have been reported to exhibit exceptional stability in water due to the stronger bonds incorporated in these frameworks [27]. The most common example of zirconium-based MOFs is UiO-66 developed by [PERSON] and coworkers in 2008 (Figure 4a), which possesses a thermal stability up to 500 \({}^{\circ}\)C and a superior chemical stability in acidic and neutral aqueous solutions [28]. Due to the remarkable stability and high structural tunability, numerous zirconium-based MOFs have been designed for a range of applications [75]. However, it is still quite challenging to find zirconium-based MOFs that can show chemical stability in strong alkaline solutions or concentrated solutions containing strongly coordinating ions such as phosphate [27]; such characteristics limit the use of zirconium-based MOFs in the electrocatalytic systems operated in phosphate buffer solutions or strong base. Except zirconium-based MOFs and their hafnium-based analogs, MOFs constructed from anionic and nitrogen-containing linkers usually exhibit stability in water. The most common example is ZIF-8, one of the zeolitic imidazolate frameworks (ZIFs), which can preserve its crystallinity in strong alkaline solutions. Such a remarkable stability is in part attributed to the small and hydrophobic pore opening of ZIF-8 that excludes the penetration of liquid water [27, 76]. Another example of MOFs constructed from nitrogen-containing linkers is pyrazolate-based MOFs. MOF constructed from nickel-based nodes and pyrazolate-based linkers has been reported to show a chemical stability in aqueous solutions ranging from pH 2 to pH 14 [77].