*3.2. Electrocatalysis*

Besides the use of POM@MOF hybrids in organocatalysis, POMs also exhibit interesting electrocatalytic properties as they can undergo fast and reversible multi-electron transfers [157]. Within this context, POMs have already shown grea<sup>t</sup> potential in the electrochemical oxygen evolution reaction (OER) in a homogenous manner [158]. Despite the remarkable progress in this field, there are only a few reports on the encapsulation of POMs in the cages of MOFs for electrocatalytic water oxidation, as can be seen in Table 2. This is probably due to the fact that the majority of MOFs possess a low electrical conductivity and high hydrophobicity. The first report on the encapsulation of an unsubstituted Keggin POM in a MOF to perform electrocatalytic water oxidation was reported by Das et al. [137]. More specifically, a one-pot synthesis was performed to include the [CoW12 O40] 6− anion in the size matching cage of ZIF-8 (see Figure 13). During the electrochemical measurements, performed at pH 1.9, the authors observed a clear shift to a less anodic potential for the redox CoIII/CoII couple in the cyclic voltammogram of POM@MOF with respect to that of the uncapsulated POM (from 1.14 V for the Keggin POM to 0.97 V for the composite material). In addition to this, the POM@MOF catalyst exhibited an excellent stability as only a very small drop in the catalytic current was observed after 1000 catalytic cycles and no leaching of Co species was observed. It is, however, important to note here that, although the catalyst exhibited a high turnover frequency (TOF = 12.5 s<sup>−</sup><sup>1</sup> based on the quantitative oxygen evolution) and an excellent faradaic e fficiency of 95.7%, a rather high overpotential was required (784.19 mV at a current density of 1 mA cm<sup>−</sup>2).

**Figure 13.** Encapsulation of an inactive Keggin POM in ZIF-8 to become an active oxygen evolution reaction (OER) catalyst. Reprinted with permission from [137]. Copyright (2018), John Wiley and Sons.

In a very recent report of the same group, a redox inactive SiW12 POM was used to lower the required overpotential [138]. The co-encapsulation of this POM together with the true catalyst, an Fe(salen) complex, within ZIF-8 resulted in a decrease of the overpotential of more than 150 mV. In the absence of the encapsulated POM, the Fe-salen@ZIF-8 required an overpotential of 672.9 mV to attain a current density of 1 mA cm<sup>−</sup>2, while in the presence of the POM, the overpotential decreased to 516 mV. The authors attributed this observation to the fact that the POM not only increased the hydrophilicity of the catalyst and facilitated the charge conduction in ZIF-8, but also ensured a higher loading of the Fe-salen complex within ZIF-8. Another way to decrease this overpotential for oxygen evolution and thus improve the hydrogen production efficiency was reported by Pang and co-workers [139]. In this work, the authors demonstrated the successful coating of ZIF-67 with a catalytically active Keggin POM, H3PW12 O40. The unique yolk/shell structure of the ZIF-67@POM catalyst ensured a high electrical conductivity and fast charge transfer, which resulted in a significant reduction of the overpotential. From all of the examined ZIF-67@POM hybrids, the 6-ZIF-67@POM catalyst exhibited the smallest Tafel slope (58 mV dec−1) and lowest overpotential values (287, 313, and 338 mV at current densities of 10, 20, and 30 mA cm<sup>−</sup>2, respectively), which are even comparable to those observed for RuO2, which is one of the most efficient and well-known electrocatalysts for the OER reaction [159].

Besides these few examples on OER, POMs have also shown grea<sup>t</sup> potential in the second half reaction for water-splitting, namely the hydrogen evolution reaction (HER) [160]. However, to solve their shortcomings, particularly the limited stability of POMs in the required highly acidic pH for HER, Zhang et al. encapsulated POMs in metal–organic nanotubes (MONTs), which can be considered as a special kind of MOF [140,141]. Upon encapsulation of the POMs, using a one-pot synthesis method, the chemical stability of both the POM and MONTs increased. The POMs served as a kind of template to construct the MONTs, while the MONTs ensured a sort of a shield to increase the chemical stability of the POM. The best POM@MONTs electrocatalyst displayed an overpotential of 131 mV (at a current density of 10 mA cm<sup>−</sup>2), which is far more superior than other POM-based MOFs (which showed overpotentials above 200 mV) [161].
