Efficient Electrocatalytic Water Oxidation by Fe(salen)–MOF Composite: Effect of Modified Microenvironment

Abstract

An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H 4 [SiW 12 O 40 ]•HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW 12 (i.e., H 4 [SiW 12 O 40 ]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW 12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s −1 ) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW 12 results in (i) faster formation of FSWZ-8 composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.

ZIF‐8 MOF Encapsulated Co‐porphyrin, an Efficient Electrocatalyst for Water Oxidation in a Wide pH Range: Works Better at Neutral pH

“…In the observed overpotential, a large contribution should be present from the resistive nature of ZIF‐8. A comparison with the previously reported ZIF‐8 based electrochemical OER catalysts by our group suggests the CTMZ‐8 to possess the lowest overpotential requirement among the host‐guest composites prepared using molecular catalysts and ZIF‐8 which can perform OER in neutral pH [2,53] . This clearly depicts the outstanding performance of CTMZ‐8 as heterogeneous electrochemical OER catalyst.…”

“…Faradic efficiency of conversion of electrical energy to chemical energy has been calculated by determining the amount of evolved gas (Figure 8) under chronopotentiometric measurement using a home‐made setup following a protocol, we have reported earlier [53] . Bulk electrolysis using a volumetric set‐up was carried out at constant current for 8 hours using a two electrode system.…”

POM@MOF Hybrids: Synthesis and Applications

“…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 coworkers [139]. In this work, the authors demonstrated the successful coating of ZIF-67 with a catalytically active Keggin POM, H3PW12O40.…”

Hierarchical CuCo2S4 Nanoflake Arrays Grown on Carbon Cloth: A Remarkable Bifunctional Electrocatalyst for Overall Water Splitting

“…[66,67] In Bode plots of Z vs log(frequency), the straight lines in the low frequency region and high frequency region are ascribed to the impedance of the resistance and capacitance, respectively. [68] In the low-frequency region (linear portion of the curves), with increasing the frequency, the great decrease of charge-transfer resistance of CuCo 2 S 4 /CC compared to that of CoS 1.097 /CC and CuS/CC showed significantly enhanced charge transfer characteristics and catalytic performance. In addition, the potential of the CuCo 2 S 4 /CC is well kept after 20 h measurements (Figure 3f), indicating that CuCo 2 S 4 /CC is an excellent catalyst with highly stable and active for HER reaction.…”

“…[66,67] With the increase of the frequency In the low-frequency region, the CuCo 2 S 4 /CC shows great decreased charge-transfer resistance than CoS 1.097 /CC and CuS/CC, indicating significantly enhanced charge transfer characteristics and catalytic performance. [68] Cyclic voltammetry (CV) was used to characterize the redox behavior of CuCo 2 S 4 /CC. electrode ( Figure S11).…”