Decorating MOF-74-derived nanocarbons with a sandwich-type polyoxometalate to enhance their OER activity: Exploring the underestimated bulk-deposition approach

Abstract

Ternary nanocomposites consisting of cobalt phosphotungstate, metal nanoparticles and a carbon matrix (Co4(PW9)2@NP@C) were synthesized by decorating four different MOF-74-derived nanocarbons with the sandwich-type polyoxometalate [Co4(H2O)2(PW9O34)2]10−, Co4(PW9)2. An unprecedented strategy based on the “bulk” deposition of the POM salt nanocrystals has been persecuted on purpose, avoiding the homogeneous dispersion of POM clusters across the nanocarbon surfaces. Thereby, virtually unaltered Co4(PW9)2 nanocrystals were supported on three of the four nanocarbons, but unexpectedly the combination of bimetallic undoped Co/Ni@C support with the POM induces partial structural modifications in both materials. Consequently, the derived Co4(PW9)2@Co/Ni@C electrocatalyst undergoes a noteworthy electroactive surface area relative increase, but at the same time, its intrinsic OER activity declines. This is not an obstacle for this nanocomposite to exhibit (along with Co4(PW9)2@N,S-Co@C) very significant nominal OER performances: low overpotentials (ca. 400 mV to develop 10 mA cm−2 of current density), fast kinetics at intermediate overpotentials (Tafel slopes ≤67 mV dec−1) and remarkable stability levels. Regardless of the specific results, this preliminary study demonstrates for the first time the POM “bulk” deposition strategy as a useful tool to prepare highly active OER catalysts, and it shows up the dramatic effect that the nanocarbon doping and metal composition has on the resultant POM loading, electrocatalytically active area and OER behavior.