Deconvolution of phase-size-strain effects in metal carbide nanocrystals for enhanced hydrogen evolution.

Understanding the descriptors of electrochemical activity and ways to modulate them are of paramount importance for the efficient structural engineering of electrocatalysts. Although, many studies separately elucidated the significance of thermodynamic and kinetic descriptors, lack of integrative approaches bars the potential utilization of these engineering tools for electrocatalytic activity enhancement. Here, through a facile post-carbonization synthetic technique using templated polyoxometalate based metal organic frameworks (POMOFs), we integrate three major structural engineering tools, viz. phase, size and strain into cost-effective Mo and W carbide electrocatalysts, and demonstrate how these factors qualitatively and quantitatively affect the critical descriptors of electrochemical activity. Deconvolution of these effects through combined experimental-theoretical analyses, shines new light on structure-activity relationships in this class of HER electrocatalysts. Optimum modulation of the structural tools culminated into the design of a superior electrocatalyst, consisting of ultrasmall γ-WC nanocrystals supported on N doped graphitic carbon that exhibited multifold activity enhancement in terms of onset potential, current density and Tafel slope compared to its structural analogues reported in this work and elsewhere. The present comprehensive study showcasing the effects of the structural engineering tools on activity will have considerable influence on future designs of more efficient nano-composite electrocatalysts.

Two Keggin-Based Isostructural POMOF Hybrids: Synthesis, Crystal Structure, and Catalytic Properties.

In this work we synthesized two novel isostructural twin hybrids Comp1: [H(C10H10N2)Cu2][PMo12O40] & Comp2: [H(C10H10N2)Cu2][PW12O40], based on the Keggin ions (PMo12O40-3 & PW12O40-3), Cu(I) cation, and 4,4'-bipyridine, by in situ hydrothermal reduction of Cu, facilitated through extensive standardizations of synthetic pH conditions. Both compounds crystallized in monoclinic P21/ c space group with similar lattice parameters and crystal structures. The structural similarity prompted us to explore comparative catalytic properties of the hybrids, to understand the relative role of the POM species in the activity. While characterization techniques like powder X-ray diffraction (XRD), single-crystal XRD, IR, adsorption studies, etc. confirmed the identical structural hierarchy in the twin polyoxometalate-based metal organic frameworks (POMOFs), critical analyses through X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and magnetic property studies elucidated the electronic and local structural properties of the two. The hybrids were highly active for heterogeneous catalysis of small-molecule oxidation, with Comp 2 showing better activity than Comp1, particularly for oxidation of ethylbenzene and cyclooctene. Comp2 also outperformed Comp1 in photocatalytic degradation of methylene blue, with higher conversion efficiency of 83% and one order higher apparent rate constant of 0.0139 min-1, which is comparable to that of the well-known photocatalyst, P25. Electrochemical pseudocapacitance studies revealed that these POMOFs are having the potential to act as good charge storage and conducting devices if their electrochemical stability can be improved.