Effect of Structural Changes at Various Length Scales in SiVOC Ceramic Nanocomposites on Electrocatalytic Performance for the Oxygen Reduction Reaction

Polymer-derived processing of ceramics (PDC) is an efficienttechniqueto prepare porous nanocomposites with precise control over their phasecomposition and in relation to the Si-based ceramic matrix containingfree carbon. The microstructure of these nanocomposites can be fine-tunedat the molecular scale for obtaining necessary properties by tailoringthe chemical configuration of the preceramic polymer. In the presentwork, vanadium-based nanocomposites were synthesized as oxygen reductionreaction (ORR) catalysts with the objective of elucidating the effectof microstructure changes on catalytic efficiency. For this purpose,a single-source precursor (SSP) was synthesized by crosslinking phenyl-and hydrido-substituted polysiloxane and vanadium acetylacetonatefollowed by pyrolysis at 1100 & DEG;C. The resulting solid was composedof sparsely distributed nanodomains of vanadium carbide (VC) crystalsprecipitated within an amorphous silicon oxycarbide (-Si-O-C-)matrix. High-temperature treatment of the pyrolyzed samples beyond1300 & DEG;C induced the crystallization of & beta;-SiC as well asVC. Furthermore, Raman spectroscopy confirmed the segregation of sp(2)-hybridized, turbostratic free carbon. The samples exposedto 1300 & DEG;C revealed a specific surface area of 239 m(2)/g. The electrocatalytic activity of the sample heat-treated at 1300 & DEG;C showed the best performance with respect to the ORR performancewith onset potential (E (o)) and half-wavepotential (E (1/2)) values of 0.81 and 0.72V, respectively. In addition, improved kinetics with a Tafel slopeof 57 mV/dec and enhanced current density in the diffusion-controlledregion (I (d)) of 3.7 mA/cm(2) wereobserved for this sample. The increase in E (o) was attributed to the optimal interfacial characteristics betweenthe VC and SiOC matrix with better embedment of VC with free carbonthrough V-C bonds. The higher E (1/2) and faster kinetics are because of the higher electronic conductivitycaused by the free carbon effectively connecting metallic VC crystallites.Besides, the higher specific surface area of this sample enhanced I (d).