Structured and Nanoparticle Assembled Co−B Thin Films Prepared by Pulsed Laser Deposition: A Very Efficient Catalyst for Hydrogen Production

Abstract

Amorphous Co−B-based catalyst powder, produced by chemical reduction of cobalt salts, was used as the target material for Co−B thin film catalyst preparation through pulsed laser deposition (PLD). A comparative kinetic analysis of the sodium borohydride (NaBH4) hydrolysis by using Co−B catalyst added to the hydride solution as powder or as thin film was carried out. Both forms of catalyst (powder and film) were heat-treated at 623 K for 2 h under various atmospheric conditions (in vacuum or by using Ar, H2, and O2 gases) in order to study their effects on H2 generation rate. Surface morphology of the catalyst was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM), while compositional and bond formation analysis were carried out using X-photoelectron (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Structural characterization of catalysts was performed using the X-ray diffraction (XRD) technique. It was observed that nanoparticles produced during laser ablation process act as active centers in the catalyst films, producing significantly higher rate (about 6 times) of H2 generation than the corresponding Co−B powder. No significant changes were observed for Co−B powder treated in an inert atmosphere (vacuum and Ar) while it caused structural changes in Co−B films. Co2B phase formation in films makes them more efficient catalysts with 28% increase in rate of H2 generation as compared to untreated film. Heat treatment in an oxygen atmosphere causes complete inactivation of powder catalyst, while film still showed excellent catalytic activity with just a longer induction time. The AFM and SEM analysis of the heat-treated films did not show drastic change in surface morphology, indicating that changes in catalytic activity of the films were possibly connected to structural modification and formation of boron oxide on the catalyst surface. We report that by using suitable thin film Co−B catalyst the maximum H2 generation rate of about 5000 mL/(min g of catalyst) can be achieved. This can generate about 0.9 kW (0.7 V) for proton exchange membrane fuel cells (PEMFC), a critical requirement for portable devices.