A phase-pure molybdenum carbide modified by Pt ALD significantly reduced Pt loading while enhancing the activity and durability of the resultant catalysts.
Due to the similar electronic and geometric structures between transition metal carbides (TMC) and Platinum (Pt), Pt supported on TMC surfaces (a.k.a., Pt/TMC) has been the focus of significant research interest. This presentation focuses on a new class of electrocatalyst fabrication in which high purity β-molybdenum carbide (referred to as Mo 2 C hereafter) was synthesized into hollow nanotubes and subsequently modified with nanoscale platinum particles (1-5 nm) deposited via rotary Atomic Layer Deposition (ALD) (referred to as Pt/Mo 2 C). X-Ray diffraction (XRD) measurements of Mo 2 C and Pt/Mo 2 C showed no molybdenum peaks and a very small amount of unreacted multi-wall carbon nanotubes (MWCNT) in the Mo 2 C samples, indicating that Pt was primarily supported on Mo 2 C nanotubes. Strong interaction between Pt nanoparticles and the Mo 2 C nanotube support was observed in lattice spacing changes from high resolution transmission electron microscopy (HRTEM) images, in addition to Pt binding energies from x-ray photoelectron spectroscopy (XPS) results of Pt/Mo 2 C. Atomic adsorption spectroscopy (AAS) measurements of Pt mass in Pt/Mo 2 C samples showed an average of 2.4% Pt loading in Pt/Mo 2 C nanoparticles. The electochemically active surface area (ECSA) of the 2.4% Pt/Mo 2 C was found to be 70% higher than commercial 20% Pt/C, indicating an increased number of electrochemically active sites, possibly resulting from the interaction between ALD-deposited Pt nanoparticles and Mo 2 C nanotube support. Cyclic voltammograms of the 2.4% Pt/Mo 2 C catalyst showed higher hydrogen oxidation reaction (HOR) activity than commercial 20% Pt/C, while bare Mo 2 C did not demonstrate any significant HOR activity. For the hydrogen evolution reaction (HER), the 2.4% Pt/Mo 2 C showed similar activity to the 20% Pt/C, while bare Mo 2 C nanotube showing noticeable, but not significant HER activity. Another focus of this talk is to present the 2.4% Pt/Mo 2 C catalyst performance in a PEMFC anode (HOR), which outperformed the commercial 20% Pt/C with up to a 403% increase in peak power density and 448% increase in current density. Consequently, the reported Pt/Mo 2 C validates a new fabrication approach for practical nanoscale electrocatalysts where the supported catalyst is deposited via atomic layer deposition onto high purity transition metal carbide nanotubes. Ongoing work in our group focuses upon long-term stability, the corrosion mechanism, and ORR activity of the Pt/Mo 2 C catalyst. Furthermore, we are conducting systematic investigations on how the number of ALD cycles affects the size and distribution of Pt on Mo 2 C nanotube and subsequent HER, ORR activity and PEMFC performance of resultant catalysts.
We recently showed that phase-pure molybdenum carbide nanotubes can be durable supports for platinum (Pt) nanoparticles in hydrogen evolution reaction (HER). In this paper we further characterize surface properties of the same Pt/β-Mo2C catalyst platform using carbon monoxide (CO)-Pt and CO-Mo2C bond strength of different Pt particle sizes in the <3 nm range. Results from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temporal analysis of products (TAP) revealed the existence of different active sites as Pt particle size increases. Correlation between the resultant catalyst activity and deposited Pt particle size was further investigated using water–gas-shift (WGS) as a probe reaction, suggesting that precise control of particle diameter and thickness is needed for optimized catalytic activity.
This work focuses on a new class of electrocatalyst fabrication in which phase pure β-molybdenum carbide (referred to as Mo 2 C hereafter) was synthesized into hollow nanotubes and subsequently modified with nanoscale platinum (Pt) particles (1-5 nm) deposited via rotary Atomic Layer Deposition (ALD) (referred to as Pt/Mo 2 C). Pt has been deposited on Mo 2 C with 15, 50 and 100 ALD cycles. Strong interaction between Pt and Mo 2 C was observed by XPS and HRTEM. In this paper, we focus on presenting comparison of eletrochemical and surface characterization data among three ALD cycles (100, 50 and 15) to elucidate how performance of the resulting co-catalyst can be tuned by the easily tunable ALD process parameter, number of ALD cycles.
