Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution.

Giovanni Valenti,Giovanni Bertoni,Matteo Cargnello,Maurizio Prato,Massimo Marcaccio,Raymond J Gorte,Stefania Rapino,Francesco Paolucci,Michele Melchionna,Marcella Bonchio,Lucia Nasi,Alessandro Boni,Paolo Fornasiero

doi:10.1038/ncomms13549

Abstract

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes. Efficient hydrogen evolution from water is one promising route towards a renewable energy economy and sustainable development. Here we show a tridimensional electrocatalytic interface, featuring a hierarchical, co-axial arrangement of a palladium/titanium dioxide layer on functionalized multi-walled carbon nanotubes. The resulting morphology leads to a merging of the conductive nanocarbon core with the active inorganic phase. A mechanistic synergy is envisioned by a cascade of catalytic events promoting water dissociation, hydride formation and hydrogen evolution. The nanohybrid exhibits a performance exceeding that of state-of-the-art electrocatalysts (turnover frequency of 15000 H2 per hour at 50 mV overpotential). The Tafel slope of ∼130 mV per decade points to a rate-determining step comprised of water dissociation and formation of hydride. Comparative activities of the isolated components or their physical mixtures demonstrate that the good performance evolves from the synergistic hierarchical structure.

Highlights

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes
Carbon nanotubes (CNTs) nanoscaffolds can template the arrangement of the metal-based catalytic interfaces by enhancing the active surface area, while connecting the phase boundaries to improve the electrocatalytic efficiency of the process
These functionalized nanoscaffolds provide a direct insight into the role of the chemical-spacer (–OH; –COOH versus benzoic residues) separating the TiO2 shell from the nanocarbon surface, while the final thermal annealing step turns out to seal the MWCNT/TiO2 interface, overriding any structural difference, as addressed by the electrocatalytic screening

Summary

Introduction

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes. Key steps for the overall process are as follows: (i) the separation of HER half-reaction (HER, 2H þ þ 2e À 1⁄4 H2) from the anodic counterpart; (ii) a tailored design of the electrocatalytic interface to promote the formation of HER intermediates (Had) via water dissociation at neutral pH (H2O þ e À -Had þ OH À , Volmer step at pH 1⁄4 7); and (iii) the correct trafficking of reducing and oxidizing species between these two compartments[20] For this purpose, an attractive strategy can be envisaged by a hierarchical design of the catalyst nanoarchitecture[21], where selected components are arranged to leverage the expected mechanistic functions, namely: (i) water dissociation; (ii) protons reduction; and (iii) electrons translocation[22]. Pd has the additional value to be nontoxic or allergenic as for other investigated systems such as the Ni-based ones

Methods

Results

Conclusion