Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support

Abstract

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON _x ) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON _x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON _x support. The Pt/TiON _x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal-support interaction (SMSI) triggered between the TiON _x support and grafted Pt nanoparticles. SMSI between TiON _x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON _x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Figure 1

Characterization of a Pt/TiON_x composite: (a) XRD spectra; (b) STEM imaging showing the overall structure of the sample; (c) high-magnification STEM imaging showing predominant anchoring of Pt NPs to TiON_x; (d) particle size distribution.

Figure 2

(a) Cyclic voltammograms of the Pt/C and Pt/TiON_x catalysts (Ar-saturated 0.1 M HClO₄, 50 mV s^–1, Pt loadings were 5 μg for Pt/C and 4.6 μg for Pt/TiON_x); (b) CO stripping voltammetry (full lines) and subsequent voltammograms (dotted lines) for Pt/C and Pt/TiON_x (0.1 M HClO₄, 20 mV s^–1, Pt loadings were 5 μg for Pt/C and 3.7 μg for Pt/TiON_x).

Figure 3

Comparison of the HER activity of the Pt/C and Pt/TiON_x catalysts: (a) HER polarization curves (0.1 M HClO₄, 10 mV s^–1); (b) intrinsic HER activity given with respect to ESA; (c) Tafel slope analysis derived from polarization curves from (a); and (d) electrochemical impedance spectroscopy (−20 mV_RHE, 50 mHz to 100 kHz, amplitude 10 mV).

Figure 4

Stability test for the Pt/C and Pt/TiON_x catalysts performed by cycling in the potential region between −0.1 and 0.2 V_RHE (5000 cycles, 100 mV s^–1, 0.1 M HClO₄): (a, c) HER polarization curves (10 mV s^–1) before and after the stress test for Pt/C and Pt/TiON_x, respectively, and (b, d) the corresponding cyclic voltammograms (200 mV s^–1) of Pt/C and Pt/TiON_x, respectively.

Figure 5

XPS characterization of the Pt 4f region: (a) normalized Pt 4f region for Pt/C and Pt/TiON_x; (b) fitted Pt 4f region for Pt/C; and (c) fitted Pt 4f region for Pt/TiON_x.

Figure 6

Average hydrogen adsorption free energy (at 298 K and 1 atm) as a function of the number of H atoms on the two Pt nanoparticles supported on TiON_x and graphene. Vertical green stripes indicate H coverages relevant under HER conditions. Only the adsorption free energy of the most stable identified structure for each number of H atoms is reported. Below the graphs, exemplar snapshots of each considered Pt NP/support system at high H coverage are shown (snapshots at other H coverages are provided in Figures S6 and S7 in the Supporting information).

Figure 7

Adhesion of Pt⟨6,9⟩ on TiON_x and graphene supports. Left: planar integrated electron charge density difference, Δρ(z) of eq 14, for Pt/TiON_x (blue curve) and Pt/C (red curve); Δρ(z) > 0 corresponds to electron excess. The respective structures are superposed with the Δρ(z) curves to facilitate interpretation (TiON_x support is shown in the upper left half, graphene support in the bottom left half); z = 0 is arbitrarily set to the bottom layer of Pt⟨6,9⟩. Right: the corresponding 3D electron charge density difference, Δρ(r) of eq 13. The Δρ(r) plots are drawn with isosurfaces of ±0.01 e Bohr; blue (red) color represents the electron-deficit (excess) regions.