Elsevier

Electrochimica Acta

Volume 228, 20 February 2017, Pages 528-536
Electrochimica Acta

Influence of nanosizing on hydrogen electrosorption properties of rhodium based nanoparticles/carbon composites

https://doi.org/10.1016/j.electacta.2017.01.109Get rights and content

Highlights

  • Rh nanohydride (1.3–2.3 nm) investigated by cyclic voltammetry in the hydrogen domain.

  • An isopotential point characteristic of surface dehydrogenation is observed on cycling.

  • Upon cycling, the nanoparticle surface converts from RhHx to Rh.

  • The amount of sorbed H increases with nanoparticle downsizing, H/Rh = 0.47 at 1.3 nm.

  • Nanoparticle downsizing creates new multi-fold adsorption (sub)surface sites for Hupd

Abstract

Composites made of ultra-small Rh hydride nanoparticles (NP) with controlled average sizes of 1.3, 1.9 and 2.3 nm dispersed in high surface area graphite powders were synthesized. The hydrogen electrosorption properties of the Rh nanohydride that is stable under ambient conditions were characterized by cyclic voltammetry under various scan rates in the hydrogen potential domain with the help of a cavity microelectrode. During the first cycles, an evolution of the voltammograms, characterized by an isopotential point, is observed that corresponds to a surface conversion of RhHx into Rh, the core of the NP remaining in the hydride phase. After stabilization, the voltammograms exhibit the classical hydrogen electrosoprtion peaks of Rh, but a shift to positive potentials indicates that H is more weakly bounded to the surface as the NP size decreases. The onset of the HER follows the same trend. Interestingly, it is observed that the quantity of electrosorbed hydrogen strongly increases when downsizing the NPs, with the H/Rh ratio reaching 0.47 ± 0.11 for NPs with an average size of 1.3 nm. This enhancement cannot be explained just by the increase in surface area by NP downsizing. It may arise from the creation of new multi-fold adsorption surface and sub-surface sites due to the presence of many corner and edge atoms in ultra-small NPs with strong surface curvature.

Introduction

Composites materials made of nanoparticles (NPs) dispersed in a porous carbon and particularly Rh/carbon composites are commonly used as electrocatalysts for oxidation or reduction reactions in fuel cells or in environmental applications, for example ethanol oxidation [1], [2], borohydride oxidation [3], [4] or nitrate reduction [5].

Various synthetic methods are reported for obtaining such Rh/carbon materials. A colloidal suspension of NPs can be synthesized by reduction with NaBH4 in the presence of PVP (poly(vinylpyrrolidone)) and then mixed with the carbon powder [1]. NPs can also be synthesized directly in the carbon host in presence of reducing and stabilizing reagents [2], [4] or by a polyol method [3]. We have recently developed an alternative route to elaborate size-controlled Pd NPs [6], [7]. It consists in impregnating the carbon powder with the metal salt in aqueous solution followed by reduction during heating under a hydrogen flow. In a recent work, we applied the same synthetic approach to produce Rh hydride (RhHx) NPs supported on a high surface area graphitic carbon [8]. The NP size is controlled by the reduction temperature. A series of composites with RhHx was synthesized with average NP sizes ranging from 1.3 to 3.0 nm. This study revealed that NPs below 2.3 nm can be synthesized as metallic Rh which forms a hydride phase (RhHx) at room temperature and H2 pressure below 0.1 MPa, in contrast to bulk Rh that can be hydrogenated only at 4 GPa. Above 2.3 nm, NPs only form solid solutions with H under the same conditions (0.1 MPa, RT). This evidences a nanoscale effect that drastically changes the H sorption thermodynamics of Rh. In addition, it was also observed that once formed, the hydride phase is stable at room temperature, a complete dehydrogenation only occurring under thermal treatment around 175 °C in vacuum.

Hydrogen electrosorption at Rh electrode surfaces has been studied for a long time, either on bulk Rh (mono or polycrystalline) or on Rh films [9], [10], [11], [12], [13], [14], [15], [16]. To our knowledge, only a few recent works have addressed the electrochemical properties of Rh NPs (supported or not) towards hydrogen sorption as well as oxygen sorption [1], [4], [5], [10], [17], [18]. Surprisingly, in none of these works, the amount of adsorbed H per Rh as a function of the particle size has been investigated.

The ability of our synthetic route to elaborate RhHx NPs with well-defined and controlled size below 10 nm offers a suitable mean to investigate, for the first time to our knowledge, the effect of nanosizing on hydrogen/oxygen electrosorption of RhHx. In the light of our recent results on the enhanced hydrogen ad/absorption in ultra-small Rh NPs under ambient conditions [8], electrochemical studies have been carried out to complement our previous solid/gas experiments. In the present article, composite powders containing RhHx NPs with different and well controlled sizes were synthesized and characterized. Electrochemical analyses were carried out using the Cavity MicroElectrode (CME) [6], [19]. Cyclic voltammetry studies in the hydrogen potential domain (in acidic medium) have been performed to characterize for the first time the hydrogen electrosorption properties of RhHx and their evolution as a function of the NP size.

Section snippets

Experimental

Composites of RhHx NPs in carbon were synthesized via the following chemical procedure. The high surface area carbon powder HSAG500 from Imerys Graphite & Carbon (hereafter designated as C) was impregnated with an aqueous rhodium chloride (RhCl3) solution under magnetic stirring. The mixture was then dried at 60 °C in an oven for one night and treated under Ar/H2 flow at different temperatures to reduce Rh(+III) to Rh(0), which further reacts with H2 gas to form RhHx NPs as detailed in [8]. We

Electron microscopy characterization.

The prepared materials have been characterized by electron microscopy. Fig. 1 shows a SEM image of the pure Rh powder (A) and TEM images of the RhHx-s/C composites synthesized by reduction of RhCl3 in Ar/H2 at 175 (B), 200 (C) and 250 °C (D).

The Rhp powder consists of large agglomerates (μm range) made of particles with sizes ranging from 10 to 100 nm. The composites exhibit RhHx NPs well dispersed in the carbon host, the average size of which increases with the temperature of the annealing

Discussion

We first discuss the hydrogenation state of the RhHx NPs under electrochemical cycling. We have shown in [8] that NPs with sizes equal or below 2.3 nm are hydrogenated, the hydride phase RhHx being stable under ambient conditions. Our previous solid/gas measurements conducted between 10‐5 atm to 1 atm indicate a very limited adsorption and solubility of H in the hydride phase, with H/Rh  0.05 at 1 atm, irrespective of the NP size. Comparatively, a larger number of H can be exchanged reversibly

Conclusion

Carbon supported Rh nanohydrides that are stable under ambient conditions have been characterized electrochemically for the first time. Three RhHx-s/C composites made of ultra-small NPs dispersed on a high surface area graphite were synthesized with well controlled NP sizes of 1.3, 1.9 and 2.3 nm. Their electrochemical response in acidic medium has been investigated with a cavity microelectrode by cyclic voltammetry in the hydrogen potential domain.

The voltammograms evolves during the first

Acknowledgements

The authors would like to gratefully acknowledge Julie Bourgon for the TEM observations, the French National Research Agency for financial support (contract GENESIS ANR-13-BS08-0004-01) and Imerys Carbon & Graphite for providing the high surface area graphite material.

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