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. 2016 Jul 22:7:12272.
doi: 10.1038/ncomms12272.

A rhodium/silicon co-electrocatalyst design concept to surpass platinum hydrogen evolution activity at high overpotentials

Lili Zhu  1 , Haiping Lin  1 , Youyong Li  1 , Fan Liao  1 , Yeshayahu Lifshitz  1   2 , Minqi Sheng  1 , Shuit-Tong Lee  1 , Mingwang Shao  1
Affiliations

A rhodium/silicon co-electrocatalyst design concept to surpass platinum hydrogen evolution activity at high overpotentials

Lili Zhu et al. Nat Commun. .

Abstract

Currently, platinum-based electrocatalysts show the best performance for hydrogen evolution. All hydrogen evolution reaction catalysts should however obey Sabatier's principle, that is, the adsorption energy of hydrogen to the catalyst surface should be neither too high nor too low to balance between hydrogen adsorption and desorption. To overcome the limitation of this principle, here we choose a composite (rhodium/silicon nanowire) catalyst, in which hydrogen adsorption occurs on rhodium with a large adsorption energy while hydrogen evolution occurs on silicon with a small adsorption energy. We show that the composite is stable with better hydrogen evolution activity than rhodium nanoparticles and even exceeding those of commercial platinum/carbon at high overpotentials. The results reveal that silicon plays a key role in the electrocatalysis. This work may thus open the door for the design and fabrication of electrocatalysts for high-efficiency electric energy to hydrogen energy conversion.

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Figures

Figure 1
Figure 1. TEM characterization.
(a) TEM image of a Rh/SiNW. Scale bar, 100 nm. (b) Enlarged HRTEM image of the red square in a showing a Rh crystallite. The 0.22 nm spacing corresponds to Rh (111). Scale bar, 5 nm. (c) HAADF-STEM image of a Rh/SiNW. Scale bar, 50 nm. (d) Its corresponding EDS mapping showing the O, Si and Rh distributions. EDS, energy dispersive spectroscopy; HAADF-STEM, high-angle annular dark field scanning TEM; HRTEM, high-resolution TEM.
Figure 2
Figure 2. The electrochemical activity of different electrocatalysts in oxygen-free 0.5 M H2SO4.
(a) LSV plots of metal/SiNW catalysts containing about 30 wt% metal. The metal type is indicated. The LSV of 40 wt% Pt/C is also shown. 29.1 wt% Rh/SiNW has the best HER activity of all metal/SiNW catalysts. (b) LSV plots of Rh/SiNW catalysts with different Rh wt% with the 59.9 wt% the best one. (c) LSV plots of SiNWs, pure Rh, 29.1 wt% Rh/SiNW, and 20 and 40 wt% Pt/C. The HER activity of 40 wt% Pt/C is better than that of 20 wt% Pt/C, so only the 40 wt% Pt/C data was used as a reference to the Rh/SiNW data. The HER activity of 29.1 wt% Rh/SiNW is worse than that of 40 wt% Pt/C for V>−160 mV but is better for V<−160 mV. (d) Tafel plots and Tafel slopes derived from c. The Tafel slopes are 145, 40, 30 and 24 mV dec−1 for SiNWs, pure Rh, 40 wt% Pt/C and 29.1 wt% Rh/SiNW, respectively. The Tafel slope of 29.1 wt% Rh/SiNW is smaller than that of 40 wt% Pt/C indicating the better HER performance of 29.1 wt% Rh/SiNW.
Figure 3
Figure 3. HER relative efficiency of 29.1 wt% Rh/SiNW and 40 wt% Pt/C in oxygen-free 0.5 M H2SO4.
(a) LSV of both catalysts for high current densities. At 1,000 mA cm−2 the electric power to hydrogen energy conversion efficiency of 29.1 wt% Rh/SiNW is larger by 7.8%. Note that the current for 29.1 wt% Rh/SiNW is more stable. (b) Hydrogen evolution by 29.1 wt% Rh/SiNW and 40 wt% Pt/C systems at −0.4 V. The hydrogen evolution using 29.1 wt% Rh/SiNW is larger by 15%.
Figure 4
Figure 4. Modelling of the HER reaction on Rh/SiNW.
(a) A schematic representation of the HER reaction mechanism: (i) adsorption of hydrogen ions on the Rh surface. (ii) Diffusion of a hydrogen atom from the Rh surface to the Si surface. (iii) The adsorbed hydrogen atom on the Si surface reacts with a proton to form a hydrogen molecule. And the Rh-adsorbed hydrogen may migrate to the surface of Si atoms (marked with a red circle): (b) IS, initial state; (c) TS, transition state; and (d) FS, final state. (e) The activation energy to get to the transition state is 0.24 eV and in the final state the adsorbed hydrogen is 0.33 eV below the transition state and 0.09 eV below the initial state.
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