Graphene Supported Rhodium Nanoparticles for Enhanced Electrocatalytic Hydrogen Evolution Reaction

Abstract

Current research on catalysts for proton exchange membrane fuel cells (PEMFC) is based on obtaining higher catalytic activity than platinum particle catalysts on porous carbon. In search of a more sustainable catalyst other than platinum for the catalytic conversion of water to hydrogen gas, a series of nanoparticles of transition metals viz., Rh, Co, Fe, Pt and their composites with functionalized graphene such as RhNPs@f-graphene, CoNPs@f-graphene, PtNPs@f-graphene were synthesized and characterized by SEM and TEM techniques. The SEM analysis indicates that the texture of RhNPs@f-graphene resemble the dispersion of water droplets on lotus leaf. TEM analysis indicates that RhNPs of <10 nm diameter are dispersed on the surface of f-graphene. The air-stable NPs and nanocomposites were used as electrocatalyts for conversion of acidic water to hydrogen gas. The composite RhNPs@f-graphene catalyses hydrogen gas evolution from water containing p-toluene sulphonic acid (p-TsOH) at an onset reduction potential, E_p, -0.117 V which is less than that of PtNPs@f-graphene (E_p, -0.380 V) under identical experimental conditions whereas the onset potential of CoNPs@f-graphene was at E_p, -0.97 V and the FeNPs@f-graphene displayed onset potential at E_p, -1.58 V. The pure rhodium nanoparticles, RhNPs also electrocatalyse at E_p, -0.186 V compared with that of PtNPs at E_p, -0.36 V and that of CoNPs at E_p, -0.98 V. The electrocatalytic experiments also indicate that the RhNPs and RhNPs@f-graphene are stable, durable and they can be recycled in several catalytic experiments after washing with water and drying. The results indicate that RhNPs and RhNPs@f-graphene are better nanoelectrocatalysts than PtNPs and the reduction potentials were much higher in other transition metal nanoparticles. The mechanism could involve a hydridic species, Rh-H^- followed by interaction with protons to form hydrogen gas.

Figure 1

H-cluster, the active site. Structure of the Hydrogen-cluster in the H₂ evolving bacterium Clostridium pasteurianum.

Figure 2

TEM images of RhNPs and RhNPs@f-graphene. (A) RhNPs at low magnification; (B) RhNPs at high magnification; (C) RhNPs@f-graphene; (D) RhNPs@rGO.

Figure 3

SEM analysis of RhNPs and RhNPs@f-graphene. (A) SEM image of RhNPs with inbox showing aggregation of nanoparticles; (B) SEM image of RhNPs@f-graphene at low magnification; (C) SEM image of RhNPs@f-graphene at higher magnification; (D) EDX analytical result of RhNPs; (E) Powder XRD spectrum of RhNPs; (F) EDX analytical result of RhNPs@f-graphene; (G) Powder XRD spectrum of RhNPs@f-graphene.

Figure 4

TEM images of the composites. (A) TEM image of PtNPs@f-graphene; (B) TEM image of CoNPs@f-graphene; (C) Powder XRD spectrum of PtNPs@f-graphene.

Figure 5

Electrocatalysis by RhNPs. Cyclic voltammograms of RhNPs as a function of increasing concentrations of added p-TsOH (red to olive green) in water. (0.5 M p-TsOH, 0.2 mL each in water). Displaying only forward reduction waves for clarity; scan rate of 100 mVs⁻¹ (Supporting electrolyte, KNO₃/H₂O (0.2 M), GCE working, Pt wire auxillary and Ag/AgCl reference electrodes).

Figure 6

H₂ evolution from acidic water by RhNPs@f-graphene. Cyclic voltammograms of RhNPs@f-graphene as a function of increasing concentrations of added p-TsOH (red to magenta) in water. (0.5 M p-TsOH, 0.2 mL each in water). Displaying only forward reduction waves for clarity; scan rate of 100 mVs⁻¹ (Supporting electrolyte, KNO₃/H₂O (0.2 M), GCE working, Pt wire auxillary and Ag/AgCl reference electrodes).

Figure 7

H₂ evolution from acidic water by f-graphene in the absence of electrocatalysts. Cyclic voltammograms (black to blue) of f-graphene as a function of increasing concentrations of added p-TsOH in water. (0.5 M p-TsOH, 0.2 mL each in water). scan rate of 100 mVs⁻¹ (Supporting electrolyte, KNO₃/H₂O (0.2 M), GCE working, Pt wire auxillary and Ag/AgCl reference electrodes).

Figure 8

Mechanism of reduction of protons to hydrogen gas on RhNPs@f-graphene. Display of RhNPs on a functionalized graphene layer which take up protons and convert to hydrogen gas.

Figure 9

Comparison of the onset potentials and electrocatalytic efficiency of RhNPs@f-graphene with other transition metal@f-graphene nanocomposites. Cyclic voltammograms of (A) RhNPs: (B) PtNPs; (C) CoNPs; (D) RhNPs@f-graphene; (E) PtNPs@f-graphene; (F) CoNPs@f-graphene as a function of increasing concentrations of added p-TsOH in water. (0.5 M p-TsOH, 0.2 mL each in water); (G) Comparison of reduction on-set potentials. black, onset potential for RhNPs@f-graphene, red, on-set potential for PtNPs@-graphene, Blue, onset potential for RhNPs. Displaying only forward reduction waves for clarity; scan rate of 100 mVs⁻¹ (Supporting electrolyte, KNO₃/H₂O (0.2 M), GCE working, Pt wire auxillary and Ag/AgCl reference electrodes).