Letter pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Enhancing Electrocatalytic Water Splitting Activities via Photothermal Effect over Bifunctional Nickel/Reduced Graphene Oxide Nanosheets Liu Gu,†,# Chao Zhang,†,# Yamei Guo,† Jian Gao,§ Yifu Yu,*,† and Bin Zhang*,†,‡
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Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University, No. 135 Yaguan Road, Haihe Education Park, Jinnan District, Tianjin 300350, China § Research Center of Heterogeneous Catalysis and Engineering Sciences, School of Chemical Engineering and Energy, Zhengzhou University, No. 100 Science Avenue, Zhengzhou 450001, Henan Province, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, No. 92 Weijin Road, Nankai District, Tianjin 300072, China S Supporting Information *
ABSTRACT: Electrocatalytic water splitting has huge potential for generating hydrogen fuel. Its wide application suffers from high energy loss and sluggish reaction kinetics. The adoption of appropriate electrocatalysts is capable of reducing the overpotential and accelerating the reaction. Present research mainly focuses on adjusting electrocatalysts, but the performances are also dependent on other parameters. Therefore, the development of an efficient strategy to enhance electrocatalytic performance through integrating with other driving force, especially a renewable driving force, is of great interest. Herein, we present a photothermal-effect-driven strategy to promote the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities of nickel/reduced graphene oxide (denoted as Ni/RGO) bifunctional electrocatalysts. The Ni/RGO composite exhibited significant enhancement of activities after exposure to light irradiation (49 mV and 50 mV decrease of overpotential at 10 mA/cm2 for HER and OER, respectively). It was found that the improved electrocatalytic activities arose from the photothermal effect of Ni/RGO, which can efficiently facilitate the thermodynamics and kinetics of electrocatalytic reactions. Furthermore, the photothermal-effectinduced enhancement for electrocatalysis showed good stability, indicating its promising potential in practical application. KEYWORDS: Bifunctional electrocatalysts, Hydrogen evolution reaction, Ni/RGO nanosheets, Oxygen evolution reaction, Photothermal effect
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sis,14−16 and solar thermal energy.17 Recently, light was introduced into an electrochemical system because it is capable of efficiently improving the electrocatalytic performance.18−22 For example, the light illumination-induced surface plasmon resonance (SPR) effect on gold nanostructures can improve HER and OER performances of MoS2 and Ni(OH)2, respectively.23,24 Our group designed and constructed a metal/semiconductor (Ni/NiO) bifunctional electrocatalyst to improve electrocatalytic HER and OER activities simultaneously through a photogenerated-carrier-driven strategy.25 However, gold nanomaterials can only utilize light in the responsive range of SPR. For semiconductors, only irradiation light with energy larger than their band gap can be absorbed and used to generate carriers. Thus, the utilization efficiency of solar energy for the enhancement of electrocatalysis is limited.
INTRODUCTION The extensive use of fossil fuels has raised serious economic and environmental concerns.1,2 Electro-driven water splitting into hydrogen and oxygen provides a promising and environmentally friendly pathway for renewable energy transformation and usage. However, the high energy loss and sluggish reaction kinetics severely retard its wide application.3 To address this issue, a lot of efforts have been invested to the exploration of electrocatalysts in the past decades to reduce the overpotential and accelerate the water splitting reaction.4,5 A series of efficient electrocatalysts, such as metal chalcogenides,6 phosphides,7−9 carbides,10 and metal alloys,11 have been developed for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). But, the efficiency of overall energy conversion cannot meet the requirement for practical application. Novel strategy is urgently needed to further improve electrocatalytic performance. As a sustainable and clean energy source, solar energy has been widely used in the form of photosynthesis,12 photovoltaic cell,13 photocataly© XXXX American Chemical Society
Received: November 23, 2018 Revised: January 21, 2019 Published: January 24, 2019 A
DOI: 10.1021/acssuschemeng.8b06117 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering As known, many materials, such as graphene,26 carbon nanotubes27 as well as metal oxides28 can absorb most sunlight to achieve efficient conversion from solar energy to thermal energy. On the other hand, the local high temperature on the surface of electrocatalysts is able to facilitate the thermodynamics and kinetics of electrocatalytic reactions.29 As far as we are aware, however, the adoption of photothermal materials with high conversion efficiency of solar energy to enhance electrocatalysis has rarely been reported. Herein, we put forward a photothermal-effect-driven strategy to improve the electrocatalytic HER and OER activities of a model bifunctional electrocatalyst, i.e. Ni nanoparticles supported on reduced graphene oxide (Ni/RGO). Ni serves as active component and RGO acts as photothermal material with the ability of utilizing the light to heat Ni. The HER and OER performances of Ni/RGO are able to be both greatly enhanced after illumination. The results demonstrated that the improved electrocatalytic activities arose from the photothermal effect of composites. Furthermore, the photothermaleffect-induced enhancement for electrocatalysis showed good stability, indicating promising potential in practical application.
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RESULTS AND DISCUSSION First, Ni(OH)2 nanoparticles supported on RGO, denoted as Ni(OH)2/RGO, was synthesized through the hydrothermal method (Figures S2, S3).30 Then, the composite of Ni/RGO was obtained after reducing Ni(OH)2/RGO at 500 °C by the H2/Ar (containing 3 vol % of H2) atmosphere for 5 h. The asprepared Ni/RGO was further characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy and infrared (IR) imaging. The TEM images in Figure 1a,b show that numerous nanoparticles with the size of 5−10 nm are distributed on the surface of RGO nanosheets. The fringe spacing of the supported nanoparticles (∼0.20 nm) matches well with the (111) lattice planes of metal Ni (inset in Figure 1b).25 In the in situ XPS spectrum (Figure 1c), only metallic Ni0 for nickel specie can be found after reduction treatment in hydrogen atmosphere.31 The XRD pattern (Figure 1d) identifies the reduction products as the composite of metallic Ni (JCPDS 04-0850) and RGO (marked as #). The Raman spectra (Figure 1e) show that GO and Ni/RGO possessed two peaks at about 1330 cm−1 (D band) and 1590 cm−1 (G band). The relative intensity ratio of ID/IG for Ni/RGO experience an obvious increase compared with that of GO, indicating the successful reduction of GO to RGO.32 These results suggested that Ni/RGO composite was successfully prepared through hydrogen reduction of the as-prepared Ni(OH)2/RGO. In order to investigate the photothermal effect of Ni/RGO, light is shined on the samples, and the temperature of Ni/RGO quickly reaches to 50 °C in 260 s, and finally maintained at 53 °C (Figure 1f), which prove that Ni/RGO has excellent ability to convert light into thermal energy.33,34 The HER performance of Ni/RGO is investigated with and without the presence of light irradiation (see details in Supporting Information). The iR-corrected polarization curves are displayed in Figure 2a. After exposure to the illumination, Ni/RGO exhibits significantly enhanced HER activities. The overpotential decreases from 233 to 184 mV at 10 mA cm−2 current density, and the mass activities at the overpotential of 50, 75, 100, and 125 mV vs RHE increase from 20, 22, 29, and 41 A g−1 to 36, 44, 68, and 120 A g−1, respectively (Figure 2b). Moreover, the Tafel plots (Figure 2c) are calculated to
Figure 1. (a) Low-magnification TEM image, (b) TEM image (inset: HRTEM image), (c) in situ XPS spectrum, and (d) XRD pattern of Ni/RGO. (e) Raman spectra of GO and Ni/RGO. (f) Timedependent temperature curve of Ni/RGO in 1 M KOH electrolyte under the irradiation of 300 W Xe lamp (intensity = 850 mW cm−2). Infrared images (inset in panel f) of Ni/RGO before irradiation (left) and after irradiation for 15 min (right).
Figure 2. (a) The iR-corrected HER polarization curves, (b) mass activity at different overpotentials, (c) Tafel plots and (d) HER durability of Ni/RGO with and without irradiation at −0.22 V(vs RHE) for HER in 1 M KOH electrolyte.
investigate the change of the kinetics for HER by light irradiation. The Ni/RGO-(dark) showed the Tafel slope of 119 mV dec−1, which is much larger than the Tafel slope of Ni/RGO-(light). These results suggested that the thermodynamics and kinetics of HER in Ni/RGO can both be efficiently B
DOI: 10.1021/acssuschemeng.8b06117 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
nickel-based oxide can only be excited by incident ultraviolet light (
