Enhanced Electrocatalytic Reduction of CO2 via Chemical Coupling

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Enhanced Electrocatalytic Reduction of CO2 via Chemical Coupling between Indium Oxide and Reduced Graphene Oxide Zhirong Zhang, Fawad Ahmad, Wanghui Zhao, Wensheng Yan, Wenhua Zhang, Hongwen Huang, Chao Ma, and Jie Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01393 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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Revised ms # nl-2019-01393v

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Enhanced Electrocatalytic Reduction of CO2 via Chemical Coupling

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between Indium Oxide and Reduced Graphene Oxide

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Zhirong Zhang,†,‖ Fawad Ahmad,†,‖ Wanghui Zhao,†,‖ Wensheng Yan,† Wenhua Zhang,*,†

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Hongwen Huang,*,§ Chao Ma,§ and Jie Zeng*,†

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†Hefei

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Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of

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Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R.

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China

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§College

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R. China

National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

of Materials Science and Engineering, Hunan University, Changsha, Hunan, 410082, P.

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*To whom correspondence should be addressed.

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E-mail: [email protected] (W. Z.)

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E-mail: [email protected] (H. H.)

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E-mail: [email protected] (J. Z.)

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‖These

authors contributed equally to this work.

ACS Paragon Plus Environment

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Abstract

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The chemical coupling interaction has been explored extensively to boost heterogenous

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catalysis, but the insight into how chemical coupling interaction works on CO2 electroreduction

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remains unclear. Herein we demonstrate the chemical coupling interaction between porous In2O3

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nanobelts and reduced graphene oxide (rGO) could substantially improve the electrocatalytic

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activity toward CO2 electroreduction. Such In2O3-rGO hybrid catalyst showed 1.4-fold and

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3.6-fold enhancements in Faradaic efficiency and specific current density for the formation of

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formate at -1.2 V versus reversible hydrogen electrode relative to the catalyst prepared by

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physically loading of In2O3 nanobelts onto rGO, respectively. The density functional theory

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calculations and electrochemical analysis together revealed that the chemical coupling

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interaction boosted CO2 electroreduction activity by improving electrical conductivity and

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stabilizing key intermediate HCOO-*. The present work not only deepens an understanding of

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chemical coupling effect but also provides an effective lever to optimize the catalytic

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performance toward CO2 electroreduction.

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Keywords: chemical coupling interaction, CO2 electroreduction, electrical conductivity,

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adsorption energy

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Table of Contents

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Electroreduction of CO2 powered by renewable electricity offers a promising approach to

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simultaneously mitigate the climate issue and store the intermittent energy sources by

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transforming CO2 to value-added chemicals/fuels.1-7 However, enabling an efficient

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electroreduction of CO2 remains an open challenge owing to the great difficulty to activate the

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stable CO2 molecule.8-9 Finding active and selective electrocatalysts thus holds the key to realize

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the economically viable conversion of CO2. Thanks to tremendous efforts from many research

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groups, the catalytic activity and selectivity of various electrocatalysts, including metals, metal

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oxides/chalcogenides, carbonaceous materials, and so on, at present can be optimized via the size

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effect, alloy effect, strain effect, sharp-tip effect, oxygen vacancy effect, etc.10-19

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The chemical coupling between different components in a hybrid material offers great

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opportunities to improve the catalytic properties.20-23 For example, a hybrid catalyst composed of

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Co3O4 nanoparticles grown on reduced graphene oxide (rGO) displayed largely enhanced

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activity and durability toward both the oxygen reduction reaction and oxygen evolution reaction

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relative to Co3O4 or graphene oxide alone.20 The synergy between Co3O4 and rGO accounted for

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such improvements. In another case, the effect of chemical coupling interaction between

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polyaniline nanodots and CoP nanowires on the improved electrocatalytic activity of hybrid

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electrocatalyst was assigned to the improved electron conductivity.21 Most of the previous

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investigations on the chemical coupling concerned with the synergetic effect between the

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different components in the hybrid, but ignored the change in electronic structure of the

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catalytically active phase. Moreover, the fundamental understanding of the chemical coupling

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interaction in the CO2 electroreduction remains relatively vague. Herein, we demonstrate the

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chemical coupling interaction between porous In2O3 nanobelts and rGO could modify the

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electron structure of In2O3 and substantially improve its electrocatalytic activity toward CO2

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reduction. We further reveal the chemical coupling effect on CO2 electroreduction by combing

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density functional theory (DFT) calculations and electrochemical analysis.

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The porous In2O3 nanobelts-rGO hybrid (denoted as In2O3-rGO hybrid) was fabricated via a

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two-step method, as schematically shown in Figure 1a. In the first step, an aqueous solution

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containing a certain amount of commercial GO nanosheets, sodium oleate and indium (III)

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chloride was subjected to a hydrothermal treatment at 150 oC for 3 h to produce the In(OH)3

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nanobelts-rGO hybrid. Afterwards, the preformed In(OH)3 nanobelts-rGO hybrid was annealed

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in air at 400 oC for 5 min to obtain the In2O3-rGO hybrid. The structural characterizations for 3


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intermediate products obtained after the hydrothermal process demonstrated the formation of

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In(OH)3 nanobelts-rGO hybrid (Figure S1). After the annealing treatment, the Raman spectrum

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with typical Raman peaks of In2O3 at 303, 360, 491, and 625 cm-1 suggested the formation of

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In2O3.

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intensity ratio of ID/IG (about 1.0) validated the presence of rGO (Figure S2). Above results

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confirmed the production of In2O3-rGO hybrid. The representative TEM images (Figure 1b and

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1c) displayed the porous In2O3 nanobelts were uniformly grown onto the rGO nanosheets. The

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total In2O3 content in the hybrid was estimated to be 58.0 wt% by the inductively coupled plasma

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atomic emission spectrum (ICP-AES).

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Meanwhile, the characteristic D- and G-band at 1350 and 1600 cm-1 as well as the

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The atomic-level structural information of the In2O3-rGO hybrid was further analyzed by

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atomic-resolution high-angle annular dark-filed scanning TEM (HAADF-STEM) (Figure 1d).

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The continuous lattice fringes and the corresponding fast Fourier transform (FFT) pattern

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confirmed the monocrystalline nature of the porous In2O3 nanobelts. Agreeing well with the

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XRD pattern shown in Figure S3, the lattice fringes matched well with the crystal planes of cubic

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In2O3 (space group Ia-3, a = 10.118 Å). The exposed surface facet of porous In2O3 nanobelts was

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further determined as the {110} plane of cubic In2O3. For comparison, we also prepared the

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porous In2O3 nanobelts using the similar procedure together with a physical mixture of In2O3

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nanobelts and rGO (denoted as In2O3/rGO) (Figure S4 and S5). The structural characterizations

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indicated that the morphology of the produced porous In2O3 nanobelts was identical to that of

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porous In2O3 nanobelts in the hybrid. The thickness of porous In2O3 nanobelts was estimated to

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be 1.4 nm by atomic force microscopy (AFM) (Figure S6). Given almost the same morphology

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of In2O3 component in these three samples, we could guess the porous In2O3 nanobelts in the

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hybrid also presented the feature of ultrathin thickness.

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The interaction between the porous In2O3 nanobelts and rGO nanosheets in the hybrid was

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revealed by X-ray absorption near edge structure (XANES) and X-ray photoelectron

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spectroscopy (XPS). The carbon K-edge XANES spectrum collected from In2O3-rGO hybrid

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presented a heightened C=O π* peak at 288.6 eV relative to the corresponding peak of rGO,

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which demonstrated the presence of chemical coupling interaction between In2O3 and rGO via

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C-O-In bonds (Figure 1e). 22 The formation of the C-O-In bonding changed the π* orbitals of

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carbon ring structure and thus leaded to a decreased peak in the range of 285-286 eV.22

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Moreover, In M2-edge XANES spectra of In2O3-rGO hybrid showed a declining intensity 4


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compared with that of In2O3, indicating the electron transfer from rGO to In2O3 originated from

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the strong interactions between In2O3 and rGO (Figure 1f). The shifted In 3d XPS peaks of

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In2O3-rGO hybrid also coincided with this trend (Figure 1g). In contrast, In2O3/rGO exhibited

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negligible interactions between In2O3 and rGO with no prominent changes in its In 3d XPS peaks

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relative to pure In2O3 (Figure S7).

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The electrocatalytic properties of In2O3-rGO hybrid toward CO2 reduction were then

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evaluated in comparison with porous In2O3 nanobelts (denoted as In2O3/C) as well as In2O3/rGO.

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The linear sweep voltammetric (LSV) curves in Figure S8 revealed the negligible electrocatalytic

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activity of the rGO nanosheets, confirming that the In2O3 was the active phase toward CO2

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electroreduction. Moreover, the In2O3-rGO hybrid catalyst exhibited the highest reduction

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current when compared with other catalysts in CO2-saturated electrolyte, hinting the improved

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activity of the In2O3-rGO hybrid catalyst. Controlled-potential electrolysis of CO2 was further

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carried out to quantify the products obtained at different applied potentials. The 1H nuclear

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magnetic resonance (1H NMR) and gas chromatography (GC) analyses identified the formation

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of formate, CO, and H2 in the potential range from -0.5 to -1.2 V versus reversible hydrogen

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electrode (vs. RHE) (all the potentials are referred to RHE in this work) for these catalysts

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(Figure S9). Figure 2a shows that the total Faradaic efficiencies (FEs) for the formation of

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formate and CO on In2O3-rGO hybrid catalyst exceeded 90% when the applied potential was

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more negative than -0.6 V, much higher than the corresponding FEs on In2O3/C and In2O3/rGO

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catalysts. The maximum FE for formate, the major product, reached 84.6% at -1.2 V for

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In2O3-rGO hybrid catalyst, which was 1.8 and 1.4 times greater than that on In2O3/C (48.2% at

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-1.2 V) and In2O3/rGO (61.9% at -1.2 V) catalysts, respectively. Furthermore, the In2O3-rGO

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hybrid catalyst exhibited a largely enhanced partial formate current density (Figure 2b). It should

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be noted that the FE and partial current density for the formation of formate reported in this work

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surpass most of the reported remarkable catalysts (Table S2). The specific activities of the

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catalysts for the formation of formate were also determined by normalizing the corresponding

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current density against the electrochemically active surface areas (ECSAs) of catalysts. The

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ECSAs of In2O3-rGO hybrid (25.5 m2/g), In2O3/rGO (41.4 m2/g) and In2O3/C (34.6 m2/g)

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catalysts were calculated by double-layer capacitance measurements (Figure S10). We then

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derived the specific activities of In2O3-rGO hybrid, In2O3/rGO, and In2O3/C catalysts for formate

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production at various potentials, demonstrating 5.3 and 3.6 times higher specific activity of the 5


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In2O3-rGO hybrid catalyst than that of In2O3/C and In2O3/rGO catalyst at -1.2 V (Figure 2c). It

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should be noted that the estimated ECSAs of In2O3-rGO hybrid and In2O3/rGO catalysts from

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double-layer capacitance measurements included the electrochemical active surface areas of both

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In2O3 and rGO. As thus, the calculated specific activities of these two catalysts were indeed

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underestimated. However, the enhancement factor should keep unchanged when comparing the

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specific activities of In2O3-rGO hybrid and In2O3/rGO catalysts due to the use of same amount of

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rGO, definitely confirming the enhanced specific activity of In2O3-rGO hybrid catalyst. The

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long-term durability test illustrated in Figure 2d exhibited inappreciable decays in the FE and

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current density for the formation of formate on In2O3-rGO hybrid catalyst over 10 h, indicating

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its remarkable stability. The retained morphological structure after the durability test shown in

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Figure S11 supported the excellent stability. Besides, the repeatability of the In2O3-rGO hybrid

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catalyst was also verified by continually examining the CO2 electroreduction performance of the

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catalyst for three times (Figure S12), demonstrating the well repeatability of the catalyst.

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The in-depth understanding of the improved specific activity was disclosed. Given the

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identical microstructure of the In2O3, the catalytically active component, for In2O3-rGO hybrid,

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In2O3/rGO and In2O3/C catalysts, the activity enhancement may thus be attributed to the

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chemical coupling interaction between rGO and porous In2O3 nanobelts. From the perspective of

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electrocatalysis, we analyzed the changes in electrical conductivity and adsorption energies of

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key intermediates that caused by the chemical coupling interaction. The electrochemical

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impedance analysis was performed to reveal the interfacial charge transfer resistance (Rct) of the

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catalysts. As shown in Figure S13, the semicircle diameter of electrochemical impedance spectra

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corresponds to a Rct value of 154 Ω, 180 Ω, and 206 Ω for In2O3-rGO hybrid, In2O3/rGO, and

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In2O3/C, respectively. The lowest Rct value of In2O3-rGO hybrid suggests its best electrical

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conductivity among the three samples. The reason for the reduction in interfacial charge-transfer

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resistance is probably related with the more delocalized electrons in In2O3-rGO hybrid due to the

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electron transfer from rGO to In2O3 resulting from strong chemical coupling interaction between

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In2O3 and rGO. Although the favored interfacial charge transfer would facilitate the CO2

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reduction, it is believed that the slightly improved interfacial charge-transfer resistance cannot

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completely account for the 5.3-fold enhancement in specific activity.

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DFT calculations were further performed to investigate the variations in electronic structure

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and adsorption of reaction intermediates on In2O3 induced by the chemical coupling interaction 6


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between In2O3 and rGO (see Supporting Information for details). Prior to determination of the

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Gibbs free energy diagrams, we first calculated the charge redistribution in In2O3-rGO hybrid

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and In2O3/rGO. The physically mixed In2O3 and rGO showed only a transfer of 0.13 electrons,

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whereas In2O3 and rGO coupled by chemical interaction presented a donation of 0.59 electrons

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from rGO nanosheets to In2O3 nanobelts (Figure 3a). The calculated results in agreement with

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the experimental observations suggested the higher extent of electron transfer with the presence

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of chemical coupling. The Gibbs free energy diagrams for CO2 reduction into formate were

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further calculated and clearly showed the much lower Gibbs free energy for the formation of key

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intermediate HCOO-* on In2O3-rGO hybrid catalyst due to the electron-rich structure (Figure S14,

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and Figure 3b). The trends in Gibbs free energies for HCOO-* are well consistent with the

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experimental trends in catalytic activities for In2O3-rGO hybrid, In2O3/rGO, and In2O3/C

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catalysts, also confirming the reasonable model construction. Besides, the protonation of CO2-*

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to form intermediate HCOO-* could be identified as the rate-determining step. Under this

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circumstance, we can conclude that the lower Gibbs free energy for the formation of key

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intermediate HCOO-* is another crucial factor to reduce the overpotential and facilitate the CO2

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electroreduction to generate formate on In2O3-rGO hybrid catalyst.

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The electrochemical microkinetic analysis was also performed to verify the rate-determining

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step for CO2 electroreduction on In2O3-rGO hybrid, In2O3/rGO, and In2O3/C catalysts. As shown

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in Figure 4a, the Tafel slopes of In2O3-rGO hybrid, In2O3/rGO, and In2O3/C catalysts were

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determined to be 67, 75, and 78 mV dec-1, which is close to the theoretical value of 59 mV dec-1,

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indicating the proton transfer step is the rate-determining step.25 Meanwhile the slightly lower

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Tafel slopes of In2O3-rGO hybrid suggests its faster reaction kinetics. Besides, electrolysis was

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performed in CO2-saturated KHCO3 electrolyte at -1.2 V (vs. RHE) with HCO3- concentrations

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ranging from 0.025 to 0.5 M, with KClO4 added to the electrolyte to maintain ionic strength. As

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showed in Figure 4b, plots of Log(jHCOOH) versus Log([HCO3-]) shows slopes of 0.98, 0.91, and

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0.87 for the In2O3-rGO hybrid, In2O3/rGO, and In2O3/C catalysts, respectively. The reaction rates

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for the CO2 electroreduction into formate on these catalysts have approximate first-order

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dependence on the concentration of HCO3-, again confirming that proton transfer step is the

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rate-determining step on the three catalysts. All evidences clearly supported that the strong

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chemical coupling interaction in In2O3-rGO hybrid catalyst caused an improved electrical

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conductivity and a reduced Gibbs free energy for the formation of key intermediate HCOO-*, in 7


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turn boosting the activity toward CO2 electroreduction.

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In summary, we have demonstrated the construction of chemical coupling interaction

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between porous In2O3 nanobelts and rGO could considerably enhance the catalytic activity

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toward CO2 electroreduction, resulting in the high FEs (more than 90%) with a potential more

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negative than -0.6 V. Specifically, the FE and specific current density for the formation of

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formate at -1.2 V over In2O3/rGO hybrid catalyst are 1.4 times and 3.6 times higher than those

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over In2O3/rGO catalyst, respectively. The insight into the great improvement in intrinsic activity

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was revealed by a combination of experimental evidences and DFT calculations, showing that

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the chemical coupling interaction accounted for the boosted CO2 electroreduction activity by

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improving electrical conductivity and stabilizing key intermediate HCOO-*. We believe the

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present work not only clearly uncovers the chemical coupling effect, but also provides an

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effective knob to maximize the catalytic activity toward electroreduction of CO2.

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ASSOCIATED CONTENT

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Supporting Information. Experimental details, TEM images of In(OH)3, In2O3, and In2O3/rGO,

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XRD patterns of In2O3, In2O3/rGO and In2O3-rGO, Raman spectra, AFM image, XPS spectra,

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LSV curves, 1H NMR spectra, CVs, Electrochemical impedance spectra, DFT models, work

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function. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Authors

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* E-mail: [email protected] (W. Z.)

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* E-mail: [email protected] (H. H.)

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* E-mail: [email protected] (J. Z.)

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Author Contributions

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‖Z.

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Notes

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The authors declare no competing financial interests.

Zhang, F. Ahmad, and W. Zhao contributed equally.

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ACKNOWLEDGMENTS

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This work was supported by NSFC (21603208，21573206，21473167), Key Research Program

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of Frontier Sciences of the CAS (QYZDB-SSW-SLH017), Anhui Provincial Key Scientific and

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Technological Project (1704a0902013), Major Program of Development Foundation of Hefei

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Center for Physical Science and Technology (2017FXZY002), the National Key Research and

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Development Program (2016YFA0200600 ， 2018YFA0208600) ， and Fundamental Research

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Funds for the Central Universities. This work was partially carried out at the USTC Center for

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Micro and Nanoscale Research and Fabrication.

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(21)Feng, J.-X.; Tong, S.-Y.; Tong, Y.-X.; Li, G.-R. J. Am. Chem. Soc. 2018, 140, 5118-5126.

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(22)Feng, K.; Zhong, J.; Zhao, B.; Zhang, H.; Xu, L.; Sun, X.; Lee, S. T. Angew. Chem. Int. Ed.

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2016, 55, 11950-11954. (23)Li, J.-S.; Wang, Y.; Liu, C.-H.; Li, S.-L.; Wang, Y.-G.; Dong, L.-Z.; Dai, Z.-H.; Li, Y.-F.; Lan, Y.-Q. Nat. Commun. 2016, 7, 11204. (24)Shanmugasundaram, A.; Ramireddy, B.; Basak, P.; Manorama, S. V.; Srinath, S. J. Phys. Chem. C 2014, 118, 6909-6921. (25)Hori, Y. Mod. Aspects Electrochem. 2008, 42, 89-18.

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Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

Figure 1. Formation process and structural characterizations. a) Schematic illustration shows the

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preparation procedure for the In2O3-rGO hybrid. b) Low-magnification TEM image. c)

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High-magnification TEM image. d) Atomic-resolution HAADF-STEM image. The inset shows

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the corresponding FFT pattern. e) C K-edge XANES spectra of In2O3-rGO hybrid and rGO. f) In

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M2-edge XANES spctra of In2O3-rGO hybrid and In2O3 nanobelts. g) In 3d XPS spectra of

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In2O3-rGO hybrid and In2O3 nanobelts.

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Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1 2 3

Figure 2. The performance of In2O3-rGO hybrid, In2O3/rGO and In2O3/C catalysts for

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electrochemical reduction of CO2. a) FEs of CO and formate for In2O3-rGO hybrid (blue),

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In2O3/rGO (olive green) and In2O3/C (red) catalysts. b) Current density of formate based on the

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electrode's geometric surace area. c) ECSA-normalized current density of formate. d) Long-term

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durability of In2O3-rGO hybrid at -1.2 V for 10 h.

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Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

Figure 3. DFT calculation. a) The differential charge diagram of In2O3-rGO hybrid catalyst.

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Yellow represents the electron accumulation area, blue represents the electron loss area. b) Gibbs

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free energy diagrams for CO2 reduction to formate on In2O3-rGO hybrid, In2O3/rGO, and

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In2O3/C catalysts.

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Nano Letters

1 2 3

Figure 4. Electrokinetics of CO2 reduction to formate over In2O3-rGO hybrid, In2O3/rGO, and

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In2O3/C catalysts. a) Tafel plots in CO2-saturated 0.1 M KHCO3 electrolyte. b) Partial current

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density of formate versus HCO3- concentration at a potential of -1.2 V.

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Enhanced Electrocatalytic Reduction of CO2 via Chemical Coupling

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