carbon black system

Accepted Manuscript Electrochemical Reduction of CO2 to Formate Ion Using Nanocubic Mesoporous In(OH)3/Carbon Black System Arash Rabiee, Davood Nematollahi PII:

S0254-0584(17)30148-7

DOI:

10.1016/j.matchemphys.2017.02.016

Reference:

MAC 19511

To appear in:

Materials Chemistry and Physics

Please cite this article as: Arash Rabiee, Davood Nematollahi, Electrochemical reduction of CO2 to formate ion using nanocubic mesoporous In(OH)3 or carbon black system,Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.02.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract Electrochemical Reduction of CO2 to Formate Ion Using Nanocubic

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Arash Rabiee, Davood Nematollahi*

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Mesoporous In(OH)3/Carbon Black System

Nanocubic In(OH)3 with mesoporous structure had a great tendency to adsorb CO2 and electrochemical formate generation. The maximum faraday efficiency was achieved in the

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range of 70-77%.

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Highlights


Electrochemical generation of formate

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In(OH)3 morphology in CO2 electroreduction.

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Optimization of current density and faraday efficiency

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Electrochemical reduction of CO2 to formate ion using nanocubic

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mesoporous In(OH)3/carbon black system

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Arash Rabiee and Davood Nematollahi*

Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran. Zip Code 65178-38683. Fax:

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0098 - 813- 8257407, Tel: 0098 - 813- 8282807

*Corresponding author. Fax: 0098-813-8272404, Phone: 0098-813-8271541. E-mail address: [email protected]

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Abstract: To investigate the importance of In(OH)3 morphology in CO2 electroreduction, a form

of

this

catalyst

was

hydrothermally

synthesized

and

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mesoporous

electrochemically evaluated toward formate generation in In(OH)3/carbon black system. Nanocubic In(OH)3 with mesoporous structure had a high surface area with

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great tendency to adsorb CO2 showing elevated electrochemical performance. The maximum faraday efficiency was achieved in the range of 70-77%.

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Keywords: Electrochemical reduction of CO2, In(OH)3/carbon black, Mesoporous structure, Morphology, CO2 adsorption 1. Introduction

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It is generally accepted that the global warming is closely associated with anthropogenic CO2 emission. The human population is growing very fast and its dependence on fossil fuel for energy sources has even aggravated this condition [1].

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Among the technologies leading to CO2 mitigation, electrochemical reduction received active interest since value added products can be produced from abundant

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resources such as carbon dioxide and water [2]. In aqueous solutions, the electrochemical reduction of CO2 leads to products like

HCOOH, CH3OH or CH4 depending on the electrode material [3]. It is also proposed that formic acid can be utilized as a convenient means of hydrogen storage [2]. Catalysts play a key role in electrochemical CO2 reduction. An efficient catalyst must mediate multiple electron and proton transfer to CO2 in a preferably low 2

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overvoltage in presence of H2O [4]. Indeed, the feasibility of a catalyst for CO2 electroreduction refers to both high energetic efficiency and high conversion rate [5]. Previously, Bocarcely et al, have shown that a hydroxyl rich indium surface

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improved the faradic efficiency for format production. It has been proposed that an anodized indium surface allows for generation of an electroactive surface carbonate and simultaneously outcompetes H2O reduction [6]. Several works utilized Indium-

Reportedly,

electrochemically

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Table 1

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based materials for CO2 electroreduction which are summarized in Table 1.

silent

In(OH)3,

as

a

n-type

wide-band

semiconductor is a promising material for CO2 photoreduction. Ye et al. have demonstrated that the CO2 photoreduction needs technologies both to capture CO2

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from atmosphere and to convert it into the fuel. Indeed, mesoporous structure of In(OH)3 with high surface area and strong ability to adsorb CO2 as a hydroxide , facilitate electron transfer to CO2 and ultimately ends up generating CH4 [10]. Besides

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inherent ability of a catalyst to take part in a reaction and its affecting physical properties, changing the catalyst shape and morphology has the significant impact on

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catalysis activity, stability, selectivity and electrochemical properties. To prepare the most efficient catalyst, the finest control on the shape and morphology of metal oxide nanocrystal has been achieved in solution–phase methods by careful selection of solvents, precursors, capping agents, temperature and time of reaction [11,12]. For example, different morphologies of CeO2 nanomaterials as an active component or active support display different catalytic activity in CO conversion, hydrogenation 3

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of organic substances, water–gas shift reaction, photocatalysis, water treatment, NO reduction, reforming reaction, via alteration in surface area, exposed planes, edge or corner sites, extension of oxygen vacancies and dispersion of active clusters [13]. In a

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detail work,Pan et al, studied morphology-dependent catalytic properties of CeO2 nanoparticles in CO oxidation process. They have found that CeO2 nanorods, among other shaped catalysts, provides the best performance because of higher surface

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area [14]. Additionally, 3D porous structures of carbon black facilitates CO2 transport

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and reduction. Recently, Meyer et al. loaded Tin oxide nanoparticles onto carbon black and reaching the maximum faraday efficiency of > 86% for formate production [15]. Based on these evidences, we hypothesize and test here that the reduction of CO2 on a high surface area of carbon support loaded with mesoporous In(OH)3 as a

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catalyst, with great ability to adsorb CO2 , may depend on morphology and presents a promising route toward improved CO2 reduction activity of this catalyst. 2. Experimental

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2.1. Amorphous In(OH)3

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A proper amount of Indium nitrate pentahydrate (Sigma-Aldrich, 99.99%) was dissolved in deionized water to obtain a solution with concentration of 0.4 M. The pH of this solution was adjusted to 9.0 using ammonium hydroxide (SurChem Product, England) while strictly controlled by a metrohm pH meter equipped with a ceramic junction pH electrode. The white precipitate collected and copiously washed with ethanol, then dried in a vacuum oven at 80°C for 24h [16].

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2.2. Nanocubic In(OH)3 A modified procedure adopted from Shanmugasundaram et al was used [17]. Exactly 0.88 g of InCl3.4H2O (Sigma-Aldrich, 97%) (3 mmol) was dispersed in 30 ml of

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deionized water by ultrasonication and then 6 ml of ethanolamine (Merck, 99%) was added to this solution dropwise under vigorous propeller stirring. After 15 min additional stirring, 30 ml of polyethylene glycol (PEG Mn = 400) (Merck) was added

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under stirring condition. This suspension transferred to closed PTFE vessels in START

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D microwave lab station (Milestone Corp.). The solution was heated to 220 °C at a controlled rate of 10 °C/min by microwave heating and then kept at 220 °C for 6 h or 24 h. After cooling to ambient temperature, the resulting product was filtered and washed with abundant deionized water to ensure removal of any impurities. Finally,

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the precipitate was dried in a vacuum oven at 80 °C for 24 h. 2.3. Carbon black supported In(OH)3

Carbon black (Vulcan XC72R, Cabot Corp) was dispersed in water by

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ultrasonication for 30 min, then appropriate amount of In(OH)3 suspension in water

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was slowly added to the vigorously stirring carbon black dispersion to afford a catalyst with 30% indium as metal bases. The resultant material filtered and washed with water, then dried in vacuum oven at 80 °C and 0.2 bar for 6 h. A uniform catalyst ink (4 mg mL-1) was prepared as follow: 20 mg of catalyst was dispersed by stirring in 5 mL of solution composed with 79.6 mL of water, 20 mL of 2-propanol (Merck, 99.9%) and 0.4 mL of Nafion® 117 solution (Sigma-Aldrich, 5% in mixture of lower aliphatic alcohols and water) as a binder. 5

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2.4. Physical characterization of materials Adsorption/Desorption isotherms of the samples acquired by BELSORP MAX physisorption system using N2 as adsorptive gas at -196 °C. The Belsorp

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Adsorption/Desorption data analysis software was employed to determined surface area and pore size via BET modeling. The samples were degassed at 80 °C for 3 h prior the experiments. CO2 chemisorption capability of samples was evaluated by the

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same BELSORP MAX analyzer at 25 °C. In situ pre-treatment prior to CO2 adsorption

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was vacuum degassing at 80 °C for 3 h. Powder X-ray diffraction (XRD) data were recorded to characterize the products on a STOE powder diffraction system equipped with Cu-target as anode material. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were conducted by a TESCAN MIRA3-XMU with the

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electron beam set to 15 keV.The materials were examined by transmission electron microscopy (TEM) on a Philips CM10 operating at 80-120 KeV. 2.5. Electrochemical measurements and product analysis

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An Autolab/PGSTAT30 was used for all CO2 reduction experiments. A portion of

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10 μL of well dispersed catalyst ink was applied onto a pre-polished 0.196 cm2 glassy carbon disk (Autolab, 5 mm in diameter). This electrode surface provided a loading of 40 μg of carbon supported In(OH)3 containing 17 μg of In(OH)3 by calculation. To prevent forming any crack on catalyst surface, the electrode was placed under an upside down beaker for 24 h with wetted sponge to slow down drying process. Linear Sweep Voltammetric (LSV) scans were recorded in CO2 saturated 0.5 M K2SO4 (Sigma-Aldrich) at pH of 4.4. Under Ar atmosphere, the pH was adjusted to 4.4 with 6

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very dilute H2SO4. All potentials were measured against an Ag/AgCl reference electrode with ceramic junction (3M KCl, Metrohm) and converted to the RHE reference scale using E(RHE) = E(Ag/AgCl) + 0.21 V + 0.0591 pH. A Pt rod (Metrohm) was

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served as a counter electrode during the electrochemical measurements.

A custom-made, two-compartment open electrochemical cell was utilized to perform bulk electrolysis at room temperature in 0.5 M K2SO4 electrolyte. The

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constant CO2 purge was carried out during the electrolysis to maintain CO2 saturation

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and solution convection. A type four sintered glass disk prevented products from reoxidizing on anode surface. The volume of catholyte and anolyte were 70 and 40 mL respectively. To fabricate working electrode, a round graphite plate (EK20, SGL Carbon Group) was dropcasted by 1 mL of catalyst ink and dried at room

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temperature overnight. The apparent electrode size used for electrolysis was about 19.63 cm2. A large Pt plate with ca. 18 cm2 in each side served as a counter electrode in anolyte solution.

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The electrolysis product (HCOOH) was analyzed on a Bruker 500 MHz NMR spectrometer to determine faraday efficiency at any desirable time. A 0.5 mL of

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catholyte solution was taken and mixed with 0.1 mL of D2O containing 6 mM of dimethylSulphoxide (Merck, 99.9%) as an internal standard. The 1H spectrum was measured with water suppression using a presaturation method allowed direct detection of product in the electrolyte at the micromolar level.

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2.6. Quantification of formate with 1H NMR After certain period of time with known current level which are necessary for calculating total coulomb, a 0.5 mL of catholyte solution was taken and mixed with

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0.1 mL of D2O containing 6 mM of dimethylsulphoxide as an internal standard. The final concentration of DMSO in NMR tube would be 1 mM due to dilution. By assigning the value of 6 to the surface area of DMSO, the concentration of product

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can be directly obtained by integration of peak manifested at 8.3 ppm. The faraday

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efficiency (FE) can be calculated using the following equation: FE% = nFMV/ Qtotal 3. Results and discussion

In(OH)3 nanocubes with appreciate control on morphology were prepared by very

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facile hydrothermal condition. To obtain the deeper insight on the effect of catalyst morphology on its electrochemical activity, a simple route was employed to produce

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In(OH)3 precipitate. As-prepared In(OH)3 nanostructure were morphologically evaluated by FE-SEM and TEM (Figure 1). It is generally agreed that

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nucleation/growth and ripening process are time dependent and consequently have crucial effects on the ultimate morphology of product [17]. Figure 1 have depicted a series of SEM and TEM images of In(OH)3 obtained at variant reaction time while other reaction conditions were similar. Figures 1a and 1b are typical FE-SEM and TEM images showing an amorphous structure of In(OH)3 synthesized via nonhydrothermal condition. It is evident that the isolated product is a clustered

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aggregate of small nanoparticles with no well-shaped structure. When hydrothermal circumstances applied for synthesizing In(OH)3, in initial stage of 6 hours (6h catalyst), semiformed nanocubes appeared among the aggregated smaller particles

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(Figures 1c and 1d). Further prolonged reaction time to 24 hours (24h catalyst) generated well-defined blocks of In(OH)3 nanocubes with almost uniformity in their

Figure 1

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shapes (Figures 1e and 1f).

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The phase purity and crystal structure of the as-obtained In(OH)3 analyzed by powder XRD (Figure 2). It is quite obvious that the degree of crystallinity is much higher when the reaction time was extended to 24 hours (Figure 2c). Sharp and intense peaks do not exist in amorphous In(OH)3 except for the most intense plane

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(200) (Figure 2a).The pattern of X-ray diffraction can be easily indexed to a pure body centered cubic phase of In(OH)3. The calculated lattice constants of a = b = c = 0.797

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concur well with standard values (JCPDS Card No.: 73-1810). Figure 2

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Surface area of various samples was determined on BET set-up using nitrogen

adsorption/desorption isotherm (Figure 3). This parameter calculated from linear region of the BET plot is about 27.7, 38.9 and 142.7 m2/gr for amorphous, 6h and 24h hydrothermally treated samples respectively. The pore size distribution diameter for 24h sample is found to have pore diameter in the range of 21 nm which falls in the category of mesoporous material. Furthermore, there is a H3 type hysteresis loop in 9

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24h product which also reaffirms the existence of slit-shaped pores [18]. Notably, the aforementioned hysteresis loop does not exist in amorphous and 6h samples.

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Figure 3

As illustrated in Figure 4, CO2 adsorption capability of hydrothermally synthesized In(OH)3(24h) is much greater than 6h and amorphous material in atmospheric

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pressure. Actually, the total CO2 adsorption on In(OH)3(24h) is about 2.2 wt% which is nearly four times greater than amorphous and 6h catalysts(0.47 w% and 0.55 w%). It

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is imaginable that higher surface area and mesoporous structure of 24h sample might pave the way to further elevate its performance. Figure 4

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In the electrochemical experiments, each catalyst sample (amorphous, 6h and 24h) was supported on Vulcan XC-72R furnace carbon black (SBET = 235 m2/g) at a loading of 30% on metal basis. The representative EDX spectrum (Figure 5) show

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Indium contents of around 30 wt% for In(OH)3(24h)/carbon black catalyst which agree well with the stoichiometric ratio used for carbon supported catalyst. The

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samples were loaded into a standard three-electrode cell for the electrochemical measurements. Figure 6, part I, depicts reductive LSV scans at a carbon black (control experiment, curve a) and In(OH)3/carbon black coated glassy carbon electrode in CO2 and Ar saturated 0.5 M K2SO4 solution (curves b and c).The pH of Ar saturated K2SO4 electrolyte was adjusted to 4.4 with little amount of dilute H2SO4 to offset the pH effect of CO2 presence. The K2SO4 electrolyte was recommended elsewhere [6]. 10

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Figure 5 Figure 6

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By comparing LSV scans obtained from a glassy carbon electrode modified with In(OH)3/carbon black in CO2 and argon saturated electrolyte, we can observe that In(OH)3 is electrochemically silent in argon atmosphere. As obvious, completely

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different response in presence and absence of CO2 specified with pronounced amplification in current is due to the catalytic reduction of CO2 on In(OH)3/carbon

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black. By considering all data together, it can decidedly be inferred that, in the presence of CO2, electrochemically inactive In(OH)3 turns into an active material to reduce CO2 even onto a carbon matrix. More importantly, CO2 electroreduction efficiency strongly depends on morphological and structural features of catalyst. The

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LSV scans in Figure 6, part II, show that 6h (curve e) and amorphous catalysts (curve d) have modest (1 mA/cm2) to negligible (0.2 mA/cm2) effect on catalytic currents for CO2 reduction at -1.1 V. From these results, we can conclude that the mesoporous

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structure of 24h catalyst with nanocubic structure (curve f) provides advantageous

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for electrochemical reduction of CO2. Supposedly, higher tendency to react with CO2 may also be attributed to six {200} planes [17] in cubic structures existed much rampantly in In(OH)3 (24h). Recently many applications emerged in the literatures exhibiting the effect of exposed facets on catalytic activity of nanocatalysts which support this idea [11,19-22]. The carbon black powder loaded with In(OH)3 nanocatalyst was immobilized as a thin film on a carbon electrode surface to achieve the preparative-scale electrolysis 11

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experiments with long-term control potential. The solution phase products quantified with 1H NMR by sampling from catholyte in a certain period of time

Figure 7

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(Figure 7).

Formate is the only aqueous product detected in these experiments. Figure 8

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shows the total geometric current densities (jtot) and faraday efficiency (FE) for HCOO- at -1.1 V. The reported value for CO2/HCOO- equilibrium potential is -0.225 V

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[23], thus, -1.1 V corresponds to 0.875 V overpotential for HCOOH production (ηHCOOH). Under this condition, the In(OH)3(24h)/carbon black electrode converts CO2 to HCOOH with a relatively high faraday efficiency between 70-77%. The total current density remained nearly stable (~5.2mA/cm2) through the electrolysis time of

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7 hours indicating that In(OH)3(24h)/carbon black system can produce formate in a sustainable and robust manner in this time frame. As shown in Figure 9, the structure

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of the In(OH)3 nanocubes supported on carbon black remained nearly unchanged

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after the electrolysis time, showing catalyst stability. Figure 8 Figure 9

It is noteworthy that even In(OH)3(6h) and amorphous nanoparticles supported

on carbon black produce formate on the level of ~13.6% and ~7.4%, respectively which are much lower than what obtained from 24h catalyst (Figure 8, curves b and c). These findings reveal that the high surface area with marked susceptibility of

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In(OH)3(24h) in CO2 adsorption, and its mesoporous structure created by a hydrothermal synthesis process are essential for achieving higher faraday efficiency toward formate at a moderate overpotential. Based on these results, electrode

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modified with In(OH)3(24h)/carbon black was selected for further studies. Figure 10 shows faraday efficiencies and total current densities for formate generation at various potentials between -0.9 V and -1.4 V. When lower over-potential applied on

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the electrode, lower currents detected, thus, smaller formate faraday efficiencies

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observed, as expected. The maximum attainable formate faraday efficiency of ~77% occurred at -1.1 V. Further increase in overpotential beyond -1.1 V caused a decline in formate FEs attributing to hydrogen evolution reaction. Figure 10

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Figure 11

The stability of current during electrolysis has spurred us to elucidate CO2 electroreduction mechanism by Tafel slope analysis (Figure 12). The value obtained

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for this parameter was 144 mV.dec-1 which is close to 120 mV.dec-1 indicating an

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initial electrochemical electron transfer as a rate determining step [24]. This finding suggests that after the formation of the CO2•- intermediate, a fast chemical step including proton transfer accompanied with an additional electron transfer will remove the poorly adsorbed intermediate finally regenerating the catalyst. Figure 12

4. Conclusion

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In n this work we have focused on improvement of physical properties of In(OH)3 which ultimately enhances its CO2 electroreduction performance as a catalyst. The mesoporous and lattice structure of the catalyst with higher ability to adsorb CO2 in

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combination with high surface area of carbon black as a conductive carbon network have shown a notable reactivity toward CO2 reduction as an inert molecule. It is worth underlining that current work have shown the feasibility and concept but

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further investigations including long-term stability test and mechanical strength may

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be required to explore possible application of this catalyst in practical use. Acknowledgements

We acknowledge the Bu-Ali Sina University Research Council and the Center of Excellence in Development of Environmentally Friendly Methods for Chemical

References

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7234.

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Figure and Table Captions Fig. 1. Representative FE-SEM and TEM images of the as-prepared In(OH)3 a)

24h.

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Amorphous and hydrothermally synthesized for different reaction times b) 6h , c)

Fig. 2. Powder X-ray diffraction of In(OH)3 a) Amorphous and for different reaction

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times b) 6h , c) 24h, d) Reference pattern (JCPDS 73-1810).

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Fig. 3. Nitrogen adsorption-desorption isotherm for a) amorphous, b) 6h and c) 24h reaction times.

Fig. 4. CO2 adsorption isotherm for a) amorphous, b) 6h and c) 24h reaction times. Fig. 5. The representative EDX spectrum of In(OH)3(24h)/carbon black catalyst.

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Fig. 6. Linear sweep voltammetric scans with CO2 and Ar saturated electrolyte at 50 mV/s. Part I) glassy carbon electrode with carbon black in CO2-saturated electrolyte

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(curve a), glassy carbon electrode modified with In(OH)3(24h)/carbon black in Arsaturated pH adjusted electrolyte (curve b) and glassy carbon electrode modified

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with In(OH)3(24h)/carbon black in CO2-saturated electrolyte( curve c). Part II) Linear sweep voltammetric scans with CO2-saturated electrolyte on glassy carbon electrode modified with amorphous In(OH)3/carbon black (curve d), In(OH)3(6h)/carbon black (curve d) and In(OH)3(24h)/carbon black (curve e) (all potentials are referenced to RHE electrode).

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Fig. 7. A representative 1H NMR spectrum of catholyte acquired for faraday efficiency calculations.

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Fig. 8. The comparison of faraday efficiency and current density in CO2 reduction. a) In(OH)3 (24h), b) In(OH)3 (6h) and c) amorphous In(OH)3 at a potential of -1.1 V vs. RHE.

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Fig. 9. TEM image of In(OH)3(24h)/carbon black after 7 hours electrolysis.

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Fig. 10. Variation of faraday efficiencies and total current densities for formate production at various over-potentials on In(OH)3(24h)/carbon black electrode. Fig. 11. CO2 electroreduction activity and faraday efficiency of In(OH)3(24h)/carbon black at various applied potentials.

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Fig. 12. Tafel plot of formate production partial current densities for

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In(OH)3(24h)/carbon black.

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Figures and Tables

Table 1

Electrode

Electrolyte

Anodized Indium

0.5 M K2SO4

Applied potential vs SCE -1.4V to -1.8V

0.5 M KHCO3

30.6 mA/cm2

Indium metal

TEAPa

-2.04 V

Indium metal

0.1 M KHCO3

-1.79 V

0.5 M K2SO4

a

Tetraethylammonium perchlorate

b

This value is -1.1 V vs RHE

15.3%

-1.6 Vb

8

94.7%

9

70-77%

AC C

EP

TE D

Table1

21

7

87.6%

M AN U

Nanocubic In(OH)3

Formate faraday Ref. efficiency 30-40% 6

SC

Indium metal

RI PT

Conditions and formate production efficiencies at Indium-based electrodes.

This work

ACCEPTED MANUSCRIPT

RI PT

b

M AN U

d

TE D

50 nm

200 nm

e

AC C

EP

f

c

SC

50 nm

200 nm

200 nm

Figure 1

22

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 2

23

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Figure 3

24

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Figure 4

25

ACCEPTED MANUSCRIPT

C Kα 5000

RI PT

4000

3000

SC

2000

InLα

1000

M AN U

InLβ 0 0

5

AC C

EP

TE D

Figure 5

26

keV 10

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 6

27

ACCEPTED MANUSCRIPT

Applied potential: -0.9 V vs RHE Current Density: 1.33 mA/cm2

M AN U

SC

RI PT

Sampling Time: 2 h electrolysis

AC C

EP

TE D

Figure 7

28

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 8

29

ACCEPTED MANUSCRIPT

nanocubes

M AN U

SC

RI PT

Carbon black

AC C

EP

TE D

Figure 9

30

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Figure 10

31

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Figure 11

32

AC C

EP

TE D

M AN U

Figure 12

SC

RI PT

ACCEPTED MANUSCRIPT

33

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 12