Highly Selective Reduction of CO2 to Formate at Low Overpotentials

Subscriber access provided by READING UNIV

Article

Highly Selective Reduction of CO2 to Formate at Low Overpotentials Achieved by a Mesoporous Tin Oxide Electrocatalyst Rahman Daiyan, Xunyu Lu, Wibawa Hendra Saputera, Yun Hau Ng, and Rose Amal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02913 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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

ACS Sustainable Chemistry & Engineering

Highly Selective Reduction of CO2 to Formate at Low Overpotentials Achieved by a Mesoporous Tin Oxide Electrocatalyst Rahman Daiyan, Xunyu Lu*, Wibawa Hendra Saputera, Yun Hau Ng, Rose Amal* Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Corresponding Author *Rose Amal ([email protected]) *Xunyu Lu ([email protected]) Mailing Address: Level 3, Particles and Catalysis Research Group, Tyree Energy Technology Building (H6), UNSW, Kensington, NSW 2052, Australia Abstract A well-ordered mesoporous SnO2 prepared by a simple and inexpensive nanocasting method was used as catalysts for the electrochemical reduction of CO2 to formate. The as-prepared catalyst exhibited a high activity towards CO2 reduction, which was capable of reducing CO2 to formate with 38% of Faradaic efficiency (FE) at an applied overpotential as low as 325 mV. The maximum FE for formate generation (75%) was achieved at an applied potential of -1.15 V (vs RHE), accompanied by a high current density of 10.8 mA cm-2. The enhanced catalytic activity obtained with the mesoporous SnO2 electrocatalyst is attributed to its high oxygen vacancy defects (promotes CO2 adsorption and lowers overpotential) and crystallinity that provides

1 Environment ACS Paragon Plus

ACS Sustainable Chemistry & Engineering 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

Page 2 of 31

sufficient active sites for CO2RR as well as its distinctive structural configurations which reduces impedance to facilitate faster CO2RR reaction kinetics. Keywords: Electrochemical Reduction, Carbon dioxide, Formate, Mesoporous, Tin Oxide Introduction Rising level of CO2 accumulation in the atmosphere has attracted considerable research interest in technologies capable of CO2 capture, storage and conversion.

1–3

The electrochemical

reduction of CO2 into high value liquid organic products could be of vital importance to mitigate this issue.

4,5

The direct conversion of CO2 to liquid fuel using renewable energy, which can

readily be integrated with the current infrastructure, will help realize the creation of a sustainable cycle of carbon based fuel that will promote zero net CO2 emissions.

6–10

Despite initial

promising findings, significant progress is required in improving the production rate, efficiency, stability and cost to make this technology realistic for large scale utilization. 7,11 The current benchmarking electrocatalysts for CO2RR to formate (HCOO-) are sp group metals, notably Pb, In and Sn.

12–19

Amongst the high performing materials, Sn based catalysts are

especially favored due to their relative low cost, abundance and non-toxic properties, compared to Pb and In catalysts.

20

Sn catalysts however exhibit certain characteristics, for instance the

local chemical structure of Sn is shown to play a major role in CO2RR, as the bulk Sn foils are reported to have inconsistent formate Faradaic efficiency (FEHCOO-) at a wide range of potentials. 16,21

To address such discrepancy in catalytic performances, numerous studies on the effect of

electrolyte, pH, morphology and catalyst deactivation for CO2RR with Sn foil based catalysts have been undertaken.

22–25

In spite of the insights and understanding into the mechanisms

obtained by such studies, Sn foil based catalysts still require large overpotentials to attain high


Page 3 of 31 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


values of FEHCOO-. For example, three-dimensional Sn foam grown on Sn foil catalysts require a large applied potential of -1.3 V (vs. RHE, applies for all potentials mentioned in this study) to achieve a FEHCOO- of 90%.

26

Similarly, the heat-treated Sn dendrite electrodeposited on Sn foil

are also reported to convert CO2 to formate with a moderate FEHCOO- of 71% but this is also done at a large negative applied potential of -1.35 V. 22 In recent times, nanoparticulate SnO2 electrocatalysts are widely considered to be practical candidates for HCOO- production. For instance, pristine SnO2 nanoparticles were reported to achieve a FEHCOO- of 69 % with a current density of 3.5 mA cm-2 at an applied potential of -0.6 V, but this is attained under extreme alkaline reaction conditions (pH=10.4).

27

Similarly, Wu et

al. reported Sn nanoparticles that were annealed at 100oC for a duration of 6 hours can exhibit moderate CO2RR activity, demonstrating a FEHCOO- of 51.5% at -1.2 V, however with an inhibited current density of ~ 2 mA cm-2. 28 Quite recently, SnO2 nanowires (containing abundant grain boundaries) were fabricated using plasma treatment and was shown to be able to convert CO2 to HCOO- with a high FEHCOO- of 80% and a current density of 6 mA cm-2 at an applied potential of merely -0.8 V vs RHE, albeit with a high mass loading of 4 mg cm-2 and also using IR compensation. 29 Moreover, nanoparticulate SnO2 electrocatalysts were also studied with Gas Diffusion electrode (GDE) systems to further enhance current densities and FEHCOO-. SnO2 nanospheres and microspheres were used in GDE setup and attained a FEHCOO- of 68% and 62% with stable current densities of 6 mA cm-2 and 12.5 mA cm-2 at applied potentials of -1.1 V and 1.2 V, respectively.

30,31

Nevertheless, pristine SnO2 nanoparticle catalysts present certain

limitations, for instance, the fabrication technique requires usage of surfactants and also the catalysts suffer from particle aggregation that would limit transport of reactants to the active sites, resulting in lower CO2RR activity.

32

Although loading the SnO2 nanoparticles on carbon



Page 4 of 31

based supports (carbon black, graphene, etc.) is a promising approach to circumvent the challenges and contribute to an improved HCOO- production, such systems cannot be used for large scale application as typically these catalysts require stringent and intricate control of the fabrication process, despite delivering high catalytic activities.

18,33–35

For instance, hierarchical

SnO2 nanosheets on carbon cloth electrocatalysts were demonstrated to attain exceptional catalytic activity towards formate production with a FEHCOO- of 87% and a current density of 45 mA cm-2 at an applied overpotential of 0.88 V. Moreover, the integration of SnO2 with carbon (including carbon black and graphene) will bring uncertainties to the CO2RR process since the products observed may originate from both CO2RR and the degradation of carbon supports. An alternative approach to fabricating catalysts with enhanced accessible surface area and improved mass transport properties is to develop mesoporous materials. Mesoporous catalysts are commonly advocated for their superior electrocatalytic performance for energy conversion reactions such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and recently for CO2RR due to their distinctive combination of high surface area, enhanced crystallinity and mesopores creating an interlinked network.

36–40

Compared to nanoparticle

catalyst systems, mesoporous materials offer larger pore volume and well-ordered channels, thereby enabling greater diffusivity for reactants and products through the catalysts.

41

In

contrast, nanoparticle catalysts typically generate inefficient packing of particles on the electrode surface that results in decreased accessibility of reactants to the active sites.

42

Moreover, the

fabrication of mesoporous materials is shown to result in increased generation of defects, notably oxygen vacancies, which are known to play an active role in improving reaction kinetics and catalytic performances.

43–45

On the basis of this understanding, the performance of the SnO2


Page 5 of 31 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


electrocatalysts for CO2RR could be further improved through the development of mesoporous SnO2 catalysts. Herein, we report the fabrication of mesoporous SnO2 via a nanocasting method using mesoporous silica (KIT-6) as a hard template.

37

The as-prepared catalyst was employed for the

first time for CO2RR, exhibiting a low onset potential for HCOO- generation among the Sn based catalysts, with HCOO- detected at -0.55 V (corresponding to 325 mV of overpotential). The ordered m-SnO2 catalyst was demonstrated to reduce CO2 to HCOO- with good FE in the low overpotential region, achieving a FEHCOO- of 38% that increases to ~50% as the potential was shifted from -0.55 V to -0.75 V (which is one of the highest among all the studies reported so far within the same potential range). Overall, the catalyst displayed a maximum FEHCOO- of 75% and a high current density of 10.8 mA cm-2 at an applied potential of -1.15 V. The enhanced catalytic activity of the mesoporous SnO2 catalyst in delivering both high values of FEHCOO- as well as large current densities is attributed to the unique configuration and improved dispersion of the catalyst in expediting greater diffusivity of the reactants (CO2 and H2O) and products (H2, CO and HCOO-) through the active sites, present on the highly crystalline electrocatalyst that consists of significant oxygen vacancy defects.

Experimental Preparation of mesoporous SnO2: Mesoporous SnO2 was prepared by nanocasting method using KIT-6 as the hard template. KIT-6 was fabricated using an established method. 46 Briefly, 2 g of P123 (Pluronic® P-123, Sigma Aldrich, 99%) was dissolved in 72 mL of deionized water, followed by the addition of 2.5 mL of HCl. 2.47 mL of butanol was then added dropwise and the



mixture was stirred for duration of one hour at 35oC. The solution was then transferred to a hydrothermal reactor and heated to 100oC for 24 hours. The solid product formed was then filtered and calcined at 600oC for 5 hours. To prepare mesoporous SnO2, 1.75 g Tin(IV) tetrachloride pentahydrate (SnCl4.5H2O, Sigma Aldrich, 99%) was dissolved in 30 mL of ethanol. 0.6 g of KIT-6 was then added and the mixture heated to 70oC to evaporate ethanol. The sample was then calcined at 600oC for 3 hours. The impregnation and the calcination process were repeated again with two-thirds of the Sn precursor used in the first step to complete the nanocasting process. The KIT-6 templates were then removed by washing the calcined powder twice in a hot 2 M NaOH solution. The resulting samples were collected by repeated centrifugation and washing with deionized water. Preparation of SnO2 nanoparticles: SnO2 nanoparticles were prepared by a facile hydrothermal method.

47

Briefly, 1.75 g of Tin(IV) tetrachloride pentahydrate (SnCl4.5H2O) was dissolved in

50 mL of deionized water and 0.64 g of the surfactant, Hydrazine hydrate (N2H4, Sigma Aldrich, 99%) was added. The mixture was stirred for a duration of 10 minutes and then placed in a Teflon-lined stainless steel autoclave which was heated to 100oC for 12 hours. The autoclave was cooled naturally to room temperature and the resulting solid powder was then filtered and washed with ethanol and deionized water using centrifugation. Preparation of SnO2 nanoparticles supported on carbon black: SnO2 nanoparticles dispersed on carbon black were prepared by a modification of an established method. 18,48 200 mg of Tin(IV) tetrachloride pentahydrate (SnCl4.5H2O) was dissolved in 40 mL of ethylene glycol and mixed with 300 mg of carbon black (VULCAN ® XC72, Cabot Corporation). The solution was then thoroughly sonicated for 30 minutes and subsequently heated to a temperature of 196oC and


Page 6 of 31

Page 7 of 31 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


refluxed under stirring for a duration of 3 hours. The solid powder was then collected by filtration and was washed with ethanol and deionized water using centrifugation. Electrochemical Measurements: 5 mg of catalyst was dispersed in 0.5 mL deionized water and ethanol solution (1:1, v/v) followed by the addition of 25 µL of Nafion solution (Sigma-Aldrich). The mixture was sonicated thoroughly to form a homogeneous ink. The working electrodes were then prepared by drop-casting the catalyst inks onto carbon paper to achieve a loading of 1 mg cm-2. All electrochemical measurements in this study were carried out with a CHI 750E (CH Instrument, Texas) electrochemical workstation using a customized two compartment gas-tight cell, with the catalyst inks loaded carbon paper and saturated calomel electrode (SCE) in the cathodic compartment and a Pt wire in the anodic compartment. The two compartments are separated by a glass frit to prevent the reduction products on the cathode from re-oxidizing on the anode. The potentials measured in this study were all converted to the reversible hydrogen electrode (RHE) reference for the purpose of simple comparison, using the following equation: ERHE (V) = ESCE (V) + 0.245 + 0.059 × pH.

(1)

The electrolyte employed in this study for CO2 reduction is 0.1 M KHCO3, which was saturated with CO2 and Ar, giving a measured pH value of 6.8 and 8.4, respectively. Before each electrochemical testing, the working compartment was purged with CO2 for 30 minutes. Constant potential electrolysis was carried out at various potentials for a duration of two hours and the experiments repeated twice and the results presented are the averaged values. For each potential, the results for the first hour of testing was discarded as the working electrode needs to be conditioned. The electrochemical surface area (ECSA) for the electrodes was estimated by measuring the double layer capacitance. This was done with the aid of Cyclic Voltammetry (CV), which was carried out in a non-Faradaic region (-0.6 V to +0.1 V), by varying the scan rate



in Ar saturated 0.1 M KHCO3 electrolyte. The cathodic and anodic currents from the double layer charge and discharge curves (CV) obtained at -0.35 V was then plotted against the scan rate to obtain capacitance. Product Detection: Gas chromatograph (Shimidzu, Model 2025), equipped with both thermal conductivity detector (TCD) and flame ionization detector (FID) was used to quantify the gas phase products after 3600s and 7200s during the two hour long bulk electrolysis. 0.5 mL of electrolyte aliquots were collected at the end of each experiment and was mixed with 0.1 mL of D2O and 7.143 ppm of internal standard dimethyl sulfoxide (DMSO, Sigma 99.99%) and were analyzed using a 600 MHz 1H 1 D liquid Nuclear Magnetic Resonance (NMR) spectrometer (Bruker Advance) at 25oC. The 1D 1H spectrum was measured with water suppression with a pre-saturation method. The amounts of formate products were calculated by comparing the integral areas of the observed formate peak with DMSO peak. The peak position for formate was calibrated using formic acid (HCOOH, 98%, Sigma Aldrich) dissolved in 0.1 M KHCO3 solution containing the internal standard solution outlined above. Physical Characterization: The surface morphology was studied using TEM with a Philips CM 200 microscopy operated at 200 kV. The structural characterization was studied using powder XRay Diffraction (XRD) pattern using PANalytical X’Pert instrument using Cu K ∝ radiation (λ=1.54 Å) and scan range from 10o to 90o. XPS was performed on a Thermo ESCALAB250i Xray Photoelectron Spectrometer. BET isotherms were measured on a Micrometrics Tristar 3030 using nitrogen adsorption at 77 K. The presence of oxygen vacancies and defects formation were evaluated using Electron Paramagnetic Resonance (EPR) spectroscopy on a Bruker EMX-plus X-Band EPR spectrometer at 9.41 GHz (X-band) at room temperature where the microwave power was set at 2 mW and the modulation amplitude at 5G. 8 Environment ACS Paragon Plus

Page 8 of 31

Page 9 of 31 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


Results and Discussion

Scheme 1. Fabrication of m-SnO2 catalyst.

Mesoporous SnO2 catalyst (referred herein as m-SnO2) was prepared using a facile and scalable nanocasting method employing KIT-6 as the hard template, as illustrated in Scheme 1. Typically, the nanocasting process involves filling the void in a mould (referred as hard template) with the desired precursors and is followed by subsequent processing and removal of the hard template. 49 Through nanocasting, the precursor powders take a strong negative replica of the hard template and thereby, the properties of the templates play an important role in guiding the catalytic activity of the replicated mesoporous powder.

50

Among various hard templates, ordered

mesoporous silica materials such as KIT-6 are widely utilized for the development of mesoporous materials. KIT-6 offers certain benefits, notably it displays a uniform pore size distribution which would allow proper dispersion of the precursors during the nanocasting process. Furthermore, due to the well-ordered nature of KIT-6, upon removal of the template, the negative replica collapses into fragments that contain well-ordered pores.



Figure 1. TEM images of the obtained m-SnO2 catalyst under different magnifications.

Figure 1 represents the transmission electron microscopy (TEM) images of the as-synthesized mSnO2. The m-SnO2 catalyst displayed an ordered porous structure, which replicates the threedimensional structure of the KIT-6 template (Figure 1a). The high magnification TEM image in Figure 1b displayed an interplanar lattice spacing of 0.347 nm and 0.251 nm that is shown to correspond to (110) and (101) facets of SnO2 species.

51

To further establish the presence of Sn

and O species in the as-synthesized m-SnO2 catalyst, energy dispersive spectroscopy (EDX), as shown in Supplementary Information Figure S1, was carried out. It can be observed from the EDX results that both Sn and O were uniformly present all throughout the whole surface tested herein, indicating the successful preparation of mesoporous tin oxide.


Page 10 of 31

Page 11 of 31 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


Figure 2. (a) N2 adsorption-desorption isotherms of m-SnO2. (b) XRD pattern for m-SnO2. High resolution XPS spectra for m-SnO2 showing (c) Sn 3d and (d) O 1s.

The m-SnO2 catalyst was further characterized by the N2 adsorption-desorption isotherm and the results are presented in Figure 2a. It can be observed from Figure 2a that the m-SnO2 catalyst displayed the typical type IV hysteresis loops, indicating the presence of mesopores. 52 The inset in Figure 2a depicts the corresponding Barret-Joyner-Halenda (BJH) pore size distribution of the catalyst. The m-SnO2 catalyst was observed to exhibit a predominant mesopore centering at 4 nm and this is in agreement with the results obtained with the TEM images. Figure 2b presents the X-ray diffraction (XRD) patterns obtained with m-SnO2. From Figure 2b, it is evident that m-SnO2 can be indexed as a polycrystalline fcc structure with no preferential



crystallographic orientation. The XRD pattern with m-SnO2 revealed major peaks at 26o, 33o and 54o that corresponded to the presence of (110), (101) and (211) facets and minor peaks at 38o, 55o, 62o, 64o, 72o and 79o that corresponded to the presence of (200), (220), (312), (112), (202) and (321) facets of SnO2, respectively.

53,54

The surface chemical composition of the m-SnO2

catalyst was then studied using X-ray photoelectron spectroscopy (XPS). Figure S2 manifests the XPS survey carried out in a wide energy range from which the peaks of Sn, C (background) and O were identified. Elemental scan spectrum for Sn 3d is also presented in Figure 2c where two peaks corresponding to Sn 3d3/2 and Sn 3d5/2 were observed at binding energies 495 eV and 487 eV, respectively.

55

The peak positions and peak separation (~ 8 eV) obtained in Figure 2c

correlated well with the results obtained with Sn4+, further confirming the presence of SnO2 in the obtained catalyst. 33,56,57 The O 1s spectrum is also presented in Figure 2d where the peak can be deconvoluted into three oxygen containing species, with fitted peaks observed at 530.2 eV, 531.6 eV and 532.6 eV which corresponds to Sn4+-O, adsorbed oxygen and oxygen vacancies in the as-synthesized m-SnO2. 33,56


Page 12 of 31

Page 13 of 31 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


Figure 3. (a) Polarization curves obtained with m-SnO2 drop-casted on carbon paper in Ar and CO2 saturated 0.1 M KHCO3 with a scan rate of 10 mV s-1, respectively. (b) Chrono-amperometric i-t curve showing the stabilization of current density with time at -0.85 V for m-SnO2 drop-casted on carbon paper in CO2 saturated 0.1 M KHCO3. Dependence of FE of (c) gaseous products (CO and H2) and (d) HCOO- for m-SnO2, SnO2 NPs and SnO2/C NPs drop-casted on carbon paper on the applied potential during constant potential electrolysis in CO2 saturated 0.1 M KHCO3.

The electrochemical performance of m-SnO2 catalyst in reducing CO2 was then tested using potentiostatic studies conducted in CO2 saturated 0.1 M KHCO3. The onset potential and the ideal potential range for CO2RR with m-SnO2 electrode was determined by linear sweep voltammetry (LSV) scans (Figure 3a), performed in both Ar and CO2 saturated 0.1 M KHCO3. The polarization curve obtained with Ar saturated 0.1 M KHCO3 revealed that the m-SnO2 electrode can effectively suppress hydrogen evolution reaction (HER), as indicated by the large onset overpotential (~550 mV). It is a vital design parameter to suppress HER during CO2RR,



Page 14 of 31

otherwise a significant portion of electricity will be diverted in generating hydrogen rather than reducing CO2 to useful fuels. 58 In hindsight, the polarization curve obtained in CO2 saturated 0.1 M KHCO3 also demonstrated a similar onset potential compared to that achieved with the Ar saturated solution, however, the current density obtained in CO2 environment was slightly higher. On the basis of this observation, we can conclude that the CO2RR and HER is taking place concurrently at potentials lower than -0.55 V with the m-SnO2 catalyst. Potentiostatic studies were then conducted at various potentials in CO2 saturated 0.1 M KHCO3 to investigate the product distribution, corresponding Faradaic efficiencies and current densities obtained with the m-SnO2, SnO2 NPs and SnO2/C NPs electrodes during CO2RR, respectively. Figure 3b presented the amperometric i-t curve obtained with the m-SnO2 electrode at -0.85 V in CO2 saturated 0.1 M KHCO3. The initial large current density (~ 4.3 mA cm-2) obtained in Figure 3b decreased dramatically during the first hour before stabilizing ~ 1.7 mA cm-2. This variation in current density was attributed to the reduction of the oxide particles to reach an equilibrium interface between SnO2 and Sn before initiating CO2RR.

16,19,22

Thus, the first hour

of testing at each potential was used for the conditioning of the electrode and was excluded from our Faradaic efficiency measurements for all the catalysts tested herein. Figure 3c revealed the dependence of Faradaic efficiency of the gas phase products within a wide range of applied potentials. For all the catalysts tested herein, only hydrogen (H2) and CO were observed and their corresponding Faradaic efficiencies depended on the potentials applied. The concurrent production of CO and H2 is also advantageous as the feed can be used as synthesis gas (syngas) in the Fischer-Tropsch (FT) process to produce liquid hydrocarbons which can be incorporated in the current infrastructure. 59,60 It is observed from Figure 3c that HER on both mSnO2 and SnO2/C NPs is effectively suppressed, with the maximum FE for H2 (FEH2) of merely


Page 15 of 31 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


40% obtained at -0.85 V and -1.15 V, respectively. As a comparison, exorbitantly high FEH2 of 90% was detected with the SnO2 NPs at low operating potentials (from -0.75 V to -0.95 V). Besides H2, the generation of CO is observed to be sluggish with all three catalysts tested herein. Also shown in Figure 3c, within the optimal potential range for HCOO- generation (from -0.95 V to -1.25 V), the maximum FE for CO (FECO) obtained with m-SnO2, SnO2/C NPs and SnO2 NPs are merely 18%, 10% and 12 % at -0.75 V, -0.95 V and -1.15 V, respectively. The collective data reveals that the m-SnO2 catalyst is capable of suppressing gaseous products (both CO and H2) generation during CO2RR, thereby facilitating the yield of the more favorable liquid products. The only liquid product detected with all the catalysts tested herein was HCOO-, which also exhibited a strong potential dependence (Figure 3d). Of these catalysts, m-SnO2 is the most active towards CO2RR to formate, which only requires an onset overpotential of 325 mV to produce formate with a reasonable FEHCOO- of 38%, while both SnO2 NPs and SnO2/C NPs call for 200 mV of additional overpotential before formate can be detected. Apart from that, it also can be seen that nearly at all applied potentials, m-SnO2 exhibits better selectivity towards HCOO- generation, comparing with both SnO2 NPs and SnO2/C NPs, attaining the maximum FEHCOO- of 75% at an applied potential of -1.15 V. It has to be mentioned that within a wide range of applied potential (from -0.75 V to -1.25 V), the FEHCOO- obtained with m-SnO2 is always higher than 50%, indicating the majority of electricity that has been consumed in CO2RR is resulting in the generation of formate. In retrospect, the SnO2 NPs demonstrated sluggish reaction kinetics towards formate generation, with merely 20% FEHCOO- at -1.05 V. Only when a substantial amount of overpotential has been applied to SnO2 NPs (> 725 mV), the FEHCOO- can be increased, however, still at a relatively low value (~52%). Combining SnO2 with carbon (SnO2/C NPs) will improve its catalytic activity towards formate generation, which can be



Page 16 of 31

ascribed to the enhanced material conductivity and the possible formation of Sn-C bonds, revealed by comparing the Sn 3d XPS spectrum for m-SnO2, SnO2 NPs and SnO2/C NPs (Figure S3). It can be observed from Figure S3 that the high resolution Sn 3d spectra for SnO2/C NPs indicated a peak shift towards higher binding energy compared with m-SnO2 and SnO2 NPs, suggesting the formation of metal support interaction between the Sn and carbon black support. 18

As a result, even though inferior to m-SnO2, SnO2/C NPs displayed much higher selectivity

towards formate generation than SnO2 NPs before reaching -1.15 V, peaking at 67 % at -1.05 V. Collectively, it can be concluded that the meso-structured SnO2 demonstrated superior catalytic properties in reducing CO2 to HCOO- with respect to pristine SnO2 nanoparticles, regardless of the presence of carbon supports. The catalytic activity of the m-SnO2 electrode was also compared with other benchmarked Sn catalysts for the conversion of CO2 to HCOO- in aqueous solutions (Table S1). Typically, the catalytic activities for CO2RR are dependent on a number of critical parameters such as cell/electrode configurations and testing conditions (e.g. electrode conditioning, pH, electrolyte, etc.) and as a result, the reporting of activities is only a trivial assessment of the catalyst performance among all the materials compared. Nevertheless, by using FEHCOO- and current density as the performance indicator, the m-SnO2 electrode was demonstrated to be a competitive catalyst for CO2RR to HCOO-. From Table S1, it is evident that the m-SnO2 electrode presented herein displayed a higher selectivity and larger current densities compared to the Sn foil based catalysts.

21,23,61

Heat-treated Sn dendrites and three-dimensional Sn foam catalysts can also

circumvent this challenge and achieve both high FEHCOO- (>70%) and current densities (>17 mA cm-2) but can be only achieved at high operating potentials (more negative than -1.3 V),

22,26

whereas the m-SnO2 is capable of achieving the maximum FEHCOO- of 75% and a current density


Page 17 of 31 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


of 10.8 mA cm-2 at -1.15 V. Moreover, the m-SnO2 electrode outperform the majority of Sn nanoparticle based catalysts, including the annealed Sn nanoparticles (51.50 % at -1.2 V), nanosphere SnO2 (68% at -1.1 V) and Sn6O4(OH)4 / SnO2 catalysts loaded on carbon black support (75% at -1.2 V).

28,30,34

Although some SnO2/carbon, Sn foam and SnO2 nanowire

catalysts perform better than the as-obtained m-SnO2 catalyst,

18,26,29,62

they require critical

fabrication steps which may hinder their potential large scale application. The major advantage of the m-SnO2 electrode over other nanoparticulate Sn catalysts (either tested herein or reported in literature) is its ability to convert CO2 to HCOO- with a reasonable selectivity as well as appreciable current densities at very low overpotentials (Figure S4). Particularly, in the potential region between -0.75 V to -0.95 V, the m-SnO2 can achieve FEHCOO> 49%, whereas for the same potential range, the FEHCOO- obtained with SnO2 NPs and SnO2/C NPs were merely < 7% and < 34%, respectively. Moreover, the partial current density for formate generation (jHCOO-) obtained with m-SnO2 is also much higher than that obtained with both SnO2 NPs and SnO2/C NPs during the entire potential range, as shown in Figure 4a. Specifically, the jHCOO- attained with the m-SnO2 at -1.15 V is 8.2 mA cm-2 whereas the SnO2 NPs and SnO2/C NPs displayed a lower jHCOO- of only ~2.5 mA cm-2 at the same potential, indicating the capability of m-SnO2 of reducing CO2 to formate with a high selectivity as well as a faster production rate.



Figure 4. (a) Partial current density for HCOO- generation for m-SnO2, SnO2 NPs and SnO2/C NPs drop-casted on carbon paper under different applied potentials in constant potential electrolysis in CO2 saturated 0.1 M KHCO3. (b) Stability of current density and Faradaic efficiency for HCOO- production of m-SnO2 drop-casted on carbon paper during long term (16 hours) electrolysis in CO2 saturated 0.1 M KHCO3 at -1.15 V.

Besides activity, stability is another critical parameter to judge the performance of a catalyst in CO2RR. Figure 4b shows the constant potential electrolysis of CO2 at -1.15 V with the m-SnO2 catalyst in CO2 saturated 0.1 M KHCO3 solution. The curves in Figure 4b conveys that the mSnO2 exhibited a prominent stability for CO2RR as revealed by a constant current density of 10.8 mA cm-2 during the 16-hour reaction period. Moreover, the FEHCOO- obtained also stabilized ~ 75% during the entire reaction session, further confirming the capability of m-SnO2 in delivering stable CO2RR catalytic performances. Generally, the improved performance for any electrocatalyst can be attributed to the increased electrochemical surface area (ECSA) that results in larger accessible sites for the reaction to take place. However, this is not the case in this study. As shown in Figure S5, all the catalysts tested in this study, including m-SnO2, SnO2 NPs and SnO2/C NPs, exhibited similar ECSA values while their catalytic activity for CO2RR varied dramatically, indicating some other factors rather than ECSA, are playing the critical roles.


Page 18 of 31

Page 19 of 31 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


The exceptional CO2RR catalytic performances obtained with the m-SnO2 catalyst can be intrinsically related to its unique structural configurations, as described in the following part. (i) The mesoporous structure provided greater pore volume compared to SnO2 NPs and SnO2/C NPs. The m-SnO2 catalyst exhibited a high cumulative volume of the BJH pores (0.32 cm3/g), which is significantly larger than that obtained with both SnO2 NPs (0.06 cm3/g) and SnO2/C NPs (0.17 cm3/g), and could facilitate greater transports of reactants (CO2 and H2O) and products (H2, CO and HCOO-) during CO2RR. (ii) The meso-structured SnO2 catalyst can effectively avoid particle agglomeration during reactions. Severe particle clustering issues were observed with the as-synthesized pristine SnO2 nanoparticles (Figure S6 and S7) that were prepared via a well-established method

47

, which

tends to prevent the transport of reactants and products as well as inhibit accessibility to the active sites, thereby leading to inferior catalytic performances. In fact, the N2 adsorption and desorption isotherms for SnO2 NPs also revealed the absence of significant pores (Figure S8). By contrast, both m-SnO2 and SnO2/C NPs can bypass such issues owing to the presence of mesopores (Figures 2a and S8) and carbon black supports (Figures S9 and S10), respectively, which allows the reactants to reach the active sites, facilitating the catalytic conversion of CO2 to HCOO-. To further confirm this understanding, post-reaction SEM characterizations were also carried out with the m-SnO2, SnO2 NPs and SnO2/C NPs electrodes (Figure S11). It can be observed from Figure S11 that m-SnO2 and SnO2/C NPs are well dispersed in the carbon fiber paper, in agreement with the TEM and BET results whereas the SnO2 NPs are shown to aggregate. Moreover, when we scratched off catalyst from the surface of post-reacted m-SnO2 electrode and carried out TEM, we can still observe the presence of pores, indicating the structural stability of the catalyst (Figure S12).



(iii) The highly crystalline nature of the m-SnO2 catalyst also contributes significantly to the superior CO2RR catalytic activity. The TEM images (Figure 1b) and the XRD pattern (Figure 2b) of m-SnO2 displayed a preferential exposure of (110), (101) and (211) of SnO2, together with several minor peaks. In a stark contrast, the XRD pattern with the SnO2 NPs (Figure S13) revealed diffraction peaks which were less pronounced compared to the m-SnO2, with two major peaks observed at 33o and 54o that are corresponding to the (101) and (211) facets of SnO2. From this comparison, it is suggested that the higher crystallinity as well as the larger presence of SnO2 (110) facets may result in more active sites that facilitates the conversion of CO2 to HCOO-. It was also observed that despite the in-situ reduction of the oxide layers during conditioning and CO2RR that generates SnO2/Sn interfaces (as indicated by the formation of Sn0 species with the high-resolution Sn 3d XPS spectra obtained with the m-SnO2 electrode after long term electrolysis in Figure S14), the bulk of m-SnO2 electrode still displayed a strong presence of SnO2 (110) facets as confirmed by the post-reaction XRD characterization (Figure S15). Moreover, previous high performing SnO2 nanoparticle catalysts for CO2RR also exhibited strong presence of SnO2 (110), indicating this facet may play a positive role in affecting formate generation during CO2 reduction. 18,33,34


Page 20 of 31

Page 21 of 31 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


Figure 5. EPR spectra for m-SnO2, SnO2 NPs and SnO2/C NPs.

The improved catalytic performances of the m-SnO2 could also be attributed to the existence of a greater amount of oxygen vacancy defects on the highly crystalline m-SnO2 catalyst. It has been well understood that mesoporous structures normally bear a high level of oxygen vacancy,

44,63

which is also the case in this study. As shown in Figure 5, m-SnO2 catalyst presented the highest intensity of oxygen vacancy (as demonstrated from the electron paramagnetic resonance (EPR) signal at g=2.0001, which corresponds to a typical signal for oxygen vacancies),

64,65

which is

much stronger than both the SnO2 NPs and SnO2/C NPs. It has been reported that oxygen vacancy tends to promote the adsorption as well as reactivity of a variety of gases, including O2, CO and CO2. 66–68 As a result, oxygen vacancies can be shown to cause a greater stabilization of the formate anion radical (generated during CO2RR) and therefore leads to lowered overpotential for formate generation.

69

The m-SnO2 also demonstrates similar CO2RR trend as it exhibits a

higher activity at lower applied overpotential as well as requiring a lower onset potential for CO2RR (compared to the SnO2 and SnO2/NPs). Therefore, it is reasonable to correlate the exceptional CO2RR catalytic activity of m-SnO2 with the abundant presence of oxygen vacancies.



Page 22 of 31

Figure 6. (a) EIS measurements at -1.15 V vs RHE and (b) Tafel plots for HCOO- production on m-SnO2, SnO2 nanoparticles and SnO2 nanoparticles on carbon black support electrodes in CO2 saturated 0.1 M KHCO3 solution.

Figure 6a represents the electrochemical impedance spectroscopy (EIS) study carried out at -1.15 V in a CO2 saturated 0.1 M KHCO3 electrolyte with the three catalysts prepared in this study. From the Nyquist plots in Figure 6a, it is evidently clear that the semi-circle obtained with mSnO2 is considerably smaller relative to that attained with SnO2 NPs and SnO2/C NPs, indicating that among the three catalysts, m-SnO2 bore the smallest impedance (contact and transfer), therefore presents the fastest reaction kinetics for reducing CO2 to formate. The reduced impedance attained with the m-SnO2 electrode can be ascribed to the higher crystallinity, improved mass transport as well as the enhanced dispersion of the mesoporous SnO2 particles on the carbon fiber paper (Figure S11). Figure 6b represents the Tafel plots for HCOO- generation attained with m-SnO2, SnO2 NPs and SnO2/C NPs, where the overpotential was sketched against the logarithm of jHCOO-. Using the Tafel plots, crucial insights into the CO2RR reaction mechanism to HCOO- on the as-synthesized catalysts can be obtained. Typically, the electrochemical reduction of CO2 to HCOO- is reported 18,35

to be governed by the following mechanism (1-4) where * represents adsorbed species:

CO2 + e- + * *CO2˙-

(2)


Page 23 of 31 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


CO2˙- + HCO3- + e- *HCOO- + CO32-

(3)

*

HCOO- * + HCOOH

(4)

CO32-+ CO2 + H2O 2HCO3-

(5)

*

As is seen from Figure 6b, the mechanistic Tafel plots for HCOO- generation with m-SnO2 displayed the smallest slope of 129 mV dec-1 compared to 136 mV dec-1 and 143 mV dec-1 obtained with SnO2/C NPs and SnO2 NPs, respectively, suggesting that the rate determining step for all the catalysts is the transfer of the first electron to form the radical intermediate *CO2˙(Step 1).

16,22

Despite demonstrating similar rate determining step, since m-SnO2 exhibits the

lowest Tafel slope, it is plausible to deduce that this catalyst will have the fastest reaction kinetics for CO2RR, which correlates well with the results obtained from EIS. Conclusion In summary, mesoporous SnO2 catalyst prepared by a simple and facile nanocasting method using a hard template was successfully employed as a novel electrocatalyst for the selective conversion of CO2 to HCOO-. The as-synthesized catalyst was capable of reducing CO2 to HCOO- with high efficiency and current density at low overpotentials and demonstrated a maximum Faradaic efficiency of 75 % and a large current density of 10.8 mA cm-2 at an applied potential of -1.15 V. The results presented herein also demonstrated the high stability of the mSnO2 electrode towards CO2RR, displaying a stable current density and Faradaic efficiency with no observable decay over 16 hours of operation. The improved catalytic activity of the m-SnO2 electrode was ascribed to: (i) preferential exposure of crystalline facets that provides sufficient active sites for CO2RR, (ii) significant presence of oxygen vacancy defects and (iii) enhancement of CO2RR reaction kinetics due to reduced impedance and greater transport of reactants and facile dissipation of products through the large mesopores and well dispersed catalyst.



ASSOCIATED CONTENT Supporting Information Includes EDX, XPS, TEM images, current density plots at different scan rates, EIS, N2 adsorption desorption isotherm, SEM images, XRD patterns and a table of comparison of high performing Sn-based catalysts. This material is available free of charge via the Internet at www.acs.org

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT All material and surface characterizations were carried out at Mark Wainwright Analytical Centre (MWAC), UNSW. We thank Dr. Bill Gong from MWAC for the XPS measurements. We also thank Dr. Jian Pan for assistance in drawing the schematic diagram. The work was supported by the Australian Research Council (ARC) under the Laurate Fellowship Scheme FL140100081 and Discovery Early Career Researcher Award DE170100375.


Page 24 of 31

Page 25 of 31 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


References (1)

Lee, S.; Lee, J. Electrode Build-Up of Reducible Metal Composites toward Achievable Electrochemical Conversion of Carbon Dioxide. ChemSusChem 2015, 333–344. DOI:10.1002/cssc.201501112

(2)

Jones, J. P.; Prakash, G. K. S.; Olah, G. A. Electrochemical CO2 Reduction: Recent Advances and Current Trends. Isr. J. Chem. 2014, 54 (10), 1451–1466. DOI: 10.1002/ijch.201400081

(3)

Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; Dubois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R. K.; Waldrop, G. L. Frontiers , Opportunities , and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113, 6621−6658. DOI: 10.1021/cr300463y

(4)

Whipple, D. T.; Kenis, P. J. A. Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. J. Phys. Chem. Lett. 2010, 1 (24), 3451–3458. DOI: 10.1021/jz1012627

(5)

Mirzaei, A. A.; Shirzadi, B.; Atashi, H.; Mansouri, M. Modeling and Operating Conditions Optimization of Fischer-Tropsch Synthesis in a Fixed-Bed Reactor. J. Ind. Eng. Chem. 2012, 18 (4), 1515–1521. DOI: 10.1016/j.jiec.2012.02.013

(6)

Centi, G.; Perathoner, S. Opportunities and Prospects in the Chemical Recycling of Carbon Dioxide to Fuels. Catal. Today 2009, 148 (3–4), 191–205. DOI: 10.1016/j.cattod.2009.07.075

(7)

Lu, X.; Tan, T. H.; Ng, Y. H.; Amal, R. Highly Selective and Stable CO2 Reduction to CO Achieved by a Graphitic Carbon Nitride/carbon Nanotube Composite Electrocatalyst. Chem. - A Eur. J. 2016, 22,11991–11996. DOI:10.1002/chem.201601674

(8)

Lu, X.; Leung, D. Y. C.; Wang, H.; Leung, M. K. H.; Xuan, J. Electrochemical Reduction of Carbon Dioxide to Formic Acid. ChemElectroChem 2014, 1 (5), 836–849. DOI: 10.1002/celc.201300206

(9)

Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy: Second Edition. Beyond Oil Gas Methanol Econ. Second Ed. 2009, 1–334. DOI: 10.1002/anie.200462121

(10)

Daiyan, R.; Lu, X.; Ng, Y. H.; Amal, R. Liquid Hydrocarbon Production from CO2: Recent Development in Electrocatalysts. ChemSusChem 2017. DOI: 10.1002/cssc.201701631

(11)

Daiyan, R.; Lu, X.; Ng, Y. H.; Amal, R. Highly Selective Conversion of CO2 to CO Achieved by a Three-Dimensional Porous Silver Electrocatalyst. ChemistrySelect 2017, 2 (3), 879–884. DOI: 10.1002/slct.201601980



(12)

Wang, J.; Wang, H.; Han, Z.; Han, J. Electrodeposited Porous Pb Electrode with Improved Electrocatalytic Performance for the Electroreduction of CO2 to Formic Acid. Front. Chem. Sci. Eng. 2014, 9 (1), 57–63. DOI: 10.1007/s11705-014-1444-8

(13)

Kwon, Y.; Lee, J. Formic Acid from Carbon Dioxide on Nanolayered Electrocatalyst. Electrocatalysis 2010, 1 (2–3), 108–115. DOI: 10.1007/s12678-010-0017-y

(14) He, Z.; Shen, J.; Ni, Z.; Tang, J.; Song, S.; Chen, J.; Zhao, L. Electrochemically Created Roughened Lead Plate for Electrochemical Reduction of Aqueous CO2. Catal. Commun. 2015, 72, 38–42. DOI: 10.1016/j.catcom.2015.08.024 (15)

Detweiler, Z. M.; White, J. L.; Bernasek, S. L.; Bocarsly, A. B. Anodized Indium Metal Electrodes for Enhanced Carbon Dioxide Reduction in Aqueous Electrolyte. Langmuir 2014, 30 (25), 7593–7600. DOI: 10.1021/la501245p

(16)

Chen, Y.; Kanan, M. W. Tin Oxide Dependence of the CO2 Reduction Efficiency on Tin Electrodes and Enhanced Activity for Tin/tin Oxide Thin-Film Catalysts. J. Am. Chem. Soc. 2012, 134 (4), 1986–1989. DOI: 10.1021/ja2108799

(17)

White, J. L.; Bocarsly, A. B. Enhanced Carbon Dioxide Reduction Activity on IndiumBased Nanoparticles. J. Electrochem. Soc. 2016, 163 (6), H410–H416. DOI: 10.1149/2.0681606jes

(18)

Zhang, S.; Kang, P.; Meyer, T. J. Nanostructured Tin Catalysts for Selective Electrochemical Reduction of Carbon Dioxide to Formate. J. Am. Chem. Soc. 2014, 136 (5), 1734–1737. DOI: 10.1021/ja4113885

(19)

Daiyan, R.; Lu, X.; Ng, Y. H.; Amal, R. Surface Engineered Tin Foil for Electrocatalytic Reduction of Carbon Dioxide to Formate. Catal. Sci. Technol. 2017. 7, 2542-2550. DOI: 10.1039/C7CY00246G

(20)

Azuma, M.; Kazuhito, H.; Watanabe, M. Electrodes in Low-Temperature Aqueous KHCO3 Media. J. Electrochem. Soc. 1990, 137 (6), 1772–1778. DOI: 10.1149/1.2086796

(21)

Kim, H.-Y.; Choi, I.; Ahn, S. H.; Hwang, S. J.; Yoo, S. J.; Han, J.; Kim, J.; Park, H.; Jang, J. H.; Kim, S.-K. Analysis on the Effect of Operating Conditions on Electrochemical Conversion of Carbon Dioxide to Formic Acid. Int. J. Hydrogen Energy 2014, 39 (29), 16506–16512. DOI: 10.1016/j.ijhydene.2014.03.145

(22)

Won, D. H.; Choi, C. H.; Chung, J.; Chung, M. W.; Kim, E.-H.; Woo, S. I. Rational Design of a Hierarchical Tin Dendrite Electrode for Efficient Electrochemical Reduction of CO2. ChemSusChem 2015, 8 (18), 3092–3098. DOI:10.1002/cssc.201500694

(23)

Wu, J.; Risalvato, F. G.; Ke, F.-S.; Pellechia, P. J.; Zhou, X.-D. Electrochemical Reduction of Carbon Dioxide I. Effects of the Electrolyte on the Selectivity and Activity with Sn Electrode. J. Electrochem. Soc. 2012, 159 (7), F353–F359. DOI: 10.1149/2.049207jes

(24)

Chiacchiarelli, L. M.; Zhai, Y.; Frankel, G. S.; Agarwal, A. S.; Sridhar, N. Cathodic Degradation Mechanisms of Pure Sn Electrocatalyst in a Nitrogen Atmosphere. J. Appl.


Page 26 of 31

Page 27 of 31 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


Electrochem. 2012, 42 (1), 21–29. DOI: 10.1007/s10800-011-0367-z (25)

Anawati; Frankel, G. S.; Agarwal, A.; Sridhar, N. Degradation and Deactivation of Sn Catalyst Used for CO2 Reduction as Function of Overpotential. Electrochim. Acta 2014, 133, 188–196. DOI: 10.1016/j.electacta.2014.04.057

(26)

Du, D.; Lan, R.; Humphreys, J.; Sengodan, S.; Xie, K.; Wang, H.; Tao, S. Achieving Both High Selectivity and Current Density for CO2 Reduction to Formate on Nanoporous Tin Foam Electrocatalysts. ChemistrySelect 2016, 1 (8), 1711–1715. DOI: 10.1002/slct.201600451

(27)

Lee, S.; Ocon, J. D.; Son, Y. Il; Lee, J. Alkaline CO2 Electrolysis toward Selective and Continuous HCOO- Production over SnO2 Nanocatalysts. J. Phys. Chem. C 2015, 119 (9), 4884–4890. DOI: 10.1021/jp512436w

(28)

Wu, J.; Risalvato, F. G.; Ma, S.; Zhou, X.-D. Electrochemical Reduction of Carbon Dioxide III. The Role of Oxide Layer Thickness on the Performance of Sn Electrode in a Full Electrochemical Cell. J. Mater. Chem. A, 2014, 2 (6), 1647. DOI: 10.1039/c3ta13544f

(29)

Kumar, B.; Atla, V.; Brian, J. P.; Kumari, S.; Nguyen, T. Q.; Sunkara, M.; Spurgeon, J. M. Reduced SnO2 Porous Nanowires with a High Density of Grain Boundaries as Catalysts for Efficient Electrochemical CO2-into-HCOOH Conversion. Angew. Chemie Int. Ed. 2017, 56 (13), 3645–3649. DOI:10.1002/anie.201612194

(30)

Fu, Y.; Li, Y.; Zhang, X.; Liu, Y.; Zhou, X.; Qiao, J. Electrochemical CO2 Reduction to Formic Acid on Crystalline SnO2 Nanosphere Catalyst with High Selectivity and Stability. Chin. J. Catal. 2016, 37, 1081–1088. DOI: 10.1016/S1872‐2067(15)61048‐8

(31)

Fu, Y.; Li, Y.; Zhang, X.; Liu, Y.; Qiao, J.; Zhang, J.; Wilkinson, D. P. Novel Hierarchical SnO2 Microsphere Catalyst Coated on Gas Diffusion Electrode for Enhancing Energy Efficiency of CO2 Reduction to Formate Fuel. Appl. Energy 2016, 175, 536–544. DOI: 10.1016/j.apenergy.2016.03.115

(32)

Sichao Ma, Jianfeng Liu, Kazunari Sasaki, S. L. and P. K. Carbon Foam Decorated with Silver Nanoparticles for Electrochemical CO2 Conversion. Energy Technol. 2017, 5, 861 – 863. DOI: 10.1002/ente.201600576

(33)

Dutta, A.; Kuzume, A.; Rahaman, M.; Vesztergom, S.; Broekmann, P. Monitoring the Chemical State of Catalysts for CO2 Electroreduction: An In Operando Study. ACS Catal. 2015, 5 (12), 7498–7502. DOI: 10.1021/acscatal.5b02322

(34)

Baruch, M. F.; Pander, J. E.; White, J. L.; Bocarsly, A. B. Mechanistic Insights into the Reduction of CO2 on Tin Electrodes Using in Situ ATR-IR Spectroscopy. ACS Catal. 2015, 5 (5), 3148–3156. DOI: 10.1021/acscatal.5b00402

(35)

Li, F.; Chen, L.; Knowles, G. P.; MacFarlane, D. R.; Zhang, J. Hierarchical Mesoporous SnO2 Nanosheets on Carbon Cloth: A Robust and Flexible Electrocatalyst for CO2 Reduction with High Efficiency and Selectivity. Angew. Chemie Int. Ed. 2017, 56, 505 –



509. DOI: 10.1002/anie.201608279 (36)

Xiao, C.; Lu, X.; Zhao, C. Unusual Synergistic Effects upon Incorporation of Fe And/or Ni into Mesoporous Co3O4 for Enhanced Oxygen Evolution. Chem. Commun. 2014, 50 (70), 10122–10125. DOI: 10.1039/c4cc04922e

(37)

Lu, X.; Ng, Y. H.; Zhao, C. Gold Nanoparticles Embedded within Mesoporous Cobalt Oxide Enhance Electrochemical Oxygen Evolution. ChemSusChem 2014, 7 (1), 82–86. DOI: 10.1002/cssc.201300975

(38)

Li, M.; Wang, J. J.; Li, P.; Chang, K.; Li, C. L.; Wang, T.; Jiang, B.; Zhang, H. B.; Liu, H. M.; Yamauchi, Y.; Umezawa, N.; Ye, J. H. Mesoporous Palladium-Copper Bimetallic Electrodes for Selective Electrocatalytic Reduction of Aqueous CO2 to CO. J. Mater. Chem. A 2016, 4 (13), 4776–4782. DOI: 10.1039/c6ta00487c

(39)

Linares, N.; Silvestre-Albero, A. M.; Serrano, E.; Silvestre-Albero, J.; García-Martínez, J. Mesoporous Materials for Clean Energy Technologies. Chem. Soc. Rev. 2014, 43 (22), 7681–7717. DOI: 10.1039/c3cs60435g

(40)

Wang, X.; Li, Z.; Li, Q.; Wang, C.; Chen, A.; Zhang, Z.; Fan, R.; Yin, L. Ordered Mesoporous SnO2 with a Highly Crystalline State as an Anode Material for Lithium Ion Batteries with Enhanced Electrochemical Performance. CrystEngComm 2013, 15 (18), 3696. DOI: 10.1039/c3ce40087e

(41)

Xu, W.; Wu, Z.; Tao, S. Recent Progress in Electrocatalysts with Mesoporous Structures for Application in Polymer Electrolyte Membrane Fuel Cells. J. Mater. Chem. A 2016, 4 (42), 16272–16287. DOI: 10.1039/c6ta05304a

(42)

Guo, Y.-G.; Hu, J.-S. J.-S.; Wan, L.-J. Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices. Adv. Mater. 2008, 20 (15), 2287–2878. DOI: 10.1002/adma.200800627

(43)

Wang, D.; Sun, J.; Cao, X.; Zhu, Y.; Wang, Q.; Wang, G.; Han, Y.; Lu, G.; Pang, G.; Feng, S. Tungsten Oxide Mesocrystals with Increased Oxygen. J. Mater. Chem. A, 2013, 1, 8653–8657. DOI: 10.1039/c3ta11506b

(44)

Luo, Z.; Miao, R.; Huan, T. D.; Mosa, I. M.; Poyraz, A. S.; Zhong, W.; Cloud, J. E.; Kriz, D. A.; Thanneeru, S.; He, J.; Zhang, Y. Mesoporous MoO3 – X Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions. Adv. Energy Mater. 2016. 6, 1600528. DOI: 10.1002/aenm.201600528

(45)

Moya, A.; Cherevan, A.; Marchesan, S.; Gebhardt, P.; Prato, M.; Eder, D.; Vilatela, J. J. Oxygen Vacancies and Interfaces Enhancing Photocatalytic Hydrogen Production in Mesoporous CNT / TiO2 Hybrids. Applied Catal. B, Environ. 2015, 179, 574–582. DOI: 10.1016/j.apcatb.2015.05.052

(46)

Choi, M.; Heo, W.; Kleitz, F.; Ryoo, R. Facile Synthesis of High Quality Mesoporous SBA-15 with Enhanced Control of the Porous Network Connectivity and Wall Thickness. Chem. Commun. (Camb). 2003, 75 (12), 1340–1341. DOI:10.1039/B303696K


Page 28 of 31

Page 29 of 31 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


(47)

Patil, G. E.; Kajale, D. D.; Gaikwad, V. B.; Jain, G. H. Preparation and Characterization of SnO2 Nanoparticles by Hydrothermal Route. International Nano Lett. 2012, 2:17. DOI: 10.1186/2228-5326-2-17

(48)

Jiang, L.; Sun, G.; Zhou, Z.; Sun, S.; Wang, Q.; Yan, S.; Li, H.; Tian, J.; Guo, J.; Zhou, B.; Xin, Q. Size-Controllable Synthesis of Monodispersed SnO2 Nanoparticles and Application in Electrocatalysts. J. Phys. Chem. B. 2005, 109 (18), 8774–8778. DOI: 10.1021/jp050334g

(49)

Deng, X.; Chen, K.; Tüysüz, H. A Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides. Chem. Mater. 2017, 29, 40−52, DOI: 10.1021/acs.chemmater.6b02645.

(50)

Lu, A. H.; Schüth, F. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials. Adv. Mater. 2006, 18 (14), 1793–1805. DOI: 10.1002/adma.200600148

(51)

Wang, J. X.; Liu, D. F.; Yan, X. Q.; Yuan, H. J.; Ci, L. J.; Zhou, Z. P.; Gao, Y.; Song, L.; Liu, L. F.; Zhou, W. Y.; Wang, G.; Xie, S. S. Growth of SnO2 Nanowires with Uniform Branched Structures. Solid State Commun. 2004, 130 (1–2), 89–94. DOI: 10.1016/j.ssc.2004.01.003

•

(52) Sing, K. S. W.; Everett, D. H.; Haul, R. a. W.; Moscou, L.; Pierotti, R. a.; Rouquérol, J.; Siemieniewska, T. International Union Of Pure Commission On Colloid And Surface Chemistry Including Catalysis * Reporting Physisorption Data For Gas / Solid Systems With Special Reference To The Determination Of Surface Area And Porosity. Pure Appl. Chem. 1982, 54 (11), 2201–2218. DOI: 10.1351/pac198254112201

(53)

Garcia, J. C.; Assali, L. V. C.; Justo, J. F. The Structural and Electronic Properties of Tin Oxide Nanowires : An Ab Initio Investigation. J. Phys. Chem. C, 2012, 116 (24), pp 13382–13387. DOI: 10.1021/jp300793e

(54)

Chen, Z.; Lai, J. K. L.; Shek, C. H.; Chen, H. Synthesis and Structural Characterization of Rutile SnO2 Nanocrystals. J. Mater. Res. 2003, 18 (6), 1289–1292. DOI: 10.1557/JMR.2003.0176

(55)

Kwoka, M.; Ottaviano, L.; Passacantando, M.; Santucci, S.; Czempik, G.; Szuber, J. XPS Study of the Surface Chemistry of L-CVD SnO2 Thin Films after Oxidation. Thin Solid Films 2005, 490 (1), 36–42. DOI: 10.1016/j.tsf.2005.04.014

(56)

Wang, C.; Wu, Q.; Ge, H. L.; Shang, T.; Jiang, J. Z. Magnetic Stability of SnO₂ Nanosheets. Nanotechnology 2012, 23 (7), 75704. DOI: 10.1088/0957-4484/23/7/075704

(57)

Jie, L.; Chao, X. XPS Examination of Tin Oxide on Float Glass Surface. J. Non. Cryst. Solids 1990, 119 (1), 37–40. DOI: 10.1016/0022-3093(90)90238-H

(58)

Zhang, Y.-J.; Sethuraman, V.; Michalsky, R.; Peterson, A. A. Competition between CO2 Reduction and H2 Evolution on Transition-Metal Electrocatalysts. ACS Catal. 2014, 4 (10), 3742–3748. DOI: 10.1021/cs5012298

(59)

Jhong, H. R. M.; Ma, S.; Kenis, P. J. Electrochemical Conversion of CO2 to Useful



Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2 (2), 191–199. DOI: 10.1016/j.coche.2013.03.005 (60) Delacourt, C.; Ridgway, P. L.; Kerr, J. B.; Newman, J. Design of an Electrochemical Cell Making Syngas (CO + H2) from CO2 and H2O Reduction at Room Temperature. J. Electrochem. Soc. 2008, 155 (1), B42–B49. DOI: 10.1149/1.2801871 (61)

Lv, W.; Zhang, R.; Gao, P.; Lei, L. Studies on the Faradaic Efficiency for Electrochemical Reduction of Carbon Dioxide to Formate on Tin Electrode. J. Power Sources 2014, 253, 276–281. DOI: 10.1016/j.jpowsour.2013.12.063

(62)

Lei, F.; Liu, W.; Sun, Y.; Xu, J.; Liu, K.; Liang, L.; Yao, T.; Pan, B.; Wei, S.; Xie, Y. Metallic Tin Quantum Sheets Confined in Graphene toward High-Efficiency Carbon Dioxide Electroreduction. Nat. Commun. 2016, 7, 12697. DOI: 10.1038/ncomms12697

(63)

Ma, T. Y.; Zheng, Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Mesoporous MnCo2O4 with Abundant Oxygen Vacancy Defects as High-Performance Oxygen Reduction Catalysts. J. Mater. Chem. A 2014, 2 (23), 8676–8682. DOI: 10.1039/c4ta01672f

(64)

Liu, Z. C.; Chen, H. R.; Huang, W. M.; Gu, J. L.; Bu, W. B.; Hua, Z. L.; Shi, J. L. Synthesis of a New SnO2/mesoporous Silica Composite with Room-Temperature Photoluminescence. Microporous Mesoporous Mater. 2006, 89 (1–3), 270–275. DOI: 10.1016/j.micromeso.2005.10.037

(65)

Konstantinova, E. A.; Pentegov, I. S.; Marikutsa, A. V.; Rumyantseva, M. N.; Gaskov, A. M.; Kashkarov, P. K. EPR Study of Nanocrystalline Tin Dioxide. Phys. Status Solidi Curr. Top. Solid State Phys. 2011, 8 (6), 1957–1960. DOI: 10.1002/pssc.201000140

(66)

Wang, D.; Doan-nguyen, V.; Gorte, R. J.; Wang, D.; Kang, Y.; Doan-nguyen, V.; Chen, J.; Küngas, R.; Wieder, N. L.; Bakhmutsky, K.; Gorte, R. J.; Murray, C. B. Synthesis and Oxygen Storage Capacity of Two- Dimensional Ceria Nanocrystals Synthesis and Oxygen Storage Capacity of Two-Dimensional Ceria. Angew. Chem. Int. Ed. 2011, 50, 4378 – 4381. DOI: 10.1002/anie.201101043

(67)

Sun, L.; Hu, J.; Qin, H.; Zhao, M.; Fan, K. Influences of Ca Doping and Oxygen Vacancy upon Adsorption of CO on the LaFeO3 (010) Surface : A First-Principles Study. J. Phys. Chem. C. 2011, 3 (10), 5593–5598. DOI: 10.1021/jp107038z

(68)

Funk, S.; Burghaus, U. Adsorption of CO2 on Oxidized , Defected , Hydrogen and Oxygen Covered Rutile (1x1)-TiO2 (110). Phys. Chem. Chem. Phys. 2006, 8, 4805–4813. DOI: 10.1039/b609768e

(69) Gao, S.; Sun, Z.; Liu, W.; Jiao, X.; Zu, X.; Hu, Q.; Sun, Y.; Yao, T.; Zhang, W.; Wei, S.; Xie, Y. Atomic Layer Confined Vacancies for Atomic-Level Insights into Carbon Dioxide Electroreduction. Nat. Commun. 2017, 8, 14503. DOI: 10.1038/ncomms14503


Page 30 of 31

Page 31 of 31 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


TOC Graph Synopsis: Mesoporous SnO2 are proposed as potential electrocatalysts for large-scale conversion of CO2 to formate owing to its high selectivity and current density.


Highly Selective Reduction of CO2 to Formate at Low Overpotentials

Recommend Documents