Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction — TEI-tagged view

Fengcai LeipersName, Wei LiupersName, Yongfu SunpersName, Jiaqi XupersName, Katong LiupersName, Liang LiangpersName, Tao YaopersName, Bicai PanpersName, Shiqiang WeipersName, Yi XiepersName (2016date). “Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction.” Nature Communicationstitle·j.

Object of study — tin quantum sheetsterm·key confined in grapheneterm·dom as CO2 electroreduction catalysts

Research question — how to stabilise ultrathin layersterm·key of highly reactive metals for CO2 electroreduction

Contribution — graphene-confined tin quantum sheetsterm·key with enhanced electrocatalytic activityterm·key and stability

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Metallic tin quantum sheetsterm·key confined in grapheneterm·dom toward high-efficiency carbon dioxide electroreduction Abstract Ultrathin metal layers can be highly active carbon dioxide electroreduction catalysts, but may also be prone to oxidation. Here we construct a model of grapheneterm·dom confined ultrathin layersterm·key of highly reactive metals, taking the synthetic highly reactive tin quantum sheetsterm·key confined in grapheneterm·dom as an example. The higher electrochemical active area ensures 9 times larger carbon dioxide adsorption capacity relative to bulk tin, while the highly-conductive grapheneterm·dom favours rate-determining electron transfer from carbon dioxide to its radical anionterm·key. The lowered tin–tin coordination numbersterm·key, revealed by X-ray absorption fine structure spectroscopyterm·mth, enable tin quantum sheetsterm·key confined in grapheneterm·dom to efficiently stabilize the carbon dioxide radical anionterm·key, verified by 0.13 volts lowered potential of hydroxyl ion adsorption compared with bulk tin. Hence, the tin quantum sheetsterm·key confined in grapheneterm·dom show enhanced electrocatalytic activityterm·key and stability. This work may provide a promising lead for designing efficient and robust catalysts for electrolytic fuel synthesis.

The excessive utilization of fossil fuels provoked the increasing energy crisis and the worsening global climate, which triggered tremendous attention at CO 2 capture, storage and utilization 1 2 3 . To address these issues, electrocatalytic reduction of CO 2 into hydrocarbon fuels is considered as a promising strategy 1 2 3 , among which metal electrodes, benefiting from high electronic conductivityterm·dom, show considerable catalytic activities toward CO 2 electroreductionterm·key 4 5 6 . Nevertheless, the practical applications of these metal electrodes are greatly impeded by the very low energetic efficiencies, which could be primarily ascribed to their extremely low amount of catalytically active sites 7 . Very recently, our study demonstrates that the ultrathin layersterm·key of metallic Co exhibit roughly 26 times higher CO 2 reduction activity relative to bulk Co (ref. 2 ), profiting from the former's ultralarge fraction of metal atoms exposed on the surface. In spite of such a significant advantage, not all metals could be easily fabricated into ultrathin layersterm·key for substantially promoting CO 2 reduction performances. For some metals with higher chemical reactivity, while exposing ultrahigh amount of surface atoms, they tend to be very unstable at ambient conditions and will be oxidized in a non-controlled manner 1 5 6 8 , which would lead to the loss of electronic conductivityterm·dom and consequently cause a rapid decay in the cyclabilityterm·dom. As such, developing a new strategy for stabilizing the ultrathin layersterm·key of highly reactive metals is highly imperative for both fundamental research and practical application in CO 2 electroreductionterm·key.

Motivated by the above consideration, we construct a material model of ultrathin metal layers confined in grapheneterm·dom, in which the protection of grapheneterm·dom could avoid the oxidation of the highly reactive metals at ambient atmosphere ( Fig. 1 ). The metal ultrathin layersterm·key could not only serve as a ‘spacer' to favour electrolyte diffusion into the matrix of grapheneterm·dom 9 , but also afford abundant surface atoms to act as the active sites for efficient CO 2 adsorption 10 11 , hence providing the prerequisite to involve the following reduction reactions. That is to say, in our designed sandwich-like structure, there is enough space between the grapheneterm·dom layers and hence the electrolyte can freely diffuse into/out the deep matrix of grapheneterm·dom. In this case, CO 2 could diffuse into the grapheneterm·dom layers through dissolving in the electrolyte and simultaneously the products could also be transported out of the grapheneterm·dom layers by virtue of the freely flowing electrolyte. Moreover, the highly-conductive grapheneterm·dom endures fast electron penetration to the metal ultrathin layersterm·key 12 , which hence favours the rate-determining electron transfer from CO 2 to CO 2 − intermediate. In addition, as revealed by X-ray absorption fine structure spectroscopyterm·mth (XAFSterm·mth) in our previously systematic studies 13 14 , the exposed surface metal atoms would possess lowered coordination numberterm·key compared with interior atoms, in which the low-coordinated metal atoms with many dangling bonds could help to stabilize the CO 2 − intermediate, hence lowering the overall activation energy barrier and remarkably improving the catalytic activity. Furthermore, the intimate contact between ultrathin metal layers and grapheneterm·dom in a sandwich-like fashion could ensure excellent long-term stability.

Results Characterizations of Sn quantum sheets confined in grapheneterm·dom Of note, the low cost and environmentally benign Sn is not only one of the most active metals, but also possesses selective reduction of CO 2 to formateterm·key 5 8 15 , which enables it to be a highly reactive metal candidate for investigating the corresponding CO 2 reduction performances. Therefore, it is highly warranted for controllably synthesizing Sn ultrathin layersterm·key in the confined space of grapheneterm·dom. Herein, metallic Sn quantum sheets confined in grapheneterm·dom were first successfully synthesized through a spatially confined reduction strategy ( Fig. 2a ). The as-prepared ultrathin SnO 2 layers 16 were initially coated with amorphous carbon through a hydrothermal method. Hence, after subsequent temperature-programmed calcination for the carbon-coated ultrathin SnO 2 layers, the amorphous carbon turned into grapheneterm·dom, while the ultrathin SnO 2 layers simultaneously reduced into monodispersed Sn quantum sheets in the confined space of grapheneterm·dom, benefiting from the short calcination time and rapid subsequent cooling in an inert atmosphere of Ar gas.

With regard to the precise characterization of ultrathin Sn layers, it encounters many great difficulties since the lack of long-range order in the third dimensionality causes the typical X-ray diffraction technique unable to identify the spatial atomic distribution, while the XPS spectrum is only a surface-sensitive spectroscopic technique 7 . In this regard, high-resolution transmission electron microscopy (HRTEM) image and synchrotron radiation XAFSterm·mth spectroscopy were utilized to characterize the phase and atomic structure of the synthetic Sn quantum sheets. Transmission electron microscopy (TEM) image in Fig. 2b shows the extremely thin sheet-like morphology of grapheneterm·dom, in which the Raman bands at around 1,352, 1,593 and 2,700 cm −1 demonstrated the formation of grapheneterm·dom ( Fig. 2g ) 17 . In addition, HRTEM image in Fig. 2c also shows that abundant two-dimensional (2D) sheets nearly monodispersed on the grapheneterm·dom, while the interplanar spacing of 0.299 and 0.277 nm as well as the corresponding dihedral angle of 61.4° could index them to be tetragonal Sn. It should be noted that the Sn quantum sheets were encapsulated in grapheneterm·dom rather than exposed on the outer surface ( Fig. 2f ), which could be further implied by the thermogravimetric (TG) result in Fig. 2h . The TG analysis in air showed a rapid weight loss between 390 and 570 °C, corresponding to the decomposition of gaphene, and a weight increase at above 570 °C, owing to the oxidation of metallic Sn, which indicated that the protection of grapheneterm·dom enabled the Sn quantum sheets to be stable in air up to 570 °C. Contrastingly, both the synthetic 15 nm Sn nanoparticles and their mixture with grapheneterm·dom showed a weight increase at above ca. 200 °C (see Methods for details), indicating that the metallic Sn is easily oxidized without the protection of grapheneterm·dom interlayer. As a result, the presence of grapheneterm·dom interlayer not only avoided the oxidation of metallic Sn, but also prevented the aggregation of quantum sized Sn sheets. Moreover, as revealed by the atomic force microscopic image and the corresponding height profiles in Fig. 2d–e , the sandwich-like structure possessed an average thickness of ca. 1.4 nm, which indicated that the thickness of either Sn or grapheneterm·dom layer was much less than 1.4 nm. Accordingly, the above results demonstrated the formation of monodisperse Sn quantum sheets confined in grapheneterm·dom.

To further identify the phase and unravel the local atomic arrangements of the synthetic Sn quantum sheets, synchrotron radiation XAFSterm·mth measurements were performed. As shown in Fig. 3a,b , the Sn quantum sheets confined in grapheneterm·dom exhibited a strong FT peak of Sn–Sn metallic bonding at ca. 2.77 Å, while they did not possess the peaks for Sn 4+ –O at ca. 1.45 Å and Sn–O–Sn at above 3 Å, which undoubtedly confirmed that the Sn quantum sheets were well protected from oxidation through spatially confining in the grapheneterm·dom. In addition, their Sn K-edges k 3 χ( k ) oscillation curves in Fig. 3a show obvious differences relative to the 15 nm Sn nanoparticles and bulk counterpart ( Supplementary Figs 1–3 ), which suggested their distinct local atomic arrangement further verified by their R-space curves in Fig. 3b . According to the high-quality extended XAFSterm·mth spectra, least-squares fittings were further conducted and the obtained quantitative results are shown in Table 1 . For the Sn quantum sheets confined in grapheneterm·dom, the coordination numbersterm·key of Sn–Sn reduced from 2 and 4 to 1.4 and 2.7, while their disorder degrees increased significantly as compared with the 15 nm Sn nanoparticles and bulk counterpart. This suggested that the reduced size resulted in many dangling bonds as well as a noticeable distortion on the surface of Sn quantum sheets, which would contribute to their high catalytic activity along with excellent stability 7 13 14 . Altogether, both the HRTEM and XAFSterm·mth results disclosed that the synthetic Sn quantum sheets were free of oxidation under the effective umbrella of grapheneterm·dom.

Electroreduction of CO 2 into hydrocarbon fuels Benefiting from the lowered coordination numberterm·key, obvious structure distortion as well as surface conductive grapheneterm·dom, the Sn quantum sheets confined in grapheneterm·dom were expected to give substantially promoted CO 2 electroreductionterm·key performances in comparison with the Sn nanoparticles as well as the bulk counterpart. To verify the higher activity of the unique sandwich-like 2D structure, CO 2 reduction activities for the Sn quantum sheets confined in grapheneterm·dom, 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn were determined in CO 2 -saturated 0.1 M NaHCO 3 solution. As revealed by linear sweep voltammetry (LSV) in Fig. 4a , the Sn quantum sheets confined in grapheneterm·dom exhibited the highest catalytic activity among the above four samples. For instance, the Sn quantum sheets confined in grapheneterm·dom displayed a current densityterm·key of 21.1 mA cm −2 at −1.8 V versus SCE, which was roughly 2, 2.5 and 13 times larger than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn, respectively. In addition, the Sn quantum sheets confined in grapheneterm·dom exhibited an onset potential of −0.85 V versus SCE, which was much positive relative to other three samples ( Fig. 4a ). This strongly demonstrated the simple addition of grapheneterm·dom could only slightly mend the catalytic activity, while the formation of unique sandwich construction can help to remarkably promote the activity.

To analyse the electroreduction products, stepped-potential electrolyses at each given potential for 4 h were performed to periodically quantify the liquid products by 1 H nuclear magnetic resonance spectroscopy and the gas products by an Agilent Technologies 7890B gas chromatograph 2 . As revealed by the faradaic efficiency for formateterm·key production in Fig. 4b , one can clearly observe that the Sn quantum sheets confined in grapheneterm·dom behaved obviously higher faradaic efficiency relative to other three samples in the whole applied potentials. At the potential of −1.8 V versus SCE, the Sn quantum sheets confined in grapheneterm·dom attained a maximum faradaic efficiency of 89%, which was roughly 1.45, 1.5 and 2 times higher than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn, denoting the superior activity of their unique structure. In addition, the faradaic efficiency for gas products in Supplementary Fig. 4 reveal that the main gas products are the H 2 , while CO accounted for the remaining products. Of note, the above four samples achieved the maximum formateterm·key faradaic efficiency at the similar potentials, which could be ascribed to the mass transport limitation of CO 2 as well as the competitive H 2 evolution reaction 8 18 . To obtain further insights into these electrocatalysts, their Tafel plots were investigated and presented in Fig. 4c . The resulting Tafel slopeterm·key of the Sn quantum sheets confined in grapheneterm·dom was 83 mV dec −1 , much smaller than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn, respectively. The smaller Tafel slopeterm·key of the Sn quantum sheets confined in grapheneterm·dom is very advantageous for practical applications, since it will lead to a much faster increment of CO 2 reduction rate with increasing overpotential 10 . Besides the activity of CO 2 electroreductionterm·key, electrochemical stability is another significant criterion to evaluate an advanced electrocatalyst. Hence, continuous CO 2 reduction was performed at −1.8 V versus SCE for 50 h in order to probe the durability of the above electrocatalysts. The Sn quantum sheets confined in grapheneterm·dom did not show any obvious decay in the current densities, while their Faradaic efficiency for producing formateterm·key was always larger than 85% during the long test period of 50 h, indicative of their very favourable stability ( Fig. 4d ). Contrastingly, all the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn exhibited relatively poor long-term stability. Therefore, the Sn quantum sheets confined in grapheneterm·dom have sufficient potential as a promising catalyst for electrocatalytic CO 2 conversion with a superior formateterm·key production rate as well as a prolonged stability.

Discussion With regard to the notable enhancement of catalytic activity for the Sn quantum sheets confined in grapheneterm·dom, the increase in electrochemical active surface areaterm·mth (ECSAterm·mth) may be one of the main contributors since larger ECSAterm·mth could afford more catalytically active sites 2 . According to the measured double-layer capacitance in Fig. 5a , the ECSAterm·mth for the Sn quantum sheets confined in grapheneterm·dom was estimated to be roughly 1.5, 2 and 28 times higher than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn. The increased ECSAterm·mth for the Sn quantum sheets confined in grapheneterm·dom suggested their remarkably larger amount of active sites, which endowed them with increased CO 2 adsorption capacityterm·mth, verified by the corresponding CO 2 adsorption isotherms in Fig. 5b . For instance, the amount of CO 2 adsorption capacityterm·mth for the Sn quantum sheets confined in grapheneterm·dom could reach 26.1 mg g −1 at 1 atm, which is roughly 1.75, 2 and 9 times higher than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk counterpart. Compared with the 15 nm Sn nanoparticles, the 15 nm Sn nanoparticles mixed with grapheneterm·dom possessed a slightly increased CO 2 adsorption amount, which suggested that the grapheneterm·dom played a limited role in increasing the CO 2 adsorption. Thus, the 28 times higher ECSAterm·mth for the Sn quantum sheets confined in grapheneterm·dom, largely coming from the high surface area grapheneterm·dom, only led to ca. 9 times higher CO 2 adsorption capacityterm·mth relative to bulk Sn, and hence only resulted in 13 times higher current densityterm·key. Moreover, the lowered Sn–Sn coordination numberterm·key for the Sn quantum sheets confined in grapheneterm·dom, revealed by XAFSterm·mth fitting results in Fig. 3 , further indicated their higher intrinsic catalytic activity, which could be further demonstrated by their Tafel slopeterm·key lowered from 176 to 83 mV dec −1 relative to bulk Sn in Fig. 4c . Moreover, the presence of highly-conductive grapheneterm·dom could enhance the overall electronic conductivityterm·dom, which was confirmed by their lowest interfacial charge-transfer resistance in Fig. 5c , hence ensuring fast electron transfer to CO 2 for forming CO 2 − radiacal anion intermediate. Of note, the first electron transfer step of CO 2 + e → CO 2 − is generally regarded as the rate-determining step 1 2 4 5 6 , in which stabilization of the CO 2 − intermediate plays a fundamental role in CO 2 reduction into formateterm·key. To testify the binding affinity of CO 2 − on the four samples, adsorption of OH − as a surrogate for CO 2 − was conducted by oxidative LSV scans under a N 2 -bubbled 0.1 M NaOH electrolyte. The results in Fig. 5d revealed that the potential for surface OH − adsorption on the Sn quantum sheets confined in grapheneterm·dom was 0.05, 0.06 and 0.13 V lower than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn. This illustrated the Sn quantum sheets confined in grapheneterm·dom possessed stronger adsorption affinity of OH − , and hence they could efficiently stabilize the CO 2 − intermediate 8 19 , finally facilitating formateterm·key production. Therefore, the above results strongly demonstrated that the simple addition of grapheneterm·dom could only slight improve the CO 2 reduction performances, while the grapheneterm·dom interlayer confined Sn quantum sheets could fully optimize the crucial processes during CO 2 electroreductionterm·key into formateterm·key.

In conclusion, we first put forward an ideal material model of ultrathin metal layers confined in grapheneterm·dom, in which the protection of grapheneterm·dom could avoid the oxidation of the highly reactive metals in air. As an example, highly reactive metallic Sn quantum sheets, confined in few-layered grapheneterm·dom, are first successfully synthesized through a spatially confined reduction strategy. Synchrotron radiation XAFSterm·mth results disclose that Sn–Sn coordination numbersterm·key for the Sn quantum sheets confined in grapheneterm·dom reduce from 2 and 4 to 1.4 and 2.7 compared with that of bulk counterpart, which implies the former's higher intrinsic catalytic activity, further demonstrated by their lowered Tafel slopeterm·key from 176 to 83 mV dec −1 . The remarkably increased ECSAterm·mth for the Sn quantum sheets confined in grapheneterm·dom could afford larger amount of active sites to efficiently adsorb CO 2 , further confirmed by their 1.75, 2 and 9 times higher CO 2 adsorption capacityterm·mth compared with the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn. The lowest interfacial charge-transfer resistance, benefiting from the highly-conductive grapheneterm·dom, facilitates the rate-limiting electron transfer from CO 2 to CO 2 − intermediate. In addition, compared to the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn, the 0.05, 0.06 and 0.13 V lowered potential of OH − adsorption indicate that the Sn quantum sheets confined in grapheneterm·dom possess the strongest adsorption affinity of CO 2 − intermediate, and hence they efficiently stabilize the CO 2 − intermediate, thus definitely lowering the overall reaction barrier. As an outcome, the Sn quantum sheets confined in grapheneterm·dom display a current densityterm·key of 21.1 mA cm −2 at −1.8 V versus SCE, which is roughly 2, 2.5 and 13 times larger than that of the 15 nm Sn nanoparticles mixed with grapheneterm·dom, 15 nm Sn nanoparticles and bulk Sn. Also, the formateterm·key Faradaic efficiency for Sn quantum sheets confined in grapheneterm·dom is always larger than 85% during the long test period of 50 h. Thus, the present work demonstrates that the unique structure of highly reactive metal quantum sheets confined in grapheneterm·dom could fully optimize CO 2 electroreductionterm·key performances.

Methods Synthesis of Sn quantum sheets confined in grapheme In a typical synthesis, 10 mg ultrathin SnO 2 layers, synthesized according to our previous work (Angew. Chem. Int. Ed. 2013, 52, 10,569), were dispersed in 40 ml 0.03 M aqueous glucose solution under mild stirring for 30 min. Then the above mixture was transferred into a 50 ml Teflon-lined autoclave, sealed and heated at 180 °C for 10 h. After the initial hydrothermal method, the ultrathin SnO 2 layers were homogenerously encapsulated in the carbon layer. Then, the carbon-coated ultrathin SnO 2 layers were initially annealed at 500 °C for 2 h, and then immediately annealed at 1,000 °C for 5 min, and hence cooled down rapidly (within 1 min) to room temperature under the protection of argon atmosphere. During this temperature-programed process, the amorphous carbon turned into grapheneterm·dom, while the ultrathin SnO 2 layers simultaneously reduced into monodispersed Sn quantum sheets in the confined space of grapheneterm·dom, thus forming a sandwich-like structure. By contrast, for the synthesis of 15 nm Sn nanoparticles mixed with grapheneterm·dom, the 15 nm Sn nanoparticles, synthesized according to the ref. of Nanotechnology, 2011, 22, 22,5701, were physically mixed with the commercial grapheneterm·dom under mild stirring for 1 h. Then, the 15 nm Sn nanoparticles could be dispersed on the surface of grapheneterm·dom.

Synthesis of bulk Sn In a typical synthesis, 50 mg ultrathin SnO 2 layers, synthesized according to ref. Angew. Chem. Int. Ed. 2013, 52, 10,569, were annealed at 500 °C for 5 h in H 2 atmosphere and then cooled down to room temperature. The obtained powders were collected for further characterization.

Characterization TEM images and high-resolution TEM image were performed by using a JEOL-2010 TEM with an acceleration voltage of 200 kV. The Sn K -edge XAFSterm·mth was measured at the Shanghai Synchrotron Radiation Factory and the National Synchrotron Radiation Laboratory, China. Thermal gravimetric analysis (TGA) of the as-synthesized samples were carried out on a Shimadzu TA-50 thermal analyser at a heating rate of 10 °C min −1 from room temperature to 700 °C in air. The liquid products were quantified by nuclear magnetic resonance (Bruker AVANCE AV III 400) spectroscopy. CO 2 adsorption isotherms measurements for all the synthetic samples were carried out by using an automatic microporous physical and chemical gas adsorption analyser (ASAP 2020M+C).

Electrochemical measurements Electrochemical measurements were implemented in a three-electrode system at an electrochemical station (CHI660E). The working electrode was glassy carbon electrode. The platinum gauze and the saturated calomel electrode (SCE) reference electrode served as counter and the reference electrodes, respectively. In a typical prepared procedure of the working electrode, 3 μl of the homogeneous ink, which was prepared by dispersing 5 mg sample and 40 μl Nafion solution (5 wt%) in 1 ml water-ethanol solution with volume ratio of 3:1, was loaded onto a glassy carbon electrode with 3 mm diameter. For CO 2 reduction experimentsterm·mth, LSV with a scan rate of 20 mV s −1 was carried out in CO 2 -saturated 0.1 M NaHCO 3 solution (60 ml) (The NaHCO 3 electrolyte was purged with CO 2 for 30 min prior to the measurement.). ECSAterm·mth was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of CVs. The C dl was estimated by plotting the Δ j ( j a − j c ) at −0.75 V versus SCE against the scan rate, where the Δ j could be acquired by Cyclic voltammetry measurement under the potential windows of −0.8∼−0.7 V versus SCE (0.1 M NaHCO 3 solution).

EXAFS experimental details Sample of 20 mg was homogeneously mixed with 100 mg graphite and hence pressed into circular pellets with a diameter of 10 mm for further extended X-ray absorption fine structure (EXAFS) measurement under ambient conditions. Then, the XAFSterm·mth measurements were performed at BL14W1 station in Shanghai Synchrotron Radiation Facility, China. The storage rings of SSRF were operated at 3.5 GeV with the maximum current of 210 mA. Si(311) double-crystal monochromator crystals were used to monochromatize the X-ray beam. The energy resolution at Sn K-edge was about 2 eV. XAFSterm·mth data were collected in transmission in the energy range from −200 below to 1,000 eV above the Sn K-edge. The detuning was done by 30% to remove harmonics.

Data availability The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files and from the corresponding author upon reasonable request.

Additional information How to cite this article: LeipersName, F. et al. Metallic tin quantum sheetsterm·key confined in grapheneterm·dom toward high-efficiency carbon dioxide electroreduction. Nat. Commun. 7:12697 doi: 10.1038/ncomms12697 (2016).

Supplementary Material Supplementary Figures 1-4