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Efficient electrochemical reduction of carbon dioxide into ethylene boosted by copper vacancies on stepped cuprous oxide
Journal of CO₂ Utilization 38 (2020) 125–131
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Efficient electrochemical reduction of carbon dioxide into ethylene boosted by copper vacancies on stepped cuprous oxide
T
Xiaona Rena, Xiangwen Zhanga,b, Xingzhong Caoc, Qingfa Wanga,b,* a
Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin, 300350, China c Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide electroreduction Copper vacancies Ethylene Cuprous oxide Overpotential
Electrochemical carbon dioxide (CO2) reduction powered by renewable energy source is a promising strategy for the sustainable carbon cycle, while the selectivity toward C2+ products is still a great challenge. Herein, a novel stepped cuprous oxide catalyst with abundant Cu vacancies (Cuv-Cu2O catalyst) is developed to enhance the selectivity and efficiency of CO2 conversion toward C2 products. This catalyst exhibits high C2H4 partial current density and production rate at a low overpotential benefiting from the stepped surface and the modified electronic structure by Cu vacancies. Furthermore, density functional theory calculations demonstrate that the Cu vacancy-enriched Cu2O surface ensures strong adsorption to *COH but weak affinities to *CO and *CH2 intermediates, which promotes CO2 electroreduction to C2H4 products. The enhanced selectivity of C2H4, accompanied with the Cu vacancies and Cu+ species, can remain stable during the long-term test. This study will provide guidance on designing efficient electrocatalysts for CO2 reduction towards hydrocarbon products.
1. Introduction Motivated by the increasing concerns about global warming and finite fossil fuel resources, developing clean and renewable energy has been a great challenge. Electrochemical CO2 reduction into hydrocarbon fuels is regarded as a promising strategy to achieve a zero-CO2 emission, which powered by sustainable and cheap energy source such as wind, solar and nuclear [1–3]. Among various CO2 electrochemical products (CO, HCOOH, CH4, C2H4, etc.), C2H4 is highly desired due to its widespread application in agriculture and polymer manufacturing [4–7]. Copper based materials are the most prone candidate to electrochemically reduce CO2 into valuable C2+ products. However, the low selectivity, high overpotential and vulnerability to the electrolysis environment hinder their being efficient electrocatalysts [8–12]. Therefore, understanding the mechanisms underlying the product selectivity becomes more important to further module the copper catalysts. Many avenues have been employed to promote the electrochemical activity for desired C2H4 products, such as tuning the surface morphology [13–15], crystal facets [16], grain boundaries [17] and electrolyte composition [18]. Particularly, copper oxide has been reported with enhanced CO2 reduction activity and high selectivity towards
C2H4 at lower overpotential [19–22]. A number of works have suggested that the residual surface oxide and/or subsurface oxygen enhance the binding of C1 intermediates required for the CeC coupling [10,11,23]. However, the effect of copper atom arrangement on the copper oxide surface is rarely involved, and need to be well designed and investigated. In addition, to develop more efficient electrocatalysts for C2H4 production, it’s crucial to adjust the surface structure to facilitate the desired products and suppress the side reaction. Introducing metal atomic vacancies in metal oxide is an effective method to tailor the electronic structure of neighboring atoms and consequently the electrochemical performance [24–26]. For example, Fe vacancies in FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reaction [27]. Similarly, metal vacancies can promote CO2 activation and lower the energy barriers of rate-limiting step during the electrochemical CO2 reduction [28]. Cuprous oxide is of highly interest but the influence of Cu vacancies on the product distribution has not been explored yet. In this work, we develop Cu vacancy-enriched Cu2O (Cuv-Cu2O) catalyst with efforts to disclose atomic-level insights between Cu vacancies and electrochemical CO2 reduction selectivity to C2H4. Stepped Cu2O electrocatalyst with copper vacancies was prepared using a two-
⁎ Corresponding author at: Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, China. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.jcou.2020.01.018 Received 11 November 2019; Received in revised form 1 January 2020; Accepted 17 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.
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METRON Inc.). Gas-phase products were online flushed into gas chromatograph (GC-2010, Shimadzu), which equipped with a barrier discharge ionization detector (BID) and a Micropacked ST column. High purity He (99.9999 %) was used as a carrier gas and the GC was calibrated using gas mixtures. Liquid-phase products were analyzed by high performance liquid chromatography (HPLC, Agilent) equipped with refractive index detector (RID) and a Carbomix H-NP column. The partial current density and faradaic efficiency of each gaseous product were calculated according to the formula as follow
step electrodeposition and in situ transformation strategy. Cu vacancies on Cu2O surface enhance CO2 electrochemical reduction to C2H4 by modifying the electronic structure and further the interaction with key intermediates. Density functional theory (DFT) calculations reveal that introducing Cu vacancies on Cu2O surface ensures the strong *COH adsorption but weak *CO and *CH2 affinities, which contributes to the high C2H4 selectivity. As a result, the Cuv-Cu2O catalyst exhibits high C2H4 partial current density of 15.7 mA cm−2 and production rate of 48.7 μmol h−1 cm−2 at −0.76 V vs RHE, which is one of the best electrocatalysts evaluated in H-type cells. Moreover, the stable Cu+ species during the CO2 electrochemical reduction is another significant factor for high C2H4 selectivity.
ipartial = vi × G ×
ipartial
nFp0 RT0
2. Experimental section
FEi =
2.1. Catalyst preparation
Where vi is the volumetric concentration of gaseous product, G is the gas flow rate, n is the number of transferred electrons for certain product, p0 is the pressure, T0 is the temperature, itotal itotal is the current density measured by the potentiostat, F is the Faradaic constant and R is the ideal gas constant. The partial current density of each liquid product was calculated according to the formula as follow
The Cuv-Cu2O catalyst was prepared on Cu foils (99.99 %, Tianjin Aida Corp.) by an electrodeposition method on an Autolab PGSTAT302 N electrochemical workstation. Before deposition, the Cu foils were electropolished in 85 % phosphoric acid at 259.7 mA cm−2 for 1 min and then thoroughly rinsed with ultrapure water (18.2 MΩ, Merck Millipore Inc.). The electrodeposition solution was composed of 0.1 M Cu(NO3)2 and 0.1 M KCl, with 0.5 M NH4Cl as an additive to control the stepped morphology. A conventional three-electrode system was used, with a saturated calomel electrode (SCE) and a platinum mesh as reference and counter electrode, respectively. The deposition process was carried out by chronoamperometry method at −0.4 V vs SCE for 30 min at 70 ℃. The electrode was removed from the electrolyte and generously rinsed with ultrapure water before the electrochemical measurements. For bulk Cu2O catalyst, the procedure was the same without adding NH4Cl. For r-Cuv-Cu2O catalyst, the Cuv-Cu2O catalyst was annealed for 30 min at 200 ℃ under a H2/Ar atmosphere.
itotal
ipartial =
× 100%
n × F × ci × V t
Where ci is the concentration of liquid product, V is the volume of electrolyte and t is the electrolysis time. The potential was compensated for iR drop using system resistance determined by electrochemical impedance spectroscopy (EIS). EIS measurements were performed in the frequency range of 1–104 Hz at −0.76 V vs RHE with an amplitude of 5 mV. The electrochemical surface areas (ECSA) were evaluated using the double-layer capacitance (Cdl) by performing cyclic voltammetry (CV) in a potential range from 0 V to 0.25 V vs RHE at different scan rates (40, 60, 80, 100 and 120 mV s−1).
2.2. Materials characterization The surface morphologies were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100F). The crystal structure was investigated by X-ray diffraction (XRD, Rigaku D/MAX-2500) at 40 kV and 140 mA equipped with Cu Kɑ radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a PHI-1600 equipped with Al Kɑ radiation, and the results were calibrated by the contamination carbon peak (284.8 eV). Positron annihilation lifetime spectra (PALS) measurements were conducted by a conventional fastlow coincidence spectroscope. The time resolution was 195 ps at room temperature, and 22Na was used as the positron source.
2.4. Computational details The computational simulations were carried out by the Vienna Ab initio simulation package (VASP). The projector augmented wave (PAW) potentials was employed to demonstrate the interaction between atomic cores and valence electrons. The generalized gradient approximation (GGA) function of the Perdew-Burke-Ernzerhof (PBE) form was used for the exchange-correlation potential. Five layered Cu2O (111) slab with a (4 × 4) unit cell was employed to model the Cu2O catalyst. The catalyst with Cu vacancy was built based on the above Cu2O (111) slab. The bottom three layers were fixed, while the other layers were fully relaxed during the DFT computation. A 15 Å vacuum space in the z-direction was set to avoid the periodic influence. The cutoff energy was 400 eV and the Brillouin zone was sampled by a 3 × 3 × 1 k-points grid. The force and energy convergence criterion were 0.02 eV Å−1 and 10−5 eV, respectively. The adsorption energy (Ead) was expressed as
2.3. Electrochemical measurements and product analysis Electrochemical measurements were performed in a two-compartment H-type cell separated by a proton exchange membrane (Nafion 117, Dupont). Cu electrode and Ag/AgCl reference electrode were placed in the cathodic compartment, and platinum foil as counter electrode was in the anodic compartment. Both compartments contained 40 ml of 0.1 M KHCO3 electrolyte saturated with CO2 for at least 30 min. The potentials were converted to reversible hydrogen electrode (RHE) using
ΔEads = Eads / surf − Eads − Esurf Where Eads/surf, Eads and Esurf are the energies of the adsorbed systems, the adsorption intermediates and the clean surface, respectively. 3. Results and discussion
ERHE = EAg / AgCl + 0.197 + 0.0591 × pH
As shown in Fig. 1a, Cuv-Cu2O catalyst was successfully synthesized by electrodepositing a CuCl coating on an electropolished Cu foil substrate in the presence of ammonium salt, followed by immersing the film into a KHCO3 solution. In the first precipitation step, the CuCl coating was formed on the Cu foil under cathodic treatment, accompanied by a selective etching process to generate abundant Cu vacancies. Specifically, NH4Cl as an additive can react with OH− ions to
Linear sweep voltammetry (LSV) curves were recorded from 0.1 to −1.1 V vs RHE at a scan rate of 5 mV s−1 under CO2 atmosphere. The electrochemical reduction of carbon dioxide was performed by chronoamperometry technique for 2 h at each fixed potential under vigorously stirring, with CO2 continuously pumped into the electrolyte at a rate of 40 sccm regulated by a mass flow controller (MFC, HORIBA 126
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Fig. 1. Catalyst design and structural characterization. a) Schematic illustration of Cuv-Cu2O catalyst preparation. b) SEM image, c) HRTEM image and d) SAED pattern of Cuv-Cu2O catalyst. e) XRD pattern, f) Cu 2p XPS spectra and g) Cu LMM Auger spectra of Cuv-Cu2O and Cuv-CuCl catalyst.
918.0 eV is absent, ruling out the formation of metallic Cu state [36,37]. These results are in agreement with XRD data, both confirming the formation of pure Cu2O phase in Cuv-Cu2O catalyst. To investigate the dimension and extent of Cu defect vacancies, a series of characterizations were further carried out. Positron annihilation lifetime spectra (PALS) measurement can be employed to query the free volume properties with a fast-slow coincidence system [38,39]. As shown in Table 1, the Cuv-Cu2O catalyst and the bulk Cu2O sample both exhibit three distinct lifetime values (τ1, τ2 and τ3). The shortest lifetime (τ1) is ascribed to the bulk, while the longest one (τ3) is related to the positron annihilation of orthopositronium atoms formed in the large pores [40,41]. The lifetime τ2 is assigned to the annihilation of Cu vacancies. As for Cuv-Cu2O catalyst, the relative concentration is ca. 0.79 (I2/I1), remarkable larger than that of the bulk Cu2O (ca. 0.46). This indicates that the Cuv-Cu2O catalyst has a higher concentration of Cu vacancies. In addition, defective sites on copper-based catalysts, which is absent on perfect copper surfaces, can be identified by their unique voltammetric feature through simple cyclic voltammetry (CV) measurement. As shown in Fig. 2a, the Cuv-Cu2O catalyst and the bulk Cu2O both present two reduction peaks at around -0.20 and −0.5 V vs Ag/AgCl, which can be assigned to the valence changes of Cu2+ to Cu+ and Cu+ to Cu°, respectively. The reduction peaks at the potential region from −0.86 to −1.08 V vs Ag/AgCl are signatures of Cu defective sites. [42–44] Considering the similar synthesis process and size of the
form ammonia molecules, which haul the surface Cu atom out of CuCl coating generating a soluble copper(Ⅱ)-amine complex based on strong Lewis acid-base interactions. [29–31] Thereby the CuCl coating containing numerous Cu vacancies is constructed, which comes with the surface consisting of rich nano-scale steps (Fig. S1). As a control, the sample prepared without adding NH4Cl presents cubic morphology with even surface (Fig. S2a and b). Moreover, the stepped surface can be tuned by varying the concentration of NH4Cl etching agent as shown in Fig. S2. The obtained XRD patterns well match with CuCl (JCPDS # 01-0793) and metallic Cu (JCPDS # 01-1242) due to the Cu foil substrate (Fig. 1e). Subsequently, the CuCl coating was in situ transformed into active Cu2O catalyst during the CO2 electroreduction in KHCO3 electrolyte, and the obtained Cu2O sample inherits the Cu vacancies as well as the rich stepped surfaces (Fig. 1b). The HRTEM image clearly shows the presence of a great number of atomic steps (Fig. 1c), which are considered as highly active sites. Moreover, the marked lattice fringe spacings correspond to that of Cu2O, and the SAED pattern verifies the polycrystalline nature of the coating (Fig. 1d). As shown in Fig. 1e, the XRD analysis further confirms the high phase purity of Cu2O with peaks indexed to the cubic Cu2O (JCPDS # 03-0892). To explore the chemical state and electronic configuration of the Cu surface during the synthesis process, X-ray photoelectron spectroscopy (XPS) study was performed. Cu 2p XPS spectra clearly show that the surface of both CuCl and Cu2O coating mainly consist of Cu+ or Cu0 species without the presence of Cu2+ (Fig. 1f). As the binding energies of Cu+ and Cu0 states in Cu 2p spectra are too similar to be distinguished, Cu state identification was further conducted by Cu LMM Auger spectra [32–35]. As shown in Fig. 1g, the as-prepared CuCl coating presents in CuCl state, which slightly increases to the higher binding energy associated with the Cu2O state after being in situ transformation in KHCO3 electrolyte. The characteristic peak of Cu0 at
Table 1 Lifetime components of Cuv-Cu2O and Cu2O catalyst.
127
Sample
τ1 (ps)
I1 (%)
τ2 (ps)
I2 (%)
τ3 (ps)
I3 (%)
Cuv-Cu2O Cu2O
119.6 123.6
55.4 68.1
193.1 215.9
43.8 31.2
2131 1892
0.7 0.7
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Fig. 2. Characterization of Cu vacancies. a) Cyclic voltammograms in Ar-saturated 0.1 M KHCO3, b) Cu 2p XPS spectra, c) Nyquist plots and d,e) calculated density of states of Cuv-Cu2O and Cu2O catalyst.
FE of 51.0 % is achieved at -0.76 V vs RHE. The trend of CO selectivity is diametrically opposed to that of C2H4, which suggests that CeC coupling is favored at large overpotentials through further conversion of CO absorbed on the surface. It is noted that CH4 production can be negligible at a large range of potentials. In comparison, the highest C2H4 FE of just 32.8 % over Cu2O catalyst is obtained at a more negative potential of −0.78 V vs RHE (Fig. 3b). In a wide potential range, H2 maintains the major product with the FE higher than 42 %. The total current density increases as the applied potential is more negative, and there is no obvious difference between Cuv-Cu2O and Cu2O catalyst (Fig. S4). Besides, the partial current density and production rate of C2H4 are shown in Figs. 3d and S5. At −0.76 V vs RHE for Cuv-Cu2O catalyst, the C2H4 partial current density reaches 15.7 mA cm−2 and the production rate is 48.7 μmol h−1 cm−2, both larger than those for bulk Cu2O catalyst. These results suggest that the introduction of Cu vacancies primarily promotes the selectivity of C2H4 and reduces the C2H4 overpotential during CO2 reduction reaction, accompanied with the suppression of side hydrogen evolution reaction. Fig. 3e shows the electrochemical performance comparison of the Cuv-Cu2O catalyst with other Cu-based electrocatalysts reported recently. Among the catalysts evaluated in the similar H-cell configuration, the Cuv-Cu2O catalyst exhibits one of the highest C2H4 partial current density and C2H4 FE at a lower overpotential [14,17,23,30,33,42,45,46]. Besides, the CH4 formation can be negligible in the applied potential range. To illustrate the role of Cu vacancies, electrochemical active area (ECSA) was measured to provide deep insight into the number of active sites (Figs. 3f and S6). Cuv-Cu2O catalyst shows a larger ECSA, which contributes to the increased current density. In order to further investigate the effect of Cu vacancies on the intrinsic activity, the C2H4 partial current density was normalized by ECSA. As shown in Fig. S7, Cuv-Cu2O catalyst still exhibits a better performance, indicating its higher intrinsic activity towards C2H4. To explore the origin of enhanced C2H4 selectivity by introducing Cu vacancies, the adsorption energies of key intermediates (*CO, *COH and *CH2) during C2H4 formation were calculated (Fig. 4). The mainly facet (111) of prepared Cu2O catalyst is selected, and the stable surface without and with Cu vacancy are displayed in Fig. 4a and b, respectively. *CO has been confirmed as an important intermediate in CO2 electroreduction and a highly selective catalyst towards C2H4 is
Cuv-Cu2O catalyst and the bulk Cu2O, it is reasonable to conclude that the intensity difference of defective peaks is generated from Cu vacancies. In particular, a relatively intense peak signal is observed for Cuv-Cu2O catalyst compared with the bulk Cu2O, which indicates more abundant Cu vacancies existing on the Cuv-Cu2O catalyst surface. Meanwhile, XPS was carried out to characterize the variation of electronic structure, which results from the existence of Cu vacancies (Fig. 2b). The peaks attributed to the Cu 2p1/2 and Cu 2p3/2 shift to higher binding energies for Cuv-Cu2O catalyst, revealing the different chemical environment of Cu+ due to the deficiency of Cu on the surface. Density of states (DOS) for Cu2O with Cu vacancy was calculated and shown in Fig. 2d and e. The Cu vacancy introduces new DOS in the band gap, which leads to the smaller band gap and facilitates the electrons to be easily excited to the conductive band. The easier electron transition on Cu2O with Cu vacancies makes it more conductive, resulting in much lower overpotential. The variation of electronic structure was further confirmed by electrochemical impedance spectroscopy (EIS) as shown in Fig. 2c. The Cuv-Cu2O catalyst demonstrates smaller charge transfer resistance, which originates from the unique defective structure. Combined with the results of PALS, CV, XPS, DOS and EIS, the existence of Cu vacancies are confirmed which further adjust the local electronic properties as well as electrocatalytic performance of the bulk Cu2O. To reveal the advantage of Cu vacancies in the electrochemical CO2 reduction reaction, the performance of Cuv-Cu2O and bulk Cu2O was evaluated in CO2 saturated 0.1 M KHCO3 electrolyte using a conventional H-cell configuration separated by a Nafion 117 proton exchange membrane. As shown in Fig. 3a, the linear sweep voltammetry (LSV) curves of Cuv-Cu2O and bulk Cu2O catalyst present similar trend in the CO2-saturated electrolyte. However, Cuv-Cu2O catalyst exhibits a larger onset potential and a lower current density than that of bulk Cu2O catalyst, ascribed to the suppression of competing side reaction HER. The product distribution was further quantified by online gas chromatography (GC) and high-performance liquid chromatography (HPLC). Figs. 3b and S3 show the Faradaic efficiency (FE) of different products versus applied potential over Cuv-Cu2O catalyst. At low potentials, H2, CO and HCOOH are the major products, and a small amount of C2H4 is also generated. As the potentials become more negative, the formation of C2H4 increases significantly and the maximum 128
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Fig. 3. Electrochemical CO2 reduction performance. a) LSV curves in CO2-saturated 0.1 M KHCO3 and b,c) Faradaic efficiency of different products at various potentials of Cuv-Cu2O and Cu2O catalyst. d) C2H4 current density and production rate of Cuv-Cu2O catalyst. e) Overview of some reported electrocatalysts with excellent C2H4 production performance from CO2 reduction in H-cell. f) Current density plots at various scan rates of Cuv-Cu2O and Cu2O catalyst.
reduction steps towards *CH2 species [48]. *COH has much larger adsorption energy on Cuv-Cu2O (111) surface than that on Cu2O (111) surface, indicating that *COH is more stable on Cuv-Cu2O and prefers further conversion into *CH2. While for the *CH2 intermediate, the adsorption on Cuv-Cu2O surface is relatively weaker so that *CH2 species can desorb from the surface and interact with each other to achieve CeC coupling. Furthermore, the reaction energy of two exothermic process during C2H4 generation, including hydrogenation of *CO and CeC coupling, are both lower for Cuv-Cu2O surface than that for bulk Cu2O surface (Fig. S8). Therefore, it can be concluded that the Cu vacancy contributes to adjust the adsorption of key intermediates (*CO, *COH and *CH2) on the Cu2O surface and decreases the required reaction energy, and hence significantly enhances the C2H4 selectivity and inhibits the side reaction. The stability is another important criterion to assess the electrocatalyst toward CO2 reduction. Therefore, long-term CO2 electroreduction experiment on Cuv-Cu2O catalyst was performed at −0.76 V vs RHE, and the corresponding current density and C2H4 FE are shown in Fig. 5a. The current density shows no obvious attenuation and the C2H4 FE kept stable during the continuous electrolysis of 10 h. The
required to immobilize the intermediate appropriately. When the binding energy is too small, *CO will be insufficient for the subsequent reaction to form hydrocarbon. Conversely when the binding energy is too large, the active sites will be poisoned and tend to release more hydrogen. The volcano plot of reaction energy for the CeC coupling as a function of the adsorption energy indicates that the optimized binding energy of *CO at ca. 0.85–1.05 eV is favorable for CeC coupling and thus supports the formation of C2 products. [47] The calculated adsorption energy of *CO on the perfect Cu2O (111) surface is −1.95 eV, which is too large for the subsequent steps. The active sites would be strongly occupied by *CO intermediate, resulting in the CO2 reduction reaction suppressed and side hydrogen evolution reaction increased. This is in well agreement with the experiment results. A more positive adsorption energy of *CO is observed for Cuv-Cu2O (111) surface, suggesting that *CO prefers to interact with protons and electrons to proceed the next reaction. It is reported that the hydrogenation of *CO to hydroxymethylidyne (*COH) or formyl (*CHO) is a key step leading to different products. Under the applied potential range in the above activity test (< -0.8 V vs RHE), C2H4 is produced via the *COH pathway followed by a series of
Fig. 4. The adsorption energy of c,d) three key intermediates (*CO, *COH and *CH2) on the a) Cu2O and b) Cuv-Cu2O catalyst surfaces. 129
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Fig. 5. a) Stability of Cuv-Cu2O catalyst at −0.76 V vs RHE. b) Cu 2p XPS spectra and c) Cu LMM Auger spectra of Cuv-Cu2O catalyst after stability test. d) Faradaic efficiency of different products at various potentials of r-Cuv-Cu2O catalyst.
morphology is also remained (Fig. S9). The density of Cu vacancies exhibits negligible change after CO2 reduction, verifying the stability of Cu vacancies (Table S1). Moreover, the valence state of Cu species after durability test was studied by XPS and the sample was stored and transferred in vacuum-sealed package prior to the measurement to minimize the change of the surface. As shown in Fig. 5b and c, there are no Cu2+ peaks observed in Cu 2p XPS spectra and no Cu° peaks existing in Cu LMM Auger spectra. The formation of Cu+ state during the sample transfer is ruled out by the control experiment of storing the used catalyst to the air ambient (Fig. S10). These results demonstrate that the Cu+ species on the surface of Cuv-Cu2O catalyst are highly stable during the continuous test. The Cuv-Cu2O catalyst was then treated in reducing atmosphere to confirm the important role of Cu+ on the C2H4 selectivity. Partially reduced Cuv-Cu2O catalyst (r-Cuv-Cu2O) exhibits the same morphology with the Cuv-Cu2O catalyst (Fig. S11) but an apparent Cu° feature appears in the Cu LMM spectra (Fig. S12). However, the r-Cuv-Cu2O catalyst shows a high onset potential and low FE for C2H4 with increased FE for H2 and CH4, which indicates that the Cu+ state is another significant factor for the enhanced C2H4 selectivity apart from the Cu vacancies.
CRediT authorship contribution statement
4. Conclusion
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2020.01.018.
Xiaona Ren: Investigation, Methodology, Formal analysis, Data curation, Writing - original draft. Xiangwen Zhang: Supervision, Conceptualization. Xingzhong Cao: Formal analysis. Qingfa Wang: Supervision, Conceptualization, Data curation, Methodology, Writing review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 21476169). Appendix A. Supplementary data
In summary, we have developed a stepped Cu2O catalyst with abundant Cu vacancies, which exhibits superior electrochemical CO2 reduction selectivity towards C2H4. The enhanced performance can be attributed to the modulated electronic structure by Cu vacancies and the stable Cu+ state. The Cu vacancies facilitate the adsorption of *COH and weaken the binding of *CO and *CH2 intermediates, which consequently lowers the free energy of *CO hydrogenation and CeC coupling steps. The Cu vacancies and Cu+ species remain stable during long-term CO2 electroreduction. This work proposes that the vacancy engineering can be a promising tactic to promote the activity and selectivity for the hydrocarbon production from CO2 reduction, providing new insights on designing efficient electrocatalysts for carbon cycle.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Efficient electrochemical reduction of carbon dioxide into ethylene boosted by copper vacancies on stepped cuprous oxide
T
Xiaona Rena, Xiangwen Zhanga,b, Xingzhong Caoc, Qingfa Wanga,b,* a
Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin, 300350, China c Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide electroreduction Copper vacancies Ethylene Cuprous oxide Overpotential
Electrochemical carbon dioxide (CO2) reduction powered by renewable energy source is a promising strategy for the sustainable carbon cycle, while the selectivity toward C2+ products is still a great challenge. Herein, a novel stepped cuprous oxide catalyst with abundant Cu vacancies (Cuv-Cu2O catalyst) is developed to enhance the selectivity and efficiency of CO2 conversion toward C2 products. This catalyst exhibits high C2H4 partial current density and production rate at a low overpotential benefiting from the stepped surface and the modified electronic structure by Cu vacancies. Furthermore, density functional theory calculations demonstrate that the Cu vacancy-enriched Cu2O surface ensures strong adsorption to *COH but weak affinities to *CO and *CH2 intermediates, which promotes CO2 electroreduction to C2H4 products. The enhanced selectivity of C2H4, accompanied with the Cu vacancies and Cu+ species, can remain stable during the long-term test. This study will provide guidance on designing efficient electrocatalysts for CO2 reduction towards hydrocarbon products.
1. Introduction Motivated by the increasing concerns about global warming and finite fossil fuel resources, developing clean and renewable energy has been a great challenge. Electrochemical CO2 reduction into hydrocarbon fuels is regarded as a promising strategy to achieve a zero-CO2 emission, which powered by sustainable and cheap energy source such as wind, solar and nuclear [1–3]. Among various CO2 electrochemical products (CO, HCOOH, CH4, C2H4, etc.), C2H4 is highly desired due to its widespread application in agriculture and polymer manufacturing [4–7]. Copper based materials are the most prone candidate to electrochemically reduce CO2 into valuable C2+ products. However, the low selectivity, high overpotential and vulnerability to the electrolysis environment hinder their being efficient electrocatalysts [8–12]. Therefore, understanding the mechanisms underlying the product selectivity becomes more important to further module the copper catalysts. Many avenues have been employed to promote the electrochemical activity for desired C2H4 products, such as tuning the surface morphology [13–15], crystal facets [16], grain boundaries [17] and electrolyte composition [18]. Particularly, copper oxide has been reported with enhanced CO2 reduction activity and high selectivity towards
C2H4 at lower overpotential [19–22]. A number of works have suggested that the residual surface oxide and/or subsurface oxygen enhance the binding of C1 intermediates required for the CeC coupling [10,11,23]. However, the effect of copper atom arrangement on the copper oxide surface is rarely involved, and need to be well designed and investigated. In addition, to develop more efficient electrocatalysts for C2H4 production, it’s crucial to adjust the surface structure to facilitate the desired products and suppress the side reaction. Introducing metal atomic vacancies in metal oxide is an effective method to tailor the electronic structure of neighboring atoms and consequently the electrochemical performance [24–26]. For example, Fe vacancies in FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reaction [27]. Similarly, metal vacancies can promote CO2 activation and lower the energy barriers of rate-limiting step during the electrochemical CO2 reduction [28]. Cuprous oxide is of highly interest but the influence of Cu vacancies on the product distribution has not been explored yet. In this work, we develop Cu vacancy-enriched Cu2O (Cuv-Cu2O) catalyst with efforts to disclose atomic-level insights between Cu vacancies and electrochemical CO2 reduction selectivity to C2H4. Stepped Cu2O electrocatalyst with copper vacancies was prepared using a two-
⁎ Corresponding author at: Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, China. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.jcou.2020.01.018 Received 11 November 2019; Received in revised form 1 January 2020; Accepted 17 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.
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METRON Inc.). Gas-phase products were online flushed into gas chromatograph (GC-2010, Shimadzu), which equipped with a barrier discharge ionization detector (BID) and a Micropacked ST column. High purity He (99.9999 %) was used as a carrier gas and the GC was calibrated using gas mixtures. Liquid-phase products were analyzed by high performance liquid chromatography (HPLC, Agilent) equipped with refractive index detector (RID) and a Carbomix H-NP column. The partial current density and faradaic efficiency of each gaseous product were calculated according to the formula as follow
step electrodeposition and in situ transformation strategy. Cu vacancies on Cu2O surface enhance CO2 electrochemical reduction to C2H4 by modifying the electronic structure and further the interaction with key intermediates. Density functional theory (DFT) calculations reveal that introducing Cu vacancies on Cu2O surface ensures the strong *COH adsorption but weak *CO and *CH2 affinities, which contributes to the high C2H4 selectivity. As a result, the Cuv-Cu2O catalyst exhibits high C2H4 partial current density of 15.7 mA cm−2 and production rate of 48.7 μmol h−1 cm−2 at −0.76 V vs RHE, which is one of the best electrocatalysts evaluated in H-type cells. Moreover, the stable Cu+ species during the CO2 electrochemical reduction is another significant factor for high C2H4 selectivity.
ipartial = vi × G ×
ipartial
nFp0 RT0
2. Experimental section
FEi =
2.1. Catalyst preparation
Where vi is the volumetric concentration of gaseous product, G is the gas flow rate, n is the number of transferred electrons for certain product, p0 is the pressure, T0 is the temperature, itotal itotal is the current density measured by the potentiostat, F is the Faradaic constant and R is the ideal gas constant. The partial current density of each liquid product was calculated according to the formula as follow
The Cuv-Cu2O catalyst was prepared on Cu foils (99.99 %, Tianjin Aida Corp.) by an electrodeposition method on an Autolab PGSTAT302 N electrochemical workstation. Before deposition, the Cu foils were electropolished in 85 % phosphoric acid at 259.7 mA cm−2 for 1 min and then thoroughly rinsed with ultrapure water (18.2 MΩ, Merck Millipore Inc.). The electrodeposition solution was composed of 0.1 M Cu(NO3)2 and 0.1 M KCl, with 0.5 M NH4Cl as an additive to control the stepped morphology. A conventional three-electrode system was used, with a saturated calomel electrode (SCE) and a platinum mesh as reference and counter electrode, respectively. The deposition process was carried out by chronoamperometry method at −0.4 V vs SCE for 30 min at 70 ℃. The electrode was removed from the electrolyte and generously rinsed with ultrapure water before the electrochemical measurements. For bulk Cu2O catalyst, the procedure was the same without adding NH4Cl. For r-Cuv-Cu2O catalyst, the Cuv-Cu2O catalyst was annealed for 30 min at 200 ℃ under a H2/Ar atmosphere.
itotal
ipartial =
× 100%
n × F × ci × V t
Where ci is the concentration of liquid product, V is the volume of electrolyte and t is the electrolysis time. The potential was compensated for iR drop using system resistance determined by electrochemical impedance spectroscopy (EIS). EIS measurements were performed in the frequency range of 1–104 Hz at −0.76 V vs RHE with an amplitude of 5 mV. The electrochemical surface areas (ECSA) were evaluated using the double-layer capacitance (Cdl) by performing cyclic voltammetry (CV) in a potential range from 0 V to 0.25 V vs RHE at different scan rates (40, 60, 80, 100 and 120 mV s−1).
2.2. Materials characterization The surface morphologies were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100F). The crystal structure was investigated by X-ray diffraction (XRD, Rigaku D/MAX-2500) at 40 kV and 140 mA equipped with Cu Kɑ radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a PHI-1600 equipped with Al Kɑ radiation, and the results were calibrated by the contamination carbon peak (284.8 eV). Positron annihilation lifetime spectra (PALS) measurements were conducted by a conventional fastlow coincidence spectroscope. The time resolution was 195 ps at room temperature, and 22Na was used as the positron source.
2.4. Computational details The computational simulations were carried out by the Vienna Ab initio simulation package (VASP). The projector augmented wave (PAW) potentials was employed to demonstrate the interaction between atomic cores and valence electrons. The generalized gradient approximation (GGA) function of the Perdew-Burke-Ernzerhof (PBE) form was used for the exchange-correlation potential. Five layered Cu2O (111) slab with a (4 × 4) unit cell was employed to model the Cu2O catalyst. The catalyst with Cu vacancy was built based on the above Cu2O (111) slab. The bottom three layers were fixed, while the other layers were fully relaxed during the DFT computation. A 15 Å vacuum space in the z-direction was set to avoid the periodic influence. The cutoff energy was 400 eV and the Brillouin zone was sampled by a 3 × 3 × 1 k-points grid. The force and energy convergence criterion were 0.02 eV Å−1 and 10−5 eV, respectively. The adsorption energy (Ead) was expressed as
2.3. Electrochemical measurements and product analysis Electrochemical measurements were performed in a two-compartment H-type cell separated by a proton exchange membrane (Nafion 117, Dupont). Cu electrode and Ag/AgCl reference electrode were placed in the cathodic compartment, and platinum foil as counter electrode was in the anodic compartment. Both compartments contained 40 ml of 0.1 M KHCO3 electrolyte saturated with CO2 for at least 30 min. The potentials were converted to reversible hydrogen electrode (RHE) using
ΔEads = Eads / surf − Eads − Esurf Where Eads/surf, Eads and Esurf are the energies of the adsorbed systems, the adsorption intermediates and the clean surface, respectively. 3. Results and discussion
ERHE = EAg / AgCl + 0.197 + 0.0591 × pH
As shown in Fig. 1a, Cuv-Cu2O catalyst was successfully synthesized by electrodepositing a CuCl coating on an electropolished Cu foil substrate in the presence of ammonium salt, followed by immersing the film into a KHCO3 solution. In the first precipitation step, the CuCl coating was formed on the Cu foil under cathodic treatment, accompanied by a selective etching process to generate abundant Cu vacancies. Specifically, NH4Cl as an additive can react with OH− ions to
Linear sweep voltammetry (LSV) curves were recorded from 0.1 to −1.1 V vs RHE at a scan rate of 5 mV s−1 under CO2 atmosphere. The electrochemical reduction of carbon dioxide was performed by chronoamperometry technique for 2 h at each fixed potential under vigorously stirring, with CO2 continuously pumped into the electrolyte at a rate of 40 sccm regulated by a mass flow controller (MFC, HORIBA 126
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Fig. 1. Catalyst design and structural characterization. a) Schematic illustration of Cuv-Cu2O catalyst preparation. b) SEM image, c) HRTEM image and d) SAED pattern of Cuv-Cu2O catalyst. e) XRD pattern, f) Cu 2p XPS spectra and g) Cu LMM Auger spectra of Cuv-Cu2O and Cuv-CuCl catalyst.
918.0 eV is absent, ruling out the formation of metallic Cu state [36,37]. These results are in agreement with XRD data, both confirming the formation of pure Cu2O phase in Cuv-Cu2O catalyst. To investigate the dimension and extent of Cu defect vacancies, a series of characterizations were further carried out. Positron annihilation lifetime spectra (PALS) measurement can be employed to query the free volume properties with a fast-slow coincidence system [38,39]. As shown in Table 1, the Cuv-Cu2O catalyst and the bulk Cu2O sample both exhibit three distinct lifetime values (τ1, τ2 and τ3). The shortest lifetime (τ1) is ascribed to the bulk, while the longest one (τ3) is related to the positron annihilation of orthopositronium atoms formed in the large pores [40,41]. The lifetime τ2 is assigned to the annihilation of Cu vacancies. As for Cuv-Cu2O catalyst, the relative concentration is ca. 0.79 (I2/I1), remarkable larger than that of the bulk Cu2O (ca. 0.46). This indicates that the Cuv-Cu2O catalyst has a higher concentration of Cu vacancies. In addition, defective sites on copper-based catalysts, which is absent on perfect copper surfaces, can be identified by their unique voltammetric feature through simple cyclic voltammetry (CV) measurement. As shown in Fig. 2a, the Cuv-Cu2O catalyst and the bulk Cu2O both present two reduction peaks at around -0.20 and −0.5 V vs Ag/AgCl, which can be assigned to the valence changes of Cu2+ to Cu+ and Cu+ to Cu°, respectively. The reduction peaks at the potential region from −0.86 to −1.08 V vs Ag/AgCl are signatures of Cu defective sites. [42–44] Considering the similar synthesis process and size of the
form ammonia molecules, which haul the surface Cu atom out of CuCl coating generating a soluble copper(Ⅱ)-amine complex based on strong Lewis acid-base interactions. [29–31] Thereby the CuCl coating containing numerous Cu vacancies is constructed, which comes with the surface consisting of rich nano-scale steps (Fig. S1). As a control, the sample prepared without adding NH4Cl presents cubic morphology with even surface (Fig. S2a and b). Moreover, the stepped surface can be tuned by varying the concentration of NH4Cl etching agent as shown in Fig. S2. The obtained XRD patterns well match with CuCl (JCPDS # 01-0793) and metallic Cu (JCPDS # 01-1242) due to the Cu foil substrate (Fig. 1e). Subsequently, the CuCl coating was in situ transformed into active Cu2O catalyst during the CO2 electroreduction in KHCO3 electrolyte, and the obtained Cu2O sample inherits the Cu vacancies as well as the rich stepped surfaces (Fig. 1b). The HRTEM image clearly shows the presence of a great number of atomic steps (Fig. 1c), which are considered as highly active sites. Moreover, the marked lattice fringe spacings correspond to that of Cu2O, and the SAED pattern verifies the polycrystalline nature of the coating (Fig. 1d). As shown in Fig. 1e, the XRD analysis further confirms the high phase purity of Cu2O with peaks indexed to the cubic Cu2O (JCPDS # 03-0892). To explore the chemical state and electronic configuration of the Cu surface during the synthesis process, X-ray photoelectron spectroscopy (XPS) study was performed. Cu 2p XPS spectra clearly show that the surface of both CuCl and Cu2O coating mainly consist of Cu+ or Cu0 species without the presence of Cu2+ (Fig. 1f). As the binding energies of Cu+ and Cu0 states in Cu 2p spectra are too similar to be distinguished, Cu state identification was further conducted by Cu LMM Auger spectra [32–35]. As shown in Fig. 1g, the as-prepared CuCl coating presents in CuCl state, which slightly increases to the higher binding energy associated with the Cu2O state after being in situ transformation in KHCO3 electrolyte. The characteristic peak of Cu0 at
Table 1 Lifetime components of Cuv-Cu2O and Cu2O catalyst.
127
Sample
τ1 (ps)
I1 (%)
τ2 (ps)
I2 (%)
τ3 (ps)
I3 (%)
Cuv-Cu2O Cu2O
119.6 123.6
55.4 68.1
193.1 215.9
43.8 31.2
2131 1892
0.7 0.7
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Fig. 2. Characterization of Cu vacancies. a) Cyclic voltammograms in Ar-saturated 0.1 M KHCO3, b) Cu 2p XPS spectra, c) Nyquist plots and d,e) calculated density of states of Cuv-Cu2O and Cu2O catalyst.
FE of 51.0 % is achieved at -0.76 V vs RHE. The trend of CO selectivity is diametrically opposed to that of C2H4, which suggests that CeC coupling is favored at large overpotentials through further conversion of CO absorbed on the surface. It is noted that CH4 production can be negligible at a large range of potentials. In comparison, the highest C2H4 FE of just 32.8 % over Cu2O catalyst is obtained at a more negative potential of −0.78 V vs RHE (Fig. 3b). In a wide potential range, H2 maintains the major product with the FE higher than 42 %. The total current density increases as the applied potential is more negative, and there is no obvious difference between Cuv-Cu2O and Cu2O catalyst (Fig. S4). Besides, the partial current density and production rate of C2H4 are shown in Figs. 3d and S5. At −0.76 V vs RHE for Cuv-Cu2O catalyst, the C2H4 partial current density reaches 15.7 mA cm−2 and the production rate is 48.7 μmol h−1 cm−2, both larger than those for bulk Cu2O catalyst. These results suggest that the introduction of Cu vacancies primarily promotes the selectivity of C2H4 and reduces the C2H4 overpotential during CO2 reduction reaction, accompanied with the suppression of side hydrogen evolution reaction. Fig. 3e shows the electrochemical performance comparison of the Cuv-Cu2O catalyst with other Cu-based electrocatalysts reported recently. Among the catalysts evaluated in the similar H-cell configuration, the Cuv-Cu2O catalyst exhibits one of the highest C2H4 partial current density and C2H4 FE at a lower overpotential [14,17,23,30,33,42,45,46]. Besides, the CH4 formation can be negligible in the applied potential range. To illustrate the role of Cu vacancies, electrochemical active area (ECSA) was measured to provide deep insight into the number of active sites (Figs. 3f and S6). Cuv-Cu2O catalyst shows a larger ECSA, which contributes to the increased current density. In order to further investigate the effect of Cu vacancies on the intrinsic activity, the C2H4 partial current density was normalized by ECSA. As shown in Fig. S7, Cuv-Cu2O catalyst still exhibits a better performance, indicating its higher intrinsic activity towards C2H4. To explore the origin of enhanced C2H4 selectivity by introducing Cu vacancies, the adsorption energies of key intermediates (*CO, *COH and *CH2) during C2H4 formation were calculated (Fig. 4). The mainly facet (111) of prepared Cu2O catalyst is selected, and the stable surface without and with Cu vacancy are displayed in Fig. 4a and b, respectively. *CO has been confirmed as an important intermediate in CO2 electroreduction and a highly selective catalyst towards C2H4 is
Cuv-Cu2O catalyst and the bulk Cu2O, it is reasonable to conclude that the intensity difference of defective peaks is generated from Cu vacancies. In particular, a relatively intense peak signal is observed for Cuv-Cu2O catalyst compared with the bulk Cu2O, which indicates more abundant Cu vacancies existing on the Cuv-Cu2O catalyst surface. Meanwhile, XPS was carried out to characterize the variation of electronic structure, which results from the existence of Cu vacancies (Fig. 2b). The peaks attributed to the Cu 2p1/2 and Cu 2p3/2 shift to higher binding energies for Cuv-Cu2O catalyst, revealing the different chemical environment of Cu+ due to the deficiency of Cu on the surface. Density of states (DOS) for Cu2O with Cu vacancy was calculated and shown in Fig. 2d and e. The Cu vacancy introduces new DOS in the band gap, which leads to the smaller band gap and facilitates the electrons to be easily excited to the conductive band. The easier electron transition on Cu2O with Cu vacancies makes it more conductive, resulting in much lower overpotential. The variation of electronic structure was further confirmed by electrochemical impedance spectroscopy (EIS) as shown in Fig. 2c. The Cuv-Cu2O catalyst demonstrates smaller charge transfer resistance, which originates from the unique defective structure. Combined with the results of PALS, CV, XPS, DOS and EIS, the existence of Cu vacancies are confirmed which further adjust the local electronic properties as well as electrocatalytic performance of the bulk Cu2O. To reveal the advantage of Cu vacancies in the electrochemical CO2 reduction reaction, the performance of Cuv-Cu2O and bulk Cu2O was evaluated in CO2 saturated 0.1 M KHCO3 electrolyte using a conventional H-cell configuration separated by a Nafion 117 proton exchange membrane. As shown in Fig. 3a, the linear sweep voltammetry (LSV) curves of Cuv-Cu2O and bulk Cu2O catalyst present similar trend in the CO2-saturated electrolyte. However, Cuv-Cu2O catalyst exhibits a larger onset potential and a lower current density than that of bulk Cu2O catalyst, ascribed to the suppression of competing side reaction HER. The product distribution was further quantified by online gas chromatography (GC) and high-performance liquid chromatography (HPLC). Figs. 3b and S3 show the Faradaic efficiency (FE) of different products versus applied potential over Cuv-Cu2O catalyst. At low potentials, H2, CO and HCOOH are the major products, and a small amount of C2H4 is also generated. As the potentials become more negative, the formation of C2H4 increases significantly and the maximum 128
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Fig. 3. Electrochemical CO2 reduction performance. a) LSV curves in CO2-saturated 0.1 M KHCO3 and b,c) Faradaic efficiency of different products at various potentials of Cuv-Cu2O and Cu2O catalyst. d) C2H4 current density and production rate of Cuv-Cu2O catalyst. e) Overview of some reported electrocatalysts with excellent C2H4 production performance from CO2 reduction in H-cell. f) Current density plots at various scan rates of Cuv-Cu2O and Cu2O catalyst.
reduction steps towards *CH2 species [48]. *COH has much larger adsorption energy on Cuv-Cu2O (111) surface than that on Cu2O (111) surface, indicating that *COH is more stable on Cuv-Cu2O and prefers further conversion into *CH2. While for the *CH2 intermediate, the adsorption on Cuv-Cu2O surface is relatively weaker so that *CH2 species can desorb from the surface and interact with each other to achieve CeC coupling. Furthermore, the reaction energy of two exothermic process during C2H4 generation, including hydrogenation of *CO and CeC coupling, are both lower for Cuv-Cu2O surface than that for bulk Cu2O surface (Fig. S8). Therefore, it can be concluded that the Cu vacancy contributes to adjust the adsorption of key intermediates (*CO, *COH and *CH2) on the Cu2O surface and decreases the required reaction energy, and hence significantly enhances the C2H4 selectivity and inhibits the side reaction. The stability is another important criterion to assess the electrocatalyst toward CO2 reduction. Therefore, long-term CO2 electroreduction experiment on Cuv-Cu2O catalyst was performed at −0.76 V vs RHE, and the corresponding current density and C2H4 FE are shown in Fig. 5a. The current density shows no obvious attenuation and the C2H4 FE kept stable during the continuous electrolysis of 10 h. The
required to immobilize the intermediate appropriately. When the binding energy is too small, *CO will be insufficient for the subsequent reaction to form hydrocarbon. Conversely when the binding energy is too large, the active sites will be poisoned and tend to release more hydrogen. The volcano plot of reaction energy for the CeC coupling as a function of the adsorption energy indicates that the optimized binding energy of *CO at ca. 0.85–1.05 eV is favorable for CeC coupling and thus supports the formation of C2 products. [47] The calculated adsorption energy of *CO on the perfect Cu2O (111) surface is −1.95 eV, which is too large for the subsequent steps. The active sites would be strongly occupied by *CO intermediate, resulting in the CO2 reduction reaction suppressed and side hydrogen evolution reaction increased. This is in well agreement with the experiment results. A more positive adsorption energy of *CO is observed for Cuv-Cu2O (111) surface, suggesting that *CO prefers to interact with protons and electrons to proceed the next reaction. It is reported that the hydrogenation of *CO to hydroxymethylidyne (*COH) or formyl (*CHO) is a key step leading to different products. Under the applied potential range in the above activity test (< -0.8 V vs RHE), C2H4 is produced via the *COH pathway followed by a series of
Fig. 4. The adsorption energy of c,d) three key intermediates (*CO, *COH and *CH2) on the a) Cu2O and b) Cuv-Cu2O catalyst surfaces. 129
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Fig. 5. a) Stability of Cuv-Cu2O catalyst at −0.76 V vs RHE. b) Cu 2p XPS spectra and c) Cu LMM Auger spectra of Cuv-Cu2O catalyst after stability test. d) Faradaic efficiency of different products at various potentials of r-Cuv-Cu2O catalyst.
morphology is also remained (Fig. S9). The density of Cu vacancies exhibits negligible change after CO2 reduction, verifying the stability of Cu vacancies (Table S1). Moreover, the valence state of Cu species after durability test was studied by XPS and the sample was stored and transferred in vacuum-sealed package prior to the measurement to minimize the change of the surface. As shown in Fig. 5b and c, there are no Cu2+ peaks observed in Cu 2p XPS spectra and no Cu° peaks existing in Cu LMM Auger spectra. The formation of Cu+ state during the sample transfer is ruled out by the control experiment of storing the used catalyst to the air ambient (Fig. S10). These results demonstrate that the Cu+ species on the surface of Cuv-Cu2O catalyst are highly stable during the continuous test. The Cuv-Cu2O catalyst was then treated in reducing atmosphere to confirm the important role of Cu+ on the C2H4 selectivity. Partially reduced Cuv-Cu2O catalyst (r-Cuv-Cu2O) exhibits the same morphology with the Cuv-Cu2O catalyst (Fig. S11) but an apparent Cu° feature appears in the Cu LMM spectra (Fig. S12). However, the r-Cuv-Cu2O catalyst shows a high onset potential and low FE for C2H4 with increased FE for H2 and CH4, which indicates that the Cu+ state is another significant factor for the enhanced C2H4 selectivity apart from the Cu vacancies.
CRediT authorship contribution statement
4. Conclusion
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2020.01.018.
Xiaona Ren: Investigation, Methodology, Formal analysis, Data curation, Writing - original draft. Xiangwen Zhang: Supervision, Conceptualization. Xingzhong Cao: Formal analysis. Qingfa Wang: Supervision, Conceptualization, Data curation, Methodology, Writing review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 21476169). Appendix A. Supplementary data
In summary, we have developed a stepped Cu2O catalyst with abundant Cu vacancies, which exhibits superior electrochemical CO2 reduction selectivity towards C2H4. The enhanced performance can be attributed to the modulated electronic structure by Cu vacancies and the stable Cu+ state. The Cu vacancies facilitate the adsorption of *COH and weaken the binding of *CO and *CH2 intermediates, which consequently lowers the free energy of *CO hydrogenation and CeC coupling steps. The Cu vacancies and Cu+ species remain stable during long-term CO2 electroreduction. This work proposes that the vacancy engineering can be a promising tactic to promote the activity and selectivity for the hydrocarbon production from CO2 reduction, providing new insights on designing efficient electrocatalysts for carbon cycle.
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