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Solid Bi2O3-derived nanostructured metallic bismuth with high formate selectivity for the electrocatalytic reduction of CO2

Xiaoyan Wang, Safeer Jan, Zhiyong Wang, Xianbo Jin

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Cite this article as:

Xiaoyan Wang, Safeer Jan, Zhiyong Wang, and Xianbo Jin, Solid Bi2O3-derived nanostructured metallic bismuth with high formate selectivity for the electrocatalytic reduction of CO2, Int. J. Miner. Metall. Mater., 31(2024), No. 4, pp.803-811. https://dx.doi.org/10.1007/s12613-023-2770-y
Xiaoyan Wang, Safeer Jan, Zhiyong Wang, and Xianbo Jin, Solid Bi2O3-derived nanostructured metallic bismuth with high formate selectivity for the electrocatalytic reduction of CO2, Int. J. Miner. Metall. Mater., 31(2024), No. 4, pp.803-811. https://dx.doi.org/10.1007/s12613-023-2770-y
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研究论文

电解固态Bi2O3制备具有高CO2电还原活性及高甲酸选择性的纳米金属铋催化剂

    通信作者:

    金先波 E-mail: xbjin@whu.edu.cn

文章亮点

(1) 通过直接固态电化学还原氧化铋电极制备出纳米结构铋催化剂。 (2) 通过降低氧化铋的粒径,可以获得粒径尺寸更小的纳米铋金属催化剂。 (3) 所制备纳米铋对还原二氧化碳表现出高活性(电流密度40 mA cm-2)和高甲酸选择性(几乎100%)。
二氧化碳(CO2)电化学还原是碳中和研究的一个重要方向。然而,目前的CO2电还原催化剂在稳定性、产物选择性以及活性等方面均有待提升。铋金属因其低成本、低毒性以及高CO2电还原活性及高甲酸选择性而备受青睐。本文中,我们采用固态电解,直接电化学还原商业氧化铋的固态电极制备了纳米多孔铋电极(粒径约80 nm)。应用于CO2电催化还原研究时,该纳米多孔铋电极在−0.78 V(相对于可逆氢电极RHE)表现出高达97.6%的甲酸选择性。当电极电势为−1.10 V vs. RHE时,该电极上CO2还原电流高达40.0 mA⋅cm−2,甲酸选择性仍保持86.0%。采用纳米尺寸的氧化铋前驱体可进一步将金属铋催化剂的原生粒径降低至30~50 nm,此时可提升低过电位下CO2还原时甲酸的选择性。例如,在−0.63 V vs. RHE时,甲酸选择性由原来的68.0%增加到81.7%。本工作中铋催化剂表现出优异的CO2电催化活性与其由互相连通的铋纳米网构成纳米多孔结构密切相关,该独特结构提供了CO2分子的扩散路径以及丰富的反应活性位点。

 

Research Article

Solid Bi2O3-derived nanostructured metallic bismuth with high formate selectivity for the electrocatalytic reduction of CO2

Author Affilications
    Corresponding author:

    Xianbo Jin E-mail: xbjin@whu.edu.cn

  • Received: 21 July 2023; Revised: 25 October 2023; Accepted: 30 October 2023; Available online: 02 November 2023
CO2 electrochemical reduction (CO2ER) is an important research area for carbon neutralization. However, available catalysts for CO2 reduction are still characterized by limited stability and activity. Recently, metallic bismuth (Bi) has emerged as a promising catalyst for CO2ER. Herein, we report the solid cathode electroreduction of commercial micronized Bi2O3 as a straightforward approach for the preparation of nanostructured Bi. At −1.1 V versus reversible hydrogen electrode in a KHCO3 aqueous electrolyte, the resulting nanostructure Bi delivers a formate current density of ~40 mA·cm−2 with a current efficiency of ~86%, and the formate selectivity reaches 97.6% at −0.78 V. Using nanosized Bi2O3 as the precursor can further reduce the primary particle sizes of the resulting Bi, leading to a significantly increased formate selectivity at relatively low overpotentials. The high catalytic activity of nanostructured Bi is attributable to the ultrafine and interconnected Bi nanoparticles in the nanoporous structure, which exposes abundant active sites for CO2 electrocatalytic reduction.

 

  • The electroreduction of CO2 to fuels is desirable for constructing a carbon-cyclable energy system, particularly because renewable electricity is used for the electrolysis process [1]. However, CO2, typically inert at room temperatures, requires a large overpotential to form the activated-state CO*2, and the following reduction process suffers from slow kinetics [2]. Complicated proton-coupled multicharge transfer reactions and the competitive reduction of protons to hydrogen in an aqueous solution over most electrocatalysts will lead to various cathodic products [3]. Thus, developing electrocatalysts with high selectivity and high activity remains crucial.

    Among various CO2 electrochemical reduction (CO2ER) products, formic acid (HCOOH) is particularly appealing, as it can serve as a fuel for combustion or in fuel cells, a hydrogen storage carrier, and a valuable raw chemical for industrial synthesis [4]. Regarding commercial viability, a techno-economic analysis revealed that HCOOH or formate was the most profitable product of CO2ER [2]. A recent study reported that many metallic catalysts with high hydrogen evolution overpotential, including Cd, Pb, Hg, and Bi, showed high activity for CO2ER [5]. Among these metallic catalysts, Bi stands out in terms of formate selectivity, and it is characterized by a relatively high elemental abundance and environmental friendliness [68]. Although Bi catalysts have shown high Faradaic efficiency (FE) for formate generation, the geometric productive rate of the Bi electrodes is still lower than the metrics required for practical application. Therefore, improving Bi catalyst activity without sacrificing selectivity is crucial.

    Compared with bulk materials, nanostructured materials typically exhibit superior electrocatalytic performance because they facilitate the construction of highly porous electrodes with numerous active sites for CO2 molecule adsorption [911]. Additionally, nanostructure metals with an integrity conductive network can improve the utilization rate of catalytic materials [12]. For example, porous Bi nanosheets have demonstrated high activity and formate selectivity in CO2ER [6,13]. Beyond creating a nanoporous structure of the catalyst, researchers have tried to reduce the size of primary particles in the porous electrodes to increase the specific surface area of catalysts. A study on porous Ag demonstrated that as primary particle size decreased from 400 to 50 nm, the FE of CO increased from 75% to 82% under similar conditions [14]. Jia et al. [15] explored the influence of bismuth nanoparticle size on formic acid production, and the optimal catalyst yielded a formate FE higher than 90% within a potential window close to 400 mV. However, preparing nanostructure catalysts while maintaining the integrity of their conductive network is a rather complicated procedure.

    Herein, nanostructured Bi (nBi) was prepared through facile electrochemical reduction of solid bismuth oxide (Bi2O3). Through the adjustment of the Bi2O3 particle size, nano Bi catalysts with different surface morphology and particle sizes were obtained. When used as an electrode catalyst for CO2ER, Bi2O3-derived nBi yielded a formate FE approaching 98% at −0.78 V vs. reversible hydrogen electrode (RHE) and high stability over a 10 h electrolysis period. The straightforward preparation method and excellent electrocatalytic performance of nBi make the Bi2O3-derived nBi a desirable catalyst candidate for CO2ER.

    The nBi catalysts were synthesized through electrochemical reduction of a bismuth oxide electrode. To prepare the oxide electrode, bismuth oxide was rolled into a membrane (~0.1 mm in thickness, 5.1 mg⋅cm−2) using 10wt% polytetrafluoroethylene (PTFE) as binder and isopropanol as solvent. Then, a rectangular piece (5 mm × 7 mm) of the membrane was pressed onto a titanium (Ti) mesh as a working electrode (WE) for constant-current reduction (5 mA) for 30 min in a 1.0 M K2CO3 solution against a graphite counter electrode (CE). The Ti mesh was used as the substrate, as it would not cause apparent currents for CO2ER and hydrogen evolution [16]. Two types of nBi from bismuth oxide of different sources were prepared. One was derived from the as-received commercial bismuth oxide (purchased from Shanghai Reagent No. 2 Factory, China), with particles in the micrometer scale (denoted as mBO), and the prepared nBi was denoted as mBO-nBi. The other was derived from the self-made nanometer bismuth oxide (nBO) and was denoted as nBO-nBi. The nBO was synthesized via precipitation between bismuth nitrate pentahydrate and potassium hydroxide in aqueous solution. Specifically, 270 mg of Bi(NO3)3⋅5H2O was added to 1.0 mol⋅L−1 HNO3, and then KOH was added under constant magnetic stirring. The resulting white precipitate was collected after water washing at least six times, and then, the nBO was dried in a vacuum oven at 60°C for further use. All chemicals were purchased from Sinopharm Chemical Reagent Company (China) unless otherwise specified.

    The electrochemical measurements were controlled by a CS350H electrochemical workstation (Wuhan, China) using a customized gas-tight H-cell. The anode and cathode chambers were separated by a Nafion 117 membrane (DuPont, the United States of America). The reference electrode was an Ag/AgCl (saturated KCl solution) electrode, and the CE was a Pt foil. A 0.5 M KHCO3 electrolyte was utilized, and before CO2ER tests, CO2 gas was bubbled into the electrolyte for saturation. Potentials were calibrated to a RHE as E vs. RHE) = E vs. Ag/AgCl + 0.197 + 0.059 × pH.

    Any gaseous products from the CO2ER were collected and analyzed via gas chromatography (GC, Shandong Lunan Ruihong Chemical Instruments Co., Ltd., China). Liquid products were investigated via nuclear magnetic resonance (NMR, Bruker AVANCE III HD 400MHz, Germany) with calibration curves. Each quantitative analysis was performed three times under ambient pressure at 25°C, and an average result was reported.

    The FEs for H2, CO, and formate generation were calculated via the following equations:

    FECO(orH2)=a×(v60)×N×F×(pRT)i×100% (1)
    FEformate=QformateQtotal×100\%=N×F×nformatei×t (2)

    where v (15 mL⋅min−1) represents the flow rate of CO2; ɑ represents the concentration of CO (or H2) determined via GC; nformate (mol) represents the molar amount of formate; N represents the electron transfer number for the generation of 1 mol formate, CO, or H2; i (A) represents the reduction current during the CO2ER process; t (s) represents the reduction time; Qformate and Qtotal (C) represent the formate charge or total charge during the electrolysis, respectively; F represents the Faraday constant 96485 C⋅mol−1; p = 1.013 × 105 Pa; T = 298.15 K; R = 8.314 J⋅K−1⋅mol−1.

    X-ray diffraction (XRD, Cu Kα radiation) analysis was conducted using the Bruker D8-advanced instrument (Bruker Corporation, Billerica, United States) or Rigaku Miniflex600 (Rigaku Corporation, Tokyo, Japan). The sample morphologies were imaged via scanning electron microscopy (SEM, SIRION 200 with a field-emission gun, the United States of America) and transmission electron microscopy (TEM, JEOL, and JEM-2100, Japan).

    Fig. 1 illustrates the nBi electrode preparation procedure. The as-received commercial micrometer Bi2O3 (mBO) or the homemade nanosize Bi2O3 (nBO) was used as the precursor. It was made into a membrane electrode and electrochemically reduced to form the nBi membrane electrode. Fig. S1(a) depicts the morphologies of the mBO and shows the aggregation of many Bi2O3 micrometer curved sheets. The commercial mBO sample was composed of Bi2O3 (PDF#, 71-2274) and trace impurities (Bi2O4 or Bi2O2CO3), according to the XRD analysis (Fig. 2(a)). The impurities were probably caused by long-time exposure to the chemical in the atmosphere. The XRD pattern of the mBO membrane electrode subjected to electrochemical reduction in 1 M K2CO3 (Fig. 2(a)) shows only peaks of Bi (PDF#, 85-1329) and Ti; the peaks of Ti likely originated from the current collector. This indicates the complete conversion of mBO into Bi metal.

    Figure  1.  Schematic of the nBi catalyst preparation process.
    Figure  2.  XRD patterns: (a) mBO and the obtained mBO-nBi electrode; (b) the homemade nanometer Bi2O3 (nBO) and the obtained nBO-nBi electrode.

    The surface of the as-rolled mBO membrane appeared dense (Fig. S1(c)). After electroreduction, the resulting mBO-nBi electrode exhibited a nanoporous structure comprising nBi dendrites with a primary particle size of ~80 nm (Fig. 3(a) and (b)). These nBi dendrites were tightly interconnected to form a nanostructure conductive network, likely advantageous for facilitating the mass transfer of CO2 in the electrolytes during CO2ER [12,1719].

    Figure  3.  Typical SEM images of the (a, b) mBO-nBi electrode and (c, d) nBO-nBi electrode.

    To explore the influence of Bi2O3 particle sizes, homemade Bi2O3 consisting of nanoparticles was used as a mBO substitute, and the prepared nBi electrode was denoted as the nBO-nBi electrode. Fig. 2(b) displays the XRD pattern of the homemade Bi2O3. All of the diffraction peaks can be indexed to Bi2O3 (PDF#, 76-1730). In contrast, the synthesized Bi2O3 (Fig. S1(b)) was composed of nanorods with a diameter of ~30 nm, confirming the successful synthesis of nano bismuth oxide (nBO). Fig. 2(b) further confirms that after the electrochemical reduction, nBO was entirely converted to metallic bismuth (PDF#, 85-1329). SEM reveals the morphology change from the nBO membrane to the nBO-nBi electrode. The nBO membrane was dense (Fig. S1(d)). In the nBO-nBi electrode, needle-shaped dendritic Bi was hierarchically aligned on the Ti mesh surface to form a highly porous Bi network (Fig. 3(c)). Compared with the mBO-nBi electrode, the nBO-nBi electrode featured a smaller Bi particle size (approximately 30–50 nm, Fig. 3(d)). The TEM image of nBO-nBi (Fig. 4(a)) confirmed that nBO-nBi was composed of nanoparticles 30–50 nm in diameter. The lattice fringe spacing of the high-resolution TEM (HRTEM) image in Fig. 4(b) was 0.33 nm, corresponding to the Bi (012) plane, which proves that the solid electrochemical reduction of nBO led to the generation of metallic Bi nanoparticles.

    Figure  4.  (a) TEM image and (b) HRTEM image of nBO-nBi.

    The electrochemical performances of the Bi2O3-derived nBi catalysts were investigated in the CO2-saturated KHCO3 electrolyte. Linear sweep voltammetry at 50 mV⋅s−1 was used to evaluate the influence of the catalysts on the reduction current. At potentials more negative than −0.7 V, the electrolyte in its natural state (i.e., without the addition of CO2 gas) began to decompose at mBiO-nBi and nBiO-nBi electrodes (Fig. 5(a)). However, upon the bubbling of the CO2 into the electrolyte, both catalysts yielded significantly larger cathodic current under the same potential as that of the case without CO2 addition. This suggests that nBi has a higher catalytic activity for CO2ER than for the hydrogen evolution reaction (HER). In the CO2-saturated KHCO3 electrolyte, the reduction at the mBO-nBi electrode started at −0.65 V. In contrast, the onset potential at the nBO-nBi electrode was −0.62 V. Simultaneously, at −0.9 V, the cathodic reduction currents were −36 and −30 mA⋅cm−2 at the nBO-nBi and mBO-nBi electrode, respectively. The earlier onset potential and the larger cathodic current at the nBO-nBi electrode compared with those at the mBO-nBi electrode reveals the higher electrocatalytic activity of the nBO-nBi catalyst. Notably, the obtained reduction current of approximately −70 mA⋅cm−2 at the nBO-nBi electrode is nearly the highest reported current for CO2ER based on an H-type cell [2021].

    Figure  5.  (a) Linear scan (50 mV⋅s−1) curves of the mBO–nBi and nBO–nBi electrodes in 0.5 M KHCO3 before and after the bubbling of CO2 gas. FEs of different products from CO2 reduction: (b) formate; (c) CO and H2. (d) Partial current density for formate production (jformate) at different nBi electrodes.

    However, high activity does not necessarily mean high catalyst selectivity, especially considering that the CO2ER is complicated and has various products. The product selectivity of the two Bi catalysts was analyzed via constant-potential electrolysis of the CO2-saturated KHCO3 solution, and the products of CO2ER based on the mBO-nBi and nBO-nBi electrodes were analyzed. The gas and liquid products were detected through GC and NMR, respectively. Three products, H2, CO, and formate were detected. After the quantitative determination of the electrolyzed products generated at different potentials, the potential-dependent FEs of the three products, FEformate, FECO, and FEhydrogen, were calculated (Fig. 5(b) and (c)). The FEformate of the nBO-nBi electrode reached 81.7% at −0.63 V (Fig. 5(b)), higher than that of the mBO-nBi electrode (68%) under similar conditions. This study suggests that constructing an interlinked catalyst layer and reducing Bi particle size can improve the electrocatalytic activity of Bi for CO2 reduction at a lower overpotential. The maximum FE achieved for formate generation was 97.2% (−0.74 V) and 97.6% (−0.78 V) for nBO-nBi and mBO-nBi, respectively, demonstrating the high formate selectivity of the nBi catalyst toward CO2ER, consistent with the previous observation that the FEformate over the metallic Bi catalyst exceeded 90% [2223]. Both nBi catalysts in the present study effectively suppressed CO generation, as indicated by an FECO of <7% (Fig. 5(c)) in the considered potential range. The nBO-nBi catalyst yielded FEformate higher than 80% at potentials of −0.63 to −0.98 V. The mBO-nBi catalyst yielded 80% FEformate at potentials of −0.65 to −1.1 V. When the potential shifted negatively to −1.18 V, the formate selectivity at the nBO-nBi electrode decreased to 52%. In comparison, the FEH2 increased to 46%. However, the FEformate at the mBO-nBi electrode remained at 74% at −1.18 V.

    The partial current of formate generation (jformate, Fig. 5(d)) at the two nBi electrodes was calculated as the product of the FEformate (Fig. 5(b)) and the total reduction current (Figs. S2–S3). The nBO-nBi electrode yielded greater jformate than mBO-nBi at potentials more positive than −0.9 V. However, the nBO-nBi and mBO-nBi electrodes displayed similar jformate values under potentials more negative than −0.98 V, and the mBO-nBi electrode even outperformed the nBO-nBi electrode at −1.18 V in terms of jformate (~40 mA⋅cm−2), owing to the sharp decrease in FEformate at the nBO-nBi electrode.

    To analyze the reason for the difference in current densities delivered at different nBi electrodes during the CO2ER, the electrochemical surface areas (ECSAs) of the two nBi electrodes were calculated according to double-layer charging capacities (Fig. 6). The double-layer charging capacity was measured via cyclic voltammetry at the double-layer potential domain (between −0.01 and −0.13 V). The double-layer charging currents for both electrodes were proportional to the scan rate, and the half-gap currents between the anodic and cathodic branches at −0.07 V were plotted against the scan rate (Fig. 6), forming perfect lines (Fig. 6(b) and (d)). The double-layer capacitance of the electrodes can be calculated via linear curve fitting, and it is linearly correlated with the ECSA of the electrode [24]. The double-layer capacitances of the nBO-nBi and mBO-nBi electrodes were approximately 4.45 and 1.98 mF⋅cm−2, respectively, indicating that the nBO-nBi electrode had an ECSA more than twice that of the mBO-nBi electrode. Hence, the decrease in bismuth nanoparticle size could effectively increase the ECSA of the electrode. However, besides the size and ECSA effects, the electrode structure and porosity may influence nBi catalysts’ activity and selectivity toward CO2ER [25]. The ECSA and current density are not in a directly proportional relationship, especially at more negative potentials [26]. For example, the ECSA ratio of the nBO-nBi electrode to the mBO-nBi electrode was ~2, but the corresponding CO2 reduction current ratio was ~1.7 at −0.74 V and 1.1 at −1.08 V. This suggests a comprehensive influence of ECSA and electrode structure on catalyst performance. The nBi electrodes used in this study were porous, presenting a potential mass transfer issue in the electrode membranes. As a result of the smaller primary particles of nBO-nBi, the pores in nBO-nBi should be smaller and more tortuous. Therefore, while the large ECSA of nBO-nBi may provide more catalytic sites, the electrode may face more mass transfer difficulties, especially at larger polarization conditions. In particular, at high overpotentials, when the mass transfer became the rate-determining step, the performance of nBO-nBi might be inferior to that of the mBO-nBi electrode (Fig. 5(d)). In such a case, the local CO2 concentration in the inner part of the electrode significantly decreased owing to the mass transfer difficulty of CO2, unlike in the case of water; consequently, the contribution of CO2ER to the overall electrochemical reaction at the electrode would decrease, and the HER would become increasingly greater with increasing polarization, resulting in a limited formate partial current and a high FEhydrogen (Fig. 5(c)–(d)) [27]. In contrast, at relatively positive potentials, in which the electron transfer step essentially controls the reaction, the nBO-nBi electrode would display a higher formate partial current than the mBO-nBi electrode. Therefore, at relatively negative potentials (high overpotentials), the mBO-nBi electrode outperformed (Fig. 5) the nBO-nBi electrode, as the latter displayed a lower limiting diffusion current for CO2 reduction to formate (Fig. 5(d)).

    Figure  6.  Cyclic voltammograms (CVs) of the (a) mBO-nBi and (c) nBO-nBi electrodes obtained in a 0.5 M KHCO3 electrolyte. Plots of current densities against the scan rates: (b) mBO-nBi and (d) nBO-nBi. For each CV, the current density was half the gap between anodic and cathodic branches at −0.07 V.

    As previous works have reported the high catalytic activity of bismuth oxide for the reduction of CO2 to formate [2829], the electrocatalytic CO2ER performance of the pristine nano bismuth oxide (nBO) electrode was tested for comparison (Fig. S4). The catalytic performance of the nBO was comparable to those reported in the literature [2829] but lower than that of nBO-nBi: (i) at low overpotentials (−0.68 V) when the Bi2O3 has not been fully reduced (Fig. S5(a)), the nBO electrode delivered an FEformate of merely 38.5%, only approximately half of the nBO-nBi electrode; (ii) at high overpotentials, the Bi2O3 was reduced to Bi (Fig. S5(a)), and the FEformate significantly increased (89% at −0.78 V and 85% at −0.88 V), but it still slightly inferior to that of nBO-nBi (90.4% at −0.78 V and 91.2% at −0.88 V). The SEM images show that the nBO electrodes exhibited markedly different morphologies after CO2ER at different potentials (Fig. S5(b)–(h)). Therefore, the solid electroreduction of bismuth oxide to nBi in advance would be beneficial for maintaining the consistency of the electrode for a high electrocatalytic CO2ER performance.

    Tafel slopes were recorded to elucidate the reduction mechanism of the CO2ER at nBi electrodes. Typically, the formate formation mechanism includes the following steps [22,30]:

    CO2+*CO*2 (3)
    CO*2+eCO*2 (4)
    CO*2+H++eHOCO* (5)
    HOCO*HOCO+* (6)

    Reaction (3) reflects the adsorption of CO2 on the catalyst. The nBO-nBi electrode displayed a Tafel slope of 93.1 mV⋅dec−1 (Fig. 7(a)). In contrast, the Tafel slope of the mBO-nBi electrode was relatively large (100.6 mV⋅dec−1), indicating that increasing the overpotential allows the nBO-nBi electrode to more easily reach a high CO2 reduction current. The comparison suggests that nBO-nBi exhibited better intrinsic catalytic activity than mBO-nBi. As reported in previous studies, a Tafel slope of 118 mV⋅dec−1 suggests that the CO2 reduction could be governed by the first electron transfer (reaction (4)) [31]. However, the Tafel slopes of both nBi electrodes were <118 mV⋅dec−1, suggesting that CO2 reduction to formate was subject to hybrid control by the first electron transfer and the following hydrogenation step [32]. Bicarbonate ion (HCO3) has been considered the main source of protons according to its pKa (i.e., lgKa, and Ka is the acid equilibrium constant of a solution) value (10.33) compared with that of water (15.7) [33]; therefore, the influence of the concentration of HCO3 on jformate was investigated. Fig. 7(b) suggests a linear relationship between jformate and HCO3 concentration, with a slope of ~1.2, indicating a first-order dependence of HCO3 in the rate equation for formate generation. This further suggests that the protonation or the proton-coupled electron transfer reaction (reaction (5)) was involved in the kinetic control of CO2 reduction to formate [12,34].

    Figure  7.  (a) Tafel plots of formate partial current against overpotential at different nBi catalysts; (b) logarithmic plot of the formate partial current and the [HCO3] concentration at the nBO-nBi electrode (measured at −0.78 V); (c) Nyquist plots of impedance measured at −0.78 V at different nBi catalysts in a CO2-saturated 0.5 M KHCO3 electrolyte (100 kHz to 1 Hz, amplitude: 10 mV); (d) equivalent circuit used for fitting the data shown in Fig. 7(c).

    Charge-transfer information was explored via electrochemical impedance spectroscopy (Fig. 7(c)), and kinetic parameters were extracted by fitting the impedance spectra with a widely used equivalent circuit (Fig. 7(d)). The charge-transfer resistance (Rct) values of two electrodes were higher than the solution and ohmic resistance, indicating that the charge-transfer step at −0.78 V was dominant in the CO2ER process (Table 1). The mBO-nBi electrode exhibited a higher Rct than the nBO-nBi electrode, consistent with the smaller ECSA of mBO-nBi. However, as discussed above, the formate FE of mBO-nBi during CO2ER was also lower than that of nBO-nBi, indicating that reducing the particle size of nBi would increase the formate selectivity of the catalyst. These findings align with the Tafel plots. Hence, constructing a three-dimensional connected catalyst layer with small size nBi could improve both reaction kinetics and product selectivity [35].

    Table  1.  Data obtained after fitting Nyquist plots in Fig. 7(c) (Rs, R2, and Rct denote solution resistance, ohmic resistance, and charge-transfer resistance, respectively.)
    Electrode Rs / Ω R2 / Ω Rct / Ω
    mBO-nBi 2.9 5.4 25.7
    nBO-nBi 3.0 4.3 22.8
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    | 显示表格

    Stability is another important criterion in developing electrode catalysts for practical applications. To evaluate the long-term stability of the prepared nBi catalysts, continuous electrolytic reduction of CO2 was performed through the application of a potential of −0.78 V for 20 h. Some slight fluctuations of current density occurred during the electrolysis process because H2 and CO were often generated during the CO2ER process (Fig. 8(a) and (b)). The adsorption and evolution of gas generally lead to current fluctuation, and the higher the gas generation current, the larger the current fluctuation [7,28]. During the long-time CO2 reduction, the total current of both electrodes revealed no pronounced attenuation, but the FEformate of the mBO-nBi electrode decreased (Fig. 8(a)), with a retention of 78%. However, the nBO-nBi electrode performed stably (Fig. 8(b)), with almost no FEformate change observed during the 20 h electrolysis.

    Figure  8.  Total current and product efficiencies during electrolysis of CO2 at −0.78 V at the (a) mBO-nBi and (b) nBO-nBi electrodes. XRD patterns of (c) mBO-nBi and (d) nBO-nBi after the 20 h electrolysis of CO2 at −0.78 V; (e, f) SEM images of the electrodes characterized in (c) and (d), respectively.

    After long-term electrolysis, the electrodes were analyzed via XRD and SEM. Before and after the electrolysis, none of the electrodes exhibited any change in XRD diffraction peaks, indicating they were phase-stable during the electrolysis (Fig. 8(c) and (d)). The SEM results indicate that the particle sizes of mBO-nBi (Fig. 8(e) vs. Fig. 3(a)) and nBO-nBi (Fig. 8(f) vs. Fig. 3(c)) increased after the 20 h electrolysis. The size increase was probably the main reason for the decrease in the formate selectivity of the mBO-nBi electrode. In contrast, while the Bi nanoparticles in the nBO-nBi electrode agglomerated and the size of the Bi particle increased, the FEformate changed little, suggesting that there was a size range for the Bi to achieve a high FEformate; for example, both the mBO-nBi (~80 nm) and nBO-nBi (30–50 nm) electrodes exhibited similar FEformate values in the potential range of −0.65 to −1.0 V. However, the stability test results suggest that a further increase in the particle size of nBi beyond the aforementioned range would result in a rapid drop in FEformate. Studies have often attributed the morphological changes of electrode catalysts after long-term electrolysis to the surface reorganization of the catalysts, probably induced by the surface adsorption/desorption, related to the strong interaction between the Bi metal and CO2 or its derivations [12,26,36]. The long-term instability of the nBi catalysts needs to be investigated further. Nevertheless, during the 20 h CO2ER test, the nBO-nBi electrode displayed high activity, durability, and selectivity. Moreover, the mBO-nBi and nBO-nBi electrodes exhibited comparable and even higher catalytic activity compared with other formate catalysts recently reported in the literature (Table S1) [3740]. This demonstrates the application potential of the oxide-derived nBi catalysts for CO2ER.

    In summary, a nanostructured Bi catalyst was prepared through the direct solid electroreduction of Bi2O3 in a K2CO3 solution. Both mBO-nBi and nBO-nBi electrodes, derived from micronized and nanosized bismuth oxide, respectively, exhibited a hierarchical structure and an integrated conductive network and demonstrated high CO2 reduction activity and formate selectivity. They achieved high formate FEs (>80%) in a wide potential window, between −0.63 and −0.98 V for the nBO-nBi electrode and between −0.65 and −1.08 V for the mBO-nBi electrode. However, at lower polarization conditions, nBO-nBi exhibited a higher formate production rate and FE than mBO-nBi. This suggests the intrinsic high catalytic activity of the smaller nanoscale Bi particles toward CO2 reduction to formate. This study provides a novel approach for fabricating interlinked nanostructured catalysts, which may prove helpful for designing high-efficiency catalysts in the future.

    This work was financially supported by the National Natural Science Foundation of China (Nos. 22072110 and 21872107) and the Key Research and Development Projects of Hubei Province, China (2022BAA083).

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