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Engineering Interfacial Aerophilicity of Nickel-Embedded Nitrogen-Doped CNTs for Electrochemical CO2 Reduction Ruida Chen, Weiliang Tian, Yin Jia, Wenwen Xu, Fanhong Chen, Xinxuan Duan, Qixian Xie, Cejun Hu, Wen Liu, Yun Kuang, Ying Zhang, Yuxin Zhao, and Xiaoming Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00395 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019
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ACS Applied Energy Materials
Engineering Interfacial Aerophilicity of Nickel-Embedded NitrogenDoped CNTs for Electrochemical CO2 Reduction †
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Ruida Chen, Weiliang Tian, Yin Jia, Wenwen Xu, Fanhong Chen, Xinxuan Duan, Qixian Xie, † † † ‡ † Cejun Hu, Wen Liu, Yun Kuang,*, Ying Zhang, *,# Yuxin Zhao*, and Xiaoming Sun .
†State Key Laboratory of Chemical Resource Engineering, College of Energy, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 (P. R. China) ‡School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shanxi 710049 (P. R. China) #School of Chemistry, Monash University, Wellington Road, Clayton 3800, VIC (Australia). KEYWORDS: Electrochemical CO2 reduction, N-doped CNTs, aerophilic, Three-phase interface, Gas-consumption process.
ABSTRACT: Electrochemical CO2 reduction reaction (CO2RR) is a promising approach for conversion of CO2 to value-added chemicals. In this contribution, we demonstrated an electrode design strategy via wettability control and fabricated a free-standing three-dimensional electrode. This electrode design strategy created more three-phase (solid−liquid−gas) contacts due to the sufficient amount of CO2 gas bubbles attached to the surface of the electrode under catalytic turnover conditions and the on-going change of electrolyte wetting and replacement of electrolyte by the gas bubble promotes the activity of CO2RR. This work exploits a new way that sheds light on electrode design for underwater gas-consumption electrocatalytic applications.
To achieve efficient electrocatalytic conversion of CO2 to value-added chemicals, researchers have devoted significant efforts to the rational design of advanced catalytic materials in order to improve their activity. In particular, they have focused on metals1, 2, alloys3, 4, reducible metal oxides5, metal dichalcogenides6, metal complexes7 and carbons8, 9. Among all the candidates, N-doped carbons (NCs) stand for a promising class of alternative materials to replace metal based catalysts for practical implementation of electrochemical CO2 reduction reaction (CO2RR) due to their high chemical stability, large surface area, tunable conductivity and chemisorption property, potential electrochemical activity, and low cost industrial scale manufacturing.10, 11 However, scientific and technical challenges for Ndoped carbon electrocatalysts still exist. For example, the use of insulating polymer binders (eg. PTFE) for fabricating electrodes hindered access of electrolyte to the active sites. In general, current available N-doped carbon electrocatalysts are mostly powder-based, thus pressing and bonding are required
to fabricate them into practical electrodes. This technological process segregates electroactive species, thereby resulting in a more severe ohmic loss and lowering CO2RR activity. Another challenge is the uncontrollable gas/liquid/solid three-phase contact state when the electrode is immersed into liquid media, especially in aqueous media. For gas-consumption reactions under water, reliable manipulation of gas bubbles is of vital importance for real applications. Unexpected low adhesion force and small contact area between the CO2 and the electrode interface prevent continuous supply of sufficient gaseous reactants to reactive catalytic sites, which, in turn, results in mass transport problem of dissolved CO2 to the active site and a decrease in electrocatalytic performance during operation. Even though notable attempts to overcome these hurdles and raise their technological potential to drive efficient CO2RR have been made, few have simultaneously addressed all aforementioned challenges owing to the difficulty of concurrently tailoring structural integrity
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Scheme 1. Schematic diagram of the “aerophilic” structured PTFE-H-NiNCNTs under reaction conditions. Continuous CO2 films can be created in the interface of the rough microstructures and the electrolyte solution, resulting in accelerated electron transfer processes and thus a superior CO2RR performance. of NCs and gas-bubble behaviours on their interfaces. Besides, the catalyst needs to exhibit long-term stability in underwater electrocatalytic environment. In this paper, aiming at improving the CO2RR performance, we demonstrate the design and synthesis of hierarchical porous nanostructures composing carbon fibre stems and secondary epitaxial N-doped carbon nanotube (NCNTs) arrays derived from Ni(OH)2 embellished CNF. Then, the surface coating of polytetrafluoroethylene (PTFE) provides an engineered interfacial affinity between gaseous reactant, electrolyte and reactive catalytic sites to accelerate the gas-diffusion process as well as electron transport (Scheme 1). In our case, direct growth of threedimensional (3D) nanotube arrays on CNF, avoids the use of polymer binders and ensures effective network conductivity for rapid electron transportation towards CO2RR.12 N-doped carbon itself can act as active sites for CO2-to-CO conversion because of their strong binding to *COOH but weak binding to *CO.13, 14 The introduction of Ni atoms can further cause the metal-N moieties synergetic effect that enhances catalytic activity and improves CO production.15 Additionally, PTFE modification on the highly rough surface effectively guarantees sufficient amount of CO2 gas bubbles attached on the surface of electrode in aqueous media.16-18 Owing to such a 3D structure and the aerophilic property, a 95.6 % of FECO was obtained at 1.4 V vs. Ag/AgCl with a total current density of 4.56 mA/cm2 (partial current density of 4.24 mA/cm2). The synergetic effect exerted by the 3D structure and the rational composition is proposed to be the origin of the improvement of the performance, which provides new insight into designing materials and fabricating electrodes for CO2RR and other gas consumption reactions. As illustrated in Figure 1a, the preparation of the Ni-incorporated NCNTs (NiNCNTs) arrays on CNF involves three steps. First, the vertically aligned Ni(OH)2 nanosheet arrays were hydrothermally constructed on CNF (Figure 1b), showing a flake
structure with a thin thickness of about 50 nm and average lateral size up to about 1 μm (Figure 1c). Subsequently, the Ni(OH)2 nanosheet arrays, as the catalyst and support, induced an in situ epitaxial growth of tortuous NiNCNTs via a calcination process in the presence of melamine as the carbon and nitrogen sources. During the calcination, the flake structure on the surface was converted to the hairy structure (Figure 1d). Transmission electron microscopy (TEM) further revealed that the Ni nanoparticles were embedded in the tip of carbon nanotubes19 (Figure S1, Supporting Information). Afterwards, the structured electrode was acid treated and modified with PTFE to obtain PTFE-H-NiNCNTs-CNF. The enlarged side-view scanning electron microscopy (SEM) images (Figure 1e, f) demonstrated that the highly porous morphology can be well maintained after acid treatment and further PTFE decoration. Further insight into the composition and crystallinity change during the synthesis procedure are analysed by X-ray diffractometry (XRD) and energy dispersive X-ray (EDX) spectroscopic mapping. Figure 2a shows the XRD patterns of Ni(OH)2CNF, NiNCNTs-CNF, H-NiNCNTs-CNF and PTFE-HNiNCNTs-CNF catalysts. All of them show diffraction peaks at about 26.1° and 54.6°, which can be assigned to the (002) and (004) crystal face of carbon, respectively20. After calcination under 650°C with the presence of melamine, the carbon reflections are slightly shifted to smaller 2θ angles due to the lattice distortion caused by the incorporated nitrogen atoms. Two prominent peaks at about 44.5° (111) and 52.2° (200) emerged and matches well with JPCDS no. 01-1258, implying the presence of crystalline metallic cubic nickel crystals. Additionally, EDX (Figure S2) and XRD analyses also demonstrate a homogeneous distribution of Ni, C, N and O elements. Except chlorine from hydrochloric acid treatment, no additional impurities were introduced during the calcination and surface modification. To provide more information about the surface electronic states and chemical composition of the as-synthesized catalysts, X-ray photoelectron spectroscopy (XPS)
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ACS Applied Energy Materials
Figure 1. Preparation process of PTFE-H-NiNCNTs and SEM images of synthesized electrode materials in each step. a) Schematic illustration of the fabrication procedure of PTFE-H-NiNCNTs-CNF. Low- and high-magnification SEM images of b) macroporous CNF, c) Ni(OH)2 nanosheet arrays that are densely and vertically formed on CNF substrates, d) porous NiNCNTs on CNF, and its derivatives after e) acid treatment and f) subsequent PTFE modification.
measurements were performed. The elemental analysis shows that for the sample synthesized before and after acid treatment, the Ni 2p spectra displayed two dominating peaks at 855.0 and 872.5 eV of 2 p3/2 and 2 p1/2, respectively21, accompanied by their corresponding satellite peaks (Figure 2b). The two pairs of characteristic peaks indicate the existence of Ni2+ in the synthesized catalysts. After HCl leaching, the Ni 2p spectrum intensity of H-NiNCNTs increased in comparison with that of NiNCNTs, suggesting the decreased electron density for residual Ni metal that might be attributed to the electronic interaction and charge transfer between Ni and carbon support22. The acid treatment also caused significant changes in the O 1s spectra at 532.4 eV, wherein the oxygen peak decreased significantly, indicating the loss of O-based functional groups, like carbonyl, on the surface of the catalyst (Figure 2c)23. The deconvolution of the C 1s spectrum also shows the decline of C=O (287.8 eV) species in H-NiNCNTs, confirming the release of O heteroatoms from carbons24 (Figure 2d). It is important to note that too much surface oxygen can considerably increase the electrical resistance of carbon materials as well as the hydrophilicity of the exposed carbon surface. High resistance impedes electron transfer during catalysis, while strong hydrophilicity leads to displacing of gaseous CO2 from the active sites, thus, impeding of CO2 mass transport, which in turn leads to more HER taking place. The N 1s spectra of NiNCNTs in Figure 2e were deconvoluted into three major peaks at 398.7 eV, 399.4 eV and 401.1 eV, corresponding to pyridinic, pyrrolic and graphitic species, respectively10. It can be seen that after acid treatment, the percentage of pyridinic, pyrrolic and graphitic nitrogen in H-NiNCNTs changed from 49.81%, 33.95% and 16.23% to 50.42%, 24.45% and 25.14%, respectively. It is well-known that pyridinic N can lead to an increase of the structural defect density for providing more catalytic active sites, and graphitic N can enhance the conductivity of carbon nanotubes(Electrochemical impedance spectroscopy of Figure S3 can also certify this). 25-27 This means that HCl leaching procedure is beneficial for achieving en-
hanced electrocatalytic activity. Further evidence can be provided from comparing the intensity ratio between defective (D) and graphitic (G) bands (ID/IG) determined in Raman spectroscopy, which reflects the degree of defectivity of the NCNT11. As illustrated in Figure S4, the ID/IG values increased from 0.78 for CNF to 0.97 for NiNCNTs, 0.98 for H-NiNCNTs and 0.98 for PTFE-H-NiNCNTs, indicating more disordered and catalytically more active structures of final products. To assess the influence of surface modification on gas-bubble wettability and mass transport of CO2, the CO2 gas-adhesion behavior for NiNCNTs-CNF, H-NiNCNTs-CNF and PTFE-HNiNCNTs-CNF electrode in a neutral electrolyte was visualized as shown in Figures 3a-c. It was demonstrated that the individual CO2 bubble with a volume of about 2.0 μL cannot be adsorbed on the surface of NiNCNTs-CNF and H-NiNCNTs-CNF (Figure 3a, b and Movie S1, S2 in the Supporting Information). In contrast, H-NiNCNTs-CNF shows a higher contact angle (CA) of CO2 bubble than that of NiNCNTs-CNF, indicating a much stronger interaction between the surface of the electrode and gas. The CO2 bubble was immediately spread out once it reached the surface of PTFE-H-NiNCNTs-CNF (Figure 3c and Movie S3 in the Supporting Information). To further explore the underwater surface wetting state on the gas/liquid/solid three-phase interface, the surface adhesion property of CO2 bubble on these electrodes was also carefully characterized using high-sensitivity micro-electromechanical balance system (details can be found in the experimental section). As revealed in Figure 3d, both NiNCNTs-CNF and H-NiNCNTs-CNF showed aerophobicity and exhibited an explicitly small adhesive force of 0.1 μN and 1.7 μN, respectively, to CO2 gas bubble underwater, affirming that the gas is hard to reach the bottom of these surface textures. Correspondingly, the measured adhesive force to gas bubble underwater is 64.0 μN for PTFE-HNiNCNTs-CNF, by over 33 times higher than that for NiNCNTs-CNF, demonstrating the gas is
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Figure 2. XRD and XPS characterizations of synthesized electrode materials. a) XRD patterns of specific products prepared from the Ni(OH)2 precursors at different stages. High resolution XPS b) Ni, c) O1s, d) C1s and e) N spectra for pristine and acid leached NiNCNTs.
preferred to penetrate into the surface textures of PTFE-HNiNCNTs-CNF. Here, the surface energy reduces due to the C– F chains derived from PTFE, primarily resulting in the aerophilicity. Figure 3e shows that high concentration of PTFE decoration increased the adhesive force of CO2 bubble underwater. To conclude, the PTFE decoration is of vital importance in realizing the large bubble adhesion and guaranteeing a strong capacity of gas absorption, which has been usually considered as a beneficial factor for gas consuming reactions12 (Figure 3f). Such a special gas-adhesion behaviour indeed led to significant differences in both the geometric current density and overall electrocatalytic CO2RR performance, as examined in a Hcell configuration (Figure S5) containing 0.5 M aqueous KHCO3 electrolytes with continuous bubbling of CO2. To figure out the CO2RR activity, NiNCNTs, H-NiNCNTs and PTFEH-NiNCNTs are evaluated for comparison (Figure 4). During CO2RR, the PTFE-H-NiNCNTs catalyst presented similar CO partial current densities but much lower partial current densities for HER when compared with NiNCNTs and H-NiNCNTs catalysts in potential range from –1.0 to –1.8 V, revealing that the surface modification of NiNCNTs can suppress competitive side reaction but maintain highly active for the conversion of CO2 to CO (Figure 4a). Among them, PTFE-H-NiNCNTs exhibited the highest FECO at −1.4 V. At this potential, a maximum FECO of 95.6 % was achieved on PTFE-H-NiNCNTs, whereas NiNCNT and H-NiNCNT showed only 53.9 % and 72.6 %. The results demonstrate the intrinsic high catalytic activity of rationally designed PTFE-H-NiNCNTs electrode and excellent mass transport at the interface between electrode and electrolyte. The electrochemical surface area (ECSA) corrected CO partial current density of PTFE-H-NiNCNTs is significantly larger then that of H-NiNCNTs, indicating a considerable enhancement of the catalytic activity after modifying with PTFE (Figure S6 and S7). This electrode design strategy creates more three-phase (solid−liquid−gas) contacts due to the sufficient amount of CO2 gas bubbles attached to the surface of the electrode and the ongoing change of electrolyte wetting and replacement of electrolyte by the gas bubble promotes the activity of CO2 reduction.
Similar observation can be found in a previous report using Cubased gas diffusion interfaces for CO2 electroreduction to alcohols 28, 29. The stability of the PTFE-H-NiNCNTs electrode for CO2RR was evaluated under optimal conditions: the electrolyte is CO2 saturated 0.5 M KHCO3 and the applied potential is −1.4 V. As shown in Figure 4c, the PTFE-H-NiNCNTs catalyst keeps a stable total current about 4.6 mA/cm2 and the FECO only slightly decreased from 95% to 94% for more than 6 h’s continuous operation, demonstrating good electrocatalytic durability. This is ascribed to the robust structural integrity of this catalyst, since no detectable morphology changes were observed after prolonged electrolysis time (Figure S8, Supporting Information). To demonstrate the role of PTFE surface modification on the H-NiNCNTs for CO2RR, control experiments with different amount of PTFE loading were carried out (Figure 4d-f). It can be found that with 0.25-0.75 wt% of PTFE loading, much lower FECO was exhibited, mainly because of the strong water adhesion (water CA
