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Size-dependent electrochemical nitrogen reduction catalyzed by monodisperse Au nanoparticles
Electrochimica Acta 335 (2020) 135708
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Size-dependent electrochemical nitrogen reduction catalyzed by monodisperse Au nanoparticles Chan Chen a, Cong Liang b, Jun Xu a, c, Jiankun Wei a, Xiangrong Li a, Ying Zheng g, Junrui Li d, **, Haolin Tang e, Junsheng Li a, f, * a
School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, PR China China Automotive Technology and Research Center, Tianjin, 300300, PR China Research Center for Materials Genome Engineering, Wuhan University of Technology, Wuhan, 430070, PR China d Department of Chemistry, Brown University, Providence, RI, 02912, USA e State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, PR China f Hubei Provincial Key Laboratory of Fuel Cell, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, PR China g College of Urban Construction, Wuchang Shouyi University, Wuhan, 430064, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2019 Received in revised form 31 December 2019 Accepted 13 January 2020 Available online 14 January 2020
Electrochemical nitrogen reduction reaction (ENRR) is a mild and environmentally friendly process that can be potentially complementary to the traditional Haber-Bosch process. Development of highperformance ENRR catalysts and understanding of the activity origin of the catalyst are essential for the practical application of ENRR process. Herein, we report the size-dependent ENRR activity of monodisperse Au nanoparticles (NPs) sized ranging from 4 nm to 10 nm. 8 nm Au NPs is found to be an excellent ENRR catalyst with an ammonia production rate and Faradic efficiency of 17.49 mg h1 mg1Au and 5.79%, respectively. The high ENRR performance of 8 nm Au NP is rationalized with DFT studies, which indicate that an optimal proportion of the surface step sites is key to suppressing hydrogen evolution reaction and improving the catalytic activity towards ENRR. The study herein may provide guidelines for tailoring the ENRR catalysis on nanostructured metal catalysts. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Electrochemical nitrogen reduction Gold nanoparticle Monodisperse Size effect
1. Introduction Ammonia is one of the most important chemical products widely used in industries ranging from fertilizer production to energy storage [1]. Electrochemical nitrogen reduction reaction (ENRR) under aqueous condition is a promising complementary process to the traditional Haber-Bosch process due to its mild reaction condition, low facility demand and environmental friendliness [2e4]. The key to cost-effective ENRR is the development of high-performance electrocatalyst for the reaction [5]. However, the search of efficient ENRR electrocatalyst is challenging due to the high activation barrier of molecular nitrogen. In addition, ENRR at aqueous condition is facing the competition from hydrogen evolution reaction (HER) because of the relatively lower overpotential
* Corresponding author. School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, PR China. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J. Li). https://doi.org/10.1016/j.electacta.2020.135708 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
of HER [6]. An ideal ENRR catalyst should not only effectively lower the activation barrier toward nitrogen reduction but also show depressed activity for HER [3]. As a stable metallic catalyst, Au has been demonstrated to be a promising ENRR catalyst [7,8] with relatively low HER activity [9,10]. A previous experimental study suggests that ENRR on Au surface possibly initiates from the adsorption of N2 molecule on Au surface, followed by successive proton and electron transfer, which results in N2H4 by-product and NH3 product [8]. These preliminary findings have inspired intensive research interests on Au-based catalysts for ENR [11,12]. Generally, the ENRR performance of Au based material can be controlled by engineering the surface chemical state or morphology of Au catalysts. To modulate the surface electronic state of the Au ENRR catalyst, tetrahexahedral gold nanorods with exposed high-index facets were synthesized [13]. Au nanostructures with rich low coordination sites, such as Au nanocluster [14], amorphous Au nanostructure [15] and single-site Au [16], were also proposed to be an active ENRR catalyst. In addition, the activity of these Au nanostructures can be enhanced by introducing synergy between
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the Au catalysts and appropriate support [15]. Tailoring the morphology of the Au nanostructure may increase the surface area of the material and improve the mass transfer for the catalytic reaction, which are favorable for an improved ENRR performance. Hollow Au nanocages with regulated cavity size and porous features have been developed and shown to be highly active for ENRR [17,18]. Au nano-flower with rich dendrite structure was also synthesized for an efficient ENRR process [19]. Despite the progress outlined above, there is still lack of understanding about the optimal morphology and active sites towards ENRR when designing highly active nanostructured Au ENRR catalysts. Herein, we present a study on the ENRR activity of monodisperse Au NPs with different sizes and the origin of their size-dependent activity through DFT studies. Our results show that mono-dispersed Au NP with a size of ~8 nm exhibits the highest catalytic activity towards ENRR. The 8 nm Au NPs possess an optimum portion of the surface step site, namely (211) facet, which greatly suppresses HER but is energetically favorable for ENRR catalysis. 2. Experimental Monodisperse Au NPs with sizes of 4, 6, 8, 10 nm were synthesized with a modified method of what was reported before [20]. Au NPs were loaded onto Ketjenblack Carbon (EC600j) by sonicating the mixture of Au NP dispersion in hexane and Ketjenblack Carbon, resulting CeAu. The CeAu was stirred overnight in glacial acetic acid to remove the surfactant. The mixture was washed for three times with 20 mL ethanol, and then vacuum dried at 120 C for 3 h to remove the solvent. The catalyst ink was prepared by dispersing 5 mg KB-Au in a mixture of 900 mL isopropyl alcohol, 20 mL DI water and 20 mL 5% Nafion solution. The dispersion was sonicated for 30 min to form a homogeneous ink. The ink was then loaded onto a carbon cloth with a size of 1 2 cm2 and dried under ambient conditions. The loading amount of Au/C catalyst on the carton cloth was controlled to be 0.42 mg cm2. Transmission electron microscopy (TEM) images were obtained on a Philips CM20 operating at 200 kV. The electrochemical nitrogen reduction reaction (ENRR) measurement was performed in an H-shape electrochemical cell, which was separated by Nafion 211 membrane. Before ENRR tests, Nafion membrane was pretreated by heating in turn in 5% H2O2 aqueous solution, 5% H2SO4 aqueous solution and ultrapure water at 80 C for 1 h, respectively. 50 mL of 0.1 M KOH solution was added into the cathode and anode compartment, respectively. Ultra-pure nitrogen (99.99%) was bubbled into the catholyte at a flow rate of 50 mL min1 for 20 min before reaction. The outlet gas tube was connected with an ammonia trap filled with 20 mL of 0.005 M H2SO4 aqueous solution as adsorbent liquid to collect any ammonia escaped from the electrolyte. The electrochemical experiments were carried out with a CHI electrochemical workstation (CHI-660D) using a threeelectrode configuration. The prepared electrode, graphite rod and salt bridge (with L-shaped sand core) Ag/AgCl electrode (3.5 M KCl electrolyte) was used as working electrode, counter electrode and reference electrode, respectively. Prior to the measurement, the working electrode was activated by cyclic voltammetry (CV) scanning between 0.74 V and 0.36 V at a scanning speed of 100 mV s1 for 50 cycle. Then, linear sweep voltammetry (LSV) curves were collected at a scanning speed of 10 mV s1. To test the products, controlled potential electrolysis tests were carried out at fixed potentials for 10000 s. The potential was converted to RHE scale via calibration. The calibration was performed using Pt wire as both working electrode and counter electrode in H2-saturated electrolyte. LSV curves were acquired at a scan rate of 0.1 mV s1. The potential at which the current crossed zero is taken to be the
thermodynamic potential (vs. Ag/AgCl) for the hydrogen electrode reactions (Fig. S1). Salicylate method was used to test the concentration of the ammonia produced. 3 mL of the testing solution (catholyte or absorbent; the pH of absorbent solution was adjusted by concentrated KOH solution to 13) was mixed with 50 mL of 2 M NaOH solution containing 0.05 M NaClO, followed by addition of 250 mL of 0.5 M NaOH aqueous solution with 5 wt% salicylic acid, 5 wt% sodium citrate. Next, 50 mL of 1 wt% C5FeN6Na2O (sodium nitroferricyanide) aqueous solution was thoroughly mixed with the mixture solution above, and then store sealed at room temperature for 2 h. The UVevis absorbance was then measured by an UVeviseNIR spectrophotometer at 660 nm (N4, Shangfen, China) The calibration curves were built using NH4Cl solution in the presence of 0.1 M KOH to quantify the produced NH3. All the DFT calculations were implemented by Dmol3 module [21,22]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed [23]. The TS method for DFT-D correction was performed to consider the van der Waals interaction [24]. The double numerical plus polarization (DNP) basis was used for higher accuracy. In addition, the DFT semicore pseudopotential (DSPP) method was adopted [25]. Self-consistent field (SCF) computations were performed with a convergence criterion of 106 au on the total energy and electronic computations. To ensure credible results, the real-space global orbital cutoff radius was chosen as big as 4.7 Å in all computations. Furthermore, we adopted the conductor-like screening model (COSMO) [26] to simulate solvent effects and the dielectric constant of the water was set as 78.54. Au(211) and Au(111) were modeled by a 3 3 1 supercell with a vacuum space of 15 Å. The slab models used a Monkhorst-Pack mesh with 5 5 1 in reciprocal space. Au13 cluster model was also modeled and the Brillion zone was sampled with a MonkhorstPack mesh with 3 3 3. The adsorption energy (DE) of intermediate species on the Au surfaces and Au13 cluster are define as follow:
DE ¼ Etotal Eslab Eads where E total, E slab and E ads are the energy of the species-adsorbed system, the catalyst slab and species, respectively. The reaction free energy (DG) of the elementary reaction in the HER and NRR were calculated according to the computational hydrogen electrode (CHE) model proposed by Nørskov et al. [27e29] The energy of a proton/electron (Hþþe) is equal to half of the chemical potential of hydrogen and the pH is set as zero in this model. DG value was calculated as follow:
DG ¼ DE þ DZPE T DS where DE was obtained by DFT calculations as above, DZPE is the change in the zero-point energies (ZPE), and TDS is the change in entropy in 298.15 K, which were computed from the vibrational frequencies and the gas-phase molecules were obtained from the standard thermodynamic database. All structures were optimized and calculations presented in the study were for a pH value of 0. 3. Results and discussion Monodisperse Au NPs in 4 nm were synthesized via an adapted method from previous publication. Briefly, 0.2 g HAuCl6$3H2O was dissolved in a mixture of 10 mL tetralin (1,2,3,4tetrahydronaphthalene) and 10 mL oleylamine (OAm). Under the protection of N2 atmosphere, 1 mmol TBAB (tert-bultylamine borane) dissolved in a mixture of 1 mL tetralin and 1 mL OAm was injected into the previous solution to initiate the nucleation. The
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Fig. 1. TEM images of Au NPs of different sizes: (A) 4 nm; (B) 6 nm; (C) 8 nm and (D) 10 nm.
reaction was kept at room temperature for 1 h for completion. Au NPs were precipitated by adding 10 mL hexanes and 40 mL ethanol into the solution and collected via centrifugation. The Au NPs were then washed with 10 mL hexanes and 40 mL ethanol before being dispersed in hexanes for further use. 6 nm Au NPs were obtained via a seed-mediated method by using 4 nm Au NPs as seeds. 0.12 g HAuCl6$3H2O was dissolved in a mixture of 10 mL 1-octadecene (ODE) and 10 mL OAm. 30 mg 4 nm Au NPs were added into the
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solution, then the reaction was heated to 80 C at a rate of 5 C min1. The reaction was kept at 80 C for 2 h before cooling down to room temperature. After washing with hexanes and ethanol, Au NPs were collected and dispersed in hexane. For 8 nm and 10 nm Au NPs, same conditions to 6 nm Au NPs were used except that 6 nm and 8 nm Au NPs were used as seeds, respectively. The monodispersity of the synthesized Au nanoparticles was quantified with TEM measurements. It was previously demonstrated that such synthesis yields polycrystalline Au of face centered structure (FCC) with multiple exposed (111) planes [30]. TEM images and a detailed particle size analysis (Fig. 1 and Fig. S2) showed that the Au NPs are uniform in size and morphologies. The Au NPs were loaded onto Ketjen black (KB) for the evaluation of their ENRR performance. High resolution TEM measurement confirms successful loading of the Au nanoparticles onto the carbon matrix (Fig. S3). The Au NPs loaded KB (Au/C) were treated with acetic acid followed by washing with ethanol several times to remove the residual oleylamine on Au nanoparticles [31]. The compositional information of the Au/C catalyst was characterized using XPS with 8 nm Au/C catalyst as an example. C and O of different chemical states (from both the carbon cloth and Nafion binder) were identified in the spectra (Fig. S4). In addition, no N was detected on the surface of the catalyst, demonstrating complete removal of residual N on the Au NPs. ENRR performance of the Au NPs was characterized in a closed H-shaped cell, which was separated by a Nafion membrane. All the electrochemical characterizations were conducted in 0.1 M KOH. LSV curves of the Au electrodes in N2 or Ar saturated electrolyte were firstly recorded. For 8 nm Au/C, a noticeable onset of ENRR was observed at a potential of ~ -0.14 V (Fig. 2A). With the negative
Fig. 2. (A) LSV curves of 8 nm Au/C. (B) Chronoamperometric curves of 8 nm Au/C catalyst at different potentials. (C) Faradaic efficiency of Au/C catalyst with different Au particle size. (D) ammonia yield of Au/C catalyst with different Au particle size. The error bar represents standard deviations of results from three independent experiments. (Error bars in the figures represents standard deviations determined from three individual experiments.)
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shift of potential, a more pronounced ENRR current was observed. When the potential was lowered below ~ -0.5 V, ENRR current became neglectable because of the favorable HER process at low potential. The LSV curves of other electrodes are shown in Fig. S5. The results showed that all the electrodes have similar onset for ENRR. The ENRR performance of the Au/C electrodes were studied in more details by electrolysis tests at given potentials with continuous N2 bubbling (Fig. 2B). The reaction currents of Au/C electrodes were highly stable during the electrolysis and the currents increased with the decrease of the potential for electrolysis, as shown in Fig. 2B and Fig. S6. The produced ammonia during the electrolysis in the catholyte and adsorbent (at the outlet) was analyzed with the salicylate method [32]. The UVevis spectra of typical standard NH4Cl solutions and corresponding calibration curves for quantification are shown in Fig. S7. The reported ammonia yield was the sum of the ammonia detected in the cathode electrolyte and in the adsorbent. A detailed list of the source of ammonia yield was shown in Fig. S8. No hydrazine was generated in the ENRR process (Fig. S9), as reflected by the Watt and Chrisp analysis [33]. The absence of hydrazine in the products suggests that ENRR on Au/C follows an associative distal pathway [34]. To identify the optimal potential for ENRR, the electrolysis and corresponding product analysis was performed at different potentials ranging from 0.04 V to 0.44 V. The average Faradic efficiency and ammonia yield over Au/C electrodes were determined from three individual experiments and shown in Fig. 2C and D. The highest Faradic efficiency (FE) was attained at 0.14 V for all the Au/ C electrodes. Among all the electrodes, 8 nm Au/C electrode exhibited a highest FE of 5.8% at 0.14 V. In addition to its high FE, 8 nm Au/C electrode also had a high ammonia yield. The ammonia yield rate of 8 nm Au/C electrode was measured to be 17.49 mg h1 mg1Au, higher than 4 nm Au/C electrode (4.24 mg h1 mg1Au), 6 nm Au/C electrode (3.92 mg h1 mg1Au) and 10 nm Au/C electrode (6.92 mg h1 mg1Au). Such a high FE and ammonia yield is comparable to the advanced Au based catalyst reported recently (Table S1). To demonstrate that the produced ammonia is generated from the dissolved N2 in the electrolyte, control electrochemical measurements were conducted. First of all, the 8 nm Au/C electrode was tested in N2 saturated electrolyte at open circuit potential. After 10000 s of test, the electrolyte from the cathode cell was collected and analyzed. No ammonia was tested in the collected electrolyte (Fig. S10A). Furthermore, the 8 nm Au/C electrode was characterized in Ar saturated electrolyte at 0.14 V (the optimal ENRR potential as shown above) for 10000 s. Ammonia products was also absent in the catholyte of the electrochemical cell after the test (Fig. S10B) These results altogether demonstrated that the N2 dissolved in the electrolyte is the only possible source for the produced ammonia. The durability of the Au/C catalyst was investigated with repeated ammonia production tests at 0.14 V using 8 nm Au/C electrode as an example. Between each test, the Au/C electrode was rinsed with DI water and dried to remove the adsorbed electrolyte from the last measurement. Our results show that both the ammonia production rate and Faradic efficiency of 8 nm Au NP gradually decrease with cycling (Fig. S11). To investigate the origin of the performance decay, XPS and TEM characterizations were performed with the samples after durability tests. As shown in the TEM images of the sample, no NP aggregation was observed after the durability test (Fig. S12). In addition, the XPS measurements revealed that the surface states of Au remain unchanged after the durability test (Fig. S13). Considering the fact that Au NP is pasted onto the carbon cloth with Nafion binder, we think that the main reason for the decrease in the observed NRR activity is the detachment of Au NP from the carbon cloth due to the long-term soaking of the electrode in the cathode cell and repeated washing
and drying between each cycle. The performance of the catalyst is determined by both its intrinsic activity and surface area. The 4 nm Au/C catalyst, with a higher surface area compared to its counterparts, showed poor ENRR performance. This result suggests that the difference in ENRR performance of the Au/C catalysts is mainly originated from their different intrinsic activities induced by the size effect, other than the surface area effect. The proportion of uncoordinated Au atom at the surface of Au NPs (with size below 10 nm) is significantly influenced by the size of the NPs. The crystallite size of the Au nanoparticles was estimated with Scherrer equation: t ¼ Kl/bcosq, where K is a dimensionless shape factor. The shape factor has a typical value of about 0.9; l is the X-ray wavelength (1.5418 Å) and b is the line broadening at half the maximum intensity (FWHM) of Au (111) peak. The crystallite size of 4 nm Au NP, 6 nm Au NP, 8 nm Au NP and 10 nm Au NP were estimated to be 1.8 nm, 2.0 nm, 2.7 nm and 3.4 nm, respectively (Fig. 3). The standard XRD pattern of Au (PDF No. 4e784) was also provided for comparison. As shown in the figure, the (111), (200), (220) and (311) facet (centered at ~38.2 , 44.4 , 77.5 and 64.6 , respectively) of Au could be clearly identified in the XRD spectra of the as-synthesized Au NPs. Considering the fact that the fraction of the exposed Au facets differs significantly for ultra-small Au nanoparticles of different sizes [20], we hypothesize that the observed size-specific ENRR activity is most probably a result of different coordination states of Au atoms in Au NPs of different sizes. To verify this hypothesis, Au cluster (Au13 with exclusive corner sites), Au (211) and Au (111) facets, which corresponded to the corner site, edge site and planar site of small Au NP, respectively, were modeled and analyzed with DFT studies. Since no hydrazine was generated on the monodispersed Au NPs (shown above), the ENRR process probably follows an associative distal pathway. Thus, we calculated the free energy diagrams of ENRR via associative distal mechanism on these models (Fig. 4). It is clear from the diagram that the rate limiting steps on the Au models is the reductive protonation of *NN to *NNH. The calculated free-energy change for this rate limiting step (△G*NNH) on Au13, Au (111) and Au (211) was 1.08, 2.16 and 1.50 eV, respectively (Fig. 4). From this result, Au (211) and Au13 could be considered as the main potential active sites for ENRR. Furthermore, the projected density of states (pDOS) of different Au models were plotted (Fig. S14). The 5d band of Au on Au (211) facets had a slightly higher tendency to overlap with 2p
Fig. 3. XRD spectra of Au nanoparticles of different sizes.
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Fig. 4. Free energy diagrams and corresponding optimized configuration for electrochemical reduction of nitrogen on Au(111), Au(211), or a 13-atom Au cluster via associative mechanism at 0 V.
orbitals of Nads in *NNH than that for Au (111), suggesting a stronger bonding of Nads with Au (211). Such a stronger bonding is beneficial for the stabilization of reaction intermediate and enhance the overall activity. The free energy changes of HER on Au13, Au (211) and Au (111) were also compared to investigate the ENRR selectivity on these model surfaces. Hydrogen adsorption on Au13 is thermodynamically favorable with a negative △G*H of 0.234 eV (Fig. 5). In
contrast, the energy barrier for the adsorption of hydrogen on Au (211) reached 0.252 eV, higher than that for Au (111). These DFT results proves that the edge site Au atoms, modeled with Au (211) facet, have relatively high ENRR activity and suppressed HER performance. As shown in Fig. S15, Au nanoparticles with crystallite size below 2 nm has a high proportion of HER active corner sites, suggesting that ENRR might be unfavorable on these nanoparticles due to the competition of HER. With the increase of crystallite size above 2 nm, the Au nanoparticle tends to show depressed HER because of lower fraction of corner sites on the nanoparticle. It should be noted the fraction of overall surface exposed Au atoms and Au atoms on the edge sites also decreases with the increase of crystallite size, which may lead to decreased ENRR performance of the Au nanoparticles. When the crystallite size of Au NPs increases above 3 nm, the proportion of the edge sites (211) decreases. Thus, the Au nanoparticle with optimal ENRR performance should have an intermediate size that ensures a large surface area and simultaneously appropriate fraction of ENRR/HER active sites. Therefore, the observed high performance of 8 nm Au/C electrode (with a crystallite size of 2.7 nm) for ENRR can be explained by its balanced proportion of edge site Au atoms and corner site atoms as well as its relatively large surface area. 4. Conclusion
Fig. 5. Free energy diagrams for electrochemical reduction of protons to hydrogen on Au(111), Au(211), or a 13-atom Au cluster at 0 V.
Model Au NPs with mono-disperse sizes ranging from 4 nm to 10 nm are synthesized and investigated in terms of their ENRR activity. Electrochemical characterizations show that carbon supported 8 nm Au NPs exhibits the highest ENRR performance, with ammonia yield rate and Faradic efficiency being 17.49 mg h1
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mg1Au and 5 .79%, respectively. DFT analysis demonstrates that the excellent ENRR performance of 8 nm Au NPs can be attributed to their optimal proportion of coordination unsaturated edge site atoms and their relatively large surface area. Our results show that the 8 nm Au NPs can be a highly potential electrocatalyst for ENRR. In addition, the fundamental understanding of the activity of Au nanoparticles unveiled in this study provides insights for the future design of high performance ENRR electrocatalyst. Credit author statement Chan Chen: Investigation; Methodology; Writing. Cong Liang: Methodology; Investigation. Jun Xu: Methodology. Jiankun Wei: Methodology. Xiangrong Li: Data Curation. Ying Zheng: Data Curation. Junrui Li: Conceptualization; Writing; Supervision. Haolin Tang: Conceptualization. Junsheng Li: Conceptualization; Writing; Supervision; Funding acquisition. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant Nos. 51972254), Fundamental Research Funds for the Central Universities (WUT: 2018IB026, 2019IB003 and 195220002), State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology, 2019-KF-10) and Fundamental Research Funds for the Central Universities for finical support (2019-HS-B1-15, 2019III048GX). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135708. References [1] A. Boulamanti, J.A. Moya, Production costs of the chemical industry in the EU and other countries: ammonia, methanol and light olefins, Renew. Sustain. Energy Rev. 68 (2017) 1205e1212. [2] M. Jewess, R.H. Crabtree, Electrocatalytic nitrogen fixation for distributed fertilizer production? ACS Sustain. Chem. Eng. 4 (2016) 5855e5858. [3] X. Cui, C. Tang, Q. Zhang, A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions, Adv. Energy Mater. 8 (2018) 1800369. [4] J. John, D.K. Lee, U. Sim, Photocatalytic and electrocatalytic approaches towards atmospheric nitrogen reduction to ammonia under ambient conditions, Nano convergence 6 (2019) 15. [5] N. Cao, G. Zheng, Aqueous electrocatalytic N2 reduction under ambient conditions, Nano Res 11 (2018) 2992e3008. [6] A.R. Singh, B.A. Rohr, J.A. Schwalbe, M. Cargnello, K. Chan, T.F. Jaramillo, I. Chorkendorff, J.K. Norskov, Electrochemical ammonia synthesis-the selectivity challenge, ACS Catal. 7 (2017) 706e709. [7] J.H. Montoya, C. Tsai, A. Vojvodic, J.K. Norskov, The challenge of electrochemical ammonia synthesis: a new perspective on the role of nitrogen scaling relations, Chemsuschem 8 (2015) 2180e2186. [8] Y. Yao, S. Zhu, H. Wang, H. Li, M. Shao, A spectroscopic study on the nitrogen electrochemical reduction reaction on gold and platinum surfaces, J. Am. Chem. Soc. 140 (2018) 1496e1501. [9] E. Skulason, V. Tripkovic, M.E. Bjorketun, S. Gudmundsdottir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jonsson, J.K. Norskov, Modeling the electrochemical hydrogen oxidation and evolution reactions on the basis of density functional theory calculations, J. Phys. Chem. C 114 (2010) 18182e18197. [10] K. Uosaki, G. Elumalai, D. Hung Cuong, A. Lyalin, T. Taketsugu, H. Noguchi, Highly efficient electrochemical hydrogen evolution reaction at insulating
boron nitride nanosheet on inert gold substrate, Sci. Rep. 6 (2016) 32217. [11] X. Wang, W. Wang, M. Qiao, G. Wu, W. Chen, T. Yuan, Q. Xu, M. Chen, Y. Zhang, X. Wang, J. Wang, J. Ge, X. Hong, Y. Li, Y. Wu, Y. Li, Atomically dispersed Au-1 catalyst towards efficient electrochemical synthesis of ammonia, Sci. Bull. 63 (2018) 1246e1253. [12] H. Wang, H. Yu, Z. Wang, Y. Li, Y. Xu, X. Li, H. Xue, L. Wang, Electrochemical fabrication of porous Au film on Ni foam for nitrogen reduction to ammonia, Small 15 (2019) 1804769. [13] D. Bao, Q. Zhang, F.-L. Meng, H.-X. Zhong, M.-M. Shi, Y. Zhang, J.-M. Yan, Q. Jiang, X.-B. Zhang, Electrochemical reduction of N2 under ambient conditions for artificial N2 fixation and renewable energy storage using N2/NH3 cycle, Adv. Mater. 29 (2017) 1604799. [14] M.-M. Shi, D. Bao, B.-R. Wulan, Y.-H. Li, Y.-F. Zhang, J.-M. Yan, Q. Jiang, Au subnanoclusters on TiO2 toward highly efficient and selective electrocatalyst for N2 conversion to NH3 at ambient conditions, Adv. Mater. 29 (2017) 1606550. [15] S.J. Li, D. Bao, M.M. Shi, B.R. Wulan, J.M. Yan, Q. Jiang, Amorphizing of Au nanoparticles by CeOx-RGO hybrid support towards highly efficient electrocatalyst for N2 reduction under ambient conditions, Adv. Mater. 29 (2017) 1700001. [16] Q. Qin, T. Heil, M. Antonietti, M. Oschatz, Single-site gold catalysts on hierarchical N-doped porous noble carbon for enhanced electrochemical reduction of nitrogen, Small Methods 2 (2018) 1800202. [17] M. Nazemi, M.A. El-Sayed, Electrochemical synthesis of ammonia from N2 and H2O under ambient conditions using pore-size-controlled hollow gold nanocatalysts with tunable plasmonic properties, J. Phys. Chem. Lett. 9 (2018) 5160e5166. [18] M. Nazemi, S.R. Panikkanvalappil, M.A. El-Sayed, Enhancing the rate of electrochemical nitrogen reduction reaction for ammonia synthesis under ambient conditions using hollow gold nanocages, Nano Energy 49 (2018) 316e323. [19] Z. Wang, Y. Li, H. Yu, Y. Xu, H. Xue, X. Li, H. Wang, L. Wang, Ambient electrochemical synthesis of ammonia from nitrogen and water catalyzed by flower-like gold microstructures, ChemSusChem 11 (2018) 3480e3485. [20] W.L. Zhu, R. Michalsky, O. Metin, H.F. Lv, S.J. Guo, C.J. Wright, X.L. Sun, A.A. Peterson, S.H. Sun, Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO, J. Am. Chem. Soc. 135 (2013) 16833e16836. [21] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508e517. [22] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000) 7756e7764. [23] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865e3868. [24] E.R. McNellis, J. Meyer, K. Reuter, Azobenzene at coinage metal surfaces: role of dispersive van der Waals interactions, Phys. Rev. B 80 (2009) 205414. [25] B. Delley, Hardness conserving semilocal pseudopotentials, Phys. Rev. B 66 (2002) 155125. [26] A. Klamt, G. Schüürmann, COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient, Journal of the Chemical Society, Perkin Transactions 2 (1993) 799e805. [27] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, nsson, Origin of the overpotential for oxygen reduction at a fuel-cell H. Jo cathode, J. Phys. Chem. B 108 (2004) 17886e17892. [28] A.A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J.K. Nørskov, How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels, Energy Environ. Sci. 3 (2010) 1311e1315. [29] J. Rossmeisl, A. Logadottir, J.K. Nørskov, Electrolysis of water on (oxidized) metal surfaces, Chem. Phys. 319 (2005) 178e184. [30] S. Peng, Y. Lee, C. Wang, H. Yin, S. Dai, S. Sun, A facile synthesis of monodisperse Au nanoparticles and their catalysis of CO oxidation, Nano Res 1 (2008) 229e234. [31] S.J. Guo, D.G. Li, H.Y. Zhu, S. Zhang, N.M. Markovic, V.R. Stamenkovic, S.H. Sun, FePt and CoPt nanowires as efficient catalysts for the oxygen reduction reaction, Angew. Chem. Int. Ed. 52 (2013) 3465e3468. [32] G.-F. Chen, S. Ren, L. Zhang, H. Cheng, Y. Luo, K. Zhu, L.-X. Ding, H. Wang, Advances in electrocatalytic N2 reduction-strategies to tackle the selectivity challenge, Small Methods 3 (2019) 1800337. [33] G.W. Watt, J.D. Chrisp, Spectrophotometric method for determination of hydrazine, Anal. Chem. 24 (1952) 2006e2008. [34] D. Yan, H. Li, C. Chen, Y. Zou, S. Wang, Defect engineering strategies for nitrogen reduction reactions under ambient conditions, Small Methods 3 (2019) 1800331.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Size-dependent electrochemical nitrogen reduction catalyzed by monodisperse Au nanoparticles Chan Chen a, Cong Liang b, Jun Xu a, c, Jiankun Wei a, Xiangrong Li a, Ying Zheng g, Junrui Li d, **, Haolin Tang e, Junsheng Li a, f, * a
School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, PR China China Automotive Technology and Research Center, Tianjin, 300300, PR China Research Center for Materials Genome Engineering, Wuhan University of Technology, Wuhan, 430070, PR China d Department of Chemistry, Brown University, Providence, RI, 02912, USA e State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, PR China f Hubei Provincial Key Laboratory of Fuel Cell, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, PR China g College of Urban Construction, Wuchang Shouyi University, Wuhan, 430064, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2019 Received in revised form 31 December 2019 Accepted 13 January 2020 Available online 14 January 2020
Electrochemical nitrogen reduction reaction (ENRR) is a mild and environmentally friendly process that can be potentially complementary to the traditional Haber-Bosch process. Development of highperformance ENRR catalysts and understanding of the activity origin of the catalyst are essential for the practical application of ENRR process. Herein, we report the size-dependent ENRR activity of monodisperse Au nanoparticles (NPs) sized ranging from 4 nm to 10 nm. 8 nm Au NPs is found to be an excellent ENRR catalyst with an ammonia production rate and Faradic efficiency of 17.49 mg h1 mg1Au and 5.79%, respectively. The high ENRR performance of 8 nm Au NP is rationalized with DFT studies, which indicate that an optimal proportion of the surface step sites is key to suppressing hydrogen evolution reaction and improving the catalytic activity towards ENRR. The study herein may provide guidelines for tailoring the ENRR catalysis on nanostructured metal catalysts. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Electrochemical nitrogen reduction Gold nanoparticle Monodisperse Size effect
1. Introduction Ammonia is one of the most important chemical products widely used in industries ranging from fertilizer production to energy storage [1]. Electrochemical nitrogen reduction reaction (ENRR) under aqueous condition is a promising complementary process to the traditional Haber-Bosch process due to its mild reaction condition, low facility demand and environmental friendliness [2e4]. The key to cost-effective ENRR is the development of high-performance electrocatalyst for the reaction [5]. However, the search of efficient ENRR electrocatalyst is challenging due to the high activation barrier of molecular nitrogen. In addition, ENRR at aqueous condition is facing the competition from hydrogen evolution reaction (HER) because of the relatively lower overpotential
* Corresponding author. School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, PR China. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J. Li). https://doi.org/10.1016/j.electacta.2020.135708 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
of HER [6]. An ideal ENRR catalyst should not only effectively lower the activation barrier toward nitrogen reduction but also show depressed activity for HER [3]. As a stable metallic catalyst, Au has been demonstrated to be a promising ENRR catalyst [7,8] with relatively low HER activity [9,10]. A previous experimental study suggests that ENRR on Au surface possibly initiates from the adsorption of N2 molecule on Au surface, followed by successive proton and electron transfer, which results in N2H4 by-product and NH3 product [8]. These preliminary findings have inspired intensive research interests on Au-based catalysts for ENR [11,12]. Generally, the ENRR performance of Au based material can be controlled by engineering the surface chemical state or morphology of Au catalysts. To modulate the surface electronic state of the Au ENRR catalyst, tetrahexahedral gold nanorods with exposed high-index facets were synthesized [13]. Au nanostructures with rich low coordination sites, such as Au nanocluster [14], amorphous Au nanostructure [15] and single-site Au [16], were also proposed to be an active ENRR catalyst. In addition, the activity of these Au nanostructures can be enhanced by introducing synergy between
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the Au catalysts and appropriate support [15]. Tailoring the morphology of the Au nanostructure may increase the surface area of the material and improve the mass transfer for the catalytic reaction, which are favorable for an improved ENRR performance. Hollow Au nanocages with regulated cavity size and porous features have been developed and shown to be highly active for ENRR [17,18]. Au nano-flower with rich dendrite structure was also synthesized for an efficient ENRR process [19]. Despite the progress outlined above, there is still lack of understanding about the optimal morphology and active sites towards ENRR when designing highly active nanostructured Au ENRR catalysts. Herein, we present a study on the ENRR activity of monodisperse Au NPs with different sizes and the origin of their size-dependent activity through DFT studies. Our results show that mono-dispersed Au NP with a size of ~8 nm exhibits the highest catalytic activity towards ENRR. The 8 nm Au NPs possess an optimum portion of the surface step site, namely (211) facet, which greatly suppresses HER but is energetically favorable for ENRR catalysis. 2. Experimental Monodisperse Au NPs with sizes of 4, 6, 8, 10 nm were synthesized with a modified method of what was reported before [20]. Au NPs were loaded onto Ketjenblack Carbon (EC600j) by sonicating the mixture of Au NP dispersion in hexane and Ketjenblack Carbon, resulting CeAu. The CeAu was stirred overnight in glacial acetic acid to remove the surfactant. The mixture was washed for three times with 20 mL ethanol, and then vacuum dried at 120 C for 3 h to remove the solvent. The catalyst ink was prepared by dispersing 5 mg KB-Au in a mixture of 900 mL isopropyl alcohol, 20 mL DI water and 20 mL 5% Nafion solution. The dispersion was sonicated for 30 min to form a homogeneous ink. The ink was then loaded onto a carbon cloth with a size of 1 2 cm2 and dried under ambient conditions. The loading amount of Au/C catalyst on the carton cloth was controlled to be 0.42 mg cm2. Transmission electron microscopy (TEM) images were obtained on a Philips CM20 operating at 200 kV. The electrochemical nitrogen reduction reaction (ENRR) measurement was performed in an H-shape electrochemical cell, which was separated by Nafion 211 membrane. Before ENRR tests, Nafion membrane was pretreated by heating in turn in 5% H2O2 aqueous solution, 5% H2SO4 aqueous solution and ultrapure water at 80 C for 1 h, respectively. 50 mL of 0.1 M KOH solution was added into the cathode and anode compartment, respectively. Ultra-pure nitrogen (99.99%) was bubbled into the catholyte at a flow rate of 50 mL min1 for 20 min before reaction. The outlet gas tube was connected with an ammonia trap filled with 20 mL of 0.005 M H2SO4 aqueous solution as adsorbent liquid to collect any ammonia escaped from the electrolyte. The electrochemical experiments were carried out with a CHI electrochemical workstation (CHI-660D) using a threeelectrode configuration. The prepared electrode, graphite rod and salt bridge (with L-shaped sand core) Ag/AgCl electrode (3.5 M KCl electrolyte) was used as working electrode, counter electrode and reference electrode, respectively. Prior to the measurement, the working electrode was activated by cyclic voltammetry (CV) scanning between 0.74 V and 0.36 V at a scanning speed of 100 mV s1 for 50 cycle. Then, linear sweep voltammetry (LSV) curves were collected at a scanning speed of 10 mV s1. To test the products, controlled potential electrolysis tests were carried out at fixed potentials for 10000 s. The potential was converted to RHE scale via calibration. The calibration was performed using Pt wire as both working electrode and counter electrode in H2-saturated electrolyte. LSV curves were acquired at a scan rate of 0.1 mV s1. The potential at which the current crossed zero is taken to be the
thermodynamic potential (vs. Ag/AgCl) for the hydrogen electrode reactions (Fig. S1). Salicylate method was used to test the concentration of the ammonia produced. 3 mL of the testing solution (catholyte or absorbent; the pH of absorbent solution was adjusted by concentrated KOH solution to 13) was mixed with 50 mL of 2 M NaOH solution containing 0.05 M NaClO, followed by addition of 250 mL of 0.5 M NaOH aqueous solution with 5 wt% salicylic acid, 5 wt% sodium citrate. Next, 50 mL of 1 wt% C5FeN6Na2O (sodium nitroferricyanide) aqueous solution was thoroughly mixed with the mixture solution above, and then store sealed at room temperature for 2 h. The UVevis absorbance was then measured by an UVeviseNIR spectrophotometer at 660 nm (N4, Shangfen, China) The calibration curves were built using NH4Cl solution in the presence of 0.1 M KOH to quantify the produced NH3. All the DFT calculations were implemented by Dmol3 module [21,22]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed [23]. The TS method for DFT-D correction was performed to consider the van der Waals interaction [24]. The double numerical plus polarization (DNP) basis was used for higher accuracy. In addition, the DFT semicore pseudopotential (DSPP) method was adopted [25]. Self-consistent field (SCF) computations were performed with a convergence criterion of 106 au on the total energy and electronic computations. To ensure credible results, the real-space global orbital cutoff radius was chosen as big as 4.7 Å in all computations. Furthermore, we adopted the conductor-like screening model (COSMO) [26] to simulate solvent effects and the dielectric constant of the water was set as 78.54. Au(211) and Au(111) were modeled by a 3 3 1 supercell with a vacuum space of 15 Å. The slab models used a Monkhorst-Pack mesh with 5 5 1 in reciprocal space. Au13 cluster model was also modeled and the Brillion zone was sampled with a MonkhorstPack mesh with 3 3 3. The adsorption energy (DE) of intermediate species on the Au surfaces and Au13 cluster are define as follow:
DE ¼ Etotal Eslab Eads where E total, E slab and E ads are the energy of the species-adsorbed system, the catalyst slab and species, respectively. The reaction free energy (DG) of the elementary reaction in the HER and NRR were calculated according to the computational hydrogen electrode (CHE) model proposed by Nørskov et al. [27e29] The energy of a proton/electron (Hþþe) is equal to half of the chemical potential of hydrogen and the pH is set as zero in this model. DG value was calculated as follow:
DG ¼ DE þ DZPE T DS where DE was obtained by DFT calculations as above, DZPE is the change in the zero-point energies (ZPE), and TDS is the change in entropy in 298.15 K, which were computed from the vibrational frequencies and the gas-phase molecules were obtained from the standard thermodynamic database. All structures were optimized and calculations presented in the study were for a pH value of 0. 3. Results and discussion Monodisperse Au NPs in 4 nm were synthesized via an adapted method from previous publication. Briefly, 0.2 g HAuCl6$3H2O was dissolved in a mixture of 10 mL tetralin (1,2,3,4tetrahydronaphthalene) and 10 mL oleylamine (OAm). Under the protection of N2 atmosphere, 1 mmol TBAB (tert-bultylamine borane) dissolved in a mixture of 1 mL tetralin and 1 mL OAm was injected into the previous solution to initiate the nucleation. The
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Fig. 1. TEM images of Au NPs of different sizes: (A) 4 nm; (B) 6 nm; (C) 8 nm and (D) 10 nm.
reaction was kept at room temperature for 1 h for completion. Au NPs were precipitated by adding 10 mL hexanes and 40 mL ethanol into the solution and collected via centrifugation. The Au NPs were then washed with 10 mL hexanes and 40 mL ethanol before being dispersed in hexanes for further use. 6 nm Au NPs were obtained via a seed-mediated method by using 4 nm Au NPs as seeds. 0.12 g HAuCl6$3H2O was dissolved in a mixture of 10 mL 1-octadecene (ODE) and 10 mL OAm. 30 mg 4 nm Au NPs were added into the
3
solution, then the reaction was heated to 80 C at a rate of 5 C min1. The reaction was kept at 80 C for 2 h before cooling down to room temperature. After washing with hexanes and ethanol, Au NPs were collected and dispersed in hexane. For 8 nm and 10 nm Au NPs, same conditions to 6 nm Au NPs were used except that 6 nm and 8 nm Au NPs were used as seeds, respectively. The monodispersity of the synthesized Au nanoparticles was quantified with TEM measurements. It was previously demonstrated that such synthesis yields polycrystalline Au of face centered structure (FCC) with multiple exposed (111) planes [30]. TEM images and a detailed particle size analysis (Fig. 1 and Fig. S2) showed that the Au NPs are uniform in size and morphologies. The Au NPs were loaded onto Ketjen black (KB) for the evaluation of their ENRR performance. High resolution TEM measurement confirms successful loading of the Au nanoparticles onto the carbon matrix (Fig. S3). The Au NPs loaded KB (Au/C) were treated with acetic acid followed by washing with ethanol several times to remove the residual oleylamine on Au nanoparticles [31]. The compositional information of the Au/C catalyst was characterized using XPS with 8 nm Au/C catalyst as an example. C and O of different chemical states (from both the carbon cloth and Nafion binder) were identified in the spectra (Fig. S4). In addition, no N was detected on the surface of the catalyst, demonstrating complete removal of residual N on the Au NPs. ENRR performance of the Au NPs was characterized in a closed H-shaped cell, which was separated by a Nafion membrane. All the electrochemical characterizations were conducted in 0.1 M KOH. LSV curves of the Au electrodes in N2 or Ar saturated electrolyte were firstly recorded. For 8 nm Au/C, a noticeable onset of ENRR was observed at a potential of ~ -0.14 V (Fig. 2A). With the negative
Fig. 2. (A) LSV curves of 8 nm Au/C. (B) Chronoamperometric curves of 8 nm Au/C catalyst at different potentials. (C) Faradaic efficiency of Au/C catalyst with different Au particle size. (D) ammonia yield of Au/C catalyst with different Au particle size. The error bar represents standard deviations of results from three independent experiments. (Error bars in the figures represents standard deviations determined from three individual experiments.)
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shift of potential, a more pronounced ENRR current was observed. When the potential was lowered below ~ -0.5 V, ENRR current became neglectable because of the favorable HER process at low potential. The LSV curves of other electrodes are shown in Fig. S5. The results showed that all the electrodes have similar onset for ENRR. The ENRR performance of the Au/C electrodes were studied in more details by electrolysis tests at given potentials with continuous N2 bubbling (Fig. 2B). The reaction currents of Au/C electrodes were highly stable during the electrolysis and the currents increased with the decrease of the potential for electrolysis, as shown in Fig. 2B and Fig. S6. The produced ammonia during the electrolysis in the catholyte and adsorbent (at the outlet) was analyzed with the salicylate method [32]. The UVevis spectra of typical standard NH4Cl solutions and corresponding calibration curves for quantification are shown in Fig. S7. The reported ammonia yield was the sum of the ammonia detected in the cathode electrolyte and in the adsorbent. A detailed list of the source of ammonia yield was shown in Fig. S8. No hydrazine was generated in the ENRR process (Fig. S9), as reflected by the Watt and Chrisp analysis [33]. The absence of hydrazine in the products suggests that ENRR on Au/C follows an associative distal pathway [34]. To identify the optimal potential for ENRR, the electrolysis and corresponding product analysis was performed at different potentials ranging from 0.04 V to 0.44 V. The average Faradic efficiency and ammonia yield over Au/C electrodes were determined from three individual experiments and shown in Fig. 2C and D. The highest Faradic efficiency (FE) was attained at 0.14 V for all the Au/ C electrodes. Among all the electrodes, 8 nm Au/C electrode exhibited a highest FE of 5.8% at 0.14 V. In addition to its high FE, 8 nm Au/C electrode also had a high ammonia yield. The ammonia yield rate of 8 nm Au/C electrode was measured to be 17.49 mg h1 mg1Au, higher than 4 nm Au/C electrode (4.24 mg h1 mg1Au), 6 nm Au/C electrode (3.92 mg h1 mg1Au) and 10 nm Au/C electrode (6.92 mg h1 mg1Au). Such a high FE and ammonia yield is comparable to the advanced Au based catalyst reported recently (Table S1). To demonstrate that the produced ammonia is generated from the dissolved N2 in the electrolyte, control electrochemical measurements were conducted. First of all, the 8 nm Au/C electrode was tested in N2 saturated electrolyte at open circuit potential. After 10000 s of test, the electrolyte from the cathode cell was collected and analyzed. No ammonia was tested in the collected electrolyte (Fig. S10A). Furthermore, the 8 nm Au/C electrode was characterized in Ar saturated electrolyte at 0.14 V (the optimal ENRR potential as shown above) for 10000 s. Ammonia products was also absent in the catholyte of the electrochemical cell after the test (Fig. S10B) These results altogether demonstrated that the N2 dissolved in the electrolyte is the only possible source for the produced ammonia. The durability of the Au/C catalyst was investigated with repeated ammonia production tests at 0.14 V using 8 nm Au/C electrode as an example. Between each test, the Au/C electrode was rinsed with DI water and dried to remove the adsorbed electrolyte from the last measurement. Our results show that both the ammonia production rate and Faradic efficiency of 8 nm Au NP gradually decrease with cycling (Fig. S11). To investigate the origin of the performance decay, XPS and TEM characterizations were performed with the samples after durability tests. As shown in the TEM images of the sample, no NP aggregation was observed after the durability test (Fig. S12). In addition, the XPS measurements revealed that the surface states of Au remain unchanged after the durability test (Fig. S13). Considering the fact that Au NP is pasted onto the carbon cloth with Nafion binder, we think that the main reason for the decrease in the observed NRR activity is the detachment of Au NP from the carbon cloth due to the long-term soaking of the electrode in the cathode cell and repeated washing
and drying between each cycle. The performance of the catalyst is determined by both its intrinsic activity and surface area. The 4 nm Au/C catalyst, with a higher surface area compared to its counterparts, showed poor ENRR performance. This result suggests that the difference in ENRR performance of the Au/C catalysts is mainly originated from their different intrinsic activities induced by the size effect, other than the surface area effect. The proportion of uncoordinated Au atom at the surface of Au NPs (with size below 10 nm) is significantly influenced by the size of the NPs. The crystallite size of the Au nanoparticles was estimated with Scherrer equation: t ¼ Kl/bcosq, where K is a dimensionless shape factor. The shape factor has a typical value of about 0.9; l is the X-ray wavelength (1.5418 Å) and b is the line broadening at half the maximum intensity (FWHM) of Au (111) peak. The crystallite size of 4 nm Au NP, 6 nm Au NP, 8 nm Au NP and 10 nm Au NP were estimated to be 1.8 nm, 2.0 nm, 2.7 nm and 3.4 nm, respectively (Fig. 3). The standard XRD pattern of Au (PDF No. 4e784) was also provided for comparison. As shown in the figure, the (111), (200), (220) and (311) facet (centered at ~38.2 , 44.4 , 77.5 and 64.6 , respectively) of Au could be clearly identified in the XRD spectra of the as-synthesized Au NPs. Considering the fact that the fraction of the exposed Au facets differs significantly for ultra-small Au nanoparticles of different sizes [20], we hypothesize that the observed size-specific ENRR activity is most probably a result of different coordination states of Au atoms in Au NPs of different sizes. To verify this hypothesis, Au cluster (Au13 with exclusive corner sites), Au (211) and Au (111) facets, which corresponded to the corner site, edge site and planar site of small Au NP, respectively, were modeled and analyzed with DFT studies. Since no hydrazine was generated on the monodispersed Au NPs (shown above), the ENRR process probably follows an associative distal pathway. Thus, we calculated the free energy diagrams of ENRR via associative distal mechanism on these models (Fig. 4). It is clear from the diagram that the rate limiting steps on the Au models is the reductive protonation of *NN to *NNH. The calculated free-energy change for this rate limiting step (△G*NNH) on Au13, Au (111) and Au (211) was 1.08, 2.16 and 1.50 eV, respectively (Fig. 4). From this result, Au (211) and Au13 could be considered as the main potential active sites for ENRR. Furthermore, the projected density of states (pDOS) of different Au models were plotted (Fig. S14). The 5d band of Au on Au (211) facets had a slightly higher tendency to overlap with 2p
Fig. 3. XRD spectra of Au nanoparticles of different sizes.
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Fig. 4. Free energy diagrams and corresponding optimized configuration for electrochemical reduction of nitrogen on Au(111), Au(211), or a 13-atom Au cluster via associative mechanism at 0 V.
orbitals of Nads in *NNH than that for Au (111), suggesting a stronger bonding of Nads with Au (211). Such a stronger bonding is beneficial for the stabilization of reaction intermediate and enhance the overall activity. The free energy changes of HER on Au13, Au (211) and Au (111) were also compared to investigate the ENRR selectivity on these model surfaces. Hydrogen adsorption on Au13 is thermodynamically favorable with a negative △G*H of 0.234 eV (Fig. 5). In
contrast, the energy barrier for the adsorption of hydrogen on Au (211) reached 0.252 eV, higher than that for Au (111). These DFT results proves that the edge site Au atoms, modeled with Au (211) facet, have relatively high ENRR activity and suppressed HER performance. As shown in Fig. S15, Au nanoparticles with crystallite size below 2 nm has a high proportion of HER active corner sites, suggesting that ENRR might be unfavorable on these nanoparticles due to the competition of HER. With the increase of crystallite size above 2 nm, the Au nanoparticle tends to show depressed HER because of lower fraction of corner sites on the nanoparticle. It should be noted the fraction of overall surface exposed Au atoms and Au atoms on the edge sites also decreases with the increase of crystallite size, which may lead to decreased ENRR performance of the Au nanoparticles. When the crystallite size of Au NPs increases above 3 nm, the proportion of the edge sites (211) decreases. Thus, the Au nanoparticle with optimal ENRR performance should have an intermediate size that ensures a large surface area and simultaneously appropriate fraction of ENRR/HER active sites. Therefore, the observed high performance of 8 nm Au/C electrode (with a crystallite size of 2.7 nm) for ENRR can be explained by its balanced proportion of edge site Au atoms and corner site atoms as well as its relatively large surface area. 4. Conclusion
Fig. 5. Free energy diagrams for electrochemical reduction of protons to hydrogen on Au(111), Au(211), or a 13-atom Au cluster at 0 V.
Model Au NPs with mono-disperse sizes ranging from 4 nm to 10 nm are synthesized and investigated in terms of their ENRR activity. Electrochemical characterizations show that carbon supported 8 nm Au NPs exhibits the highest ENRR performance, with ammonia yield rate and Faradic efficiency being 17.49 mg h1
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mg1Au and 5 .79%, respectively. DFT analysis demonstrates that the excellent ENRR performance of 8 nm Au NPs can be attributed to their optimal proportion of coordination unsaturated edge site atoms and their relatively large surface area. Our results show that the 8 nm Au NPs can be a highly potential electrocatalyst for ENRR. In addition, the fundamental understanding of the activity of Au nanoparticles unveiled in this study provides insights for the future design of high performance ENRR electrocatalyst. Credit author statement Chan Chen: Investigation; Methodology; Writing. Cong Liang: Methodology; Investigation. Jun Xu: Methodology. Jiankun Wei: Methodology. Xiangrong Li: Data Curation. Ying Zheng: Data Curation. Junrui Li: Conceptualization; Writing; Supervision. Haolin Tang: Conceptualization. Junsheng Li: Conceptualization; Writing; Supervision; Funding acquisition. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant Nos. 51972254), Fundamental Research Funds for the Central Universities (WUT: 2018IB026, 2019IB003 and 195220002), State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology, 2019-KF-10) and Fundamental Research Funds for the Central Universities for finical support (2019-HS-B1-15, 2019III048GX). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135708. References [1] A. Boulamanti, J.A. Moya, Production costs of the chemical industry in the EU and other countries: ammonia, methanol and light olefins, Renew. Sustain. Energy Rev. 68 (2017) 1205e1212. [2] M. Jewess, R.H. Crabtree, Electrocatalytic nitrogen fixation for distributed fertilizer production? ACS Sustain. Chem. Eng. 4 (2016) 5855e5858. [3] X. Cui, C. Tang, Q. Zhang, A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions, Adv. Energy Mater. 8 (2018) 1800369. [4] J. John, D.K. Lee, U. Sim, Photocatalytic and electrocatalytic approaches towards atmospheric nitrogen reduction to ammonia under ambient conditions, Nano convergence 6 (2019) 15. [5] N. Cao, G. Zheng, Aqueous electrocatalytic N2 reduction under ambient conditions, Nano Res 11 (2018) 2992e3008. [6] A.R. Singh, B.A. Rohr, J.A. Schwalbe, M. Cargnello, K. Chan, T.F. Jaramillo, I. Chorkendorff, J.K. Norskov, Electrochemical ammonia synthesis-the selectivity challenge, ACS Catal. 7 (2017) 706e709. [7] J.H. Montoya, C. Tsai, A. Vojvodic, J.K. Norskov, The challenge of electrochemical ammonia synthesis: a new perspective on the role of nitrogen scaling relations, Chemsuschem 8 (2015) 2180e2186. [8] Y. Yao, S. Zhu, H. Wang, H. Li, M. Shao, A spectroscopic study on the nitrogen electrochemical reduction reaction on gold and platinum surfaces, J. Am. Chem. Soc. 140 (2018) 1496e1501. [9] E. Skulason, V. Tripkovic, M.E. Bjorketun, S. Gudmundsdottir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jonsson, J.K. Norskov, Modeling the electrochemical hydrogen oxidation and evolution reactions on the basis of density functional theory calculations, J. Phys. Chem. C 114 (2010) 18182e18197. [10] K. Uosaki, G. Elumalai, D. Hung Cuong, A. Lyalin, T. Taketsugu, H. Noguchi, Highly efficient electrochemical hydrogen evolution reaction at insulating
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