Oxygen vacancy engineering in spinel-structured nanosheet wrapped hollow polyhedra for electrochemical nitrogen fixation under ambient conditions

State Key Laboratory for Modication of College of Materials Science and Engineeri and Technology, Donghua University, Shang dhu.edu.cn Christopher Ingold Laboratory, Department 20 Gordon Street, London WC1H 0AJ, UK. E uk Hefei National Laboratory for Physical S Science and Technology of China, Hefei, An Department of Materials Science and Eng University, Beijing 100871, P. R. China † Electronic supplementary information TGA, and XPS results, adsorptive prope curves, electrochemical properties of sam DOI: 10.1039/c9ta11408d Cite this: J. Mater. Chem. A, 2020, 8, 1652


Introduction
Ammonia (NH 3 ) is one of the most important carbon-free energy intermediates with low liquefying pressure and high hydrogen density 1,2 and is widely used in producing numerous chemicals, such as nitrogen fertilizers or pharmaceuticals, to satisfy the increasing demand from a booming world population. [3][4][5] Up to now, over 1% of the world's energy supply is used to produce ammonia by the traditional Haber-Bosch process which is responsible for 1.6% of global CO 2 emissions and requires harsh operating conditions (400-600 C and 20-40 MPa) due to the chemical inertness of N 2 with a high bond energy of 940.95 kJ mol À1 . [6][7][8][9] In contrast, the electrochemical N 2 reduction reaction (NRR, N 2 + 6H + + 6e À / NH 3 ) occurs at ambient temperature and pressure and is regarded as a more sustainable and energy-efficient process for ammonia generation. [10][11][12][13][14] Although some electrocatalysts have recently been investigated for the NRR, including noble metal electrocatalysts (such as Ru, Au, and Rh), [13][14][15][16] non-noble metal-based electrocatalysts (such as Fe-, Co-, and Mo-based materials), 7,12,17,18 and conducting polymers (such as polyaniline and polypyrrole), 19,20 their electrocatalytic properties are still limited by the low faradaic efficiency due to the conned electro-active sites in the corresponding bulk. 10,11,21 Therefore, constructing multilevel nanostructures (such as one-dimensional nanobers/nanotubes, 22,23 two-dimensional ultrathin nanosheets, 24,25 and threedimensional hollow nanostructures 26,27 ) has become an efficient solution to obtain outstanding NRR electrocatalysts with a high specic surface area, thus resulting to an increased number of active sites for efficient reactant adsorption.
Apart from this, the impeded NRR process is deeply rooted in the chemical inertness of N 2 molecules. Then, the question on how to capture and activate N 2 gas much more efficiently using electrocatalysts arises. Atomic surface engineering (such as defect engineering 28,29 and doping engineering 30 ) is the best choice to regulate the electronic structure and alter the charge density distribution. Among various types of defects, the vacancy-type defect (such as oxygen vacancies) is regarded as a kind of point defect, which is caused by independent atoms escaping from the atomic surface. [31][32][33] Up to now, several possible strategies have been discovered for generating oxygen vacancies in metal oxides, including chemical reduction, 34 annealing in an oxygen poor atmosphere, 35,36 and heterogeneous atom substitution. 37 Furthermore, the oxygen vacancies created on the metal oxide surface could serve as trapping sites to capture and activate inert gas molecules of N 2 . However, to our knowledge, the proper control of the oxygen vacancy content in the electrocatalytic N 2 reduction reaction is still in the rudimentary state.
To gure out the contribution of the oxygen vacancies in NRR applications, we conducted density functional theory (DFT) calculations by using NiCo 2 O 4 with a perfect surface and oxygen vacancy-introduced surface as two models. As shown in Fig. 1, the introduction of oxygen vacancies in NiCo 2 O 4 is benecial for enhancing its stability towards NNH groups due to the presence of fewer coordination sites of Ni/Co cations around the vacancies. As further proved by the differential charge density of NiCo 2 O 4 with oxygen vacancies, the bonded electrons can be delocalized to electron donors (Ni and Co elements) near the oxygen vacancies and be activated dramatically. As a result, the NiCo 2 O 4 with oxygen vacancies exhibits a lower stabilization energy of 0.30 eV for NNH groups than NiCo 2 O 4 with a perfect surface (0.61 eV), indicating the positive role of oxygen vacancies in the following nitrogen reduction reactions. In addition, the delocalized electrons on the surface of NiCo 2 O 4 with oxygen-vacancies are easier to excite to the conduction band, which is favorable not only for enhanced conductivity 35,38 but also for activation of N-related groups, by transferring its electrons into their antibonding orbitals. At the same time, the hollow carbon polyhedron is benecial not only for electron transfer from buried carbon to the oxygen vacancyrich NiCo 2 O 4 surface for the fast NRR process, but also for increasing the number of active sites for the capture of NNH groups.
In this study, we design a series of highly efficient oxygen vacancy-rich spinel-structured nanosheets on hollow N-carbon polyhedra to facilitate the electrocatalytic N 2 reduction reaction. Taking the oxygen vacancy-rich NiCo 2 O 4 on hollow Ncarbon polyhedra (V o -rich NiCo 2 O 4 @HNCP) as an example, it exhibited a high production yield (NH 3 : 4.1 mg h À1 cm À2 /17.8 mg h À1 mg À1 and faradaic efficiency: 5.3%), good selectivity at À0.25 V versus RHE, and high stability during the NRR process and is much more outstanding than the corresponding oxygen vacancy-poor NiCo 2 O 4 on hollow N-carbon polyhedra (V o -poor NiCo 2 O 4 @HNCP). Furthermore, this oxygen vacancy engineering can also be applied in other spinel-structured nanosheets (such as ZnCo 2 O 4 and Co 3 O 4 ), which leads to a general solution for the design of advanced NRR electrocatalysts.

Results and discussion
According to the theoretical work, oxygen vacancy-rich NiCo 2 O 4 on hollow N-carbon polyhedra (V o -rich NiCo 2 O 4 @HNCP) was synthesized (Fig. S1 †). As shown in Fig. 2a, the V o -rich NiCo 2 -O 4 @HNCP displays a regular geometrical shape with a maximum length of $ 850 nm. A blurry interface between HNCP and V o -NiCo 2 O 4 nanosheets can be observed in the corresponding HRTEM image (Fig. 2b), with mixed phases of amorphous carbon and crystalline NiCo 2 O 4 , which is benecial for the charge accumulation and transfer. As a result, the two parts of HNCP and V o -rich NiCo 2 O 4 nanosheets can be well connected with a shared electron transfer region. Meanwhile, the fringe spacing of V o -rich NiCo 2 O 4 is revealed to be $0.23 nm in its HRTEM image (corresponding to the (311) plane), which demonstrates a negative role of oxygen vacancies in the crystalline NiCo 2 O 4 . As shown in the X-ray diffraction (XRD) pattern of V o -rich NiCo 2 O 4 @HNCP (Fig. 2c), three typical peaks at 2q ¼ 36.9 , 43.0 , and 62.7 can be well indexed to the (311), (400) and (440) planes of spinel-structured NiCo 2 O 4 (JCPDS card no. 73-1702), demonstrating its successful loading on the hollow nitrogen-doped carbon polyhedra. The nine-fold higher specic surface area (82.1 m 2 g À1 ) of V o -rich NiCo 2 O 4 @HNCP compared to the V o -rich NiCo 2 O 4 bulk (9.0 m 2 g À1 ) (Fig. 2d) indicates its higher number of active sites, leading to shortened diffusion paths for reagents during the NRR process.
Further, for comparison, oxygen vacancy-poor NiCo 2 O 4 on hollow N-carbon polyhedra (V o -poor NiCo 2 O 4 @HNCP) was obtained by calcining the corresponding nickel-cobalt layered double hydroxide nanosheet wrapped hollow nitrogen-doped carbon polyhedra (Ni-Co LDH@HNCP) in an oxygen atmosphere, and a series of characterization experiments was conducted ( Fig. S2-S8 †). In the N 1s X-ray photoelectron spectroscopy (XPS) spectrum ( Fig. 2e), the peaks at 400.6 eV, 399.7 eV, and 398.7 eV corresponded to the graphitic N, pyrrolic N, and pyridinic N of the HNCP template, which is benecial for the efficient connection of both V o -rich and V opoor NiCo 2 O 4 nanosheets to form a charge-rich region. In addition, the nitrogen content was calculated to be 11.6 wt% ( Fig. S9 †). The binding energies of Co 2p 3/2 (780 eV) and Co 2p 1/2 (795 eV) for V o -rich NiCo 2 O 4 @HNCP show negative shis compared to those for V o -poor NiCo 2 O 4 @HNCP (Fig. 2f). This indicates that some Co 3+ ions in the NiCo 2 O 4 nanosheets are reduced to Co 2+ aer introduction of oxygen vacancies, 39-42 due to the charge transfer from the V o to the Co(Ni) 3+ with a decreased number of Co(Ni)-O bonds at the surfaces (Fig. S10 †). In the O 1s XPS spectra (Fig. 2g), the peaks at 529.5, 531.1 and 532.5 eV are associated with Co(Ni)-O bonds (O1), oxygen vacancies or defects with lower oxygen coordination (O2), and surface adsorbed water molecules (O3), respectively. [43][44][45] The proportion of the integral-area of the O2 peak in V o -rich NiCo 2 O 4 @HNCP is much larger than that in V o -poor NiCo 2 O 4 @HNCP, indicating the successful introduction of abundant oxygen vacancies in V o -rich NiCo 2 O 4 @HNCP. 35,39 In addition, the V o (O2) proportion in V o -rich NiCo 2 O 4 @HNPC increases from 33.0% to 68.2% as the annealing temperature increased from 200 to 300 C (Fig. S11 †), which means that the incorporated V o amount can be well controlled by adjusting the annealing temperature. This nding could be further conrmed using the X-ray absorption near edge structure (XANES) of the O K-edge and Co L-edge. Fig. 2h shows the O K-edge XANES spectra of V o -rich and V o -poor NiCo 2 O 4 @HNCP with two main folds. In the rst fold at lower energy, another two sub-folds are centered at about 529.7 and 531.4 eV, which are denoted as t 2g and e g by the ligand eld, respectively. A relatively high intense ratio of e g to t 2g can be observed for V o -rich NiCo 2 O 4 @HNPC, indicating the successful generation of abundant V o , which can be further proved in the magnied version of t 2g and e g at the O K-edge (Fig. S12-S13 †). Meanwhile, attributed to the Co 2p to 3d transition and spin-orbit coupling, the Co L-edge spectra can be split into two parts, L3-edge and L2-edge (Fig. 2i). 46,47 The lower peak intensity for both the Co L3-edge and L2-edge in V o -rich NiCo 2 O 4 @HNCP indicates that more electrons would occupy the Co 3d orbitals, providing more crucial evidence for the presence of abundant V o on the surface of V o -rich NiCo 2 O 4 @-HNCP. A similar phenomenon is observed in the Ni L-edge spectrum (Fig. S14 †). The generation of oxygen vacancies is due to the inadequate oxidation of NiCo 2 O 4 in an oxygen-decient atmosphere as compared to an oxygen atmosphere, leading to abundant partially unoxidized metal domains in the NiCo 2 O 4 structure. 48,49 To reveal the role of oxygen vacancies in tuning the electronic structures of the NiCo 2 O 4 nanosheets, DFT calculations were performed for different types of oxygen vacancies. Considering that there are two types of oxygen (e.g. surface and sub-surface oxygens) in the perfect crystallographic lattice of NiCo 2 O 4 (Fig. 3a), we separately studied each situation regarding their density of states, formation possibility and adsorption ability towards the key NRR intermediates of NNH groups. The surface oxygen was removed rst for building the surface oxygen vacancy (sur-V o ) model, as shown in Fig. 3b. We found that the d-band center of the sur-V o model is À0.28 eV, much closer to the Fermi level compared with that of the pristine NiCo 2 O 4 model (À0.34 eV). This result indicates that oxygen vacancies can induce more activity of the metal atoms, that is, the adsorption strength of molecules and radicals over the metal atoms will be enhanced. As shown in Fig. 3c, we tested another type of oxygen vacancy, the sub-surface oxygen vacancy (sub-V o ) model, which has the closest d-band center (À0.24 eV) to the Fermi level among the three models. In the subsequent calculation, we found that the possibility of the formation of these two types of oxygen vacancies is not the same (Fig. 3d). As a more positive formation energy denotes that a vacancy is more difficult to form, the results show that sub-V o (2.47 eV) has a higher stability than sur-V o (3.25 eV). Finally, we calculated the adsorption strength of the three models by using NNH as a probe and found that the sub-V o model presents the most favorable energy (À0.03 eV). The adsorption energy of the sur-V o (0.30 eV) is also more negative than that of the pristine model (0.61 eV), indicating its enhanced adsorption ability, which is consistent with the d-band center results. According to the trend of formation energy, we can deduce that the sub-V o is preferentially formed when the vacancy density is relatively low. Therefore, the adsorption enhancement will be signicantly dependent on the vacancy density in the region of low vacancy density.
To illustrate the role of oxygen vacancies in the NRR, a twocompartment cell was assembled by employing V o -rich NiCo 2 -O 4 @HNCP as the cathode catalyst with continuous N 2 bubbling in a 0.1 M Na 2 SO 4 electrolyte. The highest average yields and corresponding faradaic efficiencies of V o -rich NiCo 2 O 4 @HNCP are achieved when the negative potential increases to À0.25 V versus the reversible hydrogen electrode (RHE) and are calculated to be about 4.1 mg h À1 cm À2 /17.8 mg h À1 mg À1 and 5.3%, respectively ( Fig. 4a and S15 †). This production yield at À0.25 V is also conrmed by the nuclear magnetic resonance (NMR) method as shown in Fig. S16-S18 and Table S1. † Beyond this negative potential, the NH 3 yields and faradaic efficiencies decrease signicantly because of the overwhelming competition from the hydrogen evolution reaction (HER). The NRR performance of the V o -rich NiCo 2 O 4 @HNCP catalyst is much more outstanding than that of the V o -poor NiCo 2 O 4 @HNCP catalyst (1.6 mg h À1 cm À2 /6.9 mg h À1 mg À1 and 1.8% at À0.25 V), as well as other results under ambient conditions or at high temperatures and pressures (Table S2 †). Note that no hydrazine is detected in the electrolyte (Fig. S19 †). In addition, the charge amounts of the experimentally quantied NH 3 and H 2 gases and the calculated one (Fig. S20 †) are roughly in agreement (S2 z S3). These results indicate the high selectivity of the V o -rich NiCo 2 O 4 @HNCP catalyst for NH 3 generation except for H 2 gas. For practical use, stability is another critical criterion to evaluate the NRR performance of a catalyst. As shown in Fig. 4b and S21-26, † the NH 3 yield, faradaic efficiency, and current density of the V o -rich NiCo 2 O 4 @HNCP catalyst are all stable without obvious uctuation. As shown in Fig. S27, † the V o -rich NiCo 2 -O 4 @HNCP catalyst displays ultra-stable properties in both NH 3 yield and faradaic efficiency even aer working for 100 h and is comparable to the recently reported catalysts but with a much longer practical life. 50 By using XRD and XPS analyses (Fig. S28 †), the V o -rich NiCo 2 O 4 @HNCP catalyst is found to exhibit an unchanged crystal structure and only slightly decreased oxygen vacancy content aer working for 100 h, which demonstrates the relatively stable structure of V o -rich NiCo 2 O 4 @HNCP as an NRR catalyst. Due to the similar results of NH 3 yield and faradaic efficiency aer varying the nitrogen ow rate at À0.25 V (Fig. 4c), the N 2 diffusion process becomes a non-rate-determining step as it is an independent gas-solid interface. Moreover, by varying the reaction temperature of the NRR, both the NH 3 yield and faradaic efficiency of the V o -rich NiCo 2 O 4 @HNCP catalyst increase simultaneously (Fig. 4d). For instance, the NRR yield is about 2.5 times higher at 60 C than at 0 C, indicating that mass transfer plays a key role in enhancing the reaction rate of the V o -rich NiCo 2 O 4 @HNCP catalyst. In accordance with the Arrhenius equation and Arrhenius plot (Fig. S29 †), the apparent activation energy of V orich NiCo 2 O 4 @HNCP for the NRR is calculated to be 11.4 kJ mol À1 . The UV/Vis absorption spectra of various samples (Fig. 4e) and photographs of NH 4 + -containing solutions before and aer staining with indophenol indicator (Fig. 4f) are exhibited. Aer comparing the EIS spectra (Fig. S30 †) and the performance of the V o -rich NiCo 2 O 4 bulk (1.4 mg h À1 cm À2 /6.1 mg h À1 mg À1 and 1.6% at À0.25 V) in Fig. S31, † it could be seen that the hollow nitrogen-doped carbon polyhedron template in V o -rich NiCo 2 O 4 @HNCP is extremely vital for its enhanced NRR performance, due to the increased specic surface area (Fig. 2d) and well-formed interface between HNCP and V o -rich NiCo 2 O 4 nanosheets for charge accumulation/transfer. For the verication of the source of ammonia, a 15 N isotope labeling experiment using NMR (600 MHz) was performed. As shown in Fig. S32, † the 1 H NMR signals of 14 NH 4 + produced 14 N triplets in the region of 6.8-7.1 ppm. Importantly, the 1 H NMR signal of 15 NH 4 + only produced an 15 N doublet without the appearance of 14 N triplets in the same region, which demonstrates that the doped nitrogen atoms in the HNCP template are stable and do not escape from the structures. Therefore, the source of ammonia is the feed N 2 gas rather than the electrocatalyst.
For further detailed insights into the structure-activity relationship between the V o -rich NiCo 2 O 4 @HNCP catalyst and its enhanced NRR properties, DFT calculation was carried out from the perspective of thermodynamic and kinetic acceleration. First, from the thermodynamic point of view, the electrons that previously occupied the O 2p orbital would partially delocalized to the neighboring Ni/Co cations aer introduction of oxygen vacancies, which suggests that a surface with oxygen vacancies is much more activated. Meanwhile, a rational model for the V orich NiCo 2 O 4 @HNCP catalyst was constructed by aligning optimized V o -rich NiCo 2 O 4 with highly conductive nitrogendoped carbon (Fig. 4g and S33 †). Aer analysis in detail, abundant charges accumulated on the blurry interface between V o -rich NiCo 2 O 4 and nitrogen-doped carbon, leading to the successful generation of highly active regions for the electrocatalytic process. As shown in Fig. S34 and S35, † the electrons are prone to transfer from nitrogen-doped carbon to V o -rich NiCo 2 O 4 due to their inconsistent charge distribution. In turn, the Ni/Co cations show increased electronic states of the dorbital around the Fermi level, which is benecial for activation of N 2 and formation of an N-catalyst bond. From the kinetic point of view, the hollow nitrogen-doped carbon polyhedron template endows the V o -rich NiCo 2 O 4 nanosheets with a higher specic surface area with more exposed active sites for adsorption/activation of N-related species. Attributed to the structural merits of the V o -rich NiCo 2 O 4 @HNCP catalyst, a possible NRR mechanism is depicted in Fig. 4h and i. Firstly, the N 2 gas can be easily adsorbed on the surface of the V o -rich NiCo 2 O 4 @HNCP catalyst to form chemisorbed Co(Ni)-N 2 bonds, which can be denoted as N 2 ðgÞ/N * 2 (here, the asterisk * denotes an adsorption site). Aer full structural relaxation, two energetically favorable congurations were found: end-on and side-on. On the V o -poor NiCo 2 O 4 surface, an N 2 molecule was more likely to anchor in the end-on conguration with a Co-N bond length of 1.84 A (Fig. S36a †), whereas, on the V o -rich NiCo 2 O 4 surface, two Co-N bonds (1.20 A in length) in the sideon conguration are formed with an elongated N-N bond length from 1.12 A to 1.16 A (Fig. S36b †). When taking DE ZPE and entropy into consideration, the DG values for N 2 adsorption are À0.05 eV for the V o -rich NiCo 2 O 4 surface and À0.57 eV for the V o -poor NiCo 2 O 4 surface, respectively. Secondly, six consecutive protonation and reduction processes on the V orich(poor) NiCo 2 O 4 surfaces were further proposed with atomic congurations at various states of each elementary step (Fig. S37 †). As discussed in Fig. 4i and Table S3, † the free energies of all these states were slightly downhill for the V o -rich NiCo 2 O 4 surface compared to the V o -poor NiCo 2 O 4 surface, which also proves that the successful introduction of oxygen vacancies in NiCo 2 O 4 is benecial for the nitrogen reduction reaction process theoretically.
We further studied the NRR activity of other spinel-structured nanosheet wrapped HNCP, such as ZnCo 2 O 4 @HNCP and Co 3 -O 4 @HNCP, in which oxygen vacancies were deliberately introduced. It should be noted that the XPS O 1s spectra (Fig. S38 †) reveal that abundant oxygen vacancies were successfully introduced into the surface of both the V o -rich ZnCo@HNCP and V orich Co@HNCP catalysts. As shown in Fig. 5a, all the catalysts with abundant oxygen vacancies showed a higher NH 3 yield rate and faradaic efficiency than their V o -poor counterparts in 0.1 M Na 2 SO 4 . Taking V o -rich ZnCo@HNCP and its V o -poor counterpart as examples, the NH 3 yield rate and faradaic efficiency for the V o -rich ZnCo@HNCP catalyst were 3.7 mg h À1 cm À2 /16.0 mg h À1 mg À1 and 3.6%, respectively, which are much more outstanding than those of the V o -poor ZnCo@HNCP catalyst (1.6 mg h À1 cm À2 /6.9 mg h À1 mg À1 and 1.7% at À0.25 V). Besides, the time-dependent curves of various catalysts at À0.25 V (Fig. 5b) exhibited excellent stability, indicating the stable vacancy structure in both the V o -rich ZnCo@HNCP and V o -rich Co@HNCP catalysts during the NRR process.

Conclusions and outlook
In summary, we uncovered the positive role of oxygen vacancies in spinel-structured nanosheets on hollow N-carbon polyhedra (e.g. V o -rich NiCo 2 O 4 @HNCP, V o -rich ZnCo 2 O 4 @HNCP, and V orich Co 3 O 4 @HNCP) towards the electrocatalysis of the nitrogen reduction reaction. For example, the V o -rich NiCo 2 O 4 @HNCP catalyst showed higher production yield (NH 3 : 4.1 mg h À1 cm À2 / 17.8 mg h À1 mg À1 and faradaic efficiency: 5.3%), good selectivity and high stability when compared with its V o -poor counterpart (NH 3 : 1.6 mg h À1 cm À2 /6.9 mg h À1 mg À1 and faradaic efficiency: 1.8%). As revealed by DFT calculations, the oxygen vacancies enhance the reactivity of the active sites, leading to reduced stabilization energy of NNH groups. Meanwhile, the HNCP template increased the number of active sites for facilitating the reaction on the surface of V o -rich NiCo 2 O 4 nanosheets and improved the conductivity for interfacial electron transfer between V o -rich NiCo 2 O 4 and HNCP. Systematic study from both theoretical and experimental aspects further conrmed the superior kinetics for NH 3 production using V o -rich NiCo 2 -O 4 @HNCP. Therefore, this work provides a guideline for the rational design of novel and highly efficient catalysts towards N 2 electrochemical reduction by increasing the surface area and introducing surface oxygen vacancies in spinel-structured nanosheets simultaneously.

Experimental section
Details of the synthetic procedures, characterization and theoretical calculation methods can be found in the ESI †.

Conflicts of interest
The authors declare no conict of interest.
Foundation and Shanghai Municipal Education Commission (16CG39) and the Engineering and Physical Sciences Research Council (EPSRC, EP/L015862/1). The computational center of the USTC is acknowledged for computational support.