Oxygen vacancies engineering in electrocatalysts nitrogen reduction reaction

Ammonia is important, both as a fertilizer and as a carrier of clean energy, mainly produced by the Haber-Bosch process, which consumes hydrogen and emits large amounts of carbon dioxide. The ENRR (Electronchemical Nitrogen Reduction Reaction) is considered a promising method for nitrogen fixation owing to their low energy consumption, green and mild. However, the ammonia yield and Faraday efficiency of the ENRR catalysts are low due to the competitive reaction between HER and NRR, the weak adsorption of N2 andthe strong N≡N triple bond. Oxygen vacancy engineering is the most important method to improve NRR performance, not only for fast electron transport but also for effective breaking of the N≡N bond by capturing metastable electrons in the antibonding orbitals of nitrogen molecules. In this review, the recent progress of OVs (oxygen vacancies) in ENRR has been summarized. First, the mechanism of NRR is briefly introduced, and then the generation methods of OVs and their applicationin NRR are discussed, including vacuum annealing, hydrothermal method, hydrogen reduction, wet chemical reduction, plasma treatment and heterogeneous ion doping. Finally, the development and challenges of OVs in the field of electrochemical nitrogen fixation are presented. This review shows the important areas of development of catalysts to achieve industrially viable NRR.


Introduction
The continued rise of the global population and excessive use of fossil fuels has led to severe environmental issues and an energy crisis. As the main constituent of air, inert nitrogen gas can be converted into ammonia, which has a large variety of applications in the industry (Galloway et al., 2008;Zamfirescu and Dincer, 2008). Nearly 80% of the ammonia produced is utilized for fertilizers, making it a significant agricultural chemical. Additionally, it can be used as a potential carrier of green fuel (Galloway et al., 2004;Christensen et al., 2006;Kitano et al., 2012;Chen et al., 2018 , 2022b). Currently, the Haber-Bosch process is used for the production of ammonia from nitrogen and hydrogen, which was invented in the early 20th century (Tanabe and Nishibayashi, 2013;Liu, 2014). However, owing to the high bond energy, lack of dipole moment, and low polarization of the molecular structure of N 2 , leads to the production of Haber-Bosch process must be carried out at high pressures (150-300 ATM) and high temperatures (400-600°C), making it an energy-intensive process that accounts for about 1-2% of the world's yearly energy supplies (Chirik, 2009;van der Ham et al., 2014;Singh et al., 2017;Sun et al., 2017;Guo et al., 2018;Song et al., 2019). There is an urgent need for researchers to find a viable and novel method of nitrogen fixation.
Recently, many methods have been proposed for nitrogen fixation in ambient circumstances, such as biochemical catalysis, photocatalysis, and electrocatalysis (Brown et al., 2016;Milton et al., 2016;Kyriakou et al., 2017;Cao and Zheng, 2018;Cui et al., 2018;Guo W et al., 2019). Among that the ENRR has been singled out as a promising method, because of their environment friendly, low pressure and moderate temperature (Zhang et al., 2019a;Zhang et al., 2019b;Li et al., 2019;Yu et al., 2019;Lazouski et al., 2020;Yang et al., 2020). However, the completion of NRR and hydrogen evolution reaction (HER) leads to the low Faraday efficiency and low ammonia yield, which limits its application (Hao et al., 2019;Qiu et al., 2019;Zhao et al., 2019). It is well-known that high Faraday efficiency and ammonia yield are requisite conditions for industrial applications of electrocatalytic reactions. Therefore, designing and producing environmentally friendly catalysts by an efficient process with low energy consumption and minimal pollution is crucial.
Oxygen vacancies engineering strategies as an effective method to improve NRR performance can tune the electronic structure and ensure successful reaction between intermediates, resulting in excellent chemical and physical properties as well as higher activity and selectivity (Yan et al., 2017;Xu et al., 2021;Gao et al., 2022;Ji et al., 2022). In recent studies, the design of catalyst materials with OVs for electrochemical NRR has drawn significant research attention Liu et al., 2019;Zhang S et al., 2019;He et al., 2021). Moreover, the introduction of OVs in electrocatalysts has been extensively employed in NRR because a large number of stable metal oxide catalysts provide a variety of carriers for OVs enriched with different structures (Hirakawa et al., 2017;Xu et al., 2019;Li P. S et al., 2020;Liu et al., 2021b). For instance, Han et al. thoroughly investigated the ENRR performance and catalysis mechanism of titanium dioxide with various OVs concentrations by theoretical calculations and experiments, including strict control of the annealing temperature during the preparation process . Accordingly, In-depth exploration of the effect of OVs on ENRR is essential to guide the design of catalysts with better catalytic performance.
In this review, we provide an overview of the most recent developments in utilizing OVs for developing catalysts for electrocatalytic nitrogen fixation. First, we briefly introduce the mechanism of electrocatalytic nitrogen fixation. We additionally summarize OVs generating methods and their applications for ENRR. Finally, the future development and possible challenges of OVs in the field of ENRR are discussed.

NRR mechanism
The complexity of the ENRR process and the catalyst's shape, microstructure, electronic structure, and density of active sites influence the effectiveness of the catalyst. Moreover, inefficient reactions also occur since most electrons unite with protons to generate hydrogen, which is the largest competitive reaction in ENRR. Therefore, it is essential to have an in-depth understanding of the NRR process.
Adsorption and activation of N 2 on the catalyst surface, along with the associated electron conversion and proton adsorption, are the first steps in the electrochemical reduction of N 2 to NH 3 . This reaction is quite difficult for the following reasons: 1) the robust triple bond of the inert N 2 molecule (Singh et al., 2017), 2) no permanent dipole, 3) a huge energy gap 10.82 eV between the highest occupied and lowest unoccupied molecular orbitals , and 4) high ionization potential (15.58 eV) and low electron affinity (−1.9 eV) of the N 2 molecule (Jia and Quadrelli, 2014;van der Ham et al., 2014). Consequently, A viable NRR catalyst requires modest binding to intermediate species and a high activation capacity relative to N 2 .
According to the intermediates involved and energy consumption, the NRR mechanism can be theoretically separated into dissociative and associative mechanisms ( Figure 1A) (Shipman and Symes, 2017). The N≡N bond is first broken by the dissociative mechanisms before hydrogenation, and then individual N atoms are adsorbed onto the catalyst surface and hydrogenated to form NH 3 . The Haber-Bosch process follows the dissociative mechanism (Wan et al., 2019). The dissociative mechanism involves overcoming the high cleavage energy of the thermodynamic N≡N bond, making NRR unfavorable under ambient conditions.
In contrast to the dissociative pathway, the N≡N triple bond partially breaks in the associative mechanism, before the hydrogenation of N atoms takes place. The associative mechanism can be further classified into the distal path and the alternating path based on the sequence in which H atoms are added to the two distinct N atoms (Shipman and Symes, 2017). The distal N atom in the distal pathway first adsorbs the H atom, Frontiers in Chemistry frontiersin.org 02 and subsequently hydrogenates until forming and releasing the ammonia molecule. Then another N atom is hydrogenated to release ammonia. The alternating path uses the alternating addition of H atoms to two different N atoms until one of them converts to NH 3 and the N≡N bond is broken .
In comparison to conventional catalysts, the introduction of OVs into a catalyst can increase the number of active sites for NRR by altering the electronic structure and surface properties. For example, Density functional theory (DFT) computations were carried out by Fu et al. on facets of Ta 2 O 5 (001) with OVs (Fu et al., 2019). The localized electrons made the Ta ions reduce, leading to an increase in the Bader charge of the two connected Ta atoms, from 2.45 e to 2.92 and 3.00 e, as shown in Figure 1B. Meanwhile, the two partly reduced Ta atoms near the OVs swapped electrons and absorbed N 2 . Their accessible d orbitals were used by the N-N π antibonding orbital to acquire electrons, which contributed to activating and adsorbing the N 2 molecule. The bond length of the successfully activated N 2 molecule increased (to 1.381 Å) due to the transfer of electrons from the Ta atom to the adsorbed N 2 , which contrasts with 1.098 Å in free N 2 . Moreover, the OV-containing Ta 2 O 5 adsorbed N 2 more easily as shown in Figure 1C. Additionally, the hydrogenation of defect-free Ta 2 O 5 had a large energy barrier. Therefore, OVs could enhance the catalyst's NRR catalytic performance.
3 Methods to generate OVs and applications in ENRR 3.1 Thermal annealing in an oxygendeficient environment A widely used method to generate OVs is annealing oxygen-containing compounds at high temperatures under anoxic conditions (e.g., He, N 2 and Ar) or vacuum. In the annealing process, the relative concentration of VOs can be adjusted by controlling the inert gas flow rate, final temperature, heating rate, annealing time and cooling rate  Frontiers in Chemistry frontiersin.org (Sarkar and Khan, 2019;Zhu et al., 2019). Wang et al. obtained In 2 O 3-x /CeO 2-y nanotubes rich in OVs by electrostatic spinning followed by vacuum annealing (Figure 2A) . According to the XPS spectra, the sample In 1 -Ce 1 , with a raw material ratio of 1:1, showed the highest concentration of OVs. With the formation of OVs, the electrochemical properties were effectively optimized, and the kinetics of NRR and electron conduction capacity were significantly enhanced, leading to a Faraday efficiency of 16.1% and NH 3 yield of 26.

Hydrothermal methods
Hydrothermal methods involve chemical reactions in a sealed vessel, where the temperature of the solvent is much higher than its boiling point due to the increase in autogenous pressure caused by heating. The ratio of the raw materials can be adjusted to generate OVs during the hydrothermal process. In recent years this method has been widely used since the powder does not require high-temperature calcination, thus, preventing re-agglomeration of nanoparticles and contamination.
BiVO 4 , containing different concentrations of OVs, was synthesized by Yao et al. by a hydrothermal reaction with adjusted pH values ( Figure 2B) (Yao et al., 2019

Hydrogen reduction
Hydrogen is a strong reducing agent and is commonly used to reduce metal oxides under high temperature or high pressure to introduce OVs. The concentration of OVs in metal oxides can be controlled effectively by adjusting parameters such as pressure, temperature and gas ratios. Zhang et al. used MoO 3 powder as the precursor and employed hydrogen reduction at 900°C for 1 h with different H 2 proportions (from 5% to 20%) in an Ar-H 2 atmosphere to obtain MoO 2 nanosheets with different OVs concentrations ( Figure 2C) . DFT calculations showed that appropriately limiting OVs in the MoO 2 layer benefits the proton transfer step by selectively stabilizing N 2 H* and destabilizing N 2 H 2 * through the distal/alternate mixing path. Consequently, in comparison to MnO 2 with free OVs, the activation barrier was lowered from 1.49 eV to 0.36 eV. Fang et al. successfully prepared two-dimensional OV-TiO 2 nanosheets by annealing TiO 2 nanocrystals in H 2 /Ar at different temperatures (Fang C. H et al., 2019). The OV-TiO 2 -400 nanosheets remained highly stable after 12 cycles and achieved NH 3 yields of 35.6 μg h −1 mg −1 , which was 2.83 times higher than that of TiO 2 without OVs.

Wet chemical reduction
Wet chemical reduction of oxides by chemical reagents such as NaBH 4 can produce OVs at temperatures lower than those required for hydrogen reduction. The reduction molecule first adsorbs on the metal oxide surface and then grabs the O atom to the surface oxygen by electron transfer, thus generating OVs.
Carbon-encapsulated MoO 2 nanoparticles (MoO 2 @C) with abundant OVs have been synthesized via a pectin-assisted hydrothermal method, followed by calcination and treatment with NaBH 4 solution (Du et al., 2021). MoO 2 @C showed a low NH 3 yield of 2.12 μg h −1 mg −1 at −0.7 V vs RHE without treatment with NaBH 4 solution. However, OV-containing MoO 2 @C showed a higher NH 3 yield 9.75 μg h −1 mg −1 at 0.5 V vs RHE and Faraday efficiency of 3.24% and inhibited the HER. Fang et al. synthesized OV-rich TiO 2 nanoparticles (NPs) grown in situ on TiO 2 /Ti 3 C 2 T x using an ethanol-based thermal technique (Fang Y. H et al., 2019). Due to the high electrical conductivity of the Ti 3 C 2 T x nanosheets, electron transport was promoted and self-aggregation of TiO 2 nanoparticles was inhibited. Thus, the TiO 2 nanoparticles increased the surface specificity (SSA) of Ti 3 C 2 T x . Moreover OVs can serve as NRR reactive sites, the TiO 2 /Ti 3 C 2 T x exhibited an excellent NRR capability with NH 3 yields of 32.17 μg h −1 mg −1 at −0.55 V vs RHE and Faraday efficiency of 16.07% at −0.45 V vs RHE in 0.1 M HCl. DFT calculations demonstrate that the N≡N triple bond at the TiO 2 (101)/Ti 3 C 2 T x surface was highly activated and showed the lowest NRR energy barrier (0.40 eV) compared to untreated Ti 3 C 2 T x or TiO 2 (101).

Plasma treatment
Efficient and rapid generation of OVs can be achieved by plasma treatment, which involves surface etching and can be Frontiers in Chemistry frontiersin.org 05 carried out at lower temperatures . Energetic particles, such as various kinds of plasma and high-energy protons, interact with the metal oxide surface during the plasma activation process, the surface structure is damaged, leading to the creation of OVs. The concentration of OVs can be precisely controlled by adjusting the plasma's power, pressure, gas flow, and irradiation period.
Li et al. used plasma technology to introduce OVs in NiO ( Figure 2D) (Li Y. B et al., 2020). DFT calculations show that the electronic structure of NiO was modified due to the introduction of OVs, further improving its electron conduction during NRR, lowering the reaction potential barrier and suppressing side reactions. In contrast to the majority of the reported NRR catalysts, the NiO nanosheets enriched with OVs showed an excellent NH 3 yield of 29.1 μg h −1 mg −1 and Faraday efficiency of 10.8% −0.5 V vs. RHE.

Heterogeneous ion doping strategy
Heterogeneous ion doping is based on the difference in the electronegativities of intrinsic atoms, and is used to introduce defects into crystal structures and adjust the physicochemical properties of materials Li et al., 2022a). For pure oxygen-containing compounds, both metallic and nonmetallic doping can create an imbalanced charge atmosphere that tends to break the long-term periodicity of the lattice oxygen in the oxide; thus, OVs are formed to maintain thermodynamic stability.

Metal-doping
Chu et al. tuned the NRR properties of CeO 2 by Fe doping (Fe-CeO 2 ) (Chu et al., 2020a). Fe doping transformed the morphology of CeO 2 from crystalline nanoparticles to partly amorphous nanosheets and significantly increased the concentrations of OVs. As a result of the abundant active sites, considerable specific surface area and high electrical conductivity, Fe-CeO 2 exhibited good catalytic activity, with an excellent NH 3 yield of 26.2 μg h −1 mg −1 (−0.5 V vs. RHE) and Faraday efficiency of up to 14.7% (−0.4 V vs. RHE).
Zhang et al. successfully prepared Cu-doped CeO 2 nanorods by a facile hydrothermal method, followed by annealing in H 2 /Ar . The synthetic Cu-doped CeO 2 nanorods were designated as Cu-CeO 2 -x, where x denotes the amount of Cu dopant. Cu-CeO 2 -3.9 exhibited excellent electrocatalytic performance due to its large surface area of 95.2 m 2 g −1 and mesoporous structure, with NH 3 yields of 5.3 × 10 -10 mol s −1 cm −2 and Faraday efficiencies of 19.1% at −0.45 V vs. RHE in 0.1 M Na 2 SO 4 , which is far beyond that of pure CeO 2 nanorods. It was found that the Ce 3+ site in Cu-doped CeO 2 was more easily replaced by Cu 2+ with the increase in Cu dopant concentration. As a result, the OVs around the Ce 3+ sites decreased; conversely, the OVs surrounding the Ce 2+ sites increased. The OVs formed around the Ce 2+ sites promoted N 2 adsorption and activation and improved NRR performance ( Figure 2E).

Nonmetal-doping
Chu et al. used a straightforward hydrothermal method to synthesize B-doped MnO 2 (Chu et al., 2020b). DFT calculations demonstrated that the asymmetric charge distribution brought on by the interaction between OVs and the B dopant enhanced the stability of the crucial intermediate *N 2 H on MnO 2 ; thus, lowering the reaction energy barrier and increasing reactivity. the B-MnO 2 /Carbon cloth in 0.5 M LiClO 4 outperformed most currently known Mn-based catalysts with an NH 3 yield of 54.2 μg h −1 mg −1 (−0.4 V vs. RHE) and Faraday efficiency of 16.8% (−0.2 V vs. RHE).
Fan et al. synthesized OV-rich P-doped potassium peroxynitrite (KNb 3 O 8 , abbreviated as P-KNO) by a simple solid-phase method followed by phosphorylation (Fan et al., 2022). The NH 3 yield of P-KNO was 23.01 μg h −1 mg −1 (at −0.45 V vs. RHE) and the FE was 39.77% (at −0.4 V vs. RHE) in 0.1 M Na 2 SO 4 electrolyte ( Figure 2F), which is twice that of unphosphorylated KNO. Additionally, due to their complementary effects, P-doping and VOs modified the electronic structure of the catalyst surface, hastening the adsorption and activation of N 2 and thus enhancing catalytic performance.

Summary and prospects
Ammonia is one of the most widely manufactured chemicals and has the potential for clean energy applications. However the conventional Haber-Bosch method requires significant amounts of energy and releases a considerable of greenhouse emissions. The efficient, cost-effective, and emission-free ammonia synthesis achieved from N 2 by ENRR under room temperature has attracted significant research attention. In order to substantially improve the NRR activity, developing and constructing novel effective NRR catalysts is vital.
Oxygen vacancies engineering is an effectively implemented strategy to enhance catalytic activity and selectivity of catalysts by altering the electronic state and creating additional active sites for NRR. In ENRR, OVs can alter the electron density and charge distribution of the catalyst and serve as reaction sites by adsorbing the reactants. By lowering the activation energy barrier, OVs inhibit the HER and increase the efficiency of the NRR.
Although introducing OVs to the catalyst has significantly advanced the development of NRR electrocatalysts, it is still difficult to accurately regulate OVs concentrations and the relationship between OVs Frontiers in Chemistry frontiersin.org 06 and NRR performance is not well understood. In addition, OVs may introduce transition layers in the electrocatalyst during the NRR process, notably in highly acidic and alkaline electrolytes. The reaction pathways are correlated with the properties of the electrolyte and the transition layer, thus, affecting the adsorption of the intermediates and the ratedetermining steps. Therefore, it is necessary to consider in situ characterization approaches that provide straightforward evidence and in-depth insight into the reaction mechanism. For example, in situ FTIR, in situ Raman spectroscopy and in situ XAFS. The performance of reported catalyst materials is unsuitable for industrial applications and needs to be further improved. We expect that improved, durable and affordable electrocatalysts for NRR can be produced by integrating experiment and theory.