DOI:
10.1039/D3QI00683B
(Review Article)
Inorg. Chem. Front., 2023,
10, 4650-4667
Recent progress in ammonia synthesis based on photoelectrocatalysis
Received
13th April 2023
, Accepted 18th June 2023
First published on 27th June 2023
Abstract
Photoelectrocatalytic (PEC) ammonia synthesis from nitrogen and water is a promising approach for energy development and N-neutralization goal under mild conditions. Although significant progress has been made in the past few decades, the mechanisms underlying the synergistic effect between light and electricity are still challenging. One particular line of study is to improve the performances of PEC catalysts, such as selectivity, yield, and stability, etc. Here we review the recent progress in PEC ammonia synthesis. We first provide a systematic description of the driven bias in PEC ammonia processes, involving electrochemical apparatus, photovoltaic voltage, and chemical potential. The various strategies, including vacancy engineering, ion doping, frustrated Lewis pair design, heterojunction construction, cocatalyst loading and single atom synthesis to fabricate new catalysts, are then outlined. The performance and mechanism of PEC N2 reduction are further summarized, followed by the current challenge and future prospects. This would guide both the productiveness and mechanism of NH3 synthesis based on advanced PEC systems.
Pengyan Li | Pengyan Li received her M.S. degree in Chemical Engineering from North China University of Science and Technology in 2020. She is now pursuing her Ph.D. degree at Beijing Normal University under the supervision of Prof. Dongpeng Yan. Her research interest is focused on energy and environmental catalysis by photo(electro)catalytic technology, such as CO2 reduction, water splitting, N2 reduction, and degradation of dye. |
Yumin Liu | Yumin Liu earned her Ph.D. degree (2021) in Chemical Engineering from Tianjin University for research on pharmaceutical crystallisation under the supervision of Prof. Jingkang Wang and Prof. Junbo Gong. In 2017–2019, she joined the Crystal Chemistry and Particle Process Engineering (CCAPPE) research group at the University of Manchester, UK, as a visiting Ph.D. student. During this period, she studied crystal growth and worked with Prof. Roger Davey and Dr Aurora Cruz-Cabeza. She currently works as a Postdoc fellow with Prof. Dongpeng Yan at Beijing Normal University and focuses on functional molecular materials. |
Muhammad Asim Mushtaq | Muhammad Asim Mushtaq received his Master's degree in 2015 from the University of Engineering and Technology Lahore, Pakistan. He obtained his Ph.D. degree in Chemical Engineering and Technology (2021) from Beijing University of Chemical Technology (BUCT), PR China. Recently, he joined as a Postdoc fellow at Shenzhen University, Guangdong, PR China. He worked under the supervision of Prof. Dongpeng Yan and Prof. Shengfu Ji at the Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University (BNU), and the State Key Laboratory of Chemical Resource Engineering, BUCT, PR China. His research focuses on the design and synthesis of catalytic materials for photo/electrochemical catalysis. |
Dongpeng Yan | Dongpeng Yan obtained his Ph.D. degree under the supervision of Prof. Acad. Xue Duan from Beijing University of Chemical Technology (BUCT, China) in 2012. Then, he became an associate professor at BUCT. In 2014, he moved to Beijing Normal University (China) as a full professor. In 2011 and 2013, as a visiting scholar, he studied at the University of Cambridge (UK) and University College London (UK). His research topics are functional molecular materials, host–guest chemistry, and photo-electrochemistry. He has received the Tang Aoqing Chemical Scholarship (2011) and the Outstanding Doctoral Dissertation Prize in Beijing (2013) amongst others. His interest focuses on the chemistry of functional materials and photo(electro)catalysis. As a corresponding author, he has published more than 150 scientific papers (such as in Chem. Soc. Rev., Angew. Chem., JACS, Adv. Mater. and Nature Commun.). He was awarded the Newton Advanced Fellowship of Royal Society and the Royal Society of Chemistry Fellowship (FRSC). |
1. Introduction
Ammonia synthesis provides an effective pathway to obtain fertilizers, medicines, chemicals and explosives from air, which is significant to human survival and industrial development.1,2 In nature, microorganisms depend on Mo–Fe protease to achieve the conversion of atmospheric N2 to multipurpose NH3.3 However, azotase is extremely sensitive to environmental oxygen, so the evolution of rhizobium cultivation technology became the key to industrialization of biological N2-fixing. As a paragon of artificial nitrogen fixation, the Haber–Bosch (H–B) reaction has contributed to the global economy in which Fe-based compounds show excellent catalytic performances under harsh conditions (temperatures: 300–500 °C, pressures: 150–300 atm) with N2 and H2 as feed gases. However, catalytic N2 hydrogenation consumes mass heat, and the production of feedstock H2 derived from methane steam reforming results in serious carbon emissions with more than 1% of global emissions,4 furthermore, the whole H–B craft requires a large plant infrastructure. Therefore, it is essential to find alternative approaches for sustainable N-neutralization under ambient conditions.5–10 Electrocatalysis and/or photocatalysis has become one of the promising strategies for realizing the N cycle due to its cost advantage, environmental friendliness, and significant N2 conversion efficiency.
Electrocatalytic N2 reduction in water involves electron-coupled proton transfer along with various product distributions (NH3, N2H2, N2H4, and H2).11–13 Generally, the bias can induce the first electron transfer progress (N2 + e− ↔ N2−: −3.37 V vs. RHE, pH = 14) and the first-H addition reaction (N2 + H+ + e− ↔ N2H+: −3.20 V vs. RHE) that are not favourable thermodynamically, and accomplish continuously the following reduction.14,15 However, the deficient sites and hydrogen evolution reaction (HER) result in minimal yield and low Faradaic efficiency (FE) in electrocatalytic ammonia synthesis.16–19 Most studies have proved that Bi-based materials and layered double hydroxides (LDHs) have the ability to undergo photocatalytic N2 reduction even if the band structures do not meet the requirements of the first electron transfer and the first-H addition, which contribute to considerable N2 activation, accelerated charge separation, and desired electron generation with strong reducibility.20 Currently, the integration of electrocatalysis and photocatalysis serves as an effective way to improve the yield and selectivity of NH3 by virtue of the bias of electrocatalysis and the abundant sites of photocatalysts (Fig. 1).21–23
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| Fig. 1 Schematic diagram of multiform PEC nitrogen fixation driven by an external bias, solar energy, chemical potential under irradiation, and coexistence of photocatalytic and electrocatalytic N2 reduction. | |
PEC N2 reduction is a process in which catalysts are excited under irradiation to generate carriers, and then carriers are separated into electrons and holes, finally, electrons migrate rapidly and participate in N2 reduction on the surface sites assisted with the bias. In addition, the photoelectric synergy is reflected in the coexistence of photochemical and electrochemical reactions. Therefore, PEC NH3 synthesis is considered as a promising approach that possesses advantages from photocatalysis and electrocatalysis simultaneously to efficiently complete N2 conversion.24–26 A classical double chamber contains a photocathode (i.e. catalysts), anode (i.e. counter electrode), electrolyte, N2 and proton exchange membrane for PEC N2 fixation. Candidate photocathodes include inorganic semiconductors with suitable bandgaps (g-C3N4, BiOI, TiO2, Cu2O, W18O49)27,28 and metal modifications,29,30 metal–organic frameworks with adjustable structures,31 and single atoms with remarkable active sites.32 In consideration of the fact that the more negative position of the conduction band (CB) minimum and the enhanced capacity of electron reduction are favourable to N2 reduction in the PEC strategy, vacancy engineering, heteroatom-doping, heterostructure construction, and single atom synthesis are promising approaches to improve N2 reduction ability. What's more, the extended light response capacity to the near-infrared region is a potential idea for solar-based economy in PEC N2 reduction.33,34
Chemisorption is a crucial step for N2 reduction because appropriate adsorption can destroy the molecular symmetry and enhance the proton affinity,35 thus promoting N2 hydrogenation. However, the bond energy (941 kJ mol−1) and cleavage energy (410 kJ mol−1) indicate that N2 activation is challenging (Fig. 2a). Many studies have investigated activation mechanisms, such as vacancy engineering,27,36 large specific surface area, transition metal (TM)/non-metal doping,25 and thermal field coupling. The empty d orbital of the TMs can capture lone pair electrons from N2, and the occupied d orbital donates electrons to the π* orbital, thus producing a weak NN bond through the ‘acceptor–donor’ route.37 In addition to metals, B-based molecules with empty orbitals and lone pair electrons have similar electronic structures to TMs, so N2 activation could be achieved through the ‘acceptor–donor’ pathway of B-containing catalysts (Fig. 2b and c).37–40
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| Fig. 2 (a) N2 molecular structure,41 Copyright 2020, American Chemical Society. (b) Mechanism of metal activation N2 (left) and boron activation N2 (right), (c) Outcomes of various reduction reactions of dihaloborane adduct 1, including generation of a transient dicoordinate borylene species (2) and its reactionwith dinitrogen. Dip, 2,6-diisopropylphenyl; Dur, 2,3,5,6-tetramethylphenyl.38 Copyright 2018, American Association for the Advancement of Science. | |
The research studies of electrocatalytic and photocatalytic ammonia synthesis are flourishing;42–45 however, systematic summarizations of PEC N2 reduction are rather rare. Here, we review the various bias-driven photocatalytic ammonia processes, involving electrochemical apparatus, photovoltaic voltage and chemical potential. The various strategies and mechanistic understanding based on calculations and experiments are then outlined, furthermore, we summarize the methods of NH3 quantification, and highlight the necessity of nitrogen source determination.
2. Merging the solar strategy and the electrical strategy for PEC N2 reduction
There are synergistic effects between light and electricity, which promote the PEC progress by taking advantage of both photocatalysis and electrocatalysis.46,47 In particular, the electro-coupled photocatalysis strategy avoids NH3 oxidation, enhances the reduction ability of electrons, and boosts the carrier separation capacity, while photo-assisted electrocatalysis is more outstanding than single electrocatalysis progress, which contributes to the equilibrium of electron distribution under irradiation.48 Besides, the synergistic effect is displayed in the form of the coexistence of photochemical and electrochemical processes, that is, the potential of electrons is determined by bias in electrocatalysis, and the reduction ability of photoelectrons depends on the minimum CB of semiconductors for photocatalysis.49
2.1 PEC N2 reduction assisted by electrochemical apparatus
Generally, in the PEC process, light is absorbed to initiate the catalytic reaction directly, and the applied bias is used to promote the migration of the photogenerated electrons by changing the band bending.49–51 Upon quenching photogenerated holes located on the valence band (VB),52 regulating the H+ concentration on the surface of the catalyst,27 and increasing the accumulation of electrons in the catalytic site, the catalytic performance of ammonia synthesis would be significantly improved. Electrocatalysis enhanced by photo-excitation and photocatalysis promoted by external bias occur simultaneously in PEC N2 reduction, which make the catalysis more effective than the individual electrocatalytic or photocatalytic nitrogen fixation. It was reported that black phosphorus on indium tin oxide showed extraordinary PEC N2 reduction properties, which benefit from the coexistence of electrocatalysis and photocatalysis.48
2.2 Solar-driven and chemical bias-driven PEC N2 reduction
Photocatalysts (such as g-C3N4, TiO2, BiVO4, etc.) exhibit inefficient electrocatalytic performance due to poor conductivity, and there is a obvious side reaction (HER) for the electrochemical apparatus-assisted PEC strategy. The current studies have proved that PEC N2 fixation can be carried out without electrochemical apparatus, and the formed anode photovoltage contributes negative potential to N2 reduction in the form of compensation voltage.53–55 The photovoltage from plasma-induced semiconductors shows huge potential for efficient ammonia synthesis in PEC cells without apparatus. Ali et al. described a solar-driven PEC cell over plasmon-assisted Si for atmospheric N2 reduction and produced a high yield (13.3 mg m−2 h−1) under 2 sun radiation for the first time.22 Lee et al. reported that the photovoltage generated from Au modified ordered Si nanoarrays can reduce nitrate to ammonia with a FE of 95.6% at 0.2 V vs. RHE.56 Li et al. fabricated a photovoltaic cell to realize PEC N2 fixation with a high NH3 yield (at the level of μmol h−1 or even mmol h−1) over the TiO2/Au photocathode.57 Inspired by the successful construction of Au-loaded black silicon, Wu's group synthesized Ag-loaded black silicon for PEC nitrogen fixation without external bias,25 and the catalysts achieved a high FE (40.6%) and NH3 yield (2.87 μmol h−1 cm−2). The perovskite-based photocathode (TiOx/CdS/Cu2ZnSnS4) achieved NH3 production from NO3− reduction with a FE of 89.1% (Fig. 3).58
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| Fig. 3 The enhanced current density or improved NH3 yield under light irradiation compared to that in the dark. (a) J–V plots of planar Si (black), Si nanowire (red), and highly ordered Si nanowire (blue) photocathodes for NO3− reduction in Ar-filled K2SO4 with KNO3. Dashed lines represent dark current,56 Copyright 2022, Wiley-VCH. (b) ammonia yield on various substrates: (i) P-type Si, (ii) black Si, (iii) Au/black Si, (iv) Au/black Si/Cr, and (v) Au/Si/Cr under photo-radiation, (vi) Au/black Si/Cr under dark conditions,22 Copyright 2016, Nature Publishing Group. (c) Linear sweep voltammetry (LSV) of the Ag/black Si electrode under N2, Ar and dark conditions,25 Copyright 2020, American Chemical Society. (d) The LSV of TiOx/CdS/Cu2ZnSnS4 in KNO3 and H2SO4 solution, (e) the LSV of TiOx/CdS/Cu2ZnSnS4 and in K2SO4 solution, (f) the chronoamperometry curves of TiOx/CdS/Cu2ZnSnS4 with different spray coating temperatures at 0 V vs. RHE.58 Copyright 2022, Wiley-VCH. | |
Photocatalytic N2 fixation is completed under the action of chemical bias induced by pH. Oshikiri's group was committed to research chemical bias-driven PEC N2 fixation over Au/SrTiO3 to improve NH3 selectivity.30,59 The photo-excited hot electrons from Au were transferred to the CB of SrTiO3 to induce nitrogen reduction, and resulted in a hole on the SrTiO3 surface for the oxidation reaction. There is an accelerated reaction under the action of chemical bias that was generated when the anode potentials move negatively and the cathode potentials move positively.
3. Synthesis and performance of advanced catalysts for PEC N2 reduction
3.1 Vacancy engineering
Thermal vibration may cause atoms to deviate from the equilibrium position or escape from the surface lattice, and form vacancies.60 As one of the most common defects on atomic-scale surfaces, vacancies can promote charge separation, anchor single atoms, and act as adsorption sites by breaking NN.61–63 Therefore, the creation of vacancies significantly improved PEC performance for ammonia synthesis.
The methods for constructing oxygen vacancies have been widely studied, and the nitrogen fixation performance has been significantly improved. BiOF, BiOCl, BiOBr and BiOI are metallic compounds consisting of a [Bi2O2]+ layer and a F/Cl/Br/I atomic layer through Van Der Waals interaction. The induced dipole action between the layered structures is favourable to form an electric field, thus passivating the recombination of photogenerated carriers.65 Due to improved electronic availability, the Ovs located on the BIOX (X = F, Cl, Br, I) surface is used as a reaction site to facilitate N2 reduction.66 Bai et al. synthesized Ovs-BiOI photocathodes by the in situ electrodeposition method (Fig. 4a),24 and the adsorption capacity of R-BiOI to N2 was significantly enhanced after introducing Ovs (Fig. 4b). The ammonia production rate is 1400 μmol m−2 h−1 (Fig. 4c) at 0.4 V vs. RHE, which is 1.3 times higher that of the anaerobic catalyst. Similarly, Lin et al. prepared an Ovs-BiOBr/TiO2 photoelectrode, and the Ovs elevated the capacity of capturing photoelectrons. The bandgap of Ovs-BiOBr was reduced to 2.76 eV, indicating that Ovs can broaden the light absorption range (Fig. 4d). The Ovs is in favor of the formation of quasi-continuous defects that transport electrons under low-energy light irradiation, and Ovs-BiOBr/TiO2 increases the NH3 yield, which is 3.3 fold of the BiOBr/TiO2 photocathode.27 Mao et al. prepared LixMoO3 nanosheets with Ovs through the lithiation strategy, and there are more abundant Ovs and Mo5+ in LixMoO3 than in MoO3 (Fig. 4e).64 Density functional theory (DFT) calculations confirmed that LixMoO3 nanosheets were favourable for the activation of N2 and the formation of *N2H (Fig. 4f). The experiments confirm that the effective NH3 yield of LixMoO3 nanosheets is 3.48 μg cm−2 h−1, 9 times that of MoO3 nanosheets. The LDH systems with an abundance of surface oxygen vacancies or coordinatively unsaturated metal cations in ammonia synthesis were popular in N2 reduction. Zhang et al. demonstrated a simple pretreatment of ZnCr-LDH, ZnAl-LDH, and NiAl-LDH nanosheets with aqueous NaOH, thus enhancing the concentration of oxygen vacancies to significantly improve photocatalytic activity for N2 reduction to NH3.67
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| Fig. 4 (a) Schematic diagram of the synthesis process, of the I-BiOI/R-BiOI, (b) N2-TPD profiles of the I-BiOI/R-BiOI, (c) production rate of ammonia,24 Copyright 2019, Elsevier. (d) the bandgap of OVS-BiOBr and BiOBr,27 Copyright 2022, Elsevier. (e) EPR of MoO3 and LixMoO3, (f) free energy on oxygen vacancies (OVS) of MoO3 (010) and Li0.33MoO3 (010), (g) Gibbs free energy difference between *N2 and *H over MoO3 (010) and Li0.33MoO3 (010).64 Copyright 2022, Elsevier. | |
Nitrogen vacancies (Nvs) exhibit enhanced performance for PEC N2 reduction. Li et al. used 3-amino-1,2,4-triazole treated with NaOH as a precursor to prepare g-C3N5 with Nvs,68 and the Nvs realized charge separation by accepting electrons of g-C3N5. BiOBr/g-C3N5 heterostructures with Nvs are synthesized to realize the double-electron transfer mechanism (DETM). The DETM delays the recombination of carriers to ensure high-quality electrons, which is manifested through increased ammonia yield (BiOBr: 2.5 μg mg−1 h−1; Nvs-g-C3N5: 14.8 μg mg−1 h−1, Nvs-g-C3N5/BiOBr: 29.4 μg mg−1 h−1).
3.2 Heteroatom doping
Metal/non-metal ion doping could adjust the electronic structure, redistribute the electron density, and construct surface defects.69,70 The unoccupied orbitals of the TMs capture lone pair electrons of N2 molecules due to matched orbital energy and symmetry, while the occupied d orbitals donate electrons in reverse to the anti-bonding orbitals of the N2 molecule. Therefore, the strong triple bond of N2 is weakened when the TM–N σ-bond is formed, thus promoting the activation of N2. The most studied metal doping for photocatalytic nitrogen fixation is an iron ion with abundant unfilled 3d orbitals, variable valence and considerable cost. Schrauzer et al. improved the nitrogen-fixing activity of TiO2 by doping Fe, Co, Mo or Cr, and found that 2%Fe-doped TiO2 exhibited the best performance in photocatalytic NH3 synthesis.71 Vu et al. synthesized Fe-W18O49 nanorods with Au modification by two-step approach, in which the Fe existed in the form of Fe3+/Fe2+(Fig. 5a).21 Fe-W18O49 enhanced light absorption ability and improved the separation capacity of electron–hole pairs, and achieved a high yield of NH3 (9.82 μg h−1 cm−1) under PEC conditions.
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| Fig. 5 (a) Fe 2p XPS of WOF-Au, (b) EPR of WO and WOF-Au,21 Copyright 2020, American Chemical Society. (c) Partial charge density of the Bi-2B (012) facet, (d) N2 reduction progress on Bi-2B (012).72 Copyright 2020, Elsevier. | |
The doping of non-metallic atoms (such as B, P, N, etc.) is used to increase the VB position and reduce the bandgap through orbital hybridization or defect engineering.73 Furthermore, non-metallic doping avoids the formation of recombination centers in N2 reduction.74,75 Xu et al. synthesized B-doped Bi from layered BiOBr nanosheets with sodium borohydride (NaBH4) as a reductant,72 and the injection of B atoms enhanced the polarization of Bi (012), and Bi provided electrons to the neighbouring B atoms, resulting in the change of the Bi oxidation state and surface geometry (Fig. 5c). The Bi surface with a high curvature promoted N2 adsorption, and the accumulation of electrons on Bi facilitated the entry of electrons into the anti-bonding orbital of *NNH, thus promoting the elongation of the N–N bond of *NNH. The energy barrier of the potential-determining step was reduced to 2.00 eV, showing that B doping had a positive role in N2 reduction (Fig. 5d). The B-doped Bi showed excellent PEC N2 reduction performance (NH3 yield: 29.2 mg gcat.−1 h−1), far exceeding that of bismuth (10.6 mg gcat.−1 h−1).
As a burgeoning class of two-dimensional materials, graphene is sought in the field of photocatalysis and electrocatalysis. Nitrogen doping can regulate the optical bandgap of graphene to endow favourable optical and electrical features through expanded π electron delocalization.76 The introduction of nitrogen affects both the crystal structure and electronic structure of graphene, in addition, the polarity, electronic donor property, conductivity and chemical stability of graphene can be improved.77,78 Nitrogen doping into carbon supporter generates regulated charge density, resulting in complex reactions related to the multi-proton and electron transfer in N2 reduction. Meanwhile, studies have shown that the graphene lattice that is rich in nitrogen dopants contributes to the formation of n-type semiconductors.79 Generally, nitrogen doped reduced graphene oxide (RGO) shows potential for p–n heterojunction construction, which is conducive to the formation of an electric field. Paramanik et al. constructed the composite of CoTiO3 and N-RGO, where an electron is confined in the atomic layer when N-RGO grows on the surface of CoTiO3. In addition, N-RGO/CoTiO3 achieves a shorter diffusion distance of electrons, rapid charge transfer and small interface resistance.80 In addition, the synergistic interaction between the rod-like structure of CoTiO3 and the rough surface of N-RGO contributes to the overall improvement of the catalytic performance. The optimized 1N-RGO/CoTiO3 has a quenching rate of 54.89% for photogenerated carriers, and possesses an electron lifetime of 3.0 ms. The photocatalytic and PEC activities are more than 1.7 mmol−1 h−1 and 16 μg cm−1 h−1, respectively.
Although nonmetallic doped catalysts have improved performance, the mechanism and stability still need to be further explored in view of the fact that the introduced elements may become the quenching sites of electrons and holes,62 so it is indispensable to study the stabilization strategy for doping non-metallic elements.
3.3 Frustrated Lewis pairs (FLPs)
Metal-free catalytic ammonia synthesis with high selectivity and ideal yield faces challenges until the breakthrough of N2 activation based on the frustrated Lewis pairs (FLPs) in 2018.81 FLPs with an electrophilic acid and nucleophilic base are favorable for the breaking of NN through the electron transfer between the unoccupied orbitals of electrophilic acid and the nonbonding orbitals of nucleophilic base (Fig. 6a). Stephan et al. reported the space blockage in inhibiting the capture of N2 in the synthesis of Ph2CN2B(C6F5)3 with carbene, N2 and borane as reactants.82 Dai et al. presented boron carbon nitride (BCN) with the abundant unsaturated B (Lewis acid sites) and N atoms (base sites), and the results showed that FLPs adsorb N2 molecules to form six membered ring intermediates, which reduces the energy of NN breaking.83 Chen et al. proposed an alternative strategy based on FLP catalysts to stabilize N2H* intermediates, that is, substituting two-dimensional black phosphorus co-doped with TMs and boron atoms for FLPs.84 Zhu et al. designed a variety of FLPs that have the ability to activate N2, and exhibit low ΔG values of −37.5 to −51.0 kcal mol−1 (Fig. 6e).81
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| Fig. 6 (a) Illustration of artificial frustrated Lewis pairs to adsorb and activate N2, C–N coupling reaction,85 Copyright 2021, the Royal Society of Chemistry. (b) Lewis acidic boranes for N–N activation,86 Copyright 2017, Wiley-VCH. (c) Schematic illustration of BCN for electrochemical nitrogen reduction,83 Copyright 2022, Wiley-VCH. (d) Mechanism of over 10B/10Mo–C3N4 in the photocatalytic N2 fixation strategy,87 Copyright 2020, the Royal Society of Chemistry. (e) Gibbs energy over carborane-based FLPs for N2 reduction.81 Copyright 2022, American Chemical Society. | |
3.4 Heterojunction construction
Different from the inherent band structure of single-phase catalysts, heterojunction materials have the advantages of regulating the threshold of light absorption, accelerating the separation of carriers by changing the band structure, and passivating the recombination of electrons and holes. A typical heterojunction is an interface formed through the contact of two different semiconductors, which is mainly divided into types I, II and III (Fig. 7a–c). In type I, the VB and the CB of one semiconductor are more negative than those of another semiconductor, and there is no photocatalytic activity because the carriers cannot be effectively separated under photoexcitation. In type III heterojunction, the band positions of one semiconductor are all above the band positions of another semiconductor, and the formed interface has almost no catalytic activity in type III. For type II, the band positions are staggered between the two semiconductors, thus forming an upward or downward bending of the band,89 which causes the charge carriers to migrate in the opposite direction, thus prolonging the life of electrons and holes and improving the redox ability. Although effective charge separation is accomplished in constructed type II systems, the main disadvantage is the lower oxidation and poorer reduction capabilities.90 The Z-scheme heterojunctions achieve rapidly separated carriers and strong redox ability, which can be attributed to the recombination of holes on the VB for one semiconductor and electrons on the CB for another semiconductor (Fig. 7d and e). The Z-scheme heterojunctions are divided into semiconductor–semiconductor (that is the S-scheme) and semiconductor–mediator–semiconductor depending on whether there is an electronic mediator between two semiconductors. Researchers have focused on the construction of different heterojunction nanocomposites,91 aiming to boost the utilization of ultraviolet light or near-infrared light and to promote carrier separation for PEC nitrogen fixation.
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| Fig. 7 Band structure of (a) type I, (b) type II, (c) type III, and (d) Z-scheme heterojunction in semiconductor–semiconductor and for example, (e) g-C3N4/W18O49,33 Copyright 2017, the Royal Society of Chemistry. (f) Z-scheme in the semiconductor–electron mediator–semiconductor. A: electron acceptor, D: electron donor; and for example, (g) Bi-Bi2O3/KTa0.5Nb0.5O3.88 Copyright 2022, the Royal Society of Chemistry. | |
Owing to the controllable chemical environment, metal–organic frameworks (MOFs) are used in the preparation of heterostructures as photoelectrodes.92,93 Liu et al. used the in situ chemical etching strategy to form Cu-MOFs on the Cu2O surface with an organic ligand (H3BTC), and the Cu-MOF amount can be determined by etching time (Fig. 8a).94 The establishment of the Cu-MOF/Cu2O heterostructure improves the carrier density and charge separation efficiency (Fig. 8b). Meanwhile, the NH3 yield of Cu-MOF/Cu2O is increased to 7.2 mmol m−2 h−1 due to unsaturated Cu(II) sites, which is 5 times that of Cu2O (3.7 mmol m−2 h−1) and 3.9 times that of Cu-MOFs (1.9 mmol m−2 h−1). Phthalocyanine copper is used for ammonia synthesis due to strong photo-responsiveness. However, the surface of phthalocyanine copper lacks an active site and does not have the desired surface reaction kinetics. Li et al. prepared a phthalocyanine copper/CeO2 heterostructure by the chemical deposition method, and CeO2 is coated on the phthalocyanine copper surface.95 UV-Vis absorption and electrochemical measurement make it clear that the phthalocyanine copper/CeO2 heterostructure has a wide absorption range and a high electrochemical activity area. The PEC N2 reduction performance reaches the best with an NH3 yield of 1.16 μmol h−1 cm−2.
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| Fig. 8 (a) TEM images and EDS spectra of Cu-MOF/Cu2O, (b) Mott–Schottky plot (frequency: 800 Hz),94 Copyright 2021, Elsevier. (c) optical bandgap energy of g-C3N4 and MoSe2@g-C3N4, (d) equivalent NH3 yield and FE at different potentials on MoSe2@g-C3N4 heterojunctions, (e) associative distal pathway for PEC N2 reduction, (f) Gibbs free energy on MoSe2@g-C3N4 heterojunctions,26 Copyright 2020, the Royal Society of Chemistry. (g) Fluctuation of temperature and energy against the time of WO3-MoS2 at 300 K and 500 K,96 Copyright 2022, American Chemical Society. (h) The image of an underwater N2 bubble on the Au-PTFE/TS surface, (i) original (black line) and deconvoluted (red line) spectra of interfacial water at the Au-PTFE/TS surface.97 Copyright 2019, Elsevier. | |
The C atom inserted into the Mo lattice shows a narrow metal band and exhibits similar Fermi level densities to noble metals, so Mo2C is expected to perform well in catalysis.98 However, Mo2C is a matrix with poor electrical conductivity,99 so it is necessary to consider the optimization of carrier migration for PEC N2 reduction. Li et al. took graphite as a unit of the core–shell heterostructure, and wrapped it on the Mo2C cluster surface based on in situ hydrothermal and calcination treatment, and effectively broke through the limitation of Mo2C conductivity.100 The graphitized carbon can accelerate electron transfer, which is conducive to the activation of N2, with an NH3 yield of 6.6 μg h−1 mg−1. In addition, MoSe2 and the two-dimensional material g-C3N4 are assembled to form heterojunctions. Mushtaq et al. synthesized layered flower-like MoSe2-g-C3N4,26 and the Mo–N bond between MoSe2 and g-C3N4 enhances the internal conductivity of MoSe2. The absorption capacity of MoSe2-g-C3N4 is enhanced (Fig. 8c), and the MoSe2-g-C3N4 hybrid shows efficient PEC N2 reduction performance with an FE of 28.91% and a yield of 7.72 mmol h−1 cm−2, respectively (Fig. 8d). The associative distal pathway and Gibbs free energy on MoSe2@g-C3N4 heterojunctions are shown in Fig. 8e and f. The results show that N2 is well adsorbed and activated on the MoSe2-g-C3N4 (7 wt%) hybrid surface.
WO3 is widely used as a photo/electrocatalyst due to its tunable composition, favourable stability and abundant availability. Guo et al. simulated a heterojunction constructed by WO3 and metal dichalcogenides (MoS2, WSe2), and proved that heterojunctions are stable in thermodynamics through calculation (Fig. 8g). In particular, optimized WO3-MoS2 has the lowest onset potential (0.25 V),96 and its catalytic performance is enhanced by re-positioning the d-band center that is precisely controlled to a higher energy level. Furthermore, in order to improve the N2 reduction activity, porous skeleton structures are used in heterojunction construction engineering. Multiple scattering and absorption of light in the porous structure are conducive to the efficient capture of sunlight.101–103 Porous CdS was obtained by growing CdS on Mo doped WO3 hollow microspheres based on the hydrothermal method. The Mo-WO3@CdS heterostructure104 achieved a FE of 36.72% and an NH3 yield of 38.99 μg h−1 mg−1. The DFT analysis suggests that Mo–W is the catalytic center of N2 reduction, and CdS promotes the provision of electrons, thus accelerating PEC N2 hydrogenation. Meanwhile, the development of porous structures provides the basis for functionalized heterogeneous structures. The Au modified porous polytetrafluoroethylene (PTFE) skeleton dispersed on a silicon-based electrode was synthesized. The silicon and the porous PTFE skeleton were used as the light absorber and gas diffusion channels, respectively, while Au was used as the reaction site.97 Attenuated total reflection and caption-bubble measurements demonstrated that the porous framework of Teflon and Au created a gas-hydrophilic structure, which promoted N2 enrichment and controlled proton activity in aqueous solution (Fig. 8h and i). The optimal ammonia production rate of the photocathode is about 18.9 μg cm−2 h−1, and the FE is 37.8% at −0.2 V vs. RHE. The LDH plays a key role in the fields of photocatalysis/electrocatalysis and energy conversion due to easily controllable composition and abundance of surface oxygen vacancies or coordinatively.105 Yan et al. constructed a CoVP@NiFeV-LDH heterojunction that has shown excellent electrocatalytic performance in the N2 reduction reaction. The new three-dimensional hollow hierarchical structure provides abundant active sites for nitrogen adsorption and reduction to NH3.106
3.5 Cocatalyst loading
Noble metal catalysts have the advantages of high catalytic activity, stability and selectivity. For effective utilization of noble metals, it is necessary to load them onto the host matrix. For example, black silicon serves as a carrier, and it can also enhance light absorption and suppress reflection.107,108 Ali et al. used plasma-enhanced black silicon decorated with Au nanoparticles (NPs) to achieve photoelectrically driven nitrogen conversion (Fig. 9a).22 When black silicon is replaced by unetched silicon, NH3 production is 11% of that generated by Au/black Si/Cr. Black silicon provides a large surface area for Au loading. Au-coated black silicon has a higher ammonia yield (more than 13 mg m−2 h−1) than black silicon. Wang et al. realized the PEC process of converting nitrogen into ammonia, when black silicon was used to enhance light absorption, and silver was used as an active site with a FE of 55.05% at −0.1 V (vs. RHE).25
|
| Fig. 9 (a) Corresponding TEM images of Au-coated silicon nanowires,22 Copyright 2016, Nature Publishing Group. (b) TEM images of Au/g-C3N4, (c) mapping of N, C, and Au,109 Copyright 2021, Wiley-VCH. (d) XRD patterns of SnO2/10MoS2, SnO2/10MoS2 (silicomolybdic acid is not a precursor), MoS2 (silicomolybdic acid is not a precursor) and SnO2,110 Copyright 2022, Elsevier. (e) The XPS spectra of Bi2S3 QDs/15MoS2, (f) photocurrent with a light on–off cycle.111 Copyright 2022, Elsevier. | |
Porous structures are also excellent to disperse and stabilize metal NPs. Peramaiah et al. deposited Au NPs on porous g-C3N4 with N defects, and then coated it on the n+np+-Si photocathode surface (Fig. 9b and c).109 Silicon plays the roles of light collection and electrical management, Au is the active center to promote the conversion of N2 to NH3, and porous g-C3N4 with N-defects provides more N2 adsorption sites. Each N atom is alternately associated with the first and second hydrogenation; then, the third hydrogen is added to destroy the N–N bond; finally, the distal N atom is reduced thoroughly with the release of the first NH3 molecule. Au/g-C3N4 exhibits excellent PEC N2 reduction performance with a FE of up to 61.8% and an NH3 yield of 13.8% μg h−1 cm−2. MoS2 is an excellent carrier due to its well-dispersed layered structure, high conductivity and large specific surface area. Ye et al. dispersed TiO2 NPs on MoS2 nanosheets by the hydrothermal method, and the addition of MoS2 leads to a smaller bandgap (2.98 eV) than that of TiO2, and expedites the separation of electrons and holes.112 Enhanced charge transfer maintains higher photoelectric voltage and slower decay behaviour. The results show that the NH3 yield and FE are 1.42 × 10–6 mol h−1 cm−2 and 65.52%, respectively, for MoS2@TiO2. Hydrophilic SnO2 is uniformly dispersed on the surface of hydrophobic MoS2, which leads to a change in the growth direction of MoS2, so as to induce the growth of the (002) crystal plane. The improved transfer and separation of carriers are attributed to the strong interaction between SnO2 and MoS2, with an NH3 yield of 19.6 at −0.3 V μg h−1 mg−1 and a FE of 40.34%, much higher than those of individual SnO2 and MoS2 (Fig. 9d).
Quantum dots (QDs) possess superior properties that play important roles in N2 reduction,113 such as small size, strong photostability, multiple excitation, exposed sites and electronic peculiarity. For example, Bi2S3 QDs have a high absorption coefficient and narrow bandgap.114 Gao et al. grew Bi2S3 QDs on MoS2 through hydrothermal and solvothermal processes for photocatalytic reduction of N2 to NH3.111 Bi2S3 QDs were dispersed on the MoS2 surface, and the agglomeration of Bi2S3 QDs can be inhibited. The shift of binding energy indicates that there is an interaction between Bi2S3 QD and MoS2 (Fig. 9e), thus leading to electron transfer from Bi2S3 to MoS2 and accelerating the carrier separation (Fig. 9f). The FE of the Bi2S3 QD/15MoS2 photocathode reaches 33%, and the NH3 yield is 19 μg h−1 mg−1. Carbon quantum dots (CQDs) are narrow-gap semiconductor materials, and Hu et al. synthesized CQD-modified hydrogenated mesoporous SrTiO3 (CQDs/STO) based on the small size and good conductivity.115 The ammonia yield of CQDs/STO is 143 μmol g−1 h−1 and is 7 times that of STO without a sacrificial agent. Furthermore, heterogeneous CQD/STO with a defective STO (200) surface can promote catalytic N2 conversion.
3.6 Other advanced catalysts and strategies for nitrogen fixation
Single atomic alloy catalysts have broad application prospects in electro-catalytic nitrogen reduction because of the highly exposed active sites. However, there are limited comprehensive experiments and theoretical research on the conversion of N2 to nitrogen-containing molecules. Chuk et al. reported that the yield of NH3 in the nitrogen reduction reaction was 111.9 μg h−1 mg−1 using single atom PdFe alloy.116 In addition, compared with the pristine Ru, the use of Ru–Cu alloy showed that the addition of Cu improved the FE of NH3 (31%) through enhanced N2 adsorption and a reduced H barrier.117 Li-mediated N2 reduction is flexible in the electrocatalytic approach, which has practical significance for realizing the nitrogen cycle; however, the yield and FE are not optimistic. Du et al. studied the properties of the interface generated by the imide-based Li salt electrolytes. The constructed interface can provide a substantial NH3 yield of 150 ± 20 nmol s−1 cm−2, and a FE of up to 100%.118 In addition, this group has realized the ammonia synthesis by the phosphonium cation and isopropanol.119 Therefore, the development of non-metallic materials with high nitrogen fixation efficiency is of great potential. Recently, both experiments and theoretical calculations have suggested that traditional semiconductor materials are promising photoelectrocatalysts for N2 reduction under ambient conditions, as listed in Table 1.
Table 1 The catalytic performance of PEC N2 reduction to NH3 over various catalysts
Catalyst |
Irradiation source |
Bias [V] |
Rate of NH3 |
Ref. |
BiVO4/PANI |
300 W Xe lamp |
−0.1 V vs. RHE |
0.055 μmol cm−2 h−1 |
52
|
BiOI |
100 mW cm−2 |
+0.4 V vs. RHE |
0.140 μmol cm−2 h−1 |
24
|
CQDs/SrTiO3 |
300 W Xe lamp |
−0.3 V vs. RHE |
1.915 μmol cm−2 h−1 |
115
|
Cu2O |
300 W Xe lamp |
+0.4 V vs. RHE |
0.423 μmol cm−2 h−1 |
49
|
PANI-ASSM/CdS-Co3S4 |
40 W LED |
−0.75 V vs. RHE |
12.33 μmol cm−2 h−1 |
120
|
Vo-BiOBr/TiO2 |
300 W Xe lamp |
−0.2 V vs. RHE |
1.475 μmol cm−2 h−1 |
27
|
Cu-MOF/Cu2O |
300 W Xe lamp |
+0.5 V vs. RHE |
0.716 μmol cm−2 h−1 |
94
|
MoSe2@g-C3N4 |
150 W Xe lamp |
−0.3 V vs. RHE |
7.72 μmol cm−2 h−1 |
26
|
Fe-doped W18O49 |
simulated solar light |
−0.65 V vs. Ag/AgCl, |
0.577 μmol cm−2 h−1 |
21
|
SnO2/MoS2 |
300 W Xe lamp |
−0.3 V vs. RHE |
19.6 μg h−1 mgcat.−1 |
110
|
B-doped Bi nanorolls |
300 W Xe lamp |
+0.48 V vs. RHE |
29.2 mg h−1 gcat.−1 |
72
|
Mo-doped WO3@CdS |
300 W Xe lamp |
−0.3 V vs. RHE |
38.99 μg h−1 mgcat.−1 |
101
|
Mo2C/C |
100 mW cm−2 |
+0.2 V vs. Ag/AgCl |
6.6 μg h−1 mgcat.−1 |
121
|
4. Mechanism of PEC N2 reduction for ammonia synthesis
It is generally believed that PEC N2 reduction on a heterogeneous catalyst surface undergoes two mechanisms, namely the associative reaction and dissociative reaction (Fig. 10a).23 For dissociation, the triple bond of N2 is cracked before hydrogenation, and two separate N atoms are reduced to form NH3 independently through hydrogenation steps, which is similar to the H–B process that requires more energy. In the association pathway, two nitrogen atom centers are combined together when the N2 molecule is hydrogenated, and NH3 is released with the breaking of the NN bond. The hydrogenation in the association mechanism can take place in two possible ways, namely distal and alternative pathways. When N2 appears in the mode of terminal coordination, hydrogenation is inclined to occur on the nitrogen far from the catalysts, thus leading to the generation of NH3 molecules, and leaving the metal nitrogen unit to react with H+ to generate another NH3 molecule. The alternative route requires that two nitrogen centers alternate between two N atoms on the catalytic surface. After the first NH3 is removed, the second NH3 will be released immediately. Our group has reported the possible route (distal pathway) of NH3 synthesis on the MoSe2-g-C3N4 surface based on the Gibbs free energy,26 that is, the adsorbed N2 is hydrogenated by H+ adsorption and electron transfer to form an N2H* group at the Mo site. And then H+ + e− reacts with N to form N2H*. The adsorption of H+ on the pre-hydrogenated N site of N2H2* yields an NH3 molecule, leaving the remaining N at the Mo atom. After that, another three H+ ions are coupled to the electron, hydrogenating the remaining N* molecule to give a second NH3.
|
| Fig. 10 (a) Dissociative, distal and alternating pathways for N2 reduction, (b) MvK mechanism on the TM nitride surface,132 (c) atom projected density of states (PDOS) of Mo-WO3,104 Copyright 2022, Elsevier. (d and e) PDOS and projected crystal orbital Hamilton populations of the N atoms (*NNH) on Bi (012) and (e) Bi-2B (012),72 Copyright 2021, Elsevier. (f) Free energy and optimized geometric structure of distal mechanisms on the Fe3C sites and Au1+ sites of the Au/SiO2/Si photocathode, (g) schematic representation of the catalytic and enhanced mechanism of the PEC NRR on the Au/SiO2/Si photocathode.133 Copyright 2021, American Chemical Society. | |
Researchers proposed a new reaction mechanism in recent years, namely Mars–van-Krevelen (MvK), based on transition metal nitrides (TMNs: V–N, Zr–N, Nb–N) as shown in Fig. 10.122 The lattice N atoms of TMNs are hydrogenated to obtain NH3 by lattice nitrogen reduction, followed by the regeneration of lattice nitrogen. Importantly, N2 activation on TMNs requires a smaller overpotential for the MvK mechanism, which is more conducive to the formation of NH3 than association and dissociation reactions.
DFT studies are used for energy assessment and catalyst design on the basis of experimental results, including the calculations of adsorption energy, Gibbs free energy,75,123,124 the density of states analysis, the Bader charge analysis, and the crystal orbital Hamilton population analysis. In addition, advanced in situ monitoring technologies are meaningful for verifying the reaction mechanisms, dynamics of molecular catalyst interactions, and revealing the structure of intermediates, especially for in situ observation of catalytic reaction intermediates under practical operation, such as synchrotron radiation, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and surface-enhanced infrared absorption spectroscopy (SEIRAS).125–128 Yang et al. used in situ XPS and in situ XAS to reveal that VN0.7O0.45 was the active site rather than VN, and they monitored that the catalyst underwent passivation due to a change in VN0.7O0.45 to form VN.129 According to in situ SEIRAS, the possible nitrogen reduction pathway on the Ru surface in acidic solution was deduced.130 Feng et al. observed that adsorbed H2O molecules were located at the Ovs site based on operando STM equipment, which accelerated the electrocatalytic HER process, and provided a rich route for revealing the kinetic process of nitrogen fixation.131 Various in situ characterization techniques can be used not only independently, but also in combination to obtain accurate and multi-dimensional research results. In the future, in situ characterization technology will make a breakthrough in cost and universality, which further deepens our understanding of catalytic mechanisms.
The selectivity of NH3 from N2 and H2O is challenging against the HER (and/or N2H4), and numerous computational simulations and experimental attempts have been conducted to elucidate the essence of selectivity. Singh et al. provided some guidance that the selectivity between NH3 and H2 can be determined through a competition between the binding energies of N and H. They found that the generation rate of ammonia is of zero-order at electron and proton concentrations, while the rate of H2 is of first-order at a potential more negative than Us (Us refers to the potential to favourable H adsorption).134 And the strategies of proton/electron availability have been proposed due to the significant impact of the proton concentration on HER performance, so employing an aprotic solvent,135 blanketing the catalyst with a hydrophobic protection layer,136 and designing photoelectrocatalysts are several potential solutions to selectively block the proton transfer to the catalyst surface, thus hindering H2 evolution. Increased loadings of H+ and e− equivalents can directly translate to greater N2H4 yields, and the powerful reductant KC8 and strong Brønsted acid can deliver H+/e− equivalents for the necessary proton-coupled electron transfer to improve the selectivity of N2H4.137 In addition, regulating the adsorption strength of N and H towards catalytic sites can alter the selectivity of NH3. It is reported that the adsorption energy of H on Ru is smaller than that on N, which indicates that N atoms are preferably adsorbed on H atoms, thus obtaining higher NH3 selectivity.138
5. Detection and quantification of nitrogen sources for ammonia
The accurate quantification of ammonia is restricted by many factors, such as trace NH3 yield intrinsically, ammonia pollution from the environment, tedious ammonia separation from electrolytes, solution pH, and interferants. Serious concerns exist on a reliable quantitative method of ammonia concentration for N2 reduction, and we should maintain a rigorous scientific attitude and strive to accurately quantify NH3 based on different catalytic systems (Fig. 11). Currently, spectrophotometry and ion chromatography are common methods with the advantage of simplicity. In addition, the use of ion-selective electrodes (ISEs) and colorimetric NH3 assay kits, and nuclear magnetic resonance (NMR) methods can significantly improve the sensitivity, however, the experimental cost is increased to a certain extent.
|
| Fig. 11 Calibration curves for NH3 quantification. (a) the UV-Vis absorption, (b) the corresponding calibration curve for the indophenol blue method,147 Copyright 2019, Wiley-VCH. (c) 1H NMR spectra of 15NH4+ standard solutions (0.1–2.0 mM), (d) the corresponding calibration curve for the NMR method,49 Copyright 2020, American Chemical Society. (e) UV-Vis of standard solutions, and (f) the corresponding calibration curve with Nessler's reagent,145 Copyright 2019, Nature Publishing Group. (g) (I) 1H NMR of NH4+ in 100 μM NH4+ in various solvents, (II) NMR spectra of a mixture of 14NH4++15NH4+, (III) NMR of the LiClO4/THF electrolyte after N2 reduction, (h) calibration curve to determine the NH4+ concentration,144 Copyright 2019, American Chemical Society. (i) UV-Vis absorption spectra and (j) the calibration curve using salicylic acid,104 Copyright 2022, Elsevier. (k) FE of ammonia measured using Nessler's reagent or an enzymatic ammonia assay kit,145 Copyright 2019, Nature Publishing Group. (l) The ion-selective electrode method.148 Copyright 1969, Springer Nature. | |
The universal chemical chromogenic approaches are indophenol blue (IB) and Nessler's reagent based on the calibration curve, and they all have limitations. The IB test is usually used under neutral and acidic conditions because the generated indophenol dye could react with other amine groups under alkaline conditions.139 Sodium salicylate is more steady than IB because salicylate can prevent the generation of toxic substances.140 Nessler's method has wide pH applicability, however, some sacrificial agents (alcohol, glucose) are oxidized to carbonyl easily, thus resulting in misleading results.35 Ion chromatography has high sensitivity and reliable results, among which the distraction of pH and sacrificial agents are avoided through appropriate detection methods.141,142 The ISE determines the concentration of ions in solution by a membrane potential based on the Nernst equation, which can be divided into the NH4+-selective electrode and the NH3-selective electrode. The concentrated solution can further improve the accuracy of NH3 detection when choosing an ISE.143 The resonance signal obtained from 1H can be qualitatively and quantitatively detected by NH4+.144 Nielander et al. reported a frequency-selective pulse NMR method for measuring NH3 concentration in electrolytes and quantified the common Berthelot method. The reliability of the results can be greatly improved by using various methods simultaneously to realize the quantitative determination of ammonia. Hao et al. used a colorimetric method with Nessler's reagent, an enzymatic NH3 assay kit (Sigma-Aldrich), and NMR to quantify ammonia accurately;145 they found that the FEs measured using Nessler's approach or the enzymatic ammonia assay kit are persuasive. Zhang et al. used Nessler's approach, ion chromatography and the indole phenol method to quantify NH3, and the results showed that the three methods had good coordination at low concentrations (0–500 μg L−1). However, Nessler's approach and ion chromatography are more suitable for the quantification of ammonia than the indole phenol method at high concentrations.146
In order to scale up the PEC ammonia synthesis technology, the most essential progress is to confirm the true role of the catalysts. The primary task of removing nitrogen pollution is to ensure that the feed gases and catalysts are not polluted by nitrogen containing substances, including nitrate, nitrite and other contaminants before the experiment. Considering serious ammonia pollution in the environment, the 15N2 labeling experiment is necessary to provide direct evidence,149 especially for N-containing catalyst systems.123 However, many researchers often ignore the problem of 15N2 pollution, so it is only used as a qualitative method. Therefore, we need to accurately present the nitrogen fixation performance of the catalysts from multiple views. In addition, the background experiment is another piece of evidence to assist in proving the role of catalysts. The development of a combined technology of ammonia synthesis and ammonia detection can avoid ammonia pollution and realize the in-situ detection and online quantification of ammonia.
6. Conclusion
The above achievements have proved that PEC N2 reduction is a promising route to realize the N-cycle goal under environmental conditions, and multiple catalytic strategies are proposed to improve the yield and selectivity of NH3. However, there are fundamental challenges as follows:
(I) Disclosure of the essence of photo-electrical synergy. The catalytic performance of PEC N2 reduction is more excellent than that of individual photocatalysis or electrocatalysis; however, this collaborative technology causes obstacles to the mechanistic understanding and revelation of photogenic/electrical effects. Most studies show that light irradiation promotes the electrocatalytic process, and bias accelerates the separation of photogenerated carriers, and few works reveal the coexistence of photocatalytic and electrocatalytic processes.
(II) Reliable methods in consideration of NH3 quantification and N-source determination. Although the spectral analysis is maneuverable, the accuracy usually is disturbed by pH, ionic strength and sacrificial agents. Therefore, it is necessary to combine it with other quantitative experiments, such as ion chromatography and NMR, to obtain accurate NH3 concentrations. In addition, researchers judge the rationality of catalytic performance by testing the recommended benchmark catalyst.
(III) Rigorous mechanistic insights. The active sites, free energies and possible reaction paths are revealed on the basis of theoretical exploration. The available theoretical methods, catalyst model and computing power are the driving force behind accurate calculations.
(IV) Advanced in situ characterization. It is difficult to predict the reconstruction of the surface structure, including the change of valence states, the identification of reaction intermediates and active centers, and real-time monitoring of reaction processes. For example, in situ surface enhanced infrared absorption spectroscopy and high-resolution electron energy loss spectroscopy showed potential for understanding the N2 reduction pathway due to its high sensitivity to the catalyst surface. In conclusion, reasonable catalysts are designed based on calculation and experiment, which will promote the rapid development of PEC N2 reduction to a large extent under environmental conditions in the future. The advantages of PEC N2 reduction are higher than those of individual photocatalysis or electrocatalysis. The correlation between PEC performance and the structures could be established at the molecular level to achieve industrial capabilities. In addition, strengthening durability testing is the key to proving the practicality at the industrial level. Finally, large-scale synthesis of catalysts should be considered.
Author contributions
Pengyan Li: methodology, investigation, and writing – original draft. Yumin Liu: supervision. Muhammad Asim Mushtaq: supervision. Dongpeng Yan: supervision and writing – review & editing. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 22288201 and 22275021) and the Beijing Municipal Natural Science Foundation (Grant No. JQ20003).
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