Global ammonia production reached 175 million metric tons in 2016, 90% of which is produced from high purity N2 and H2 gases at high temperatures and pressures via the Haber-Bosch process. Reliance on natural gas for H2 production results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to ammonia production and reduce its carbon footprint. Electrocatalytic N2 reduction to ammonia is an attractive alternative that can potentially enable ammonia synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theoretical and experimental studies on electrochemical nitrogen fixation with a focus on the low selectivity for reduction of N2 to ammonia versus protons to H2. A detailed introduction to ammonia detection methods and the execution of control experiments is given as they are crucial to the accurate reporting of experimental findings. The main part of this review focuses on theoretical and experimental progress that has been achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.

Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia.

Electronic structure greatly determines the band structures and the charge carrier transport properties of semiconducting photocatalysts and consequently their photocatalytic activities. Here, by simply calcining the mixture of graphitic carbon nitride (g-C3 N4 ) and sodium borohydride in an inert atmosphere, boron dopants and nitrogen defects are simultaneously introduced into g-C3 N4 . The resultant boron-doped and nitrogen-deficient g-C3 N4 exhibits excellent activity for photocatalytic oxygen evolution, with highest oxygen evolution rate reaching 561.2 µmol h-1 g-1 , much higher than previously reported g-C3 N4 . It is well evidenced that with conduction and valence band positions substantially and continuously tuned by the simultaneous introduction of boron dopants and nitrogen defects into g-C3 N4 , the band structures are exceptionally modulated for both effective optical absorption in visible light and much increased driving force for water oxidation. Moreover, the engineered electronic structure creates abundant unsaturated sites and induces strong interlayer C-N interaction, leading to efficient electron excitation and accelerated charge transport. In the present work, a facile approach is successfully demonstrated to engineer the electronic structures and the band structures of g-C3 N4 with simultaneous introduction of dopants and defects for high-performance photocatalytic oxygen evolution, which can provide informative principles for the design of efficient photocatalysis systems for solar energy conversion.

Synergy of Dopants and Defects in Graphitic Carbon Nitride with Exceptionally Modulated Band Structures for Efficient Photocatalytic Oxygen Evolution.

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Semiconductor‐based Z‐scheme heterojunction photocatalysts have received considerable attention for solar energy conversion and environmental purification due to their spatially separated reduction and oxidation sites, effective separation and transportation of photo‐excited charge carriers and strong redox ability. With their wide visible‐light responsive range and high photocatalytic activity, metal sulphide is an important material in developing photocatalysts. This review summarizes and highlights recent research progress in sulphide‐based direct Z‐scheme photocatalysts, followed by analysis on the limitations over all‐solid‐state Z‐scheme photocatalyst. Furthermore, the applications and characterization methods of sulphide‐based direct Z‐scheme photocatalyst are summarized. Finally, the challenges and perspectives of sulphide‐based Z‐scheme photocatalyst are discussed.

Review on Metal Sulphide-based Z-scheme Photocatalysts

Electrocatalytic Nitrogen Reduction at Low Temperature

Heterojunction and direct Z-scheme nanostructures are two typical representatives of an efficient photocatalyst, which is composed of two semiconductors. However, it is a great challenge to construct each of them on purpose. The photodeposition technique can be a potentially powerful tool to regulate the electron flow direction for constructing these nanostructures. In this report, CdS nanoparticles were deposited on the g-C3N4 nanosheets by photodeposition and chemical deposition methods for comparison. In the photodeposition case, PL and charge flow tracking demonstrate that a type II heterojunction is constructed because CdS is selectively deposited at the electron transfer site of g-C3N4, which leads to the photoexcited electron from g–C3N4 tending to transfer to CdS in the composites. In the latter, the CdS is randomly deposited onto the g-C3N4 nanosheets through chemical deposition. There is no preferred site for deposition or charge transfer in the composite. The results illustrate that the electro...

Consciously Constructing Heterojunction or Direct Z-Scheme Photocatalysts by Regulating Electron Flow Direction

Phosphides exhibit relatively low overpotential for electrical hydrogen evolution reaction (HER), thus they have great potential to be used for cocatalyst for photocatalyst. Cu3P, as a p-type semiconductor, tends to form a p-n junction with an n-type photocatalyst. Typically, it is treated as a sensitizer to extend the light absorption. However, its function and work mechanism are not fully understood in the catalyst system. In this report, we synthesized g-C3N4 and loaded Cu3P nanoparticle on its surface. The photoluminescence (PL) spectra, photocurrent and electrochemical impedance spectra confirm the Cu3P greatly enhance the charge separation process. Electrochemical HER results indicate that the composites have lower over-potential for HER. These results confirm the Cu3P works as a cocatalyst in the system, not a sensitizer. Further, we tracked the photogenerated electron transfer direction via photodeposition of Pt nanoparticles. The Pt nanoparticles tend to deposit near the Cu3P nanoparticles. That illustrates the photogenerated electron will be left on Cu3P nanoparticles. On the other hand, the photocatalytic decomposition of Rhodamine B (RhB) illustrates that the holes are left on the g-C3N4 due to both g-C3N4 and Cu3P/g-C3N4 have similar decomposition rate, but the Cu3P cannot decompose RhB. Based on these, we proposed the photogenerated electron of g-C3N4 recombine with the hole of Cu3P, the photogenerated electron of Cu3P will be left for HER. That reasonably explain the cocatalyst function of Cu3P in the composite catalyst system.

Highly efficient p-type Cu3P/n-type g-C3N4 photocatalyst through Z-scheme charge transfer route

Photocatalytic ammonia synthesis is another important reaction to mimic natural nitrogen fixation, which has attracted more and more attention. In recent reports, sacrificial agents are often used to promote charge separation, and high-activity photocatalysts are discovered by using Nessler’s reagent method as a detection technique of ammonia production. However, there is an open question on the rationality and accuracy of the ammonia production amount in the presence of the sacrificial agent and Nessler’s reagent detection method. In this report, P25 TiO2 is employed as a model photocatalyst and alcohol as sacrificial agent, and both Nessler’s reagent and cation exchange chromatography are employed as ammonia detection methods. The different ammonia production amount was found by the different detection method. HPLC and 1H NMR results indicate that carbonyl compounds (formaldehyde, acetaldehyde, and acetone) are produced in the reaction. When the carbonyl compound was added to the ammonia standard soluti...

Interference Effect of Alcohol on Nessler’s Reagent in Photocatalytic Nitrogen Fixation

Constructing Z-scheme type photocatalysts is preferred to achieve efficient photocatalyst owing to the high oxidative and reductive capabilities of composite catalysts. The direct Z-scheme (DZS) is composed of two semiconductors contacting each other, which eliminates the disadvantages of using an electron mediator in liquid and solid-state Z-schemes. However, deliberately constructing a DZS remains challenging. In this report, two semiconductors were denoted as SC-I and SC-II according to the natural photosynthesis process. Two routes for constructing DZS catalysts through photodeposition were demonstrated, which effectively regulated the photogenerated charge transfer direction. Starting from SC-I, SC-II was selectively deposited at the hole-rich site of SC-I by photooxidation. Starting from SC-II, SC-I was selectively grown at the electron-rich site of SC-II through photoreduction. This selective deposition promoted directional charge recombination to form DZS catalysts. Two examples, Fe2O3/g-C3N4 and CdS/TiO2, were synthesized by photooxidation and photoreduction, respectively. Charge-transfer tracking and reactive oxygen species were used to check the charge transfer direction and radical type. The results showed that both systems indeed formed DZS catalysts. Based on the above results, photodeposition can be used to deliberately construct DZS catalysts.

Deliberate construction of direct Z-scheme photocatalysts through photodeposition

Defects play a significant part in promoting photocatalytic activity for H2 production. Various methods such as chemical reduction have been performed to metal oxide based photocatalysts. Herein, we present the NaBH4 reduction route to introduce the defects into the graphitic carbon nitride (g-C3N4) to enhance photocatalytic activity. A new −C≡N group is observed in the FTIR spectra of treated g-C3N4 nanosheets indicating the presence of structural defects. At the same time, the B signal appears in the X-ray photoelectron spectroscopy analysis, suggesting that B is doped in the g-C3N4 during the treatment. All these results manifested that multiple types of defects are introduced in the g-C3N4 during the NaBH4 treatment. The UV–vis spectra illustrate that the absorption band edge of g-C3N4 is extended from 420 to 450 nm after NaBH4 treatment. This demonstrates that the band gap of g-C3N4 turns narrow owing to the introduction of defects. Photocatalytic H2 production of defective g-C3N4 is ∼5-fold better t...

Yuanjing Wen

Papers

Consciously Constructing Heterojunction or Direct Z-Scheme Photocatalysts by Regulating Electron Flow Direction

Highly efficient p-type Cu3P/n-type g-C3N4 photocatalyst through Z-scheme charge transfer route

Interference Effect of Alcohol on Nessler’s Reagent in Photocatalytic Nitrogen Fixation

Deliberate construction of direct Z-scheme photocatalysts through photodeposition

Defective g-C3N4 Prepared by the NaBH4 Reduction for High-Performance H2 Production