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.

The use of hydrogen as a fuel, when generated from water using semiconductor photocatalysts and driven by sunlight, is a sustainable alternative to fossil fuels. Polymeric photocatalysts are based on Earth-abundant elements and have the advantage over their inorganic counterparts in that their electronic properties are easily tuneable through molecular engineering. Polymeric photocatalysts have developed rapidly over the past decade, resulting in the discovery of many active materials. However, our understanding of the key properties underlying their photoinitiated redox processes has not kept pace, and this impedes further progress to generate cost-competitive technologies. Here, we discuss state-of-the-art polymeric photocatalysts and our microscopic understanding of their activities. We conclude with a discussion of five outstanding challenges in this field: non-standardized reporting of activities, limited photochemical stability, insufficient knowledge of reaction mechanisms, balancing charge carrier lifetimes with catalysis timescales and the use of unsustainable sacrificial reagents. Solar-driven photocatalytic water splitting provides a clean pathway for production of hydrogen fuel. This Review examines both amorphous and crystalline polymeric materials for water splitting, exploring polymer design strategies, theoretical understanding and challenges for the field.

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Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts

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

Inside-and-Out Semiconductor Engineering for CO2 Photoreduction: From Recent Advances to New Trends

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

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

A photocatalyst system is generally comprises a catalyst and cocatalyst to achieve light absorption, electron-hole separation, and surface reaction. It is a challenge to develop a single photocatalyst having all functions so as to lower the efficiency loss. Herein, the active GaN4 site is integrated into a polymeric carbon nitride (CN) photocatalyst (GCN), which displays an excellent H2 production rate of 9904 μmol h-1  g-1 . It is 162 and 3.3 times higher than that of CN with the absence (61 μmol h-1  g-1 ) and presence (2981 μmol h-1  g-1 ), respectively, of 1.0 wt % Pt. Under light irradiation the electron is injected and stored at the GaN4 site, where the LUMO locates. The HOMO distributes on the aromatic ring resulting in spatial charge separation. Transient photovoltage discloses the electron-storage capability of GCN. The negative GaN4 promotes proton adsorption in the excited state. The positive adsorption energy drives H2 desorption from GaN4 after passing the electron to the proton. This work opens up opportunities for exploring a novel catalyst for H2 production.

Photocatalyst for High-Performance H 2 Production: Ga-Doped Polymeric Carbon Nitride

Conjugated polymers (CPs) have been demonstrated to be excellent solar energy conversion materials, however, it is considered that pure CPs are not able to photocatalyze water splitting because of the fast recombination of photogenerated charge carriers. Here, we found polythiophene (PTh) nanofibrous structure can have a decent photocatalytic activity (2.19 mmol h(-1) g(-1)) and stability in ascorbic acid aqueous solution under visible light irradiation. Furthermore, PTh synthesized at high temperature will introduce structure defects, such as, alpha-beta' coupling and end groups, which are beneficial to the charge separation. In addition, PTh synthesized at low temperature has long polymer chain and ordered arrangement of chain, which enhanced the light adsorption. Considering both light adsorption and charge separation, PTh compound nanostructure was rationally designed, PTh synthesized at low temperature as core to obtain good light adsorption and PTh prepared at high temperature as sheath to form more structure defects to promote charge separation. This PTh compound nanostructure exhibit higher photocatalytic H-2 production performance (2.8 mmol h(-1) g(-1)). Our finding may open a door for conjugated polymer for photocatalyst. (C) 2017 Elsevier B.V. All rights reserved.

Structure defects assisted photocatalytic H 2 production for polythiophene nanofibers

Building a donor-acceptor (D–A) type polymeric carbon nitride is a promising way for synthesizing high-performance photocatalysts. Compared with introducing an acceptor unit, incorporating donor blocks can effectively extend the absorption range without altering H2 production sites. Herein, we report a facile method to construct a d -A structure by introducing the creatinine-derived moiety (CDM) into melem networks via copolymerization of creatinine and urea. CDM units arouse a new absorption band at 500 nm and result in the LUMO and HOMO distribute at melem and CDM unit, which promotes the charge spatial separation. The optimal photocatalytic activity of 9454 μmolg−1 h−1 can be achieved. That does not only improve the apparent quantum yield (AQE) at 420 nm from 3.80 % to 8.41 % but also raises the AQE at 500 nm from 0.71 % to 4.57 %. This strategy provides the potential to extend the further visible light response with a decent performance.

Xupeng Zong

Papers

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

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

Photocatalyst for High-Performance H 2 Production: Ga-Doped Polymeric Carbon Nitride

Structure defects assisted photocatalytic H 2 production for polythiophene nanofibers

Constructing creatinine-derived moiety as donor block for carbon nitride photocatalyst with extended absorption and spatial charge separation