Metal-Free Carbon Materials for CO2 Electrochemical Reduction

The rapid increase of the CO₂ concentration in the Earth's atmosphere has resulted in numerous environmental issues, such as global warming, ocean acidification, melting of the polar ice, rising sea level, and extinction of species. To search for suitable and capable catalytic systems for CO₂ conversion, electrochemical reduction of CO₂ (CO₂ RR) holds great promise. Emerging heterogeneous carbon materials have been considered as promising metal-free electrocatalysts for the CO₂ RR, owing to their abundant natural resources, tailorable porous structures, resistance to acids and bases, high-temperature stability, and environmental friendliness. They exhibit remarkable CO₂ RR properties, including catalytic activity, long durability, and high selectivity. Here, various carbon materials (e.g., carbon fibers, carbon nanotubes, graphene, diamond, nanoporous carbon, and graphene dots) with heteroatom doping (e.g., N, S, and B) that can be used as metal-free catalysts for the CO₂ RR are highlighted. Recent advances regarding the identification of active sites for the CO₂ RR and the pathway of reduction of CO₂ to the final product are comprehensively reviewed. Additionally, the emerging challenges and some perspectives on the development of heteroatom-doped carbon materials as metal-free electrocatalysts for the CO₂ RR are included.

Charge transport, interfacial interactions and synergistic mechanisms in BiNbO4/MWO4 (M = Zn and Cd) heterostructures for hydrogen production: insights from a DFT+U study

In the 21st century, the growing demand of global energy is one of the key challenges. The photocatalytic generation of hydrogen has attracted extensive attention to discuss the increasing global demand for sustainable and clean energy. However, hydrogen evolution reactions normally use the economically expensive rare noble metals and the processes remain a challenge. Herein, low-cost BiNbO₄/MWO₄(010) heterostructures are studied for the first time to check their suitability towards photocatalytic hydrogen production. A theoretical study with the aid of density functional theory (DFT) is used to investigate the synergistic effect, ionisation energy, electron affinities, charge transfer, electronic properties and the underlying mechanism for hydrogen generation of BiNbO₄/MWO₄(010) heterostructures. The experimental band gaps of bulk ZnWO₄, CdWO₄ and BiNbO₄ are well reproduced using the DFT+U method. The calculated band edge position shows a type-II staggered band alignment and the charge transfer between BiNbO₄ and MWO₄ monolayers results in a large interfacial built-in potential, which will favour the separation of charge carriers in the heterostructures. The effective mass of the photoinduced holes is higher compared to the electrons, making the heterostructures useful in hydrogen production. The relatively low ionisation energy and electron affinity for the heterostructures compared to the monolayers make them ideal for photocatalysis applications due to their small energy barrier for the injection of electrons and creation of holes. The BiNbO₄/MWO₄(010) heterostructures are more suitable for photocatalytic hydrogen production due to their strong reducing power relative to the H⁺/H₂O potential. This study sheds light on the less known BiNbO₄/ZnWO₄(010) heterostructures and the fully explored electronic and optical properties will pave way for future photocatalytic water splitting applications.

Cobalt Nanoparticles Encapsulated in Porous Carbons Derived from Core-Shell ZIF67@ZIF8 as Efficient Electrocatalysts for Oxygen Evolution Reaction

The synthesis of electrocatalysts consisting of selectively functionalized parts is an effective strategy to prepare nonprecious electrocatalysts with excellent performance for oxygen evolution reaction (OER). Herein, we synthesized core-shell structured ZIF67@ZIF8 and converted it into Co decorated porous carbons (CS-Co/Cs) consisting of the ZIF67 derived uniformly dispersed Co nanoparticles encapsulated in graphitic carbon as cores and the ZIF8 derived porous carbon as shells. Compared to individual ZIF67 derived samples (Co/Cs), the unique structure of CS-Co/Cs leads to the larger surface area and more hydrophilic surface, both of which facilitate the mass transfer, contributing to the enhanced OER performance. The optimized CS-Co/C sample presents the low overpotential of 290 mV to deliver 10 mA cm^-2 toward OER in 1 M KOH, which is among the best of the reported nonprecious OER electrocatalysts. The CS-Co/C exhibits no obvious current attenuation at 1.53 V for 30 000 s, demonstrating its robust stability.

Ultrathin Two-Dimensional Nanostructured Materials for Highly Efficient Water Oxidation

Water oxidation, also known as the oxygen evolution reaction (OER), is a crucial process in energy conversion and storage, especially in water electrolysis. The critical challenge of the electrochemical water splitting technology is to explore alternative precious-metal-free catalysts for the promotion of the kinetically sluggish OER. Recently, emerging two-dimensional (2D) ultrathin materials with abundant accessible active sites and improved electrical conductivity provide an ideal platform for the synthesis of promising OER catalysts. This Review focuses on the most recent advances in ultrathin 2D nanostructured materials for enhanced electrochemical activity of the OER. The design, synthesis and performance of such ultrathin 2D nanomaterials-based OER catalysts and their property-structure relationships are discussed, providing valuable insights to the exploration of novel OER catalysts with high efficiency and low overpotential. The potential research directions are also proposed in the research field.

3D Nitrogen-Anion-Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting

Developing non-noble-metal electrocatalysts with high activity and low cost for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of paramount importance for improving the generation of H₂ fuel by electrocatalytic water-splitting. This study puts forward a new N-anion-decorated Ni₃ S₂ material synthesized by a simple one-step calcination route, acting as a superior bifunctional electrocatalyst for the OER/HER for the first time. The introduction of N anions significantly modifies the morphology and electronic structure of Ni₃ S₂ , bringing high surface active sites exposure, enhanced electrical conductivity, optimal HER Gibbs free-energy (ΔG_H* ), and water adsorption energy change (ΔG_H2O* ). Remarkably, the obtained N-Ni₃ S₂ /NF 3D electrode exhibits extremely low overpotentials of 330 and 110 mV to reach a current density of 100 and 10 mA cm^-2 for the OER and HER in 1.0 m KOH, respectively. Moreover, an overall water-splitting device comprising this electrode delivers a current density of 10 mA cm^-2 at a very low cell voltage of 1.48 V. Our finding introduces a new way to design advanced bifunctional catalysts for water splitting.

Porphyrinic Metal-Organic Framework-Templated Fe-Ni-P/Reduced Graphene Oxide for Efficient Electrocatalytic Oxygen Evolution

The sluggish kinetics of oxygen evolution reaction (OER) hampers the H₂ production by H₂O electrolysis, and it is very important for the development of highly efficient and low-priced OER catalysts. Herein, a representative metalloporphyrinic MOF, PCN-600-Ni, integrated with graphene oxide (GO), serves as an ideal precursor and template to afford bimetallic iron-nickel phosphide/reduced graphene oxide composite (denoted as Fe-Ni-P/rGO-T; T represents pyrolysis temperature) via pyrolysis and subsequent phosphidation process. Thanks to the highly porous structure, the synergetic effect of Fe and Ni elements in bimetallic phosphide, and the good conductivity endowed by rGO, the optimized Fe-Ni-P/rGO-400 exhibits remarkable OER activity in 1 M KOH solution, affording an extremely low overpotential of 240 mV at 10 mA/cm², which is far superior to the commercial IrO₂ and among the best in all non-noble metal-based electrocatalysts.

Metal–Organic Frameworks and Their Derivatives for Photocatalytic Water Splitting

Amongst many strategies for renewable energy conversion, light-driven water splitting to produce clean H2 represents a promising approach and has attracted increasing attention in recent years. Owing to the multi-electron/multi-proton transfer nature of water splitting, low-cost and competent catalysts are needed. Along the rapid development of metal–organic frameworks (MOFs) during the last two decades or so, MOFs have been recognized as an interesting group of catalysts or catalyst supports for photocatalytic water splitting. The modular synthesis, intrinsically high surface area, tunable porosity, and diverse metal nodes and organic struts of MOFs render them excellent catalyst candidates for photocatalytic water splitting. To date, the application of MOFs and their derivatives as photocatalysts for water splitting has become a burgeoning field. Herein, we showcase several representative MOF-based photocatalytic systems for both H2 and O2 evolution reactions (HER, OER). The design principle of each catalytic system is specifically discussed. The current challenges and opportunities of utilizing MOFs for photocatalytic water splitting are discussed in the end.

Black Phosphorus Revisited: A Missing Metal-Free Elemental Photocatalyst for Visible Light Hydrogen Evolution

Metal-free elemental photocatalysts for hydrogen (H₂ ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H₂ evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H₂ evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h^-1 g^-1 , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C₃ N₄ ).

Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4 Nanosheets/Amorphous NiS Heterojunctions for Efficient Photocatalytic H2 Generation

The construction of exceptionally robust and high-quality semiconductor-cocatalyst heterojunctions remains a grand challenge toward highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-C₃N₄) nanosheets decorated with multifunctional metallic Ni interface layers and amorphous NiS cocatalysts were fabricated via a facile three-step process: the loading of Ni(OH)₂ nanosheets, high-temperature H₂ reduction, and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron cocatalysts to markedly boost the visible-light H₂ evolution over g-C₃N₄ semiconductor. The optimized g-C₃N₄-based photocatalyst containing 0.5 wt % Ni and 1.0 wt % NiS presented the highest hydrogen evolution of 515 μmol g^-1 h^-1, which was about 2.8 and 4.6 times as much as those obtained on binary g-C₃N₄-1.0%NiS and g-C₃N₄-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multifunctional roles in enhancing the visible-light H₂ evolution, which could first collect the photogenerated electrons from g-C₃N₄, and then accelerate the surface H₂-evolution reaction kinetics over amorphous NiS cocatalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron cocatalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-C₃N₄. Comparatively speaking, the multifunctional metallic Ni layers are dominantly favorable for separating and transferring photoexcited charge carriers from g-C₃N₄ to amorphous NiS cocatalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanosheets are mainly advantageous for decreasing the thermodynamic overpotentials for surface H₂-evolution reactions. It is hoped that the implantation of multifunctional metallic interface layers can provide a versatile approach to enhance the photocatalytic H₂ generation over different semiconductor-cocatalyst heterojunctions.

One-Dimensional Earth-Abundant Nanomaterials for Water-Splitting Electrocatalysts

Hydrogen fuel acquisition based on electrochemical or photoelectrochemical water splitting represents one of the most promising means for the fast increase of global energy need, capable of offering a clean and sustainable energy resource with zero carbon footprints in the environment. The key to the success of this goal is the realization of robust earth-abundant materials and cost-effective reaction processes that can catalyze both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with high efficiency and stability. In the past decade, one-dimensional (1D) nanomaterials and nanostructures have been substantially investigated for their potential in serving as these electrocatalysts for reducing overpotentials and increasing catalytic activity, due to their high electrochemically active surface area, fast charge transport, efficient mass transport of reactant species, and effective release of gas produced. In this review, we summarize the recent progress in developing new 1D nanomaterials as catalysts for HER, OER, as well as bifunctional electrocatalysts for both half reactions. Different categories of earth-abundant materials including metal-based and metal-free catalysts are introduced, with their representative results presented. The challenges and perspectives in this field are also discussed.

A Composite Polymeric Carbon Nitride with In Situ Formed Isotype Heterojunctions for Highly Improved Photocatalysis under Visible Light

Introducing heterojunction is an effective way for improving the intrinsic photocatalytic activity of a graphitic carbon nitride (GCN) semiconductor. These heterostructures are mostly introduced by interfacing GCN with foreign materials that normally have entirely different physicochemical properties and show unfavorable compatibility, thus resulting in a limited improvement of the photocatalytic performance of the resultant materials. Herein, a composite polymeric carbon nitride (CPCN) that contains both melon-based GCN and triazine-based crystalline carbon nitride (CCN) is prepared by a simple thermal reaction between lithium chloride and GCN. Thanks to the intimate contact and good compatibility between GCN and CCN, an in situ formed heterojunction acts as a driving force for separating the photogenerated charge carriers in CPCN. As a result, CPCN exhibits a significantly improved photocatalytic performance under visible light irradiation, which is, respectively, 10.6 and 5.3 times as high as those of the GCN and CCN alone. This well designed isotype heterojunction by a coupling of CCN presents an effective avenue for developing efficient GCN photocatalysts.

Nickel Nanoparticles Encapsulated in Few-Layer Nitrogen-Doped Graphene Derived from Metal-Organic Frameworks as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Nickel nanoparticles encapsulated in few-layer nitrogen-doped graphene (Ni@NC) are synthesized by using a Ni-based metal-organic framework as the precursor for high-temperature annealing treatment. The resulting Ni@NC materials exhibit highly efficient and ultrastable electrocatalytic activity toward the hydrogen evolution reaction and the oxygen evolution reaction as well as overall water splitting in alkaline environment.

Regulating the Catalytic Function of Reduced Graphene Oxides Using Capping Agents for Metal-Free Catalysis

Reduced graphene oxide (rGO) functionalized with organic capping agents has gained increasing attention as a promising metal-free catalyst. To optimize the properties of rGO for target applications, comprehending the link between the catalytic function of rGO and the chemical and structural characteristics of capping agents is critical. Herein, we report a systematic study on the effect of capping agents on the catalytic function of rGO for redox reactions using nitrogen-containing surface modifiers with distinctly different chemical structures, such as poly(diallyldimethylammonium chloride), cetyltrimethylammonium chloride, and poly(allylamine hydrochloride), which have the capability to endow rGO with improved suspension stability, enhanced reactant adsorption, and modified electronic properties. Functionalized rGOs were facilely prepared by the reduction of graphene oxide with hydrazine in the presence of the capping agents. The results of model redox reactions, that is, 4-nitrophenol and ferricyanide reduction reactions, catalyzed by the functionalized rGOs corroborated that the way the capping agents functionalize rGO, which is highly correlated with their chemical structure, drastically influences the overall reaction kinetics, including induction time, reduction rate, total reaction time, and reaction order. This strongly suggests that the judicious selection of capping agents is crucial to fully harness the catalytic function of rGO and thus to design novel rGO-based non-metallic catalysts with controllable reaction kinetics.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

Self-template synthesis of CdS/NiSx heterostructured nanohybrids for efficient photocatalytic hydrogen evolution

A heterostructured nanohybrid photocatalyst composed of ultrasmall CdS and NiS_x nanoparticles has been prepared by a self-template synthetic strategy.

Same titanium glycolate precursor but different products: successful synthesis of twinned anatase TiO2 nanocrystals with excellent solar photocatalytic hydrogen evolution capability

Exploiting a synthesis protocol to tailor TiO₂ with a unique morphology and crystal phase has received considerable interest in the energy and environmental fields.

Enhanced visible-light photocatalytic H2-generation activity of carbon/g-C3N4 nanocomposites prepared by two-step thermal treatment

Brown carbon/g-C₃N₄ nanocomposites synthesized by two-step calcination exhibited a wide visible light response range and improved photocatalytic H₂-generation performance.

A novel metal-free two-dimensional material for photocatalytic water splitting – phosphorus nitride (γ-PN)

Two-dimensional phosphorus nitride (γ-PN) is designed to be a potential photocatalyst for water splitting.

Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review

This review provides a comprehensive survey and critical comments on the development of photocatalysts with a focus on the metal-free materials.