Energy Conversion Reactions Research Articles

Interfacial Polarization Triggered by Covalent‐Bonded MXene and Black Phosphorus for Enhanced Electrochemical Nitrate to Ammonia Conversion

AbstractMXenes have recently emerged as a new class of electrocatalysts for diverse energy conversion reactions, but they are generally considered to be catalytically inert. Herein, a strategy of interfacial polarization is put forward to improve the catalytic activity of MXenes, with the construction of an interfacial structure consisting of black phosphorus (BP) nanoflakes and Nb2C MXene nanosheets for electrochemical nitrate reduction to ammonia as a proof of concept. The experimental and computational results reveal that a strong interfacial polarization is built between BP nanoflakes and Nb2C nanosheets in the prepared BP/Nb2C composite. The polarization of Nb centers by adjacent BP sites gives rise to the formation of positively centered Nb atoms and BP‐polarized Nb atoms, which synergistically catalyze the cleavage of N–O bonds in HNO2* to form NO*. In addition, the stabilization of monatomic *N by the BP/Nb2C composite is also enhanced compared with BP nanoflakes and Nb2C nanosheets. As a result, both the ammonia yield rate and Faraday efficiency of the BP/Nb2C composite are much higher than that of BP nanoflakes and Nb2C nanosheets. Overall, this work shows the great potential for constructing a strong interfacial polarization to improve the catalytic performance of MXenes‐based electrocatalysts.

Low‐dimension confinement effect in COF‐based hetero‐photocatalyst for energy‐conversion application

AbstractCovalent organic framework (COF) materials have aroused tremendous interest in photocatalytic applications due to their tunable pore structure and photoelectric properties. The regular nanopore of COF itself presents a strongly confinement effect, which provides a unique regulatory effect for photons, electrons, protons, and other quantum‐scale reaction groups. However, due to the weak surface electron coupling and transfer ability between the reactive groups and basic elements of its structural units, the activity of pure COFs photocatalyst is still not satisfactory. Therefore, the confinement modification strategy of confining low‐dimension entities within COFs has been proposed, thus realizing new active sites construction and band structure regulation has been intensively studied, but yet to be summarized systematically. In this paper, the semi‐conductivity of COFs is discussed dialectically based on photocatalytic thermodynamics, and the influence of internal linkage motifs and stacking behaviors on the band structure is collected. Then, the basic understanding of confinement characteristics and their influence on photocatalytic performance in dynamics is further explained according to the spatial dimension classification of low‐dimension entities. And the application and mechanism of these COF‐based confined catalysts in energy conversion reactions are discussed in detail. Lastly, the current challenges and development prospects of COF‐based confined hetero‐photocatalysts are discussed.

Engineering morphology and nitrogen content on graphitic carbon nitrides for efficient solar to hydrogen conversion reaction

Semiconducting polymeric graphitic carbon nitride (g-C3N4; GCN) has emerged as low-cost and robust photocatalyst to convert solar to chemical energy. Unfortunately, the preparation of GCN is carried out via thermal polymerization of melamine, results the GCN with low specific surface area and poor charge separation efficiency. Herein, we report the synthesis of high-quality one-dimensional g-C3N4 nanobelts with rich amino groups and higher surface area by treating melamine precursors with hydrochloric acid and subsequent thermal polymerization reaction. The GCN nanobelts produced after optimizing the HCl treatment can effectively generate H2 evolution rate of 2912.6 μmol/g/h, which is six times higher than that of bulk GCN obtained via direct thermal polymerization reaction without any treatment. Meanwhile, the quantum efficiency of 3.8% at 420 nm toward solar to hydrogen conversion was achieved, prominently surpassing the many g-C3N4 nanostructures. Such one-dimensional g-C3N4 nanobelt semiconductors are predicted to hold great potential for solar energy conversion reactions.

Novel CoMn2O4-ZnIn2S4 hollow heterostructure cage for efficient photocatalytic hydrogen evolution

Semiconductor photocatalytic hydrogen evolution (PHE) is used to convert solar energy to hydrogen energy. Therefore, a low-cost, effective, and dependable photocatalyst could be a viable solution to environmental and energy crises. However, performing energy-conversion reactions by preparing photocatalysts with suitable photoabsorption and effective charge separation is challenging. Herein, a novel CoMn2O4-ZnIn2S4 (CMO-ZIS) hollow microcube composite catalyst with a p-n heterojunction is constructed, in which ZnIn2S4 nanosheets are grown in situ on the surfaces of CoMn2O4 hollow microcubes. Surprisingly, 10% CoMn2O4-ZnIn2S4 (CMO-ZIS-10) exhibits a PHE rate of up to 11.04 mmol h−1 g−1, which is approximately 3.6 times higher than that of pure ZnIn2S4. In addition, to maintain the excellent visible light absorption efficiency of ZnIn2S4 nanosheets, the close-contact heterogeneous interface of CMO-ZIS enables easy separation and transfer of the carriers. Furthermore, the hollow structure increases the absorption efficiency of sunlight and provides more active sites, thus increasing the reactivity of photocatalyst. In this paper, we report a workable approach for building heterojunction photocatalysts powered by water splitting.

Low-Dimensional High-Entropy Alloys for Advanced Electrocatalytic Reactions.

Low-dimensional high-entropy alloy (HEA) nanomaterials are widely employed as electrocatalysts for energy conversion reactions, due to their inherent advantages, including high electron mobility, rich catalytically active site, optimal electronic structure. Moreover, the high-entropy, lattice distortion, and sluggish diffusion effects also enable them to be promising electrocatalysts. A thorough understanding on the structure-activity relationships of low-dimensional HEA catalyst play a huge role in the future pursuit of more efficient electrocatalysts. In this review, we summarize the recent progress of low-dimensional HEA nanomaterials for efficient catalytic energy conversion. By systematically discussing the fundamentals of HEA and properties of low-dimensional nanostructures, we highlight the advantages of low-dimensional HEAs. Subsequently, we also present many low-dimensional HEA catalysts for electrocatalytic reactions, aiming to gain a better understanding on the structure-activity relationship. Finally, a series of upcoming challenges and issues are also thoroughly proposed as well as their future directions.

Reductive chemistry of pyrrolic macrocycles: A PCET dichotomy between metal and ligand

Proton-coupled electron transfer (PCET) is central to the reactivity of porphyrins. The coupling of the electron to the proton is central to a porphyrin’s ability to catalyze energy conversion reactions of which the hydrogen evolution reaction (HER) is exemplary. To understand the mechanistic details of the PCET chemistry of porphyrins and related macrocyclic congeners, we have designed hangman constructs that allow a proton, placed in the secondary coordination sphere (off of the hangman backbone), to be coupled to redox transformations at the macrocycle. For metals whose reduction potentials are positive of the porphyrin macrocycle, such as Co and Fe, HER catalysis is confined to PCET transformations of the metal center where the active catalyst for HER is a reduced metal hydride. Alternatively, the reduction potentials of Ni, Zn, and 2H (freebase) porphyrins allow for redox non-innocence of the macrocycle; here the active “hydridic” catalyst is a phlorin, which gives rise to elaborate HER reaction sequences. Beyond HER catalysis, redox non-innocence of Ni, Zn, and 2H porphyrins and related compounds has been informative for providing detailed mechanistic insight into the multi-site PCET hydrogenation of olefinic bonds of the macrocycle. This mini-review unravels the PCET dichotomy between the metal and macrocycle in promoting HER catalysis and novel chemical transformations that give rise to unusual macrocyclic structures.

Direct and Indirect Effects of Fluorine on the Photocatalytic Performance of Titania‐Based Photocatalysts

TiO2 used as a light‐absorbing semiconductor represents the classic benchmark for photocatalytic solar energy conversion and many other photocatalytic reactions. Various strategies are developed to improve the photoresponse of TiO2‐based materials, such as bandgap engineering or surface sensitization in combination with nanostructuring and geometry optimization. The present feature article is focused on direct and indirect approaches involving bulk or surface fluorine used to tune the surface chemistry, electronic structure, and the morphology of TiO2 photocatalysts. A comprehensive overview is provided on fluorine effects on TiO2, involving morphology modifications, and how surface or bulk fluorine affect the photocatalytic performance of TiO2. After outlining some basic interaction principles of F and TiO2, characterization techniques for different fluorine species are discussed. It is reviewed how fluorine during crystal growth mediates the morphology of TiO2; then how surface fluorination and doping fluorine effects can be beneficially utilized in photocatalysis are discussed. Finally, synergistic effects between fluorine and cocatalyst are discussed, for example, the use of lattice fluorine species to stabilize Pt single‐atom cocatalysts. Finally, the challenges and outlook on further advancing the development of fluorine effects are highlighted.

Concerted Proton-Coupled Electron Transfer to a Graphite Adsorbed Metalloporphyrin Occurs by Band to Bond Electron Redistribution.

Surface immobilized catalysts are highly promising candidates for a range of energy conversion reactions, and atomistic mechanistic understanding is essential for their rational design. Cobalt tetraphenylporphyrin (CoTPP) nonspecifically adsorbed on a graphitic surface has been shown to undergo concerted proton-coupled electron transfer (PCET) in aqueous solution. Herein, density functional theory calculations on both cluster and periodic models representing π-stacked interactions or axial ligation to a surface oxygenate are performed. As the electrode surface is charged due to applied potential, the adsorbed molecule experiences the electrical polarization of the interface and nearly the same electrostatic potential as the electrode, regardless of the adsorption mode. PCET occurs by electron abstraction from the surface to the CoTPP concerted with protonation to form a cobalt hydride, thereby circumventing Co(II/I) redox. Specifically, the Co(II) d-state localized orbital interacts with a proton from solution and an electron from the delocalized graphitic band states to produce a Co(III)-H bonding orbital below the Fermi level, corresponding to redistribution of electrons from the band states to the bonding states. These insights have broad implications for electrocatalysis by chemically modified electrodes and surface immobilized catalysts.

Enhanced CO Functionality on Carbon Papers Ensures Lowering Nucleation Delay of ALD for Ru towards Unprecedented Alkaline HER Activity.

The success in lowering the nucleation delay for Atomic Layer Deposition (ALD) of Ru on carbon surfaces is mitigated by constructive pretreatments resulting enhancement of CO functionality. Treatment of the carbon papers (CP) allowed Ru species deposition for minimum number of ALD cycles (25 cycles) with good conformality. The development of electrocatalysts from single atoms to nanoparticles (NPs) on conductive supports with low metal loadings, thus improving performance, is essential in electrocatalysis. For alkaline hydrogen evolution reaction, ALD decorated CPs with Ru exhibit low onset potentials of ≈4.7mV versus reversable hydrogen electrode (RHE) (at 10mAcm-2 ) and a high turnover frequency of 1.92 H2 s-1 at 30mV versus RHE. The Ru decorated CPs show comparable to higher catalytic activity than of Platinum (Pt) decorated CP also developed by ALD. The current representation of unfamiliar catalytic activities of Ru active centers developed by ALD, pave a bright and sustainable path for energy conversion reactions.

Perspective on Biomass-Based Cotton-Derived Nanocarbon for Multifunctional Energy Storage and Harvesting Applications

The demand for renewable-energy-based efficient systems is a practical indication to develop sustainable energy nanomaterials for energy storage and conversion to reduce the use of pollution-based nonrenewable energy systems. The use of biomass-derived energy components is a new age for the development of sustainable energy electrode nanomaterials. Among biomass, cotton-based electrodes are more popular, as they are cost-effective and efficient energy materials. Cotton-plant-derived porous or pristine nanocarbons have good textural and morphological properties with high specific surface area, hierarchical porous structure, and large pore volume. The functionalization, addition of active moieties, and easy synthetic routes improve the properties and enhance the performance of various electronic devices. The functionalization of derived materials with a heteroatom perturbs the electrical neutrality of the framework. The incorporation of active sites enhances the surface area and creates defects in the framework. These properties speed up the charge storage and energy conversion reaction for better performance of fabricated devices. This Review gives a detailed understanding and advancements about cotton-derived energy nanomaterial. Also, it gives insight about the key factors affecting the performance of cotton-based electrodes, electrolytes, and other components of energy devices. This Review concludes with the cotton-plant-derived supercapacitor, fuel cell, and battery, with a deep insight into their physicochemical properties. This Review also discusses the role of the active mass of the material used for the different device fabrications, which alters the performance. Further, a deep insight has been provided with a brief discussion of other electronic devices like photovoltaic, triboelectric, and piezoelectric devices based on cotton-derived nanocarbon.

In situ facile fabrication of ultrathin Co(OH)2-CoO/graphene oxide nanosheet hybrids with superior oxygen evolution reaction performance

As representative electrocatalysts for the oxygen evolution reaction (OER), Co-based hydroxides and oxides generally suffer from poor electron conductivity and severe self-agglomeration, substantially hindering their wide application in catalysis. To overcome such issues, we herein report a convenient electrostatic adsorption-assisted solvothermal method for the in situ fabrication of ultrathin Co(OH)2-CoO/graphene oxide nanosheet hybrids (denoted as Co-O-H NSs/GO-x) as efficient electrocatalysts for the OER in alkaline media. Notably, the optimal Co-O-H NSs/GO-1.4 mg sample with a thickness of 3.5–3.8 nm demonstrated outstanding electrocatalytic OER performance, with a low overpotential of only 283 mV at 10 mA cm−2 and a small Tafel slope of 57 mV dec−1, making it comparable to state-of-the-art Co-based non-self-supporting electrocatalysts. Comprehensive analysis revealed that the superior OER electrocatalytic performance of Co-O-H NSs/GO-1.4 mg could be ascribed to the synergistic contribution from the enlarged electroactive surface area (approximately 146 mF.cm−2 for its double-layer capacitance), higher electron concentration (approximately 8.1 × 1017 cm−3), enriched oxygen vacancies (accounting for approximately 74.3 % of the oxygen content) and the upshift of the 3d-band centre caused by the high-quality coupling between Co-O-H NSs and GO NSs. This work provides a universal synthetic strategy for the facile construction of high-performance hybrid nanosheet-based catalysts, showing particular importance for establishing efficient energy conversion reactions.

Crystalline-Amorphous Interface Coupling of Ni3S2/NiPx/NF with Enhanced Activity and Stability for Electrocatalytic Oxygen Evolution.

The rational design of highly efficient and stable electrocatalysts for the oxygen evolution reaction (OER) is an urgent need but remains challenging for various sustainable energy systems. How to adjust the atomic structure and electronic structure of the active center is a key bottleneck problem. Accelerating the electron transfer process and the deep self-reconstruction of active sites could be a cost-effective strategy toward electrocatalytic OER catalyst development. Here, a crystalline-amorphous (c-a) coupled Ni3S2/NiPx electrocatalyst self-supported on nickel foam with an intimate interface was developed via a feasible solvothermal-electrochemistry method. The coupling interface of the crystalline structure with high conductivity and amorphous structure with numerous potential active sites could regulate the electronic structure and optimize the adsorption/desorption of O-containing species, ultimately resulting in high OER catalytic performance. The obtained Ni3S2/NiPx/NF presents a low OER overpotential of 265 mV to obtain 10 mA·cm-2 and a small Tafel slope of 51.6 mV·dec-1. Also, the catalyst with the coupled interface exhibited significantly enhanced long-term stability compared to the other two catalysts, with <5% decay in OER activity over 20 h of continuous operation, while that of Ni3S2/NF and NiPx/NF decreased by about 30 and 50%, respectively. This study provides inspiration for other energy conversion reactions in optimizing the performance of catalysts by coupling crystalline-amorphous structures.

Sphere-like Naphthalene-Based Microporous Nickel Phosphonate Facile for Asymmetric Supercapacitor Devices and Bifunctional Oxygen Electrocatalysts

To produce clean energy due to the lack of fossil fuels, the development of cost-effective and highly efficient energy storage devices and multifunctional electrocatalysts is essential to substitute noble metal-based electrocatalysts. In this context, transition metal-based nickel phosphonate material (NiNAP) has been synthesized using 1,1′-binaphthyl-2,2′-diyl hydrogenphosphate and nickel nitrate hexahydrate in a hydrothermal reaction pathway in the absence of a templating pathway. The organic–inorganic hybrid structure with microporous arrays as a potential electrode material facilitates energy storage and conversion reactions. For the first time, a sphere-like NiNAP material has been developed, which is a potential electrode material for asymmetric supercapacitor applications with high specific capacitance and both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) with a low overpotential and higher current density. Moreover, the NiNAP material exhibits a specific capacitance of 4456 F g–1 in three-electrode assembly and 455 F g–1 in asymmetric supercapacitor at a scan rate of 1 mV s–1, where the energy and power density are estimated to be 204 W h kg–1 and 409 W kg–1, respectively. Interestingly, the NiNAP material displays outstanding catalytic activity toward OER with a low overpotential of 242 mV at the current density of 10 mA cm–2 and excellent ORR activity with an onset potential of 0.74 V vs RHE. Also, it shows long-term stability without significant changes in the current density for OER and ORR.

Electrocatalysis Mechanism and Structure-Activity Relationship of Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Electrocatalytic Reactions.

Atomically dispersed metal-nitrogen-carbon catalysts (M-N-C) have been widely used in the field of energy conversion, which has already attracted a huge amount of attention. Due to their unsaturated d-band electronic structure of the center atoms, M-N-C catalysts can be applied in different electrocatalytic reactions by adjusting their own microscopic electronic structures to achieve the optimization of the structure-activity relationship. Consequently, it is of great significance for the revelation of electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts. Thus, this review first introduces the relative research methods, including in situ/operando characterization techniques and theoretical calculation methods. Furthermore, clarifying the electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts in different electrochemical energy conversion reactions is focused. Moreover, the future research directions are pointed out based on the discussion. This review will provide good guidance to systematically study the catalytic mechanism of single-atom catalysts and reasonably design the single-atom catalysts.

Application of Time-Resolved Synchrotron X-ray Absorption Spectroscopy in an Energy Conversion Reaction

The rational design of high-efficiency catalysts is hindered by the knowledge of active sites, which always experience dynamic transformations within different time scales. In this regard, tracking these time-dependent processes is essential to building the correlation between the active site and catalytic performance. Achieving this goal requires powerful characterization techniques to overcome the obstacle induced by the time mismatch. By virtue of the local structure sensitivity, synchrotron X-ray absorption spectroscopy (XAS) comprising step-scanning XAS, quick-scanning XAS, and energy-dispersive XAS has been widely applied to record structural evolution events. In this Perspective, we highlight the substantial accomplishments achieved by these time-resolved XAS techniques. Their principles, advantages, and limitations involved in monitoring energy-involving electrocatalysis were also introduced. Meanwhile, the key challenges that we are encountering and the further directions of time-resolved XAS are also provided. We sincerely hope that these insights could offer a reliable guideline for other researchers to design more efficient in situ experiments.

Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway.

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from IrxCly precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts─IrCl3, IrCl4, H2IrCl6, or Na2IrCl6─colloidal nanoparticles as small as Ir∼55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl3 or IrCl4, metallic iridium nanoparticles form rapidly from IrxClyn- complexes, whereas using H2IrCl6 or Na2IrCl6, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H2IrCl6, the formation of different Irn (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from IrxCly is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.

Electronegativity-Induced Valence State Augmentation of Ni and Co through Electronic Redistribution between Co-Ni3N/CeF3 Interfaces for Oxygen Evolution Reaction

The development of high-performance and inexpensive electrocatalysts for the oxygen evolution reaction is one of the most desired objectives in energy conversion reactions. Due to sluggish reaction kinetics, the commercialization of such processes could not be achieved yet. Choosing an appropriate coupling interface to boost the catalyst performance is essential for the creation of high-efficiency electrocatalysts. Herein, metal-nitride and metal-fluoride heterostructures are reported with an extremely low overpotential of 180 mV at a current density of 10 mA cm–2 for OER application. The Co–Ni3N/CeF3 catalyst shows strong interfacial interaction, causing significant electronic redistribution between Co–Ni3N and CeF3 phases. X-ray photoelectron spectroscopy studies reveal the charge transfer from the Co–Ni3N to the CeF3 phase, resulting in the augmentation in the valence states of Co and Ni and making them highly active sites for the adsorption of intermediates (O*, OH*, and HOO*). This phenomenon is possibly driven by the high polarity of CeF3 due to the presence of highly electronegative F atoms. The stability study of the catalyst was performed for 120 h at large current densities of 100 and 200 mA cm–2. The detailed analysis of the surface reconstruction of Co–Ni3N/CeF3 is also carried out after a long-term stability test. This work offers a fresh look at the possibilities of the design and fabrication of an efficient and low-cost catalyst.

Dual-Cation-Coordinated CoFe-Layered Double-Hydroxide Nanosheets Using the Pulsed Laser Ablation Technique for Efficient Electrochemical Water Splitting: Mechanistic Screening by In Situ/Operando Raman and Density Functional Theory Calculations

Cation modulation engineering is employed to tune the intrinsic activity and electronic structure of electrocatalysts for water electrolysis. Here, we designed two-dimensional cobalt–iron-layered double-hydroxide (CoFe-LDH) ultrathin nanosheets by pulsed laser ablation in an aqueous medium. The CoFe-LDH nanosheets exhibited abundant electrochemically active sites and a large surface area. The optimal Co0.5Fe0.5-LDH exhibited a low overpotential of 270 mV during half-cell oxygen evolution reactions (OERs), whereas Co0.25Fe0.75-LDH delivered 365 mV at 10 mA/cm2 during hydrogen evolution reactions (HERs). The bifunctional electrocatalyst exhibited an outstanding water electrolyzer performance at a cell voltage of ∼1.89 V at 10 mA/cm2 and admirable stability for long-run repetitive cycles. The synergistic effect between the modulated cations resulted in better conductivity, and the mass transfer facilitated the HER and OER. We demonstrated that this facile approach can facilitate the engineering of a highly stable and efficient electrode for renewable electrochemical energy conversion reactions.

Microscale Combinatorial Libraries for the Discovery of High-Entropy Materials.

Polyelemental material systems, specifically high-entropy alloys, promise unprecedented properties. Due to almost unlimited combinatorial possibilities, their exploration and exploitation is hard. This challenge is addressed by co-sputtering combined with shadow masking to produce a multitude of microscale combinatorial libraries in one deposition process. These thin-film composition spreads on the microscale cover unprecedented compositional ranges of high-entropy alloy systems and enable high-throughput characterization of thousands of compositions for electrocatalytic energy conversion reactions using nanoscale scanning electrochemical cell microscopy. The exemplary exploration of the composition space of two high-entropy alloy systems provides electrocatalytic activity maps for hydrogen evolution and oxygen evolution as well as oxygen reduction reactions. Activity optima in the system Ru-Rh-Pd-Ir-Pt are identified, and active noble-metal lean compositions in the system Co-Ni-Mo-Pd-Pt are discovered. This illustrates that the proposed microlibraries are a holistic discovery platform to master the multidimensionality challenge of polyelemental systems.

Progress on 2D–2D heterostructured hybrid materials for efficient electrocatalysis

2D–2D hybrids have large heterointerfaces and strong interactions between the two components, leading to strong coupling effects that influence the electronic structure of active sites and benefit charge transfer during energy conversion reactions.