Catalytic hydrogen evolution reaction on "metal-free" graphene: key role of metallic impurities

Facile Aqueous Route to Large-Scale Superhydrophilic TiO2-Incorporated Graphitic Carbon Nitride-Coated Ni(OH)2 and Ni2P Nano-Architecture Arrays as Efficient Electrocatalysts for Enhanced Hydrogen Production

Water splitting by an electrochemical method to generate hydrogen gas is an economic and green approach to resolve the looming energy and environmental crisis. Designing a composite electrocatalyst having integrated multichannel charge separation, robust stability, and low-cost facile scalability could be considered to address the issue of electrochemical hydrogen evolution. Herein, we report a superhydrophilic, noble-metal-free bimetallic nanostructure TiO2/Ni2P coated on graphitic polyacrylonitrile carbon fibers (g-C/TiO2/Ni2P) using a facile hydrothermal method followed by phosphorylation. In an aqueous-based route, PAN is dissolved in water in the presence of ZnCl2, followed by wet-spinning to prepare scalable PAN/ZnCl2 fibers. The nitrogen-contained porous graphitic carbon fibers are prepared via the pyrolysis of PAN/ZnCl2 fibers; now ZnCl2 acts as a volatile porogen to form porous matrix structures. Finally, the as-prepared graphitic carbon fibers are electrochemically activated by incorporating TiO2/Ni2P active sites. The materials formed in this work show excellent electrocatalytic activity for the hydrogen evolution reaction. The as-synthesized g-C/TiO2/Ni2P catalyst shows a low overpotential, its electrocatalytic activity is improved, and its efficiency is better than that of the commercial Pt/C catalyst. At a current density of -10 mA/cm2, the g-C/TiO2/Ni2P catalyst shows an overpotential of 55 mV, while the commercial Pt/C catalyst shows an overpotential of 77 mV. Our work provides a facile aqueous scalable route with no need for noble metals that can be considered as a potential alternative for the commercial Pt/C catalyst.

Synergistic Catalysis of SO42-/TiO2-CNT for the CO2 Desorption Process with Low Energy Consumption

To address the issue of high energy consumption associated with monoethanolamine (MEA) regeneration in the CO2 capture process, solid acid catalysts have been widely investigated due to their performance in accelerating carbamate decomposition. The recently discovered carbon nanotube (CNT) catalyst presents efficient catalytic activity for bicarbonate decomposition. In this paper, bifunctional catalysts SO42-/TiO2-CNT (STC) were prepared, which could simultaneously catalyze carbamate and bicarbonate decomposition, and outstanding catalytic performance has been exhibited. STC significantly increased the CO2 desorption amount by 82.3% and decreased the relative heat duty by 46% compared to the MEA-CO2 solution without catalysts. The excellent stability of STC was confirmed by 15 cyclic absorption-desorption experiments, showing good practical feasibility for decreasing energy consumption in an industrial CO2 capture process. Furthermore, associated with the results of experimental characterization and theoretical calculations, the synergistic catalysis of STC catalysts via proton and charge transfer was proposed. This work demonstrated the potential of STC catalysts in improving the efficiency of amine regeneration processes and reducing energy consumption, contributing to the design of more effective and economical catalysts for carbon capture.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Tailoring unique neural-network-type carbon nanofibers inserted in CoP/NC polyhedra for robust hydrogen evolution reaction

Three-dimensional catalysts have attracted great attention in the field of the hydrogen evolution reaction (HER).However, great challenges remain in structural innovation and performance enhancement. Herein we designed and tailored a unique three-dimensional cross-linked neural network-like CoP-based composite, that is, carbon nanofibers inserted in CoP/NC polyhedra derived from in situ self-assembled bacterial cellulose (BC) wired ZIF-67 polyhedra via high-temperature carbonization and subsequent phosphorization. The obtained integrated catalyst (3-D CNF@CoP/NC) consists of CoP/NC polyhedra with abundant active sites as the "neurons" and carbon nanofibers as the "axons", and displayed remarkable activity with an overpotential of 64.5 mV and 105.6 mV at 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH respectively and good stability with negligible current change after 80 h of chronoamperometric measurement or 4000 CV cycles. This work offers a high-performance HER catalyst and paves a new way for the rational engineering of unique 3-D interconnected hierarchical porous networks featuring ultrafast charge transfer and mass transport.

Monitoring the active sites for the hydrogen evolution reaction at model carbon surfaces

Carbon is ubiquitous as an electrode material in electrochemical energy conversion devices. If used as a support material, the evolution of H2 is undesired on carbon. However, recently, carbon-based materials have aroused significant interest as economic and eco-conscious alternatives to noble metal catalysts. The targeted design of improved carbon electrode materials requires atomic scale insight into the structure of the sites that catalyse H2 evolution. This work shows that electrochemical scanning tunnelling microscopy under reaction conditions (n-EC-STM) can be used to monitor the active sites of highly oriented pyrolytic graphite for the hydrogen evolution reaction. With down to atomic resolution, the most active sites in acidic medium are pinpointed near edge sites and defects, whereas the basal planes remain inactive. Density functional theory calculations support these findings and reveal that only specific defects on graphite are active. Motivated by these results, the extensive usage of n-EC-STM on doped carbon-based materials is encouraged to locate their active sites and guide the synthesis of enhanced electrocatalysts.

Ionic liquid in situ functionalized carbon nanotubes as metal-free catalyst for efficient electrocatalytic hydrogen evolution reaction

The development of cost-effective and high-performance catalysts for the production of hydrogen via electrocatalytic water splitting is crucial for meeting the increasing energy demand and expanding the hydrogen economy. In this study, a series of metal-free carbon nanotube (CNT) catalysts were designed and in situ functionalized by imidazolium ionic liquids (ILs) for enhanced electrocatalytic hydrogen evolution reaction (HER). The theoretical calculations and experimental results reveal that the functionalization of CNTs with imidazolium ILs facilitated the electron transfer process and exhibited superior hydrogen adsorption, thereby enhancing the performance of the HER. In particular, CNT-IM-Cl displays excellent electrocatalytic activity and shows a low onset overpotential and Tafel slope of 80 mV and 38 mV dec-1, respectively. This study highlights the significant potential of IL in situ functionalized metal-free CNTs for the electrocatalytic HER and provides insight into the structure design of highly efficient electrocatalysts.

Advances in the Applications of Graphene-Based Nanocomposites in Clean Energy Materials

Extensive use of fossil fuels can lead to energy depletion and serious environmental pollution. Therefore, it is necessary to solve these problems by developing clean energy. Graphene materials own the advantages of high electrocatalytic activity, high conductivity, excellent mechanical strength, strong flexibility, large specific surface area and light weight, thus giving the potential to store electric charge, ions or hydrogen. Graphene-based nanocomposites have become new research hotspots in the field of energy storage and conversion, such as in fuel cells, lithium-ion batteries, solar cells and thermoelectric conversion. Graphene as a catalyst carrier of hydrogen fuel cells has been further modified to obtain higher and more uniform metal dispersion, hence improving the electrocatalyst activity. Moreover, it can complement the network of electroactive materials to buffer the change of electrode volume and prevent the breakage and aggregation of electrode materials, and graphene oxide is also used as a cheap and sustainable proton exchange membrane. In lithium-ion batteries, substituting heteroatoms for carbon atoms in graphene composite electrodes can produce defects on the graphitized surface which have a good reversible specific capacity and increased energy and power densities. In solar cells, the performance of the interface and junction is enhanced by using a few layers of graphene-based composites and more electron-hole pairs are collected; therefore, the conversion efficiency is increased. Graphene has a high Seebeck coefficient, and therefore, it is a potential thermoelectric material. In this paper, we review the latest progress in the synthesis, characterization, evaluation and properties of graphene-based composites and their practical applications in fuel cells, lithium-ion batteries, solar cells and thermoelectric conversion.