Graphene Dots Embedded Phosphide Nanosheet-Assembled Tubular Arrays for Efficient and Stable Overall Water Splitting

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Graphene quantum dots induced defect-rich NiFe Prussian blue analogue as an efficient electrocatalyst for oxygen evolution reaction

High energy resource demand has led to the rapid development of hydrogen as a clean fuel through electrolytic water splitting. The exploration of high-performance and cost-effective electrocatalysts for water splitting is a challenging task to obtain renewable and clean energy. However, the sluggish kinetics of oxygen evolution reaction (OER) greatly hindered its application. Herein, a novel oxygen plasma-treated graphene quantum dots embedded Ni-Fe Prussian blue analogue (O-GQD-NiFe PBA) is proposed as a highly active electrocatalysts for OER. Furthermore, the defect induced by GQD can provide an abundant lattice mismatch in the matrix of NiFe PBA, which further facilitates faster electron transport and kinetic performance. After optimization, the as-assembled O-GQD-NiFe PBA exhibits excellent electrocatalytic performance towards OER with a low overpotential of 259 mV for reaching a current density of 10 mA cm^-2 and impressive long-term stability for 100 h in an alkaline solution. This work broadens the scope of metal-organic frameworks (MOF) and high-functioning carbon composite as an active material for energy conversion systems.

Structure and Interface Modification of Carbon Dots for Electrochemical Energy Application

Carbon dots (CDs) as new nanomaterials have attracted much attention in recent years due to their unique characteristics. Notably, structure and interface modification (carbon core, edge, defects, and functional groups) of CDs have been considered as valid methods to regulate their properties, which contain electron transfer effect, electrochemical activity, fluorescence luminescent, and so on. Additionally, CDs with ultrasmall size, excellent dispersibility, high specific surface area, and abundant functional groups can guarantee positive and extraordinary effects in electrical energy storage and conversion. Therefore, CDs are used to couple with other materials by constructing a special interface structure to enhance their properties. Here, diverse structural and interfacial modifications of CDs with various heteroatoms and synergy effects are systematically analyzed. And not only several main syntheses of CDs-based composites (CDs/X) are summarized but also the merit and demerit of CDs/X in electrical energy storage are discussed. Finally, the applications of CDs/X in energy storage devices (supercapacitors, batteries) and electrocatalysts for practical applications are discussed. This review mainly provides a comprehensive summary and future prospect for synthesis, modification, and electrochemical applications of CDs.

Plasma-engineered bifunctional cobalt-metal organic framework derivatives for high-performance complete water electrolysis

Metal-organic framework (MOF) derivatives are among the most promising catalysts for the hydrogen evolution reaction (HER) for clean hydrogen energy production. Herein, we report the in situ synthesized MOF-derived CoPO hollow polyhedron nanostructures by simultaneous high temperature annealing and Ar-N₂ radio frequency plasma treatment in the presence of a P precursor and subsequent oxygen incorporation from open air at lower temperature. The optimum Ar-N₂ gas flow rates are used to precisely tune the P/O ratio, cut Co bonds within the MOFs and reconnect Co with P. Consequently, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance are enhanced. Meanwhile, the filling of P elements can effectively change the electronic structure around the catalyst to ensure the uniform distribution of catalytically active sites. The resultant CoPO hollow nanocages with large specific surface areas show excellent bifunctional electrocatalytic activity towards both HER and OER with a low overpotential of 105 and 275 mV and a small Tafel slope of 48 and 52 mV dec^-1, respectively. Our results open a new avenue for precise plasma-assisted engineering of MOF-derived hybrid hetero-structured electrocatalysts with rich oxygen vacancies and P dopants to simultaneously boost both half reactions in water electrolysis.

Recent Advances in Energy Conversion Applications of Carbon Dots: From Optoelectronic Devices to Electrocatalysis

Exploitation and utilization of sustainable energy sources has increasingly become the common theme of global social development, which has promoted tremendous development of energy conversion devices/technologies. Owing to excellent and unique optical/electrical properties, carbon dots (CDs) have attracted extensive research interest for numerous energy conversion applications. Strong absorption, downconversion photoluminescence, electron acceptor/donor characteristics, and excellent electron conductivity endow CDs with enormous potential for applications in optoelectronic devices. Furthermore, excellent electron transfers/transport capacities and easily manipulable structural defects of CDs offer distinct advantages for electrocatalytic applications. Recent advances in CD-based energy conversion applications, including optoelectronic devices such as light-emitting diodes and solar cells, and electrocatalytic reactions including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, and carbon dioxide reduction reaction, are summarized. Finally, current challenges and future prospects for CD-based energy conversion applications are proposed, highlighting the importance of controllable structural design and modifications.

CoO Promoted the Catalytic Activity of Nitrogen-Doped MoS2 Supported on Carbon Fibers for Overall Water Splitting

Non-noble metal electrocatalysts have recently witnessed increasing attention for the hydrogen evolution reaction (HER) in acidic electrolytes. However, in alkaline electrolytes, the slow kinetics of water splitting leads to poor HER activities. In this study, we describe the preparation of a hybrid material consisting of cobalt oxide (CoO) decorated on nitrogen-doped MoS₂ supported on carbon fibers (CoO/N-MoS₂/CF) through a two-step process combining hydrothermal technique and electrochemical deposition. The electrochemical properties of the CoO/N-MoS₂/CF electrocatalyst were assessed in alkaline medium. The results revealed that CoO/N-MoS₂/CF exhibits excellent bifunctional electrocatalytic activity for the HER and oxygen evolution reaction (OER). The CoO/N-MoS₂/CF delivered a current density of 10 mA/cm² at an overpotential of only 78 mV for the HER and a current density of 50 mA/cm² at 458 mV for the OER in 1.0 M KOH, performing better than many noble metal-free electrocatalysts. The enhanced catalytic properties of the hybrid nanomaterial could be ascribed to its hierarchical structure, and increased number of active sites, as well as the synergetic cooperation between its different components. Additionally, the CoO/N-MoS₂/CF nanomaterial was investigated as both cathode and anode for full water splitting in 1.0 M KOH. The water electrolyzer delivered a maximum current density of 53 mA cm^-2 at an applied cell voltage of 1.5 V, which is very favorable for overall water-splitting applications.

The oxygen evolution reaction enabled by transition metal phosphide and chalcogenide pre-catalysts with dynamic changes

Dynamic morphological, structural and compositional changes will occur when transition metal phosphides and chalcogenides are used to catalyze the oxygen evolution reaction, which can substantially enhance their electrocatalytic performance.

C60-Decorated nickel-cobalt phosphide as an efficient and robust electrocatalyst for hydrogen evolution reaction

High-activity electrocatalysts play a crucial role in energy conversion through splitting water to produce hydrogen. Here we report the synthesis of a bimetallic phosphide of Ni-Co-P coupled with C60 molecules which acts as an electrocatalyst for the hydrogen evolution reaction (HER). Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization reveals that the synthesized C60-decorated Ni-Co-P nanoparticles have an average diameter of ∼4 nm with rich structural defects. Electrochemical tests show that the as-synthesized C60-decorated Ni-Co-P catalyst with a C60-content of 3.93 wt% presents a low onset overpotential of 23.8 mV, a small Tafel slope value of 48 mV dec-1, and excellent hydrogen-evolution stability with a slight increase of its η10 from 97 mV to 102 mV after 500 cycles. Additionally, electrochemical impedance spectroscopy (EIS) confirms that the C60-decorated Ni-Co-P electrode possesses faster charge-transfer kinetics and hydrogen-adsorption kinetics than the C60-free Ni-Co-P electrode during the HER process. The synthesis of a C60-decorated composite is feasible and the composite can be used as an efficient and robust Pt-free catalyst.

Self-Assembly-Assisted Facile Synthesis of MoS2-Based Hybrid Tubular Nanostructures for Efficient Bifunctional Electrocatalysis

In this work, MoS₂-based hybrid tubular nanostructures are facilely synthesized via a self-assembly-assisted process and evaluated as a bifunctional electrocatalyst for hydrogen evolution reactions (HERs) and oxygen reduction reactions (ORRs). By simply mixing the reactants under ambient conditions, (NH₄)₂MoS₄/polydopamine (PDA) hybrid nanospheres are formed. The protonated dopamine is linked to tetrahedral [MoS₄]^2- via weak N-H···S and O-H···S interactions, causing the PDA nanospheres merging together and forming nanorods under stirring-induced shear force. Moreover, the oxidative polymerization of dopamine proceeds on the surface of the nanorods, whereas it is prohibited inside the nanorods owing to lack of oxygen, leading to outward diffusion of dopamine and hence cavitation. After annealing, the tubular morphology is perfectly retained, while ultrafine MoS₂ monolayers are formed due to the confinement of the framework. Benefiting from these unique structural features, the MoS₂/C hybrid nanotubes possess abundant active sites and high surface area, as well as boost electronic and ionic transport, remarkably enhancing their electrocatalytic activities. The onset and half-wave potentials are 0.91 and 0.82 V, respectively, for ORR, close to those of Pt/C. Moreover, low onset potential and small Tafel slope are also observed for HER, demonstrating the potential of the hybrid nanotubes as a promising non-noble metal bifunctional electrocatalyst.

Active Sites Intercalated Ultrathin Carbon Sheath on Nanowire Arrays as Integrated Core-Shell Architecture: Highly Efficient and Durable Electrocatalysts for Overall Water Splitting

The development of active bifunctional electrocatalysts with low cost and earth-abundance toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a great challenge for overall water splitting. Herein, metallic Ni₄ Mo nanoalloys are firstly implanted on the surface of NiMoO_x nanowires array (NiMo/NiMoO_x ) as metal/metal oxides hybrid. Inspired by the superiority of carbon conductivity, an ultrathin nitrogen-doped carbon sheath intercalated NiMo/NiMoO_x (NC/NiMo/NiMoO_x ) nanowires as integrated core-shell architecture are constructed. The integrated NC/NiMo/NiMoO_x array exhibits an overpotential of 29 mV at 10 mA cm^-2 and a low Tafel slope of 46 mV dec^-1 for HER due to the abundant active sites, fast electron transport, low charge-transfer resistance, unique architectural structure and synergistic effect of carbon sheath, nanoalloys, and oxides. Moreover, as OER catalysts, the NC/NiMo/NiMoO_x hybrids require an overpotential of 284 mV at 10 mA cm^-2 . More importantly, the NC/NiMo/NiMoO_x array as a highly active and stable electrocatalyst approaches ≈10 mA cm^-2 at a voltage of 1.57 V, opening an avenue to the rational design and fabrication of the promising electrode materials with architecture structures toward the electrochemical energy storage and conversion.