Agent-free synthesis of graphene oxide/transition metal oxide composites and its application for hydrogen storage

Graphene oxide (GO) wrapped transition metal oxide composite materials were synthesized by a very simple route without any additional agents and the hydrogen adsorption properties of the materials were investigated. The morphologies of GO/V₂O₅ and GO/TiO₂ were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that single- or few-layered GO sheets wrapped throughout the V₂O₅ and TiO₂ particles. According to X-ray photoelectron spectroscopy (XPS), the C–OH species of GO and the surface-adsorbed oxygen of the transition metal oxide bond together via a dehydration reaction. The wrapping phenomenon of GO causes the enhancement of hydrogen storage capacity at liquid nitrogen temperature (77 K) compared with those of the pristine transition metal oxides and GO. The enhancement of hydrogen storage capacity of GO-wrapped transition metal oxide composite materials results from the existence of interspaces between the transition metal oxide particles and the thin GO layers.     

Ruthenium stabilized on transition metal-on-transition metal oxide nanoparticles for naphthalene hydrogenation

The multi-metallic nanocatalysts of ruthenium nanoclusters-on-transition metal/transition metal oxide nanoparticles (TM/TMO NPs) then supported on carbon (Ru/Ni/NiO/C or Ru/Co/Co₃O₄/C) were designed and synthesized. The Ni/NiO or Co/Co₃O₄ NPs strongly stabalized the ruthenium nanoclusters by the interfacial interaction among them. These catalysts exhibited high catalytic activity and 100% selectivity to decalin for naphthalene hydrogenation due to the synergy effect of multiple catalytic sites, where naphthalene was absorbed and activated at the TMO sites (NiO or Co₃O₄), H₂ was activated at the Ru sites and it produced the activated H* species, H* was transferred to the surface of NiO or Co₃O₄ by the hydrogen spillover effect of TM (Ni or Co), reacting with the activated naphthalene and forming decalin. The nanostructures and synergetic effect of the Ru/Ni/NiO/C and Ru/Co/Co₃O₄/C catalysts were revealed by a series of techniques, such as high-resolution transmission electron microscope (HRTEM), temperature-programmed reduction (TPR), scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) mapping, high-sensitivity low-energy ion scattering (HS-LEIS) and X-ray absorption spectroscopy (XAS). It is promising that the hydrogen storage can proceed at room temperature via catalyzing naphthalene hydrogenation over the Ru/Ni/NiO/C or Ru/Co/Co₃O₄/C catalyst.     

Reactivity of TiH2 hydride with lithium ion: Evidence for a new conversion mechanism

The electrochemical reactivity of the face centered cubic (fcc) TiH₂ hydride with lithium ion was studied. A full discharge capacity of 1072 mAh/g at an average potential of 0.2 V can be achieved when the TiH₂ hydride electrode is ground with 10wt% of carbon. From X-ray diffraction (XRD) characterization of the electrodes, dehydrogenation of the titanium hydride via an electrochemical process occurs following different reaction steps. From 0 to 0.34 Li, an fcc δ-TiH_2−x solid solution is formed according to the reaction δ-TiH₂ (fcc) + 0.34 Li → δ TiH_1.66 (fcc) + 0.34 LiH. Pursuing the dehydrogenation process from 0.34 to 1, the cubic solid solution δ-TiH_2−x reacts with lithium ion and transforms partially in a distorted face centered orthorhombic phase δ-TiH (fco). At this stage, the absence of hexagonal close-packed (hcp) α-Ti formation is attributed to the peritectic transformation: hcp α-Ti(H) + fcc δ-TiH_2−x → δ-TiH. From 1 to 2 Li, a usual conversion mechanism is observed leading to the formation of hcp α-Ti and LiH according to the reaction δ-TiH_2−x (fcc) ↔ δ-TiH (fco) + Li → α-Ti (hcp) + LiH.     

Recent progress on self-supported two-dimensional transition metal hydroxides nanosheets for electrochemical energy storage and conversion

Transition metal hydroxides (TMHs) nanosheets have attracted wide attention in electrochemical energy storage and conversion because of their superior surface area, highly tunable composition, and low cost. Moreover, the self-supported electrode has been extensively studied for electrochemical devices due to its fast electron transfer and mass transport, resulting in enhanced stability and electrode performance. Hence, reviewing the recent advances in self-supported TMHs nanosheets is crucial for developing high-performance electrodes for electrochemical devices. In this review, we first introduce the fundamental properties of TMHs in terms of layered single metal hydroxides (LSHs) and layered double hydroxides (LDHs). Then, we review various synthetic approaches utilized to construct self-supported TMHs nanosheets with tunable compositions and structures. Afterwards, the electrode performance and durability of self-supported TMHs nanosheets in various electrochemical applications (water electrolysis, zinc-air battery and supercapacitor) are comprehensively summarized. Finally, the further perspectives on current challenges and research directions of self-supported TMHs nanosheets towards electrochemical energy storages and conversion applications are proposed.     

A Fe Mössbauer spectroscopy study of cobalt ferrite conversion electrodes for Li-ion batteries

The reverse micelles method has been employed to obtain cobalt ferrite samples. The effect of the type of surfactant and volumetric proportion of the aqueous and organic phases on the electrochemical behavior has been evaluated. The sample prepared using Span 80 as a surfactant and equivalent volumes of the aqueous and organic phases showed the highest capacity values and rate capabilities. It has been correlated to the better stability of the faradaic conversion process upon cycling for this sample. Based on the ⁵⁷Fe Mössbauer spectra of discharged electrodes, this result has been associated to the preservation of reduced iron atoms into the core of the particles. The metallic atoms are ready to be oxidized, thus sustaining the reversible electrochemical reaction in further cycles.     

Future cathode materials for lithium rechargeable batteries

Lithium rechargeable batteries are now well established as power sources for portable equipment, such as portable telephones or computers. Future applications include electric vehicles. However before they can be used for this, or other price-sensitive applications, new cathode materials of much lower cost are needed. Possible cathode materials are reviewed.     

Application of transition metal high entropy boride in electrocatalytic hydrogen evolution reaction

For the growth of hydrogen energy, it is essential to create electrocatalysts that are affordable, effective, and stable. The transition metal high-entropy boride electrocatalyst is synthesized with the molten salt-assisted boron thermal reduction method. It discusses the influence of element composition, temperature, and the ratio of salt to material on electrocatalytic hydrogen evolution. The results show that WMoVNbMnB synthesized at the ratio of salt to material of 15:1 and sintering temperature of 1000 °C has nano-flake structure. The overpotential at a current density of 10 mA cm⁻² at 1 M KOH is as low as 115 mV, with the Tafel slope of 92 mV·dec⁻¹. According to theoretical study, the transition metals (V, Mo, and Mn) are a considerable contributor to the electron around the Fermi level. Under the synergistic effect of multiple components, the V 3d orbit possesses excellent polarization, which thus improves the migration ability of the carrier. In the process of water decomposition, WMoVNbMnB electrocatalyst only needs to cross the 0.24 eV energy barrier to successfully dissociation, with the excellent properties of a glass carbon catalyst. It offers a workable reference for creating water electrolysis technology because of its highly effective catalytic activity.     

Nanostructured manganese oxide electrodes for lithium-ion storage in aqueous lithium sulfate electrolyte

Nanostructured manganese oxide electrodes are fabricated directly by electrochemical deposition. Surface morphology of the electrode deposited at high-current density shows nanowires with diameter 12–16 nm distributed randomly. Nanowires tend to aggregate to clumps when the deposition current density is low. Both annealing temperature and deposition current density affect the electrochemical performance of the deposited manganese oxide electrode in an aqueous lithium sulfate electrolyte. An optimal annealing temperature is found to be 300 °C in terms of the electrode's specific capacity during high-rate charging/discharging. An electrode with thinner nanowires deposited at high-current density has a high-specific capacity because thinner nanowires shorten the diffusion of lithium ions and in favor of high-rate charging/discharging.     

Electronic modulation of multi-element transition metal phosphide by V-doping for high-efficiency and pH-universal hydrogen evolution reaction

Developing highly efficient, durable and noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) in a wide pH range is the key to achieve sustainable energy cycle. Modulating the electronic characteristics of catalysts is of great important to enhance their electrocatalytic performance. Herein, we successfully design a self-supported multi-element transition metal phosphide (V-CoP/Ni₂P/NF) electrocatalyst by V-doping and interface engineering. According to DFT calculations, V dopants and the synergistic effect between CoP and Ni₂P can bring optimal hydrogen/water adsorption free energies. Meanwhile, the 3D cross-linked network structure could expose abundant active sites and facilitate ions diffusion/electron transfer. As a result, the V-CoP/Ni₂P/NF catalysts exhibit superior electrocatalytic activities with extremely low overpotentials of 20, 58 and 79 mV to deliver a current density of 10 mA cm^?2 for HER in alkaline, neutral and acidic electrolytes, respectively. Moreover, they also display outstanding long-term stability. This electronic modulation strategy provides a promising pathway for energy-related catalysis processes.     

Recent progress in rechargeable nickel/metal hydride and lithium-ion miniature rechargeable batteries

VARTA is searching for alternative battery solutions for memory back-up and bridging applications, and for this, it is developing nickel/metal hydride and lithium-ion button cells. Presented are the results on different sizes and forms of lithium-ion cells (621, 1216 and 2025) containing different electrode materials and shapes. Presently, the most favoured cathode material is lithiated manganese dioxide. The electrodes are made from both solid and porous materials and, together with an organic electrolyte, result in a cell system with a voltage level of approximately three. Included are results, both from these lithium-ion cells, and also from ones using the nickel/metal hydride system.     

The strain and transition metal doping effects on monolayer Cr2O3 for hydrogen evolution reaction: The first principle calculations

Chromic oxide (Cr₂O₃) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr₂O₃ monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr₂O₃ monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔG_H1) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr₂O₃ monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr₂O₃ monolayer.     

Three-demensional NiCoNiCo2O4/NF as an efficient electrode for hydrogen evolution reaction

At present, there is an urgent need for plentiful non-noble metal catalyst to substitute for valuableness platinum based metal catalyst in electrochemical water splitting. Here, we fabricated a three-demensional (3D) NiCoNiCo₂O₄ nanosheets electrocatalyst that directly grew on Ni foam firstly and then were reduced in 0.1 mol dm⁻³ sodium borohydride solution. This electrode exhibited high activity in 1.0 mol dm⁻³ KOH solution with an onset potential of ∼40 mV and a tafel slope of 77 mV dec⁻¹. Furthermore, the NiCoNiCo₂O₄/NF electrode showed a splendid durability during long-playing electrochemical test. Our work may provide an inexpensive, easy-to-obtain and excellent catalyst candidate for future electrolytic water research and industry studies that may involve hydrogen applications in the future.     

Low-temperature flue gas denitration with transition metal oxides supported on biomass char

Several transition metals (Mn, Ce, V and Fe) were loaded on nitric acid modified biomass char (BC) using an impregnation method for the selective catalytic reduction (SCR) of NO with NH₃ at low-temperature. The series of prepared catalysts were characterized by BET, SEM, FT-IR and XRD. Results showed the sequence of NO conversion within the temperature of 125–225 °C was Mn/BC > Ce/BC > V/BC > Fe/BC > BC, and Mn/BC exhibited the highest NO conversion of 87.6% at 200 °C. BC supports provided high surface area, which could contribute to the better dispersion of transition oxides on biomass char, revealing rich oxygen containing groups. Besides, the BC worked not only as a promising support, but also provided high activity adsorption sites for NH₃ and conducted as oxidizing agent for NO due to the existence of graphite crystallite structure. Owing to the existence of “fast SCR” reaction process, the transition metal oxides supported on BC catalysts exhibited superior denitration efficiency in low temperature.     

Activity engineering to transition metal phosphides as bifunctional electrocatalysts for efficient water-splitting

At present, water splitting has been regarded as one of the most promising ways for hydrogen production. Therefore, exploitation of cost-effective electrocatalysts is essential to realize the industrialization of electrocatalytic techniques. In recent years, transition metal phosphides (TMPs) as non-noble metal electrocatalysts have gained a great deal of attention owing to their multifunctional active sites, tunable structure and composition, as well as unique physicochemical properties. However, the poor electrical conductivity of TMPs and high adsorption energy of hydrogen intermediates during the hydrogen evolution severely restrict its large-scale application. Therefore, it is of great importance to develop effective activity engineering to TMPs. Herein, the reaction mechanisms of water splitting including hydrogen evolution and oxygen evolution reactions and the key performance parameters are briefly clarified. Then, the strategies to improve the performance of TMPs are summarized in four aspects, including modulation of electronic structure, tailoring microstructures, selection of working electrode, and replacing OER with an energy-saving reaction. Finally, a summary and perspective for further opportunities and challenges are highlighted for the TMPs from the point of characterization methodologies, theoretical calculation and practical application.     

Taming transition metals on N-doped CNTs by a one-pot method for efficient oxygen reduction reaction

Various transition metals have been incorporated into nitrogen-doped carbon nanotubes (M-N-C/CNT, M = Fe, Co, Mn, and Ni) via a one-pot method using dopamine as nitrogen source and metal salts as precursors for oxygen reduction reaction (ORR). Raman spectra and XRD patterns of the catalysts were collected to characterize the graphitization degree and metal state. XPS was employed to determine atom state and element fraction. The electrochemical performance of the catalysts for ORR were evaluated in alkaline media at ?0.8 V to 0.2 V. The E_onset and E_1/2 with the values of ?100 mV and ?170 mV (vs Ag/AgCl) are achieved on Mn-N-C/CNT-800. The superior selectivity toward the 4e^? pathway are obtained on Mn-N-C/CNT-800 and Co-N-C/CNT-800 with the transferred electron numbers per O₂ molecule of 4.12 and 3.94, respectively. Results show that the states of doped transition metal play a key role on determining ORR performance. The electron transfer number of Ni-N-C/CNT at ?0.5 V is increased from 3.22 (Ni-N-C/CNT-800) to 3.99 (Ni-N-C/CNT-600) when the metallic Ni has been eliminated at lower pyrolysis temperature.     

Recent development of transition metal doped carbon materials derived from biomass for hydrogen evolution reaction

Achieving high catalytic performance with the lowest cost is critical for hydrogen evolution reduction. In recent years, biomass-derived carbon catalysts have triggered huge interest in catalytic reactions owing to the low cost, high energy conversion efficiency and environmental friendliness. A rapid growth of novel electrocatalysts is witnessed especially those based on non-precious metals, some of which approach the activity of precious metals. Synergistic interactions between metals and heteroatoms can significantly improve the electrocatalytic activity, thus transition metal-decorated biomass-based carbon materials were commonly adopted to improve HER performance. The resulting electrocatalytic activities are introduced and compared to conventional Pt/C-based electrocatalysts in present research. Moreover, the remaining challenges in the development process and future prospects of hydrogen evolution reduction catalysts are discussed.     

Transition metal sulfides for electrochemical hydrogen evolution

Hydrogen is identified as the most promising zero-carbon fuel of the future. Naturally, in this regard, the hydrogen evolution reaction (HER), being a first critical step of the hydrogen technology and economy, attracts much attention. Conventionally, noble metals have been used as the electrocatalyst for HER, which in part holds back the hydrogen technology to become a large scale and heavily distributed energy technology. This has encouraged scientists to study cost-effective strategies for HER. Transition metal disulfides, being a low-cost material system with a great degree of engineering versatility, have recently emerged as a potential candidate that can significantly promote hydrogen evolution. Several studies have demonstrated that the control and manipulation of the structure and morphology of these materials can improve their proton reduction performance. This review covers many of the decisive factors and strategies to advance transition metal sulfides for HER applications.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

Group IV transition metal based phospho-chalcogenides@MoTe2 for electrochemical hydrogen evolution reaction over wide range of pH

Electrochemical water splitting proved to be one of the most attractive sources of green fuel energy but the easy and time-consuming synthesis of highly active, low-priced, steady and earth-abundant metals electrocatalysts is still a challenge among the researcher. In this work, we have reported the fabrication of Group IV transition metal (M = Ti, Hf, and Zr) based phospho-chalcogenides and their composite with MoTe₂ via simple hydrothermal synthesis process. The prepared nanocomposites (MP₂S₆@MoTe₂) were compared with respect to their best performance towards HER over the entire pH range. Among the different kind of prepared nanocomposite, TiP₂S₆@MoTe₂ exhibited special almond like shape with self-layered morphology and henceforth selected as best catalyst on the basis of their low Tafel slope, small onset potential, low charge transfer resistance and small overpotential value over the wide range of pH. It was also found that nanocomposites modified working electrode showed long cycle durability in the acidic pH in controlled potential electrolysis study.     

Improved hydrogen cycling kinetics of nano-structured magnesium/transition metal multilayer thin films

A magnesium-based nano-structured multilayer material was developed by magnetron-assisted physical vapour deposition with promising hydrogen storage properties. The material has a reversible capacity of 4.6 wt% at temperatures between 250 and 350 °C and hydrogenates in <10 min at 250 °C. An activation energy of the dehydrogenation reaction of E_a = 71.6 kJ mol⁻¹ was measured by differential scanning calorimetry. Structural analysis by TEM and SEM showed that the thin magnesium layers of 16.5 nm thickness interspersed with 2.5 nm of an amorphous, nickel-rich transition metal mix resulted in a favourable nanostructure after hydrogen cycling at up to 350 °C. The material also retained its fast kinetics and capacity for the 50 cycles that the material underwent. Comparison of XRD data with TEM shows that the layer thickness of such nano-structured, directional Mg layers in PVD multilayers can be reliably estimated by XRD. In addition the XRD texture relates to the microstructural evolution of the multilayered structure pre- and post-cycling.