CoFe Prussian Blue Analogue Research Articles

Polyaniline-Encapsulated Hollow Co-Fe Prussian Blue Analogue Nanocubes Modified on a Polypropylene Separator To Improve the Performance of Lithium-Sulfur Batteries.

Recent studies of lithium-sulfur (Li-S) batteries have identified that a modified separator plays a critical role in challenging the capacity fading and shuttle effect of lithium polysulfides (LiPSs). Herein, we report a polyaniline-encapsulated hollow Co-Fe Prussian blue analogue (CFP@PANI) for separator modification. The open frame-like hollow CFP was synthesized via oriented attachment (OA). To improve the catalytic effect and electrical conductivity, PANI was coated on the synthesized CFP. The resulting CFP@PANI was applied on the conventional polypropylene (PP) separator (CFP@PANI-PP) with vacuum filtration. With a ketjen black/sulfur (KB/S) cathode with 66% of the sulfur load, the CFP@PANI-PP exhibited an initial capacity of 723.1 mAh g-1 at a current density of 1 A g-1. Furthermore, the CFP@PANI-PP showed stable cycling performance with 83.5% capacity retention after 100 cycles at 1 A g-1. During the 100 cycles, each cycle maintained high coulombic efficiency above 99.5%, which indicates that the CFP@PANI-PP could inhibit LiPS migration to the anode side without a Li+ transport disturbance across the separator. Overall, the CFP@PANI-PP efficiently suppressed LiPSs, resulting in enhanced electrochemical performance. The current study provides useful insight into designing a nanostructure for separator modification of Li-S batteries.

Systematic constructing CoFe-Prussian blue analogue on NiCo-layered double hydroxide to obtain heterostructure two-bimetallic phosphide composite as efficient self-supported eletrocatalyst for overall water and urea electrolysis

It remains a challenge to explore economical, high-effective and long term stability electrocatalysts toward large-scale hydrogen production. This work utilizes surface engineering strategy to in-situ CoFe-Prussian Blue Analogues on NiCo-layered double hydroxides to obtain 3D hierarchical heterostructure precursor (NiCo–CoFe-PBA). After phosphatization, this precursor can be further transform into tri-metallic phosphide (NiCoP/CoFeP@NF) and directly act as efficient self-supported electrode for Water and Urea Electrolysis. Impressively, the obtained NiCoP/CoFeP@NF-12 (±) electrode shows excellent catalytic performance with only requires the cell voltage of 1.61 and 1.46 V to deliver 10 mA cm−2 in overall water splitting and urea electrolysis, respectively, which benefiting from the porous Ni foam (NF) substrate, large catalytic activity area, remarkable mass/electron transfer property, the synergistic effect of components as well as superhydrophilicity and superaerophobicity of electrode surface. In addition, the experimental results also confirm that urea-assisted system has energy saving advantage superior than traditional water splitting in alkaline electrolyte. Moreover, the hierarchical strategy can also be introduced to the construction of other intricate composites for the utilization in energy conversion and storage.

Temperature-controlled fabrication of Co-Fe-based nanoframes for efficient oxygen evolution

Transition metal phosphides (TMPs) have emerged as promising electrocatalysts to enhance the slow kinetic process of oxygen evolution reaction (OER). Framelike hollow nanostructures (nanoframes, NFs) provide the open structure with more accessible active sites and sufficient channels into the interior volume. Here, we report the fabrication of bimetallic Co-Fe phosphide NFs (Co-Fe-P NFs) via an intriguing temperature-controlled strategy for the preparation of precursors followed by phosphidation. The precursors, Co-Fe Prussian blue analogues (Co-Fe PBAs) are prepared by a precipitation method with Co2+ and [Fe(CN)6]3−, which experience a structural conversion from nanocubes to NFs by increasing the aging temperature from 5 to 35°C. The experimental results indicate that this conversion is attributable to the preferentially epitaxial growth on the edges and corners of nanocubes, triggered by intramolecular electron transfer at an elevated aging temperature. The as-prepared Co-Fe-P NFs catalyst shows remarkable catalytic activity toward OER with a low overpotential of 276 mV to obtain a current density of 10 mA cm−2, which is superior to the reference samples (Co-Fe-P nanocubes) and most of the recently reported TMPs-based electrocatalysts. The synthetic strategy can be extended to fabricate Co-Fe dichalcogenide NFs, thereby holding a great promise for the broad applications in energy storage and conversion systems.

Size-Dependent Synthesis of Hollow Co3O4 Nanocubes Derived from Co-Fe PBA as Battery-Type Electrode Materials for Hybrid Supercapacitors

Hollow nanostructures with enlarged surface areas are highly attractive electrode materials for supercapacitors. In this work, the size-dependent synthesis of hollow Co3O4 nanocubes via a facile ionic exchange reaction between Co-Fe Prussian blue analogues (PBAs) and alkali solution is reported. By adjusting the concentration of sodium citrate to control the reaction kinetics during nucleation and growth, four different sizes of Co-Fe PBAs were synthesized. It was also found that a Co-Fe PBA of about 140 nm can be easily converted into a well-defined internal hollow structure, while Co-Fe PBAs with smaller or larger sizes are challenged in generating a hollow structure. Benefitting from the inner voids and thin shell architecture, the derived hollow Co3O4 nanocubes exhibit a high specific capacity of 296.6C cm−2 at 2 mA cm−2, and a rate capability of 64.5% when the current density is increased to 60 mA cm−2. Furthermore, a hybrid supercapacitor (HSC) was fabricated with hollow Co3O4 nanocubes as the cathode and activated carbon as the anode, respectively. The HSC provided a maximum energy density of 14.1 Wh kg−1 at 464.7 W kg−1. Moreover, it retained the excellent cycling stability of 85.7% of the original capacity over 5000 continuous charging and discharging processes.

Hierarchically Assembling CoFe Prussian Blue Analogue Nanocubes on CoP Nanosheets as Highly Efficient Electrocatalysts for Overall Water Splitting.

Efficient and durable electrocatalysts are highly desirable for overall water splitting. Herein, a facile strategy is demonstrated to rationally construct CoFe Prussian blue analogues (PBA)@CoP cube-on-sheet hierarchical structure by etching reaction with intermediated CoO to form PBA nanocubes. Benefitting from the heterostructured engineering, the as-synthesized CoFe PBA@CoP presents remarkable electrocatalytic performance in 1.0m KOH, only requiring overpotentials of 100 mV for hydrogen evolution reaction (HER) and 171mV for oxygen evolution reaction (OER) to reach the 10 mA cm-2 current density with good stability. Extraordinarily enhanced electrocatalytic performance is ascribed to not only the rapid charge transfer of active species, but also the synergistic effect between each component to achieve tuned electronic structure and abundant electrocatalytic active sites. Especially, the assembled two-electrode cell using CoFe PBA@CoP as both cathode and anode delivers the current densities of 10 mA cm-2 at a relatively low cell voltage of 1.542 V, outperforming most of low-cost bifunctional electrocatalysts reported to date. The controllable and versatile strategy will open up an avenue to prepare hybrid films for advanced electrochemical energy storage and conversion.

Photocatalytic degradation of oxytetracycline under visible light by nanohybrids of CoFe alloy nanoparticles and nitrogen-/sulfur-codoped mesoporous carbon

Efficient photodegradation of antibiotics in water systems is significant for environmental restoration and water purification. In this work, a series of catalysts for oxytetracycline (OTC) degradation under visible light was proposed. The hybrids (denoted by CoFe@NSC) of CoFe alloy and N/S-codoped mesoporous carbon were obtained by annealing CoFe Prussian blue analogue (CoFe PBA) nanocubes under different temperatures using thioacetamide as the N and S resource. The CoFe@NSC photocatalysts show substantially boosted photocatalytic activity for OTC degradation compared with that of the CoFe PBA catalyst. Among the three CoFe@NSC hybrids obtained by pyrolyzing at 800 °C, 900 °C, and 1000 °C, the CoFe@NSC-1000 hybrid at 1000 °C exhibits the highest photocatalytic activity toward OTC degradation under visible light irradiation for its highly conjugated nanostructure, large specific surface area (733.0 m2 g−1), enhanced visible light absorption capacity, and improved separation/migration rate for photogenerated charge carriers. Approximately 90% of OTC was degraded by CoFe@NSC-1000 catalyst after visible light irradiation for 150 min. Electron spin resonance (ESR) and trapping tests indicate that the photoinduced superoxide and hydroxyl radicals are the main active species for photodegradation. The possible photocatalytic mechanism was proposed based on the active species, trapping experiments and ESR analysis. The biotoxicity of photocatalytic degradation products and residual antibiotic were also explored using E. coli as the reference organism. This work offers new insight into the development of novel catalysts for photodegradation of antibiotics.

Atrazine removal by peroxymonosulfate activated with magnetic CoFe alloy@N-doped graphitic carbon encapsulated in chitosan carbonized microspheres

Magnetic CoFe alloy@N-doped graphitic carbon (CoFe@NC) encapsulated in chitosan carbonized microspheres (CoFe@NC/CCM) was synthesized, then coupled with peroxymonosulfate (PMS) for atrazine (ATZ) removal. CoFe@NC derived from Co-Fe Prussian blue analogues were embedded into CCM by the alkaline gel-carbonization process, in which CCM formed a protective layer for CoFe@NC. The morphology, crystal structure, and material composition of CoFe@NC/CCM were characterized. The effects of catalyst dosages, PMS dosages, initial pH, temperature, coexisting anions, natural organic matter (NOM), and different practical water matrices were systematically investigated. The productivity test was performed under the conditions of [PMS] = 0.4 mM, pHi = 5.9, T = 25 °C, ATZ = 30 mg/L and [CoFe@NC-600/CCM] = 4 mg/L, the stability upon reuse for 8 days was studied and the accumulated turnover number was determined to be 46.8 after an 8-day cycle. Electron paramagnetic resonance (EPR) technology and radical quenching tests revealed ·OH and SO4·- were major radicals, with contribution rates of 49.2% and 42.9%, respectively. Fe0 → ≡FeII ↔ ≡FeIII and Co0 → ≡CoII ↔ ≡CoIII redox cycles were involved in the catalytic reaction. The toxicity effect of transformation products (TPs) on activated sludge was also evaluated. Based on quantitative structure–activity relationship model prediction, the comprehensive toxicity of ATZ and TPs was analyzed and compared by the toxicity estimation software tool. Results showed that most TPs toxicity decreased or disappeared. This work proposes an environmentally friendly and easy-to-recycle heterogeneous catalyst to remove refractory organics by activating PMS.

General Synthesis of Hierarchically Macro/Mesoporous Fe,Ni-Doped CoSe/N-Doped Carbon Nanoshells for Enhanced Electrocatalytic Oxygen Evolution.

Constructing hierarchical porosity and designing rational hybrid composition are effective strategies for enhancing the electrocatalytic performance of hybrid catalysts for electrochemical energy conversion. Here, we develop a multistep "molecule/ion-exchange" strategy toward the synthesis of hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon nanoshells with tunable pore structures and compositions. Polystyrene (PS)@Co-based amorphous coordination polymer (Co-CP) core-shell particles with hierarchically macro/mesoporous nanoshells are first prepared by ligand-molecule-exchange etching of the outer layers in PS@Co-based metal-organic framework precursors. Afterward, a liquid-solid dual-ion-exchange reaction of PS@Co-CP particles with [Fe(CN)6]3- and [Ni(CN)4]2- ions leads to the formation of PS@Co-CP/Co-Fe Prussian blue analogue (PBA)/Co-Ni PBA particles, which are further transformed into hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon particles via a vapor-solid selenization reaction. Moreover, this approach could be extended to synthesize different hierarchically porous core-shell composites with various morphologies and tailored compositions. Because of their unique hierarchically porous nanoarchitecture, these Fe,Ni-doped CoSe/N-doped carbon particles with optimized composition show enhanced performance for electrocatalytic oxygen evolution.

Three-dimensional porous ultrathin carbon networks reinforced PBAs-derived electrocatalysts for efficient oxygen evolution

Prussian blue analogues (PBAs), identified with a face-centered cubic (fcc) unit cell with transition-metal ions bridged by cyano ligands, can be used as precursors in nanomaterials for catalysis applications, but the performance is usually limited by the original large size of PBAs. We provide here a controllable thermal decomposition route for PBAs to obtain highly dispersed small-sized nanoparticles with high catalytic activity. CoFe PBA nanocubes as catalyst precursors are grown within three-dimensional porous ultrathin N-doped carbon networks (3DNC), which is constructed using mixed soluble salts as templates. The use of 3DNC as supporting matrix can not only increase the electrical conductivity and transport properties of reaction-relevant species within the catalyst material, but also supply huge specific surface and restrict the size of loaded nanoparticles. Followed by calcination in the reductive atmosphere, the PBA nanocubes are decomposed to form numerous small-sized CoFe/CoFeOx nanoparticles that are uniformly dispersed. It is interesting to find that this thermal decomposition process is assisted by 3DNC matrix because of its excellent thermal conductive property. Benefiting from the synergetic bimetal alloy/oxide composition, advantageous structural feature and favorable interfacial integration of the hybrid material, the CoFe/CoFeOx@3DNC exhibits remarkable electrocatalytic activity and stability for the oxygen evolution reaction (OER) in alkaline media. The decomposition of PBAs precursor to small-sized nanoparticles and the use of 3DNC as supporting matrix and thermal conductive agent are appealing. The integration of these two components provides an interesting hybrid catalyst system with controllable composition, delicate morphology and favorable interfacial property.

Surface selenium doped hollow heterostructure/defects Co-Fe sulfide nanoboxes for enhancing oxygen evolution reaction and supercapacitors

The strategy of constructing a three-dimensional hierarchically porous architecture with heterointerface and defects to rich the electrochemically active site, and rapid electron transfer has been considered as an effective strategy to achieve the excellent electrochemical performance in electrochemical energy storage and conversion. Herein, a Se-doped hollow heterostructure/defects Co-Fe sulfide (Se-(CoFe)S2) box has been developed via a facile sulfurization-selenization treatment of hollow Co-Fe Prussian blue analogues precursor. Remarkably, the abundant hierarchical macro-mesopores of hollow Se-(CoFe)S2 nanobox with excellent structural stability and unique electrical properties can not only effectively expose the electrochemically accessible active sites but also significantly improve the electrolyte transport and electron transfer, thus boosting electrochemical performances in oxygen evolution reaction (OER) and supercapacitors in alkaline media. For OER, the Se-(CoFe)S2 electrocatalyst shows a small overpotential of 281 mV at 10 mA cm−2 and a low Tafel slope of 51.8 mV dec−1 as well as high stability. Furthermore, it also shows good performance for supercapacitors, suggesting its great potential as an efficient electrode for sustainable energy applications. Superb properties highlight the great prospects of PBA-based derivatives in applications of energy storage and conversion.

High yield electrooxidation of 5-hydroxymethyl furfural catalysed by unsaturated metal sites in CoFe Prussian Blue Analogue films

Unsaturated Co metal sites in CoFe Prussian blue analogues are introduced by pulse electrodeposition on Ni foam. The PBA electrode achieved 94% yield of 2,5-furandicarboxylic acid from 5-hydroxymethylfurfural at 1.43 V vs. RHE.

Photocatalytic water oxidation with a Prussian blue modified brown TiO2

A recently emerging visible light-absorbing semiconductor, brown TiO2 (b-TiO2), was coupled with a CoFe Prussian blue (PB) analogue to prepare an entirely earth-abundant semiconductor/water oxidation catalyst hybrid assembly. PB/b-TiO2 exhibits a sevenfold higher photocatalytic water oxidation activity compared to b-TiO2. An elegant band alignment unified with the optical absorption of b-TiO2 and excellent electronic dynamics of PB yield a high-performance photocatalytic system.

Self-templating construction of hollow Fe-CoxP nanospheres for oxygen evolution reaction

Rational design of catalysts with well-controlled structures and compositions has been demonstrated effective to optimize the electrochemical performance. Metal-organic frameworks (MOFs) with tunable composition, morphology as well as porous structure are desirable for constructing various functional materials acting as precursors. In this work, a facile self-templating strategy is developed to synthesize hollow Fe-CoxP nanospheres. The solid Co-glycerate nanospheres are synthesized and used as self-sacrificing templates, which are converted to Co-Fe Prussian blue analogue (PBA) via an anion-exchange process by reacting with K3[Fe(CN)6] at room temperature, and then followed by phosphorization treatment to produce the Fe-CoxP. The transformation process endows the resultant product not only the hollow nanostructure, but also Fe atoms doping into the CoxP. By taking advantages of structural and compositional benefits as well as improved conductivity, the Fe-CoxP exhibits excellent electrocatalytic activity and stability toward the oxygen evolution reaction (OER), reaching the current density of 10 mA cm−2 at an overpotential of 300 mV. The post-OER catalyst is carefully characterized and the results indicate the phase transformation of the crystalline Fe-CoxP to the amorphous FeCo (oxy)hydroxides acting as the catalytically active species. The novel self-templating strategy avoids the challenge from the removal of the template, which can possibly be employed to produce a sequence of PBA-based functional nanomaterials for applications in electrochemical energy conversion and storage.

Charge transfer driven by ultrafast spin transition in a CoFe Prussian blue analogue.

Photoinduced charge-transfer is an important process in nature and technology and is responsible for the emergence of exotic functionalities, such as magnetic order for cyanide-bridged bimetallic coordination networks. Despite its broad interest and intensive developments in chemistry and material sciences, the atomic-scale description of the initial photoinduced process, which couples intermetallic charge-transfer and spin transition, has been debated for decades; it has been beyond reach due to its extreme speed. Here we study this process in a prototype cyanide-bridged CoFe system by femtosecond X-ray and optical absorption spectroscopies, enabling the disentanglement of ultrafast electronic and structural dynamics. Our results demonstrate that it is the spin transition that occurs first on the Co site within ~50 fs, and it is this that drives the subsequent Fe-to-Co charge-transfer within ~200 fs. This study represents a step towards understanding and controlling charge-transfer-based functions using light.

Precious Metal-Free Photocatalytic Water Oxidation by a Layered Double Hydroxide-Prussian Blue Analogue Hybrid Assembly.

The development of earth-abundant photocatalytic assemblies has been one of the bottlenecks for the advancement of scalable water splitting cells. In this study, a ZnCr layered double hydroxide and a CoFe Prussian blue analogue are combined to afford an earth-abundant photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). Compared to bare ZnCr-LDH, the SC-WOC hybrid assembly exhibits a threefold enhancement in photocatalytic activity, which is maintained for 6 h under photocatalytic conditions at pH 7. The band energy diagram was extracted from optical and electrochemical studies to elucidate the origin of the enhanced photocatalytic performance. This study marks a straightforward pathway to develop low-cost and precious metal-free assemblies for visible light-driven water oxidation.

Structure–Property Relationships in Redox-Derivatized Metal–Insulator–Semiconductor (MIS) Photoanodes

Metal–insulator–semiconductor (MIS) junctions based on n-Si have proven to be effective electrodes in terms of electrocatalysis activity and durability for performing photoelectrochemical water oxidation. Here, we show that the modification of n-Si MIS systems with CoFe Prussian blue (CoFePB) and NiRu Prussian blue (NiRuPB) analogues can modify their properties and allow a direct probing of the interfacial energetics through their redox feature. Our investigations demonstrate the importance of the preparation route and attribute the large upward photovoltage variation found in n-Si/SiOx/Ni/NiRuPB to the increasing inhomogeneity of the metal thin film. Finally, the optimal photoanode was tested for oxygen evolution and urea oxidation reactions. Our findings provide important insights on MIS photoanodes for future development in the field of solar fuel production.

Unconventional bi-vacancies activating inert Prussian blue analogues nanocubes for efficient hydrogen evolution

Prussian blue analogues (PBA) have been explored as electrocatalyst for the hydrogen evolution reaction, however, they suffer from poor electrical conductivity and blocked active sites and thus usually operate at overpotentials drastically above thermodynamic requirement. Herein, we report a creation of unconventional anion (cyano group) and cation (cobalt) bi-vacancies to boost CoFe-PBA for HER through a rapid tannic acid (TA) etching strategy. The combined experimental and theoretical investigations indicate that the introduction of bi-vacancies would be favorable to exposure of accessible active sites, optimizing the electron density of CoFe PBA, and lowering the reaction energy barrier. As a result, the TA-etched CoFe PBA nanocubes supported on nickel foam (NF) ([email protected]/NF) exhibit outstanding HER catalytic activity with an overpotential of only 86 mV at 10 mA cm−2, which is not only significantly lower than bare PBA supported on NF (158 mV), but also comparable to the commercial Pt/C catalyst.

Vanadium nitride based CoFe prussian blue analogues for enhanced electrocatalytic oxygen evolution

The development of efficient and stable nonprecious-metal catalysts remains a challenge for electrochemical water oxidation in practical applications. Prussian blue analogues (MM′-PBAs, AxMy [M′ (CN)6]z·nH2O: A = alkali metal; M/M’ = transition metals), as a kind of potential electrocatalyst candidate for oxygen evolution reaction (OER), are rarely studied. Herein, by a facile coprecipitation method, CoFe–PBAs were in-situ grown on the surface of highly conductive vanadium nitride (VN) particles. The hybrid CoFe–PBAs/VN achieves the amelioration of the poor conductivity of bare CoFe–PBAs and the increase in the density of Co2+ active sites due to electron interaction. Compared with bare CoFe–PBAs (overpotential of 398 mV at 10 mA cm−2 and Tafel slope of 78.35 mV dec−1), the CoFe–PBAs/VN manifests a remarkably enhanced electrocatalytic activity for OER (lower overpotential of 290 mV at 10 mA cm−2 and a smaller Tafel slope of 39.72 mV dec−1), which is superior among PBAs and PBAs-derived materials reported as electrocatalysts for OER. Besides, this hybrid material behaves an excellent long-term durability. High conductivity of VN and electron interaction between VN and CoFe–PBAs contribute to the promoted catalytic performance. This study provides a novel strategy using VN substrates to support PBAs-based catalysts so as to obtain highly active and stable electrocatalysts towards practical applications.

Hydrogen evolution under large-current-density based on fluorine-doped cobalt-iron phosphides

Developing the transition metal-based electrocatalysts for hydrogen evolution reaction (HER) under large-current-density is crucial for industrial electrolysis of water. However, most of reported HER electrocatalysts in alkaline solution are tested under the current density of less than 100 mA cm−2. In this work, CoFe Prussian blue analogues (PBA) supported on iron foam (IF) as precursor can (has been) be utilized to construct the fluorine-doped cobalt-iron phosphides (F-Co2P/Fe2P/IF). At the large-current-density at 100, 500, 1000 and 2000 mA cm−2, we systematically investigate the HER performance of F-Co2P/Fe2P/IF. Compared to the electrocatalyst without F− doping, F-Co2P/Fe2P/IF only need the low overpotential of 229.8, 260.5, 292.2 and 304.4 mV, respectively, when reaching the large current of 500, 1000, 2000 and 3000 mA cm−2. The F doping may induce the valence state and electronic structure of Co and Fe ions for enhanced HER activity. The role of F doping is furtherly analyzed by introducing barium ions as precipitation agent. At the same time, it can maintain for a long-term stability in 1.0 and 6.0 M KOH at the different high current density. The stableF-Co2P/Fe2P/IF may bear the high current density and high temperature during industrial water electrolysis. This work enables it possible at the large current density for industrial water splitting.

Elucidation of Active Sites on S, N Codoped Carbon Cubes Embedding Co–Fe Carbides toward Reversible Oxygen Conversion in High‐Performance Zinc–Air Batteries

The development of high-performance but low-cost catalysts for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of central importance for realizing the prevailing application of metal-air batteries. Herein a facile route is devised to synthesize S, N codoped carbon cubes embedding Co-Fe carbides by pyrolyzing the Co-Fe Prussian blue analogues (PBA) coated with methionine. Via the strong metal-sulfur interaction, the methionine coating provides a robust sheath to restrain the cubic morphology of PBA upon pyrolysis, which is proved highly beneficial for promoting the specific surface area and active sites exposure, leading to remarkable bifunctionality of ORR and OER comparable to the benchmarks of Pt/C and RuO2 . Further elaborative investigations on the activity origin and postelectrolytic composition unravel that for ORR the high activity is mainly contributed by the S, N codoped carbon shell with the inactive carbide phase converting into carbonate hydroxides. For OER, the embedded Co-Fe carbides transform in situ into layered (hydr)oxides, serving as the actual active sites for promoting water oxidation. Zn-air batteries employing the developed hollow structure as the air cathode catalyst demonstrate superb rechargeability, energy efficiency, as well as portability.