Operational Potential Window Research Articles

Synthetic melanin facilitates MnO supercapacitors with high specific capacitance and wide operation potential window

Although tremendous efforts have been devoted to manganous oxide (MnO)-based hybrid electrode materials, the rational design and facile preparation of high-performance MnO hybrid supercapacitors are still full of challenges, due to the inevitable agglomeration and limited stability of MnO within carbon matrixes. In this work, we developed a direct one-pot polymerization method to synthesize a series of Mn ion doped polylevodopa [P(L-DOPA)] synthetic melanin nanoparticles (Mn@SMNPs) for further transforming into small-sized hybrid carbon nanoparticles (MnO@CNPs, 20–80 nm) consisting of many stable Mn-related chemical bindings (C–N–Mn). The obtained MnO@CNPs electrode materials with physically spatial protection and well distribution of MnO could display high specific capacitance values (e.g., 545 F g−1 at 0.5 A g−1 in 6 M KOH electrolyte) within a wide operation voltage of 1.3 V. This work will continue to provide an inspiration towards the fabrication of the next-generation of high-performance synthetic melanin-based energy storage materials and devices.

Stabilization of a 4.7 V High‐Voltage Nickel‐Rich Layered Oxide Cathode for Lithium‐Ion Batteries through Boron‐Based Surface Residual Lithium‐Tuned Interface Modification Engineering

AbstractResidual lithium on the surface and the resulting side reactions for high‐energy‐density Ni‐rich layered oxide cathodes principally impede their industrial application and trigger safety concerns. Herein, the successful construction of LiBO2−B2O3 co‐modified single‐crystal LiNi0.6Co0.2Mn0.2O2 (SC‐NCM) as a lithium‐ion battery (LIB) cathode is reported. Boric acid reacts with the surface residual lithium species to form such uniform coating on the SC‐NCM particles, which presents advanced rate and cycling capabilities. As the cathode materials for LIBs, LiBO2−B2O3 co‐modified SC‐NCM delivers a 141.9 mAh g−1 discharge specific capacity at 5 C between 3.0 and 4.5 V versus Li+/Li with 61.4 % capacity retention after 500 cycles, superior to the 20.8 % retention for the pristine SC‐NCM cathode. Besides, the LiBO2‐B2O3 protective layer substantially inhibits the unexpected phase transformation, effectively alleviates the mechanical microcracks, and stabilizes the cathode‐electrolyte interface, even at an extended operational potential window. The proposed microstructure‐modified SC‐NCM cathode provides an affordable and feasible design strategy for Ni‐rich SC‐NCM cathodes towards stable electrochemical performance and prolonged service life at high potential.

Electrochemical Synthesis of Polyaniline/metal-based Anodes and Their Use in Microbial Fuel Cell

A limited number of metals may be suitable as bioanode material: noble metals, such as gold and platinum, could be the optimum choice being electrochemically inert in the operational potential window of the bio-electrochemical system. However, high costs limit their wide scale application. Even though its antimicrobial nature, copper is being considered as a promising alternative anode material, due to its high conductivity, that allows minimising the electrode material costs. Literature research indicated that high-performing electrochemically active biofilms may be grown on this metal.In the present work, gold and copper substrates have been coated by a conductive polymer (PANI), using a layer – by – layer procedure: surface grafting by reduction of 4-nitrobenzendiazonium salt was followed by reduction of nitro- to amino-groups; PANI was electrodeposited on this under-layer.The synthesized anodes were tested as working electrodes (WE) in a microbial fuel cell fed with anaerobic sludge and acetate; to assess the growth of the biofilm on the WE surface, the trend of the bioelectrocatalytic current of acetate oxidation was monitored over time. Cyclic voltammetries reveal the presence of typical redox couples related to the presence of electroactive microorganisms on the electrode surface. Preliminary data show bioelectrochemical activity on polyaniline-coated metal surfaces.

Heteroatoms doped porous carbon derived from waste potato peel for supercapacitors

In terms of supercapacitor electrode materials, biomass derived carbon electrodes are gaining much attention as a viable option for practical applications. Obtaining unique carbon nanostructures can significantly promote the electrochemical performance. Waste potato peel (WPP) derived hierarchical porous carbon materials with heteroatoms doping can boost the development of efficient and economical electrodes with improved charge storage capabilities. Here, we report sulfur and phosphorus co-doped porous activated carbon (S, P/PAC) as an effective supercapacitor electrode material with a large specific surface area. More importantly, the S, P/PAC not only acquires dual heteroatoms doping for promoting capacitive performance but also possesses fascinating porous features which are beneficial to shorten ion adsorption distances and enlarge the contact area. The electrode exhibits a high specific capacitance of 323 F g−1 at 1 A g−1, acceptable rate capability and good cycling stability after 5000 cycles. The constructed symmetric supercapacitor device with a wide operational potential window of 1.6 V achieves a maximum energy density of 45.5 Wh kg−1 at a power density of 800 W kg−1 with an improved long cycle life (94.3 retention after 10 000 consecutive cycles).

A new formula for the faradaic fraction used to estimate the coulombic hysteresis in the charge/discharge profiles of electrochemical energy storage systems

The present work is concerned with the charge hysteresis frequently observed in the charge/discharge profiles of electrochemical capacitors (ECs) at the full charge voltage. In energy storage applications, the concept of operating potential window (OPW), envisioned as the potential window in which ECs are not deteriorated, is a key parameter for addressing the problems of both the life cycle and the energy density, but its determination remains arbitrary in nature due to its experimental determination based on kinetic parameters. For ECs, it was proposed to define the OPW as the closed gap in potential limited by a preset faradaic fraction at both ends of the stability window to ensure that the faradaic degradation of the electrode/electrolyte interface is commensurate with the charge stored in the electrochemical double layer. By implication, the potential limits so obtained are closely related to the potential range covered to reach a preset value of the faradaic fraction, so that the initial potential from which the stability is evaluated is expected to have an impact on the OPW. Here, this source of error was explored in detail and a voltage-correction is proposed to make the faradaic fraction less sensitive to the initial potential.

Self-Growing Graphite Felt/Vanadic Oxide/Polyindole Ternary Composite as Binder-Free Electrode for Supercapacitor with 1.8 V Operating Potential Window and Excellent Electrochemical Performance

Self-Growing Graphite Felt/Vanadic Oxide/Polyindole Ternary Composite as Binder-Free Electrode for Supercapacitor with 1.8 V Operating Potential Window and Excellent Electrochemical Performance

Ag+ preintercalation enabling high performance AgxMnO2 cathode for aqueous Li-ion and Na-ion hybrid supercapacitors

Cations preintercalation is an effective way to optimize the crystal structure and improve the electrochemical behavior. An Ag+ pre-intercalated AgxMnO2 material with ultrafine nanowire structure is prepared by a facile hydrothermal method. With optimized crystalline microstructure including expanded interlayer space and introduced multivalence of Mn, the as-prepared material exhibits superior performance as cathode for both Li-ion and Na-ion hybrid supercapacitors (HSCs). High specific capacitance of 424.7 F g−1 in Li2SO4 electrolyte and 450.1 F g−1 in Na2SO4 system are achieved. The assembled HSCs using AgxMnO2 and activated carbon (YEC-8A) as electrode materials deliver a wide operation potential window of 0–2.2 V and a high energy density of 204.30 Wh kg−1 at the power density of 167.15 W kg−1 for Li-ion HSCs and 131.08 Wh kg−1 at 107.25 W kg−1 for Na-ion HSCs. Furthermore, the as-fabricated HSCs deliver excellent cyclic stability. The capacitance retention is 79.4% after 20,000 cycles for Li-ion HSC and 84.9% after 10,000 cycles for Na-ion HCS.

V2O5 nanosheets assembled on 3D carbon fiber felt as a free-standing electrode for flexible asymmetric supercapacitor with remarkable energy density

As an emerging energy storage device, supercapacitor is widely investigated owing to its excellent capability, quick charge-discharge and tremendous cycle life. The operation potential window, energy density and mass loading of supercapacitor must be taken into deep consideration for its practical application. In this work, an outstanding electrode based on CFF@V2O5 nanosheets was prepared. Then a free-standing asymmetric supercapacitor with CFF@V2O5 composite as positive electrode and CFF@AC as negative electrode was assembled. Owing to the functional groups produced on CFF after the activation, V2O5 nanosheets was immobilized. The composite exhibits remarkable specific capabilities of 1465 mF cm−2 (492 F g−1). The energy density of the assembled free-standing asymmetric supercapacitor achieves 0.928 mWh cm−3 when the power density is 17.5 mW cm−3. After 6000 charging-discharging cycles as under normal, bended and anti-bended conditions for respective 2000 cycles, the device retains 89.7% of the initial capacitance, exhibiting fascinating cycle stabilization. Finally, two devices linked series can lighten a LED of 1.8 V for 2 min after charging for 2.5 min, which is inspiring for the practical application and production of self-supporting asymmetric supercapacitors.

Exploring the interdependence between the coulombic, voltage and energy efficiencies

Recently, a consensus emerges on the relevance of using the energy efficiency (EE) as a metric to evaluate the operational potential window (OPW) of the electrochemical energy storage systems. Unfortunately, EE alone is not comprehensive enough to provide a full understanding of the electrochemical degradation at the electrode/electrolyte interface since both the coulombic efficiency (CE) and the voltage efficiency (VE) have an impact on EE. Here it is proposed to deconvolute EE into its CE and VE contributions by plotting the variations of CE with VE after their Neperian logarithmic transformation (Ln) in a polar coordinate system (ρ, α) where the modulus ρ can be approximated to Ln(EE) and the argument α represents the relative importance of the individual contributions of CE and VE. In the present case, results obtained with YP80F based electrodes in 1 M H2SO4 electrolyte demonstrate that the decrease in EE at both ends of the OPW is different in nature depending on whether the electrode serves as positive or negative electrode.

A generalized one-step in situ formation of metal sulfide/reduced graphene oxide nanosheets toward high-performance supercapacitors

Metal sulfides are promising candidates for supercapacitors, but their slow reaction kinetics hinders their electrochemical performance. Large electrochemical surface area and combination with conductive carbon are potential methods to improve their capacitive performance. However, seeking for a generalized and simple approach to prepare two-dimensional composites of metal sulfide and conductive carbon for supercapacitors is challengeable. Herein, a generalized and facile one-step pyrolysis method was designed for in situ growth of cobalt nickel sulfides (CoNi2S4) on reduced graphene oxide (rGO) nanosheets (CoNi2S4/rGO) under mild conditions. The as-prepared CoNi2S4/rGO materials possess the nanoparticles-on-nanosheets structure, which is effective to provide a myriad of active sites and optimized electron/ion diffusion pathway. Benefiting from those advantages, the resultant CoNi2S4/rGO electrodes exhibit impressed specific capacitances of 1526 and 988 F g−1 at 2 and 20 A g−1, respectively. The supercapacitors based on CoNi2S4/rGO showcase an operation potential window of 1.6 V, and energy density of 54.8 W h kg−1 at the power density of 798 W kg−1. The capacitance retention of the supercapacitor is about 93.7% after 8000 cycles at 3 A g−1. Moreover, a series of metal sulfide/rGO hybrids are obtained by this generalized strategy, which could be extended to construct electrode materials for various energy devices.

Supercapacitors with Extended Operating Potential Window

Electrolytes in supercapacitors (SCs) play an important role on their energy storage capacity. The electrochemical stability of electrolytes defines the operating potential of the SCs and their viscosity defines a power density of the SCs. In the present work, a new method for composing an electrochemically stabile but high viscous ethylene glycol (EG) based electrolyte has been discussed. A viscosity of the EG then reduced by dissolving an inorganic lithium tetrafluoroborate (LiBF4) salt. The affect of the concentration of the salt on EG viscosity has been investigated. The best capacitive performance of activated carbon (AC) based electrodes with large operating potential (-1.7V to 1.7V) was found in 3M LiBF4/EG electrolyte.

Electrolyte Reactivity in the Double Layer in Mg Batteries: An Interface Potential-Dependent DFT Study.

The electrochemical degradation of two solvent-based electrolytes for Mg-metal batteries is investigated through a grand canonical density functional theory (DFT) approach. Both electrolytes are highly reactive in the double layer region where the solvated species have no direct contact with the Mg-surface, hence emphasizing that surface reactions are not the only phenomena responsible for electrolyte degradation. Applied to dimethoxyethane (DME) and ethylene carbonate (EC), the present methodology shows that both solvents should thermodynamically decompose in the double layer prior to the Mg2+/Mg0 reduction, leading to electrochemically inactive reaction products. Based on thermodynamic considerations, Mg0 deposition should not be possible, which contrasts with experiments, at least for DME-based electrolytes. This apparent contradiction is here addressed through the rationalization of the electrochemical mechanism underlying solvent electroactivation. An extended operation potential window (OPW) is extracted, in which the Mg2+/Mg0 reduction can compete with electrolyte decomposition, thus enabling battery operation beyond the solvated species thermodynamic stability. The chemical study of the degradation products is in excellent agreement with experiments and offers rationale for the Mg-battery failure in EC electrolyte and capacity fade in DME electrolyte. The potential-dependent approach proposed herein is thus able to successfully tackle the challenging problem of interface electrochemistry. Being fully transferable to any other electrochemical systems, this methodology should provide rational guidelines for the development of viable electrolytes for multivalent batteries and, more generally, energy conversion and storage devices.

FeP Quantum Dots Confined in Carbon‐Nanotube‐Grafted P‐Doped Carbon Octahedra for High‐Rate Sodium Storage and Full‐Cell Applications

AbstractTransition metal phosphides (TMPs) possess high theoretical sodium storage capacities, but suffer from poor rate performance, due to their intrinsic low conductivity and large volume expansion upon sodiation/desodiation. Compositing TMPs with carbon materials or downsizing their feature size are recognized as efficient approaches to address the above issues. Nevertheless the surface‐controlled capacitive behavior is generally dominated, which inevitably compromises the charge/discharge platform, and decreases the operational potential window in full‐cell constructions. In this work, a novel architecture (FeP@OCF) with FeP quantum dots confined in P‐doped 3D octahedral carbon framework/carbon nanotube is rationally designed. Such structure enables a simultaneous enhancement on the diffusion‐controlled capacity in the platform region (2.3 folds), and the surface‐controlled capacity in the slope region (2.9 folds) as compared to that of pure FeP. As a result, an excellent reversible capacity (674 mAh g−1@ 0.1 A g−1) and a record high‐rate performance (262 mAh g−1 @ 20 A g−1) are achieved. A full‐cell FeP@OCF// Na3V2(PO4)3 is also constructed showing an outstandingly high energy density of 185 Wh kg−1 (based on the total mass of active materials in both electrodes), which outperforms the state‐of‐the art TMP‐based sodium‐ion battery full cells.

Improving the rate capability of ultrathin NiCo-LDH nanoflakes and FeOOH nanosheets on surface electrochemically modified graphite fibers for flexible asymmetric supercapacitors

A fiber asymmetric supercapacitor system is designed with NiCo-LDH nanoflakes and FeOOH nanosheets anchored on electrochemically activated graphite fibers as positive electrode and negative electrode, respectively. Due to the formation of COMetal bonding, the oxygen-functionalized carbon on electrochemically activated graphite fibers can bind strongly with NiCo-LDH and FeOOH, which assists in establishing the fast electron transfer routes and fluent ion transport avenues. Both NiCo-LDH and FeOOH anchored on electrochemically activated graphite fibers display a high rate performance, 80% and 87.3% of the electric capacity can be reserved with the current density increasing from 2 to 20 A g−1 and 2 to 10 A g−1, respectively, while the NiCo-LDH and FeOOH deposited on untreated graphite fibers can only retain 45% and 40%. The fabricated novel solid-state fiber asymmetric supercapacitor device exhibits an expanded operation potential window of 1.8 V with a maximum energy density (130 W h kg−1) when the power density is 1.8 kW kg−1. Furthermore, a high energy density (81 W h kg−1) is still achieved at a superhigh power density (10.8 kW kg−1). Additionally, a good cycling stability of the solid-state fiber asymmetric supercapacitor can be obtained (90% capacity retention after 10,000 cycles).

Direct growth of nickel-cobalt oxide nanosheet arrays on carbon nanotubes integrated with binder-free hydrothermal carbons for fabrication of high performance asymmetric supercapacitors

A high performance asymmetric supercapacitor (ASC) has been fabricated by using nickel oxide-cobalt oxide nanosheets (NiO–CoO NSs), which were directly grown on carbon nanotubes (CNTs) and hydrothermal carbon spheres (HTCs) as positive and negative electrodes, respectively. Both electrode materials are binder-free prepared by using a catalytic chemical vapour deposition (CVD) approach followed by a facile hydrothermal method for cathode and a one-step environmental-friendly route called hydrothermal carbonization for anode. Using NiO–CoO NSs@CNTs and HTCs, which were directly grown on Ni foam, not only leads to a very small equivalent series resistance, but also provides an impressive capacitive performance. The assembled ASC exhibits remarkable capacitive performance over a broad operational potential window ensuring outstanding energy densities (84.625 Wh. kg−1 at 3 A g−1). Other noteworthy features of the prepared supercapacitor include its superior power density (7810 W kg−1 at 9.5 A g−1) and cycling stability with capacitance retention almost over 80% after 6000 cycles. These electrochemical results point to NiO–CoO NSs@CNTs-Ni//HTCs-Ni based asymmetric supercapacitors as a promising remedy for energy crisis and environmental deterioration problems.

Comparison of Two Sustainable Counter Electrodes for Energy Storage in the Microbial Rechargeable Battery

AbstractRecently, the microbial rechargeable battery (MRB) has been proposed as a potentially sustainable and low‐cost electrical energy storage technology. In the MRB, bioelectrochemical CO2 reduction and subsequent product oxidation has successfully been combined in one integrated system. However, finding a suitable counter electrode is hindering its further development. In this work, we have tested two alternative counter electrodes in duplicate‐namely, i) oxygen/water and ii) a capacitive electrode‐for use in the MRB platform. During daily charge/discharge cycling over periods of 11 to 15 days, experimentally obtained energy efficiencies of 25 and 3.7 % were reported when using the capacitive and the oxygen/water electrodes, respectively. Large overpotentials, resulting in a voltage efficiency of 15 % and oxygen crossover leading to coulombic efficiencies of 25 % caused the considerably lower efficiency for the oxygen/water systems, despite the theoretical higher voltage efficiency. Although the capacitive electrode equipped systems performed better, energy density is limited by the operational potential window within which capacitive systems can operate reliably. Microbial community analysis revealed dominant presence of Geobacter in the bioanode and Selenomonadales in the biocathode. These results do not necessarily bring practical application of the MRB closer, but they do provide new insights in the working principle of this new technology.

Ultra-small Ni-VN nanoparticles co-embedded in N-doped carbons as an effective electrode material for energy storage

As a class of emerging anode material, much attention has been dominated to vanadium nitride (VN) in view of its high specific capacitance, exceptional conductivity and proper operating potential window (OPW). Herein, ultra-small Ni nanoparticles (NPs) and VN NPs have been embedded in N-doped carbons (NCs) nanosheets as an anode material for supercapacitors (SCs), making the good use of the conductivity of Ni NPs and NCs nanosheets, as well as the excellent specific capacitance of VN NPs, the novel 3-Ni/VN/NCs-7 composite electrode exhibits an admirable specific capacitance of 236 F g−1 at 1 A g−1. Furthermore, the (−) 3-Ni/VN/NCs-7//CuCo2O4 (+) asymmetrical supercapacitor (ASC) and 3-Ni/VN/NCs-7 symmetrical supercapacitor (SSC) have been assembled, respectively. The (−) 3-Ni/VN/NCs-7//CuCo2O4 (+) ASC device shows a capacitance of 65 F g−1 at a large OPW of 1.8 V. More importantly, it achieves an energy density of 25 Wh kg−1 at 930 W kg−1 and a superb cycling stability (73.5% retention after 5000 cycles). Strikingly, the 3-Ni/VN/NCs-7 SSC device not only presents surpassing cycling behavior, but also possesses favorable energy density of 13.4 Wh kg−1 at a power density of 600 W kg−1, which is superior to most of the carbon-based materials.

Flexible and Conductive Carbonized Cotton Fabrics Coupled with a Nanostructured Ni(OH)2 Coating for High Performance Aqueous Symmetric Supercapacitors

Flexible and wearable supercapacitor (SC) fabrics have received considerable research interests recently. However, their high hydrophobicity, poor conductivity, inferior capacitance, and low energy density remain a bottleneck to be solved. Herein, a highly flexible and conductive carbonized cotton fabric (CCF) covered by a unique nanostructured Ni(OH)2 layer is fabricated via a facile high-temperature carbonization process, followed by an electrochemical deposition (ED) treatment. The nanostructured Ni(OH)2 greatly improves the hydrophilicity of CCF to promote electrolyte penetration and offers abundant electroactive sites, leading to dramatically increased specific capacitance and operating potential window (OPW). The resultant Ni(OH)2@CCF is then applied as the electrode for an aqueous symmetric SC device. This device has an OPW of 1.4 V and exhibits a high specific capacitance of 131.43 F g–1 at the current density of 0.25 A g–1 with a high energy density (35.78 Wh kg–1 at a power density of 0.35 kW kg...

High-performance flexible MnO2@carbonized cotton textile electrodes for enlarged operating potential window symmetrical supercapacitors

MnO2-based composite has attracted wide attention because of its high power density, and long cycle life as electrodes for energy storage devices. However, its instability at lower electrochemical potential limits its operating potential window (OPW) and impedes its further development as a promising electrode material for symmetrical supercapacitors. Herein, flexible MnO2@carbonized cotton textile electrodes (MCCT) is designed and demonstrated via a facile in-situ low-temperature chemical reaction method. Remarkably, an excellent electrochemical stability under negative polarization is obtained, delivering a large OPW of 1.6 V. In addition, the flexible MCCT exhibits a high areal capacitance of 526.25 mF cm−2 (751.78 F g−1) at 1 mA cm−2 coupled with good rate performance (237.6 mF cm−2 at 50 mA cm−2), as well as high cycle retention (99.7% after 10000 cycles). The operation voltage of symmetrical supercapacitors is expanded to 1.6 V, revealing a maximum energy density of 5.71 mWh cm−3 (0.057 mWh cm−2) and a high power density of 3.97 W cm−3 (39.71 mW cm−2). As a representative of high operation voltage MnO2 composite electrode, the flexible MCCT has potential for state-of-the-art energy storage devices.

Potential-Dependent, Switchable Ion Selectivity in Aqueous Media Using Titanium Disulfide.

The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl2 . We obtained a molar selectivity of Mg2+ over Cs+ of 31 (strong Mg preference) in the potential range between -396 mV and -220 mV versus Ag/AgCl. By adjusting the operational potential window from -219 mV to +26 mV versus Ag/AgCl, Cs+ was preferred over Mg2+ by 1.7 times (Cs preference).