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The design and construction of active centres are key to exploring advanced electrocatalysts for oxygen evolution reaction (OER). In this work, we demonstrate the in situ construction of point defects on CrOOH by Ni doping (Ni-CrOOH/NF). Compared with pure CrOOH/NF, Ni-CrOOH/NF showed enhanced OER activity. The effect of the amount of Ni introduced on the OER performance was investigated. Ni0.2-CrOOH/NF, the best introduction of Ni, uses a low overpotential of 253 mV to achieve a current density of 10 mA cmâ€"2 with a high turnover frequency of 0.27 sâ€"1 in 1.0 M NaOH. In addition, the electrocatalytic performance of Ni0.2-CrOOH/NF showed little deterioration after 1000-cycle cyclic voltammetry scanning. In the potentiostatic test, activity was stable for at least 20 h. [ABSTRACT FROM AUTHOR]

The article reports that measuring the nucleation over potential in Lithium Metal Batteries, and researchers have studied rechargeable Li metal batteries (LMBs) as a promising reversible energy storage system due to the high theoretical specific capacity of the lithium metal negative electrode.

The potassium-oxygen battery (KOB) is a new type of metal-oxygen battery with high rechargeability and long cycle life. Currently, the energy density is rather limited and must be improved for KOB to become a viable energy storage technology for practical applications. In this study, a two-dimensional, multiphase KOB model is developed to design an optimized cathode structure. The model is validated and is used to study the influence of cathode porosity, surface area, and thickness on the discharge behavior. Higher cathode porosity and surface area are found to increase the discharge capacity and lower the discharge overpotential. However, using a microporous cathode may not be ideal for KOB. The electronic transport properties of the discharge product KO2 are assessed, suggesting an effectively higher conductivity of KO2 than previously predicted. In consequence, the formation of large KO2 deposits with several μm thickness may effectively inhibit oxygen transport in microporous materials. Thus, a hierarchical cathode porosity together with an optimized current collector design may be the key to significantly higher discharge performance. [ABSTRACT FROM AUTHOR]

A major drawback of today's Li-ion batteries is inadequate performance at low temperatures, which slows down the userfriendliness and thus market expansion of electromobility. Due to the complexity of the system, many possible low-temperature limitations and various dependencies on the operating conditions exist. As a result, the origin of the performance limitations at low temperatures is still controversial and not completely clarified to date. We herein demonstrate a comprehensive analysis of the performance limitations at low temperatures using a LiNi0.5Co0.2Mn0.3O2-based cathode as an example. To separate the overpotential phenomena, the complex system is decomposed as much as possible and individual aspects are investigated separately. Complementary electrochemical methods are employed to quantify the C-rate and SOC dependence of the individual overpotential phenomena. Based on the comprehensive analysis of the intercalation kinetics, mass and charge transport, we obtain a coherent picture of the performance limitations as a function of operating conditions. This can serve for targeted optimization or parameterizing models to simulate battery behavior. However, the present work is not only concerned with identifying the lowtemperature limits of the system studied but also shows how the rate-determining step of the electrode reaction can be efficiently identified as a function of temperature, SOC, and C-rate, which can serve as a guide for future work. [ABSTRACT FROM AUTHOR]

UO2 is an important nuclear fuel, also has potentials in the field of functional materials. In this work, the electrochemical behavior of UO2 2+ crystal appeared. The average current density had a positive correlation with the grain size of cathodic products, and it increased first and then decreased with the increase of overpotential, having a maximum at −0.6 V (vs Pt(II)/Pt). On the other hand, the uranyl concentration also had a positive correlation with the grain size of the cathodic product without changing the integrity of grain morphology. For the preparation of large and granular UO2 crystal appeared. The average current density had a positive correlation with the grain size of cathodic products, and it increased first and then decreased with the increase of overpotential, having a maximum at −0.6 V (vs Pt(II)/Pt). On the other hand, the uranyl concentration also had a positive correlation with the grain size of the cathodic product without changing the integrity of grain morphology. For the preparation of large and granular UO2 crystal, the reduction potential of −0.2to −0.3 V (vs Pt(II)/Pt), as well as the uranyl concentration of greater than 2.0 wt% were recommended, and UO2 crystal with average grain size greater than 500 μm and high uniformity could be finally obtained. [ABSTRACT FROM AUTHOR]

Modifying TiO2 with noble metal nanocrystals via photodeposition is an efficient strategy for improving its photoelectrochemical performance. The photodeposition process is still an open question and needs to be further clarified. Herein, we selectively deposited Pb or Cu on Au nanocrystals supported by the branched TiO2 nanostructure to prepare the Au@metal/semiconductor photocatalyst. The results revealed the dependence of the photodeposition mechanism on the type of electrolyte. In an acetic acid-acetate system Pb deposition on Au undergoes both underpotential and overpotential deposition processes, while Cu deposition on Au only displays overpotential deposition behavior. In a nitric acid-nitrate system, however, only underpotential deposition occurs during Pb deposition on Au. Surface microstructure analysis shows that the Cu-Au bimetallic nanocrystals form a typical core/shell structure, while Pb-Au has a Janus structure. The lower ideality factor of Cu/Au-TiO2 than that of Pb/Au-TiO2 indicates an ideal photoelectrode/electrolyte junction for photoelectrochemical reaction, further enhancing carrier transport with a short recombination lifetime of 0.899 ms. Finally, the evaluation of photoelectrochemical performance further confirmed Cu/Au-TiO2 (Au loading amount of about 0.55 mass%) as the optimal photocatalyst, with an efficiency as high as 0.29% or about 1.93 times that of Au-TiO2 (0.15%). [ABSTRACT FROM AUTHOR]

Amorphous FeOOH nanosheets were electrodeposited onto nickel foam (NF)-supported NiCo2S4 nanotube arrays to form a threedimensional FeOOH@NiCo2S4/NF core-shell heterostructure, which is a bifunctional electrocatalyst for overall water splitting (OWS). FeOOH@NiCo2S4/NF pre-catalyst showed a good electrocatalytic activity and stability for the oxygen evolution reaction in a 1 M KOH solution, with an overpotential of 228 mV at 10 mA cm-2, a Tafel slope of 44.03 mV dec-1 and a durability of more than 50 h. The overpotential of the hydrogen evolution reaction was 112 mV at 10 mA cm-2. The OWS performance was very good because it only required a voltage of 1.56 V at 10 mA cm-2. The 3D core-shell structure with a large electrolyte contact interface, the good electronic conductivity, the large surface exposure of active sites by the NiCo2S4 nanotube arrays, and the strong electron interaction at the heterogeneous interface between FeOOH and NiCo2S4 are the key factors for the excellent performance. [ABSTRACT FROM AUTHOR]

The selectivity of CO2 electrolyzers has hitherto mainly been associated with the cathode selectivity. A few recent studies have shown that the nature of the polymer membrane can impact the system ionic selectivity, with anion exchange membranes (AEM) leading to high crossover of (bi)carbonates during operation and a CO2 pumping effect. In the present work, we investigate and compare CO2 crossover during operation through an AEM and a bipolar membrane (BPM) in a flow cell fed with gaseous CO2. With AEM, starting with 1 M KHCO3 catholyte and 1 M KOH anolyte, the anolyte pH rapidly drops from 14 to 8. This triggers an increase of 1.2 V in cell voltage at 45 mA·cm-2, due to increased OER overpotential and anolyte resistance. Steady-state operation at 45 mA·cm-2 with the AEM results in a CO2/O2 ratio of 3.6 at the anode. With BPM, the anolyte pH decreases more slowly, and the CO2/O2 ratio at the anode under steady-state at 45 mA·cm-2 is only 0.38. Overall, the cell voltage is lower with the BPM than with the AEM at steady-state. These results show the potential of BPMs to mitigate carbon crossover, which could be further reduced by optimizing their design. [ABSTRACT FROM AUTHOR]

Oxygen vacancies are considered to be an important factor to influence the electronic structure and charge transport of electrocatalysts in the field of energy chemistry. Various strategies focused on oxygen vacancy engineering are proved to be efficient for further improving the electrocatalytic performance of Co3O4. Herein, an optimal Co3O4 with rich oxygen vacancies have been synthesized via a two-step process combining solution reduction and Ni2+ impregnation. The as-prepared electrocatalyst exhibits an enhanced oxygen evolution performance with the overpotential of 330 mV at the current density of 10 mA cmâˆ'2 in alkaline condition, which is 84 mV lower than that of pristine one. With the increasing of oxygen vacancies, the charge transfer efficiency and surface active area are relatively enhanced reflected by the Tafel slope and double-layer capacitance measurement. These results indicate that combination of solution reduction and heteroatom doping can be a valid way for efficient metal oxides-based electrocatalyst development by constructing higher concentration of oxygen vacancy. [ABSTRACT FROM AUTHOR]

Development of non-noble multi-metallic electrocatalyst with high oxygen evolution reaction (OER) activity via a simple and low-cost method is of great importance for improving the efficiency of water electro-chemical splitting. Herein, a solution impregnation strategy was proposed to synthesize novel FeNi-doped Co-ZIF-L trimetallic hybrid electrocatalyst using Co-ZIF-L as sacrificial templates and Fe and Ni ions as etchants and dopants. This synthetic strategy could be realized via the etching-coprecipitation mechanism to obtain an amorphous hybrid containing multi-metal hydroxides. The as-prepared electrocatalyst loaded on Ni foam displays a low overpotential of 245 mV at 10 mA·cmâˆ'2, a small Tafel slope of 54.9 mV·decâˆ'1, and excellent stability at least 12 h in the OER process. The facile and efficient synthetic strategy presents a new entry for the fabrication of ZIFs-derived multi-metallic electrocatalysts for OER electrocatalysis. [ABSTRACT FROM AUTHOR]

In order to reduce the overpotential of hydrogen evolution reaction (HER), the ternary coating Coâ€"Wâ€"P was deposited on the surface of the nickel foam by electrochemical deposition to obtain a highly active electrode. Based on the measured double layer capacitance (C dl) and HER activity, there is volcanic behavior between the intrinsic activity of Coâ€"Wâ€"P and the Co:W ratio in the electrolyte. W and P play different roles in the formation of nanoparticles, and work together to achieve the large electrochemical surface area and excellent activity. When applied to the modification of other catalysts (Niâ€"P and Feâ€"P), the higher intrinsic activity was obtained after the introduction of W. [ABSTRACT FROM AUTHOR]

An electrochemical catalyst with efficient, stable, inexpensive energy storage for oxygen evolution and hydrogen evolution has raised global concerns on energy, calling for high-performance materials for effective treatments. In this paper, novel amorphous polymetallic doped CeO2 particles were prepared for an electrochemical catalyst via homogeneous phase precipitation at room temperature. Metal ions can be easily embedded into the oxygen vacancies formed by CeO2, and the the electron transport capacity of the CeO2/NiFeCo electrocatalyst is improved owing to the increase in active sites. In addition, the amorphous CeO2/NiFeCo composite material is in a metastable state and will transform into different active states in a reducing or oxidizing environment. Furthermore, the amorphous material drives oxygen evolution reaction (OER) through the lattice oxygen oxidation mechanism (LOM), while LOM can effectively bypass the adsorption of strongly related intermediates in the adsorbate release mechanism, thus promoting OER procedure in a timely manner. As a result, CeO2/NiFeCo exhibits a lower oxygen evolution overpotential of 260 mV at 10 mA cm−2 current density, which shows a predatorily competitive advantage compared with commercially available RuO2 and the reported catalysts. [ABSTRACT FROM AUTHOR]

We perform first principles calculations to investigate the catalytic behavior of C9N4 nanosheet for water splitting. For the pristine C9N4, we find that, at different hydrogen coverages, two H atoms adsorbed on the 12-membered ring and one H atom adsorbed on the 9-membered ring show excellent performance of hydrogen evolution reaction (HER). Tensile strain could improve the catalytic ability of C9N4 and strain can be practically introduced by building C9N4/BiN, and C9N4/GaAs heterojunctions. We demonstrate that the HER performance of heterojunctions is indeed improved compared with that of C9N4 nanosheet. Anchoring transition metal atoms on C9N4 is another strategy to apply strain. It shows that Rh@C9N4 exhibits superior HER property with very low Gibbs free energy change of –30 meV. Under tensile strain within ∼2%, Rh@C9N4 could catalyze HER readily. Moreover, the catalyst Rh@C9N4 works well for oxygen evolution reaction (OER) with an overpotential of 0.58 V. Our results suggest that Rh@C9N4 is favorable for both HER and OER because of its metallic conductivity, close-zero Gibbs free energy change, and low oneset overpotential. The outstanding performance of C9N4 nanosheet could be attributed to the tunable porous structure and electronic structure compatibility. [ABSTRACT FROM AUTHOR]

This paper reports a simple, unique, and eco-friendly fabrication method for composites of molybdenum carbide nanoparticles dispersed on nitrogen-doped carbon supports (Mo2C/NC) by annealing of MoCl5, kraft lignin, urea at 750 °C under N2 flow in one pot. Kraft lignin, a by-product from the pulping industry, acted as carbon source for formation of Mo2C and carbon support and a cheap urea provided a nitrogen source for nitrogen-doping to carbon supports. The resultant Mo2C/NC catalysts exhibited high activity with small overpotential (131 mV at 10 mA cm-2) and excellent stability over 3000 potential cycles for hydrogen evolution reaction (HER) in alkaline media. Enhanced HER activity of Mo2C/NC catalyst was attributed to highly active Mo2C nanoparticles and nitrogen-doped carbon (N-doped carbon) providing large surface area and high electrical conductivity. [ABSTRACT FROM AUTHOR]

Na–O2 batteries have been attracting attention owing to their intrinsically high theoretical energy density. Several Na–O2 systems can produce various discharge products with different electrochemical performances. For example, sodium superoxide (NaO2) batteries have a low overpotential, and sodium peroxide (Na2O2) batteries have a high capacity. Studies of Na2O2 batteries are relatively scarce, owing to the difficulty of forming pure Na2O2 discharge products. A pure Na2O2 battery system is highly desirable for fully exploring the formation and decomposition of Na2O2 in Na–O2 batteries and evaluating their potential. This model of a Na2O2 battery should also be compatible with in situ characterization. To this end, we constructed a simple rechargeable all-solid-state Na2O2 battery. Using a nanoporous gold film as the cathode and Na–β″-Al2O3 as a solid electrolyte, we assembled a Na–O2 battery that can produce and decompose Na2O2. The all-solid-state Na–O2 battery is a simple model for conducting in situ ambient-pressure x-ray photoelectron spectroscopy (APXPS) investigations. The battery can be cycled at a low overpotential (≈450 mV). Qualitative and quantitative analyses of the APXPS and Raman results demonstrated that Na2O2 was the main discharge product and its transformation occurred during the charge and discharge periods. The operando investigation of this type of all-solid-state Na2O2 battery can help in the comprehensive exploration of the potential of Na–O2 batteries. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) is a critical reaction in electrolysis and photoelectrolysis of water to generate and store clean energy. Therefore, the development of low-cost and efficient electrocatalysts for the OER is of great scientific and technological importance. Although promising iron oxide-based electrocatalysts have been recently developed for the OER, an in-depth experimental and theoretical analysis of the OER mechanism on iron oxide-based electrocatalysts is still needed to provide guidelines to optimize the performance of iron oxide-based electrocatalysts further. To address this need, we synthesized a series of monodisperse iron oxide nanoparticles to analyze their intrinsic OER activities. Using nanoparticles of the same size but different crystallinity, we show that amorphous iron oxide nanoparticles have better OER activity than crystalline ones. The size effect studies further revealed that the edge/defect sites are the active sites for the OER. Density functional theory calculations demonstrated that the edge/defect sites provide bridge sites to adsorb OER intermediates, resulting in low OER overpotential. These calculations confirm that the high OER activity of amorphous nanoparticles results from a high concentration of defect sites on their surface. These results provide novel strategies to increase the performance of iron oxide-based and likely other oxide-based OER electrocatalysts. [ABSTRACT FROM AUTHOR]

The onset of lithium nucleation and subsequent plating on the graphite particles is a challenging problem in the design of fast charging batteries. A variational phase field formulation has been developed to initiate the nucleation of lithium particles based on the local plating overpotential conditions and allowing to predict the lithium plating under fast charge conditions. The model demonstrates that (1) at small charging current density, there is no nucleation and plating of metallic lithium, and no effect on the intercalation kinetics; (2) at intermediate charging current densities, initially a stagnated growth from isolated lithium nuclei is observed followed by a continuous growth of metallic lithium; (3) at large charging current densities, a continuous growth of metallic lithium resulting from coalescence of small nuclei into large nuclei decreases the excess interfacial energy, which in turn slows down the intercalation kinetics. The formulation sets the stage for predicting and evaluating the influence of various electrolyte compositions, graphite particle morphologies and electrode loadings on the nucleation and plating dynamics under fast charge conditions. [ABSTRACT FROM AUTHOR]

Composites of flake-shaped Si nanopowder from swarf treated as an industrial waste and ultrathin graphite sheets (GSs) (Si:C = 5:1 wt) are used in Li-ion battery electrodes. Si nanopowder is dispersed and wrapped between GSs fabricated from expanded graphite. The delithiation capacity of the Si/GS composite electrode during 300 cycles is 1.69 - 0.83 mAh cm-2 (0.5 C), while that of the electrode with C-coated Si nanopowder (Si:C = 10:1 wt) fabricated in C2H4 is 1.55 ~ 0.72 mAh cm-2. The series resistances (Rs) for the Si/GS electrode are a half and two-thirds of those for the C-coated Si electrode at the 6th and 300th cycles, respectively. The charge transfer resistance (Rct) for the Si/GS electrode is two-thirds of that for the C-coated Si electrode at the 300th cycle. GS bridges are formed across cracks, and suppress cracking and peeling-off of Si. Agglomerated GSs wrap Si/GS composites, and work as stable frameworks that secure electrolyte paths and buffer spaces for Si volume change. In the C-coated Si electrodes, Si frameworks fuse after the 300th cycle, leading to low delithiation capacities. The delithiation capacity of 4 mAh cm-2 for more than 75 cycles is achieved by the Si/GS electrode at the current density of 5 mA cm-2 with delithiation limitation at 1200 mAh g-1. [ABSTRACT FROM AUTHOR]

Core-shell and core-gradient hybrid cathode materials for lithium-ion batteries display enhanced rate capability over their homogeneous counterparts. The apparent enhancement of transport is explained herein as resulting from advective flow of Li+ from the higher free-energy core towards the lower free-energy shell compositions. First-principles analysis of a planar model of these hybrid structures concludes that the inbuilt free-energy gradient enhances the Li+ de-intercalation process by reducing the average overpotential during extreme fast-charging. Analysis of representative LiNi0.8Co0.1Mn0.1O2||LiNi0.4Co0.2Mn0.4O2 core/shell reveals: (i) an optimal components ratio exists that maximizes storage capacity during fast-charging and (ii) components should be selected with appreciably large chemical potential difference between the core and shell to further exploit the free-energy gradient effects provided volume ratios are optimized against the potential gradient. In the case of NCM811||NCM424 studied herein, a balanced (ca. 40/60 vol.%) structure appears optimal. This finding indicates that the shell must not necessarily be confined to a thin chemically-protective coating; higher relative volumes of the lower free-energy shell may provide performance benefits at high-rates. The presented insights will serve towards optimizing and developing high capacity, more rate capable core-shell particles for extreme fast charging batteries. [ABSTRACT FROM AUTHOR]

Developing robust and low-cost electrocatalysts for efficient hydrogen evolution reaction (HER) through facile methods is of great significance to the energy and environment sustainability. Herein, a simple one-pot solvothermal method has been developed to synthesize a hybrid material of nickel@nickel carbide (Ni@Ni3C) nanochain embedded in a network of single-walled carbon nanotubes (SWCNTs). The electrochemical experiments indicate that the composite of Ni@Ni3C-SWCNTs has the HER electrocatalytic performance comparable to that of Pt/C in an acidic medium (0.5 M H2SO4). The overpotential to reach a current density of 10 mA cm−2 in 0.5 M H2SO4 is only −74 mV with a Tafel slope of 46.9 mV dec−1. Compared to Ni@Ni3C in the absence of SWCNTs, the enhanced HER performance of Ni@Ni3C-SWCNTs can be attributed to the increased intrinsic activity, improved electronic conductivity, and the electron coupling effect between Ni@Ni3C and SWCNTs. The Ni@Ni3C-SWCNTs composites also show high stability at the overpotential of −0.2 V for 24 h. This work provides guidance for the synthesis of high-performance non-precious metal electrocatalysts for HER. [ABSTRACT FROM AUTHOR]

Development of high-power lithium-ion batteries requires the optimization of the electrode kinetics of lithium-insertion materials to improve the rate capability of these devices. The rate capability of lithium-insertion electrodes is controlled by the concentration overvoltages that arise from changes in the Li+ concentration at the electrode/electrolyte interface. Two distinct rate-capability behaviors prevail depending on whether charge transport is limited by Li-ion mass transfer within the solid particles or within the interstitial spaces of the electrode. In this study, the diluted electrode method is employed to characterize the two types of ratecapability behavior exhibited by Li[Ni1/2Mn3/2]O4 (LiNiMO) electrodes. Low-LiNiMO electrodes exhibit better rate capabilities than high-LiNiMO electrodes indicating that lithium-ion transport is more effective (i.e., much faster) in the solid active material than in liquid electrolyte within the electrode pore. The results provide useful insights for understanding the electrode kinetics of lithium-insertion materials and designing electrodes for high-power lithium-ion batteries. [ABSTRACT FROM AUTHOR]

A Co–Mo–P–TiO2 composite electrocatalyst is electrodeposited for the first time and the hydrogen evolution reaction (HER) was characterized. The HER Tafel slope and exchange current density on the Co–Mo–P–TiO2 surface were compared with Co–Mo, Co–Mo–TiO2 and Co–Mo–P electrodeposits similarly prepared with comparable Co/Mo composition ratio. The Co–Mo–P–TiO2 exhibited a very low overpotential at −10 mA cm−2 of 31 mV in a 1 M NaOH electrolyte. The electrochemical active surface area was characterized, to show that the improvement is not merely due to surface roughness but is an intrinsic effect. The role of TiO2 and P differed in the Co–Mo electrodeposit. [ABSTRACT FROM AUTHOR]

Accurate determination of electrode overpotentials is essential to assess the performance of the electrode and understand the ratelimiting steps involved. Three-electrode configuration with the use of a reference electrode at a fixed potential is a standard way to measure overpotential of a specific electrode in liquid electrochemical systems. However, application of such three-electrode configuration to solid electrochemical cells for overpotential determination is not straightforward and requires extra caution. Here we report a theoretical Finite Element Analysis on the geometrical requirements for which the reference electrode can be applied to anode- or electrolyte-supported solid oxide button cells. The modeling results suggest that the symmetry of the working and counter electrodes is the key factor determining if a reference electrode is suitable to use. For anode-supported fuel cells with asymmetrical working and counter electrodes, reference electrode cannot be used under all circumstances. To use reference electrode for overpotential measurements, electrolyte-supported cells with symmetrical semicircular-shaped electrodes are preferred. A data processing procedure has also been presented to obtain the electrode overpotential from the measured potential using the three-electrode scheme in solid oxide electrochemical cells. [ABSTRACT FROM AUTHOR]

Membrane electrode assemblies (MEAs) with various catalyst-loading were made using a standard 46.5%Pt/KB (TEC10E50E) with the aim of developing low Pt-loading PEFCs. Since increase in concentration overvoltage for low catalyst-loading MEAs was observed, the mechanism of such increase in concentration overvoltage was considered, and then reducing concentration overvoltage was tried through two methods, by adding extra carbon and using a more hydrophobic carbon support, graphitized Ketjen black, within the cathode layer (CL). As a result, we have experimentally proven that increase in hydrophobicity of CL can promote water drain and reduce the concentration overvoltage for low catalyst-loading MEAs, and hydrophobicity is an important factor to be controlled within CL in addition to porosity. [ABSTRACT FROM AUTHOR]

Efficient and noble metal-free co-catalyst loading is an effective solution for separating and transferring photo-generated carriers and lowering the overpotential in photocatalytic H2 evolution activity. In this work, we designed and prepared a series of novel NiCoP/Mn0.3Cd0.7S (NCP/MCS) composites by modifying MCS nanorods with the co-catalyst NCP using a simple calcination method. Notably, the 10-NCP/MCS composite displays the optimum photocatalytic H2 evolution rate of 118.5 mmol g−1 h−1 under visible-light irradiation. This is approximately 3.39 times higher than that of pure MCS. The corresponding apparent quantum efficiency is 10.2% at 420 nm. The superior photocatalytic activity of the NCP/MCS composites can be attributed to the efficient separation of photogenerated carriers caused by the intimate heterojunction interface between NCP and MCS, smaller transfer resistance, and lower overpotential of NCP. Moreover, the NCP/MCS composites exhibit remarkable photostability. A plausible mechanism is proposed. [ABSTRACT FROM AUTHOR]

Ammonia synthesis, one of the most challenging chemical synthesis processes, plays a vital role in the development of human industry and agriculture. Compared with the industrial Harber–Bosch ammonia process with huge energy input and high CO2 emissions, the search for a resource-saving, environmentally-friendly ammonia synthesis alternative is extremely urgent. Electrocatalytic nitrogen reduction appears to be a good candidate. In this communication, we report the development of ruthenium nanoparticles as a highly efficient and durable nitrogen reduction reaction (NRR) electrocatalyst in acidic electrolyte under ambient conditions. Such electrochemical NRR catalyst exhibits a large NH3 formation rate (24.88 μg h–1 mg–1cat.) with Faradaic efficiency (0.35%) at −0.15 V versus reversible hydrogen electrode, outperforming many reported NRR electrocatalysts. Note that it exhibits high durability and stability during the entire electrochemical NRR process. [ABSTRACT FROM AUTHOR]

The construction of low-cost, high-performance electrocatalysts instead of platinum catalysts is critical to solving the energy crisis. Here, using simple electrospinning and hydrothermal methods, new MoS2 nanosheets on SrTiO3 nanofibers (NFs) and 2D SrTiO3@MoS2 heterostructure NFs are synthesized. In addition, SrTiO3@MoS2 heterostructure NFs are compared with bare SrTiO3 NFs and MoS2 nanosheets. Importantly, the prepared SrTiO3@MoS2 heterostructure shows better hydrogen-evolution reaction performance than other MoS2-based electrocatalysts with an overpotential of 165 mV at 10 mA cm−2, a Tafel slope of 81.41 mV dec−1, and long-term electrochemical durability of 3000 cycles. Therefore, the present work strongly demonstrates the positive synergy between SrTiO3 NFs and layered MoS2, and also provides a strategy for preparing low-cost and high-activity water-decomposition electrocatalysts. [ABSTRACT FROM AUTHOR]

PbO2-polyelectrolyte composite materials with a content of polyelectrolyte up to 13 wt% are formed by electrochemical deposition of PbO2, depending on the solution composition and the deposition conditions. Several assumptions concerning the mechanism of the formation of composites that taking into account the oxygen-containing species of lead non-fixed on the surface, located in the near-electrode zone have been proposed. The shape and size of the PbO2 crystals is altered by the presence of polyelectrolyte. The surface is more uniform and the crystals are smaller, there is not a pronounced crystallographic orientation, however the phase content remains practically the same as in the undoped sample. When using composite materials based on lead dioxide containing polyelectrolyte, selective changes in the overvoltage of oxygen evolution and electrochemical conversion of organic substances into aliphatic non-toxic compounds are observed. In this case, the electrocatalytic activity of composite materials depends on the nature and content of the polyelectrolyte additive in the oxide. [ABSTRACT FROM AUTHOR]

Current interrupt and galvanostatic EIS techniques were utilized in a complementary fashion to characterize the different sources of overpotential during anodic gas evolution. Room temperature anodic evolution of oxygen at a nickel working electrode in aqueous potassium hydroxide and the high temperature (348°C) anodic evolution of chlorine at a glassy carbon working electrode in molten (LiCl)57.5-(KCl)13.3-(CsCl)29.2 where investigatd. Combining of the two techniques enables to separate the total measured overpotential into its ohmic, charge transfer, and mass transfer components. Potential decay curves indicated that natural convection (due to both bubble evolution and density driven flow) was a major driving force in reestablishing equilibrium conditions at the working electrode surface. During oxygen evolution, charge transfer resistance dominated the total overpotential at low current densities, but as the current density approached ~100mA/cm², mass transfer overpotentials and ohmic overpotential became non-negligible. The mass transfer overpotential during chlorine evolution was found to be half that found during oxygen evolution. [ABSTRACT FROM AUTHOR]

The numerical analysis conducted in this study proposes a guideline to maximize the high-temperature effect, which is expected to reduce the electrolysis voltage of the polymer electrolyte membrane water electrolyzer. High-temperature operation is intuitively thought to reduce activation overvoltages. However, a further consideration predicts that high temperature, especially a temperature higher than the saturated temperature regulated in the operation pressure, decreases the liquid saturation and causes shortage of water, leading to a large increase in overvoltages. This high temperature problem is analyzed using the developed theoretical model, which considers gas/liquid behavior. The analysis suggests that, if the gas saturation in the anode catalyst layer is kept at or below 0.3 by increasing the pressure, liquid water in the catalyst layer is sufficient to OER catalytic ability regulated by exchange current density, demonstrating that the high-temperature effect works. According to this guideline, increasing the temperature with pressurization can monotonically reduce the anode activation overvoltage. For instance, raising the temperature from 100 to 120°C and raising the pressure from 0.13 to 0.22 MPa can prevent the gas saturation from increasing beyond 0.3 and allows the lower electrolysis voltage to vary from 1.57 to 1.51 V. [ABSTRACT FROM AUTHOR]

A triple phase boundary reaction site was formed in a direct carbon fuel cell (DCFC) when the char wetted by molten carbonate was in contact with the anode. It has been reported that gases such as CO2 covered the anode during discharge, leading to high anode overpotential. In this study, a press-type DCFC was developed to overcome this issue. By pressing the perforated anode on the carbon/carbonate-packed bed, it was expected that the carbon particles would remain in contact with the anode, while the gas products would be released through the perforation due to their buoyancy. As a result, the power output achieved with the press-type DCFC was higher and more stable as compared with the conventional DCFC. The anodic impedance spectra showed that the arc size of the press-type DCFC was much smaller than that of the conventional DCFC. This was particularly noticeable at the low frequencies, indicating the anode resistance caused by the mass transfer processes including the gas products and the contact between the anode and carbon particles to be significantly less in the press-type DCFC. [ABSTRACT FROM AUTHOR]

In a lithium ion battery, balancing of active materials is an essential requirement with respect to safety and cycle life. However, capacity oversizing of negative electrodes is associated with decrease of specific energy/energy density. In this work, the required trade-off between maximized specific energy and minimized risk of lithium plating is thoroughly investigated by evaluating underlying potential/voltage curves. The adjustment of targeted state of charge (SOC) for both, positive and the negative electrode, can be achieved by intentional selection of only two parameters: negative/positive electrode active mass ratio and charge cutoff voltage. For investigation and controlling reasons, specific charge capacity reveals to be a simple but effective tool to indirectly predict electrode potentials. While cell kinetics/overvoltage are influenced by both electrodes, specific capacity losses are affected by a single electrode. The latter only correlate with negative electrode's BET surface area as long as specific capacity losses of negative electrodes are higher compared to positive electrodes. Based on these insights, a more systematic performance and safety optimized handling of the trade-off between specific energy and safety risk can be realized. [ABSTRACT FROM AUTHOR]

Alkaline iron electrodes present some challenges for use in secondary batteries that are associated with low coulombic efficiency and discharge utilization. Low coulombic efficiency is correlated to the hydrogen evolution reaction that takes place during charge. In this work, we demonstrate rechargeable alkaline iron electrodes with significant capacity retention over 150 cycles with high efficiency by suppressing the hydrogen evolution with stannate. Adding stannate to the alkaline electrolyte when cycling the iron electrode drastically changes the electrochemistry. The additive brings on two advantageous attributes for the iron electrode: increased hydrogen evolution overpotential, and a flat and prolonged discharge curve at typical battery operation. These attributes were provided by a novel intermediate phase that was detected from in situ neutron diffraction measurements. This phase was only detected in situ while it decomposed ex situ, and indicated a solid solution constituted by some of the elements present in the electrode. [ABSTRACT FROM AUTHOR]

Recently, a new process for machining glassy carbon using electrochemical oxidation has been introduced. The process is effective in reducing cracks on the machined surface of glassy carbon, unlike conventional mechanical processes. This paper proposes a method for increasing the electrochemical oxidation rate in this process by applying a high overvoltage to improve the material removal rate. Suitable electrochemical conditions for applying high overvoltage were investigated. Experiments were conducted by varying the electrochemical conditions, including high overvoltage, to compare the material removal rates. The results showed that the material removal rate could be increased by using a high overvoltage. [ABSTRACT FROM AUTHOR]

The practically available specific energy of Li ion batteries (LIB) is highly depending on the used specific charge/discharge current, since the respective overpotentials of each electrode affect the two vital specific energy parameters, specific capacity and voltage. Focusing on the positive composite electrode as the specific energy bottleneck, the overall nature of the overpotential is discussed for the LiNi1/3Co1/3Mn1/3O2 (NCM) active material. It is shown that the characteristic overpotentials during charge (delithiation) and discharge (lithiation) is state of charge (SOC) and depth of discharge (DOD) dependent, respectively. It was demonstrated that the discharge characteristics are intertwined with the previous charge conditions, particularly with the charging time and the specific charge capacity. Increasing both in parallel can even lead to a deterioration of the subsequent specific discharge capacity. Furthermore, Li+ transport pathways within the NCM composite electrode are discussed and their influence on the observed overpotential evaluated. Changes of the overpotential are found to be mainly associated with changes within the NCM crystal structure, which is experimentally supported by the correlation of the SOC dependent overpotential with the XRD determined c-axis lattice parameter. Consequently, the Li+ transport within the active material is mostly responsible for limiting the practically available specific energy. [ABSTRACT FROM AUTHOR]

Underpotential deposition (UPD) of Cd on polycrystalline Au, Au(poly), has been found to catalyze the reduction of selenate in 0.1 M HClO4 to yield, following stripping of the Cd-UPD layer, elemental Se. Under certain conditions, the rates of this process, as measured by the charge under the peak associated with Se oxidation, were found to be nearly constant, with values which increased with the applied overpotential. [ABSTRACT FROM AUTHOR]

We consider a PEM fuel cell with concentric circular electrodes: the small anode and the large cathode. A model for in-plane distributions of the cathode overpotential ɳc and the membrane potential Φ in the anode-free region of the cell is developed. Mathematically, the problem reduces to the axially symmetric Poisson-Boltzmann equation for ɳc. An approximate analytical solution shows that ǀɳcǀ exhibits rapid decay to zero with the radius, while ǀΦǀ grows to the value of ǀɳ0cǀ the cathode overpotential in the working domain of the cell. For typical ɳ0c, the radial shape of qc far from the anode edge only weakly depends on ɳ0c; this effect is analogous to Debye screening in plasmas. The smaller the anode radius, the faster Φ approaches ɳ0c with the distance from the anode. It follows, that a reference electrode for measuring the cathode overpotential in the working area can be placed at a small distance from the curved anode edge. [ABSTRACT FROM AUTHOR]

Instabilities in electrodeposition that lead to formation of dendrites on metal surfaces place a significant limitation on important technologies such as batteries and microcircuits. In this paper, we show that a structured electrolyte with immobilized anions can improve the stability of electrodeposition as well as enhance the conductivity of the electrolyte at large overpotentials. A linear stability analysis of electrodeposition incorporating the effect of surface tension shows that the electrode surface is more stable at higher fractions of immobilized anions. Immobilizing even a small fraction (10%) of anions significantly improves stability compared to conventional liquid electrolytes. The analysis shows that the structured electrolyte stabilizes a wider range of perturbations than surface tension alone and reduces the growth rate of the most unstable perturbation. The stabilizing effects arise primarily at overpotentials large enough so that the metallic electrode is nearly depleted of ions in a liquid electrolyte. Under these conditions, the structured electrolyte maintains immobilized anions and mobile cations providing a stable current density at the metal electrode. [ABSTRACT FROM AUTHOR]

We present an in situ electrochemical technique for the quantitative measurement and resolution of the ohmic, charge transfer and diffusion overvoltages at the negative electrode of an all-vanadium redox flow battery (VRFB) using electrochemical impedance spectroscopy (EIS). The mathematics describing the complex impedance of the V+2/V+3 redox reaction is derived and matches the experimental data. The voltage losses contributed by each process have been resolved and quantified at various flow rates and electrode thicknesses as a function of current density during anodic and cathodic polarization. The diffusion overvoltage was affected strongly by flow rate while the charge transfer and ohmic losses were invariant. On the other hand, adopting a thicker electrode significantly changed both the charge transfer and diffusion losses due to increased surface area. Furthermore, the Tafel plot obtained from the impedance resolved charge transfer overvoltage yielded the geometric exchange current density, anodic and cathodic Tafel slopes (135 ± 5 and 121 ± 5 mV/decade respectively) and corresponding transfer coefficients α = 0.45 ± 0.02 and β = 0.50 ± 0.02 in an operating cell. [ABSTRACT FROM AUTHOR]