Electrochemical Impedance Spectra Measurements Research Articles

Reactivity of beryllium in aqueous solution from acidic to basic pH

Beryllium wastes produced by nuclear industry could be managed by encapsulation in cements. The main risk is the aqueous corrosion, which leads to the hydrogen production and cracks causing a loss of radioactivity confinement. However, the corrosion can be limited by the formation of the hydroxide solid phase Be(OH)2(s). A study on the influence of the pH on the reactivity of beryllium was carried out in aqueous solution. The solubility diagram was drawn and showed that the hydroxide solid phase is stable until very high basic pH of 14. The pH variation of solution at different initial pH was measured after immersion of metallic beryllium. The results are in good agreement with the calculated solubility diagram. The open circuit potential (OPC) and the electrochemical impedance spectra (EIS) were recorded at a beryllium electrode from pH 2 to 15. The simulation of the EIS by using an electric equivalent circuit allowed to evaluate the Be reactivity by measuring the charge transfer resistance as a function of the pH. The experimental results confirm that the corrosion of beryllium is limited at neutral to basic pH and this low reactivity was attributed to the formation of the hydroxide solid phase Be(OH)2. The OCP and EIS measurements put also in evidence that the corrosion of beryllium is the lowest in solution having pH around 12. In this pH range, the Be metal would be protected against the corrosion both by the formation of the hydroxide solid Be(OH)2 and by the adsorption of hydroxide ions (OH−) at the metal surface.

Ultrafine Ni0.85Se nanoparticles encapsulated inside hollow porous carbon spheres and their excellent Na storage performance

As a novel anode material for sodium-ion batteries, ultrafine Ni0.85Se nanoparticles encapsulated inside hollow porous carbon spheres (HPCS@Ni0.85Se) are prepared by impregnating Ni(NO3)2 into HPCSs, calcination and selenization, which is based on the strong capillary effect of HPCSs. Material characterization reveals that Ni0.85Se nanoparticles with a diameter of 5–15 nm are evenly dispersed and intimately coupled to the internal wall of HPCSs, and the content of Ni0.85Se reaches 52.81%. HPCSs as unique reactors and Ni0.85Se carriers improve electronic conductivity of Ni0.85Se, facilitate electrolyte penetration and storage, buffer volume variation of Ni0.85Se, effectively confine Ni0.85Se during long-term cycles. Consequently, HPCS@Ni0.85Se exhibits excellent cycling durability and extraordinary rate capability. High reversible capacities of 341 mA h g−1 at 1 A g−1 after 1000 cycles, 280 mA h g−1 at 5 A g−1 after 2000 cycles and 305 mA h g−1 at 10 A g−1 after 540 cycles are achieved. The reaction kinetics and Na + storage mechanism are further analyzed by galvanostatic intermittent titration technique, cyclic voltammetry and electrochemical impedance spectra measurements. Ex-situ characterizations confirm outstanding structural stability of HPCS@Ni0.85Se. It is demonstrated that HPCS@Ni0.85Se is an excellent anode material, and this unique HPCS-based nanoencapsulation structure is an effective composite strategy for transition metal selenides.

Correlation Between Microstructure and Performance of GDC-Based Electrodes

Gadolinium doped ceria (GDC) gains increasing attention as a promising material for both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). The GDC-based electrodes demonstrate positive features, e.g. enhanced electrochemical performance in low operation temperatures, resistivity to sulfur poisoning and carbon deposition, etc. As GDC is a mixed electronic-ionic conductor in reducing atmosphere, electrochemical reactions can be supported by the double phase boundary (DPB) at the GDC surface. Several studies demonstrated that the DPB reaction may dominate the electrochemical performance over triple phase boundary (TPB) reaction in Ni-GDC electrodes, where it can account for as much as 95 % of the total reaction depending on the cermet microstructure [1]. Recent studies even show that pure ceria electrodes can achieve substantial power density [2,3]. In this context, the microstructure design principle for GDC-based electrodes should focus on maximizing GDC surface area. Particularly, GDC electrodes with infiltrated GDC nanoparticles or electro-catalytic nanoparticles may achieve competitive performance compared with the conventional Ni-YSZ electrodes. Although many strategies have been proposed to improve performance of GDC-based electrodes, it is difficult to quantitatively compare all the reported results. Furthermore, the correlation between microstructure and performance of pure GDC electrode are still scarce.Several different types of GDC-based electrodes are fabricated and tested in the unified characterization procedure in this study. Four basic designs are considered, i.e. Ni-GDC, GDC-perovskite composites, GDC electrodes with deposited Ni nanoparticles and pure GDC electrodes. Several strategies to modify the GDC surface area are discussed. In addition, the substantial enhancements in the DPB density and electrochemical performance were achieved by precipitating Ni-nanoparticles directly on sub-micron GDC powders at 2wt.% loading. The performance of electrodes was evaluated by electrochemical impedance spectra measurements with various humidity and temperature conditions. The results showed clear dependance between GDC surface area and electrode performance. At the same time, it should be noted that the electrode active thickness depends on the electrode microstructure. In particular, it is in the range of 5 µm for the pure nano-GDC electrode. When the GDC electrode is thicker than the active thickness, it results in large Ohmic resistance due to the electronic conduction in the porous electrode.[1] A. Nenning, M. Holzmann, J. Fleig, A.K. Opitz, Mater. Adv. 2 (2021) 5422–5431.[2] M. Ouyang, A. Bertei, S.J. Cooper, Y. Wu, P. Boldrin, X. Liu, M. Kishimoto, H. Wang, M. Naylor Marlow, J. Chen, X. Chen, Y. Xia, B. Wu, N.P. Brandon, J. Energy Chemistry. 56 (2021) 98–112.[3] W. Jung, K.L. Gu, Y. Choi, S.M. Haile, Energy Env. Sci. 7 (2014) 1685–1692.

Study on Commercially Available Membranes for Alkaline Direct Ethanol Fuel Cells.

This study provides a comparison of different commercially available low-cost anion exchange membranes (AEMs), a microporous separator, a cation exchange membrane (CEM), and an anionic-treated CEM for their application in the liquid-feed alkaline direct ethanol fuel cell (ADEFC). Moreover, the effect on performance was evaluated taking two different modes of operation for the ADEFC, with AEM or CEM, into consideration. The membranes were compared with respect to their physical and chemical properties, such as thermal and chemical stability, ion-exchange capacity, ionic conductivity, and ethanol permeability. The influence of these factors on performance and resistance was determined by means of polarization curve and electrochemical impedance spectra (EIS) measurements in the ADEFC. In addition, the influence of two different commercial ionomers on the structure and transport properties of the catalyst layer and on the performance were analyzed with scanning electron microscopy, single cell tests, and EIS. The applicability barriers of the membranes were pointed out, and the ideal combinations of membrane and ionomer for the liquid-feed ADEFC achieved power densities of approximately 80 mW cm-2 at 80 °C.

Advanced Online Broadband Impedance Spectrum Acquisition of Fuel Cells by S-Transform

Electrochemical impedance spectrum (EIS) applied in fuel cell characterization and diagnosis facilitates its system management in the vehicular application, but the first consideration of impedance-based management is how to obtain impedance online. The traditional laboratory method based on sine sweep is unsuitable for vehicular EIS measurement as limited by measured time and equipment price. In this article, we design an excitation device that can generate the square wave signal containing many harmonics as the excitation current. Besides, introduce S-transform (ST) for online broadband fuel cell impedance spectrum fast calculation, whose window width can be adaptively changed with frequency, and meanwhile, preserving high computational efficiency by fast Fourier transform operation. Based on this, the performance of ST and the previous method based on Morlet wavelet transform (MWT) is compared under different operating conditions. Extensive experimental results show that the ST can calculate impedance with the mean relative error of less than 3% based on appropriate excitation, while the computed time can be saved by nearly 95% as compared to MWT. Furthermore, the effectiveness and robustness of ST against different current densities, cell temperatures, fault scenarios with various fault degrees, and stable time are also verified through experimentation.

Online Monitoring of Lithium-Ion Battery Internal Temperature Using PWM Switching Oscillations

Online monitoring of Lithium-ion (Li-ion) battery internal temperature by electrochemical impedance spectrum (EIS) is important for the system safe and reliable operation. However, it is challenging in the high temperature region due to the limited thermal sensitivity. Moreover, additional signal injection or disturbance for online EIS measurement may interact with the operating system. To address these challenges, a Li-ion battery internal temperature continuous monitoring method using the self-excited ultrahigh frequency (UHF) PWM switching oscillations (e.g., several hundred kHz) is proposed in this article. First, the UHF EIS characteristics of a Li-ion battery are explored to find a novel thermal sensitive electric parameter applicable for high temperature measurement. Then, an PWM switching oscillation based online battery temperature monitoring scheme is presented, including the UHF oscillation sensor, resonance impedance extraction algorithm, and battery internal temperature estimation procedure. Finally, experimental work was carried out, and the results demonstrate an excellent performance, such as high sensitivity over 90 °C, ms-level fast response, and noncontract continuous monitoring.

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1

WO3/Ag2S type-II hierarchical heterojunction for improved charge carrier separation and photoelectrochemical water splitting performance

In the present work, WO3/Ag2S heterojunction was fabricated to achieve an improved photoelectrochemical (PEC) water splitting performance. To prepare the working electrodes, a two-step method was adopted which includes, a thin film of WO3 deposited using DC sputtering and well-separated Ag2S nanorods fabricated by glancing angle deposition. The PEC response was studied for bare WO3, Ag2S, and WO3/Ag2S heterojunction. The as-prepared WO3/Ag2S heterojunction samples revealed higher absorption as well as a higher photocurrent density of 2.40 mA/cm2 (at 1 V Ag/AgCl) as compared to bare WO3 thin film (0.34 mA/cm2). The enhancement in the photocurrent density of WO3/Ag2S electrodes could be ascribed to the formation of the type-II heterojunction between WO3 and Ag2S which effectively separates and transfers the charge carriers at the interface. In addition, increased trapping of light due to vertically tilted Ag2S nanorods structures results in effective absorption of light. Furthermore, electrochemical impedance spectra measurements showed that WO3/Ag2S samples have lower charge transfer resistance at the semiconductor electrolyte interface with high flat band potential. This work provides a deeper insight into the role of the interface formed between WO3 and Ag2S for the photoelectrochemical water splitting response.

(Digital Presentation) Fabrication of Conductive Carbon Composite Films for Freestanding Lib Anodes Using Cellulose Nanofiber Binder

Fig.1 Cycle performance and Coulomb efficiency of graphite/CNT/CNF composite electrodes Introduction Currently, lithium-ion batteries (LIBs) have been the primary power source for electric vehicles and portable devices. LIBs consist of anodes (e.g. graphite), cathodes (e.g. LiCoO2: LCO), and electrolytes,[1] and are dis-/ charged as Li+ de-/ intercalate between the electrodes. Since LIBs require expensive fluorinated polymer as binders (e.g. poly (vinylidene fluoride): PVDF) and a volatile and toxic organic solvent such as N-methyl pyrrolidone (NMP), cheaper and easier-to-handle materials are necessary. Furthermore, the metal substrates (Cu, Al foil, etc.) as the current collectors are heavy and reduce the weight energy density of electrodes. Therefore, providing alternatives to the conventional components of LIB electrodes is a key challenge. Developing functional nanocomposites using sustainable natural resources is one of the most importance strategies.[2] Conductive thin films of cellulose nanofibers (CNFs) combined with carbon nanotubes (CNTs) have been reported as novel carbon composites.[3] Environmentally friendly and only carbonaceous electrodes without PVDF and organic solvents, and graphite are normally used for anodes. As an allotrope of graphite, CNTs have been approved to be a suitable anode material due to their unique structure (one-dimensional cylindrical tubule of graphite sheet), high conductivity (106 Sm-1 at 300 K), low density, high rigidity (Young’s modulus 1 TPa), and high tensile strength (up to 60 GPa).[4] In this study, the free-standing carbon composite anodes were successfully fabricated by using ultrasonication of CNF/ CNT or graphite mixture and vacuum filtering. Graphite slurry as the active material layer was dispersed in DI-water as the solvent containing CNF binder. This composite film has a dual-layer structure consisting of graphite/CNF active materials layer and CNT/CNF layer; here, the CNT/CNF layer with a thickness of about 10 μm is utilized as a current collector replacing the conventional metal substrate. These fabricated electrodes were evaluated by structural evaluation and dis-/ charge measurements. Experimental method CNF gel (2 wt%, Rheocrysta, DKS Co. Ltd.) and functionalized (carboxylic acid) multi-walled CNT (f-MWCNT, Sigma-Aldrich) were suspended in DI-water and sonicated for 2 h. Synthetic graphite was dispersed in DI-water with 5 wt% CNF and 10 wt% Carbon Black (CB) as a conductive material and sonicated for 30 min. Nanocomposite dual-layer films were formed by vacuum-filtering the dispersion using PVDF (pore size: 0.1 μm) membrane filters. These films were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and Raman spectroscopy. The conductivity was measured by four probe method. The electrochemical properties of the free-standing electrode were measured using coin cells at room temperature. The coin cells were assembled in an argon-filled glovebox using Li as a counter electrode, glass fiber filter as a separator, and 1 M LiPF6 in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. Charge-discharge characteristics were recorded from 0.01 to 3 V at charging speeds of 0.1 – 2 C. The electrochemical impedance spectra (EIS) were measured in the frequency range from 100 mHz to 100 kHz with a potential perturbation at 5 mV. Results CNF combined with CNT constructs a robust conductive fibrous network, and CNT linked with graphite builds electronic conductive paths to improve the electronic conductivity. Specific capacities of the electrode were 44 mAhg-1 at 2 C and 272 mAhg-1 at 0.5 C, respectively (Fig.1). The discharge capacity of the first cycle was 363 mAhg-1 at 0.1 C. After 125 cycles, the discharge capacity was 255 mAhg-1 at 0.5 C. The ratio of the charge and discharge capacities (Coulombic efficiency) suggests that the charge-discharge behavior is stable in the cycle measurement, indicating that the irreversible capacity was small. Other information obtained by cyclic voltammetry (CV), and EIS measurement and further results will be shown in the presentation. In summary, we successfully fabricated the graphite/CNT/CNF composite film as environmentally friendly and lightweight anodes for the first time. The present results indicate that graphite/CNT/CNF composite film is a good candidate of flexible free-standing anodes. This research focused on CNFs as a promising alternative to conventional battery materials, and CNFs were successfully used to fabricate electrodes without using expensive PVDF binders and toxic organic solvents. Moreover, the gravimetric energy density was improved by replacing the conventional heavy Cu foil with a lightweight and thin composite film of CNT/CNF.

Electrochemical biosensor with electrokinetics-assisted molecular trapping for enhancing C-reactive protein detection

C-Reactive protein (CRP) is an essential biomarker relevant to various disease prognoses. Current biosensors require a significant amount of time for detecting CRP. To address this issue, this work proposes electrokinetic flow-assisted molecule trapping integrated with an impedance biosensor, where a driving signal in terms of a gated sine wave is provided to circularly arranged electrodes which detect proteins. To verify the biosensor's efficacy, protein aggregation on the electrode surface was evaluated through a fluorescence analysis and measurement of the electrochemical impedance spectrum (EIS). The fluorescence analysis with avidin showed that target samples largely accumulated on the electrode surface upon provision of the driving signal. The EIS measurement of CRP accumulation on the electrode surface further confirmed a significant electrokinetic phenomenon at the electrode/electrolyte interface. Even at the low CRP concentration of 10 pg/ml, the proposed device's sensitivity and reliability were as high as 3.92 pg/ml with a signal-to noise ratio (SNR) of ≥3, respectively. In addition, the protein detection time (without considering the preparation time) was minimized to as low as 90 s with the proposed device. This device's advantage is its minimal time consumption, and simple drop-analysis process flow; hence, it was used for monitoring clinical serum samples.

Experimental Optimization of Metal Foam Structural Parameters to Improve the Performance of Open-Cathode Proton Exchange Membrane Fuel Cell

Using metal foam as a flow field structure is an attractive route to improve the performance of open-cathode PEMFC. Metal foam has shown great potential in improving the uniformity of reactants, but optimized structure parameters that can more effectively transfer gas and remove excess water are needed. Here we experimentally investigate the effect of metal foam structure parameters on cell performance using polarization curves, power density curves, and electrochemical impedance spectrum (EIS) measurements. By optimizing the pore density, thickness, and compression ratio of the metal foam, the performance of the fuel cell is improved by 49.8%, 42.1%, and 7.3%, respectively. The optimum structure value of metal foam is the pore density of 40 PPI, the thickness of 2.4 mm, and the compression ratio of 4:2.4. In this configuration, the cell could achieve a maximum power density of 0.485 W cm−2. The findings of this work are beneficial for the application of metal foams in open-cathode PEMFC.

Amorphous phosphorus chalcogenide as an anode material for lithium-ion batteries with high capacity and long cycle life

The ever-increasing demands for modern energy storage applications drive the search for novel anode materials of lithium (Li)-ion batteries (LIBs) with high storage capacity and long cycle life, to outperform the conventional LIBs anode materials. Hence, we report amorphous ternary phosphorus chalcogenide (a-P4SSe2) as an anode material with high performance for LIBs. Synthesized via the mechanochemistry method, the a-P4SSe2 compound is endowed with amorphous feature and offers excellent cycling stability (over 1500 mA h g−1 capacity after 425 cycles at 0.3 A g−1), owing to the advantages of isotropic nature and synergistic effect of multielement forming Li-ion conductors during battery operation. Furthermore, as confirmed by ex situ X-ray diffraction (XRD) and transmission electron microscope (TEM), the a-P4SSe2 anode material has a reversible and multistage Li-storage mechanism, which is extremely beneficial to long cycle life for batteries. Moreover, the autogenous intermediate electrochemical products with fast ionic conductivity can facilitate Li-ion diffusion effectively. Thus, the a-P4SSe2 electrode delivers excellent rate capability (730 mA h g−1 capacity at 3 A g−1). Through in situ electrochemical impedance spectra (EIS) measurements, it can be revealed that the resistances of charge transfer (Rct) and solid electrolyte interphase (RSEI) decrease along with the formation of Li-ion conductors whilst the ohmic resistance (RΩ) remains unchanged during the whole electrochemical process, thus resulting in rapid reaction kinetics and stable electrode to obtain excellent rate performance and cycling ability for LIBs. Moreover, the formation mechanism and electrochemical superiority of the a-P4SSe2 phase, and its expansion to P4S3−xSex (x = 0, 1, 2, 3) family can prove its significance for LIBs.

Enabling ultra-fast charging for Ni-rich LiNi0.88Co0.09Mn0.03O2 cathode by bulk W-doping

W-doped Ni-rich LiNi0.879Co0.088Mn0.030W0.003O2 cathode material for lithium-ion batteries (LIBs) is synthesized from a bulk W-doped Ni0.879Co0.088Mn0.03W0.003(OH)2 precursor. Comparative study between the W-doped cathode and its undoped counterpart LiNi0.88Co0.09Mn0.03O2 is implemented. X-ray diffraction indicates that the W-doping shows no obvious influence on the crystallographic structure. Scanning electron microscope measurements indicate that the W-doped material has coarsened primary particles. Electrochemical evaluations reveal that the electrochemical performance is significantly improved by a small amount of W-doping, especially for the rate capability. The W-doped cathode delivers an initial discharge capacity of 201.7 mAh g−1, which is slightly higher than that of the undoped one (197.5 mAh g−1). When tested at 20C, the W-doped sample still has a discharge capacity of about 140 mAh g−1, whereas the value of the undoped one is only 104 mAh g−1. Furthermore, cyclic voltammetry and electrochemical impedance spectra measurements suggest that the W-doping can stabilize the H2 → H3 phase transition, as well as suppress the impedance increase during cycling process.

Prolonging lifetime of photogenerated carriers in WO3 nanowires by oxygen vacancies engineering for enhanced photoelectrocatalytic oxygen evolution reaction

WO3 nanowires (NWs) have emerged as a promising alternative electrocatalyst for the photoelectrochemical (PEC) oxygen evolution reaction (OER), due to their nontoxicity, low cost, good stability, and strong photocatalytic oxidation ability. However, a significant challenge is limited by the poor electrical conductivity and the rapid recombination rate of photogenerated carriers. This paper reports a facile and effective way to synthesize the WO3 NWs with oxygen vacancies (Ov-WO3 NWs), which have high PEC OER activity and good stability. Photoelectric measurements indicate that the Ov-WO3 NW-based device shows a good light-harvesting property under visible light and a prolonged photoresponse time. Electrochemical impedance spectra measurements reveal a reduced value of Rct denoting an improved electrical conductivity, which should be responsible for the superior PEC OER performance. Our work provides a strategy for fabricating efficient water-splitting electrodes to help establish rational design principles for future OER catalysts.

An effective route for growth of WO3/BiVO4 heterojunction thin films with enhanced photoelectrochemical performance

The unsatisfactory solar light absorption of WO3 and poor charge separation of BiVO4 are main limits for their use in photoelectrochemical (PEC) water oxidation. Coupling WO3 with BiVO4 has been considered as a feasible way to improve PEC performance by taking complementary advantages of them. In this work, we obtained nanoflake-structured WO3 by hydrothermal growth with post-annealing. The effect of process variables on morphology and resultant performance were investigated. Electrodeposition growth was utilized to deposit BiVO4 onto WO3 forming WO3/BiVO4 heterojunction thin films. Porous BiVO4 with wormlike morphology was tightly coupled and well-distributed onto WO3 nanoflakes. The optimized best-performing WO3/BiVO4 photoanode exhibits higher photocurrent density than that summation of bare WO3 and BiVO4 over entire range of applied potential. This enhancement is mainly attributed to the effective charge separation at WO3/BiVO4 interface, which is confirmed through electrochemical impedance spectra (EIS) measurements, respectively. Our work provides a referable approach for the growth of WO3/BiVO4 heterojunction photoanode with enhanced PEC performance.

Preparation of NdBaCo2O5+δ–Ce0.9Gd0.1O1.95 composite cathode by in situ sol-mixing method and its high-temperature electrochemical properties

NdBaCo2O5+δ-xCe0.9Gd0.1O1.95 (NBCO-xCGO, x = 0 ~ 8 wt%) composite materials are prepared by novel sol-mixing method and evaluated as cathodes for intermediate-temperature solid oxide fuel cells. The chemical compatibility, thermal expansion behavior and electrochemical performance of NBCO-xCGO composite cathodes are studied. There is no chemical reaction between NBCO and CGO after sintering at 1100 °C for 12 h. Introducing CGO leads to the decrease of both thermal expansion coefficient and conductivity. Among all the composites, NBCO-5CGO shows the lowest polarization resistance of 0.034 Ω cm2 at 700 °C. The anode-supported single-cell with NBCO-5CGO cathode exhibits a maximum power density of 1.0 W cm−2 at 700 °C. The electrochemical impedance spectrum measurement combining with the distribution of relaxation times analysis proves that the addition of CGO nanoparticles significantly improves the surface oxygen dissociation process, and the charge transfer process is identified to be the reaction limiting step to control the oxygen reduction kinetics on NBCO-5CGO composite cathode.

Strengthened passivation and mechanism of 316L stainless steel in hot diluted sulfuric acid solution by connecting to Pd electrode

Purpose The purpose of this paper is to study the variations of composition and properties of the passive film on 316 L stainless steel surface in 80°C, 0.5 mol L-1 H2SO4 + 2 mg L-1 NaF solution, is helpful to understand the mechanisms of corrosion resistancethe of plated Pd on 316 L ss. Design/methodology/approach The variations of composition and properties of the passive film on 316 L stainless steel surface in 80°C, 0.5 mol L-1 H2SO4 + 2 mg L-1 NaF solution after connected to Pd electrode were studied with methods of potential monitor, X-ray photoelectron spectroscopy analysis and electrochemical impedance spectrum (EIS) measurement. Findings By connecting to a Pd electrode, the potential of the SS sample increased from the active region to the passive region. By connecting to the Pd electrode, the contents of Cr, Cr(OH)3 and Fe3O4 in passive film increased obviously. With increased Pd/SS area ratio, the Cr(OH)3 content in passive film increased but the Fe3O4 content changed little. The results show that after connecting to Pd the corrosion resistance of the passive film on 316 L stainless steel increases obviously, which may be attributed to the more compact passive film because of higher Cr, Cr(OH)3 and Fe3O4 contents and less point defects in the film. Originality/value The effects and mechanism of Pd on passivation of SS was studied.

Surface modification of the p-type Cu2ZnSnS4 photocathode with n-type zinc oxide nanorods for photo-driven salt water splitting

The sulfurization of co-sputtering Cu–Zn–Sn metal precursors was employed to prepare the quaternary copper-zinc-tin-sulphide (Cu2ZnSnS4, CZTS) photocathodes on substrates. Influence of [Zn]/[Sn] ratios in CZTS photocathodes on their phases, morphologies, and the efficiencies of photo-driven salt-water splitting was examined. Pristine p-type CZTS photocathodes showed the highest photo-driven performance of 0.61 mA cm−2 in an electrolyte containing 1 M sodium chloride with the external bias kept at −1.0 V vs. Ag/AgCl. An n-type zinc oxide (ZnO) nanorod arrays layer was then coated on the CZTS photocathode to improve its photo-driven salt-water splitting performance. The CZTS/ZnO photoelectrode had the best photo-driven performance of 1.87 mA cm−2 in the 1 M NaCl solution under illumination with the external bias set at −1.0 V vs. Ag/AgCl. From results of electrochemical impedance spectra measurements for the samples in the electrolyte, the CZTS/ZnO sample had good photo-driven salt-water splitting performance due to its lowest charge transfer resistance and p-n junction formed at the sample. Intensity modulated photocurrent spectroscopy and electrochemical impedance spectra results of samples indicated that the surface states at the CZTS/ZnO interface were the recombination centers with the electrons from the CZTS sample and holes from the ZnO and therefore improved its photo-driven salt-water splitting performance.

Enhanced CO2 photocatalytic conversion into CH3OH over visible‐light‐driven Pt nanoparticle-decorated mesoporous ZnO–ZnS S-scheme heterostructures

In the present contribution, the design and fabrication of Pt nanoparticle-decorated mesoporous ZnO–ZnS heterostructures were described and used effectively for photocatalytic CO2 conversion to yield CH3OH. TEM images of the mesoporous Pt/ZnS–ZnO heterostructure demonstrated spherical ZnO NPs ~20 nm, and Pt NPs ~3 nm were well dispersed on the porous ZnS–ZnO heterostructure. The formation of CH3OH over the Pt/ZnS–ZnO heterostructure was 78, 39 and 20 times larger than that bare ZnS, ZnO NPs and ZnS–ZnO, respectively. The optimal Pt/ZnO–ZnS heterostructure exhibited a high CH3OH formation rate of 81.1 μmolg-1h-1, which is about 44, 22 and 20 times larger than that of bare ZnS (1.86 μmolg-1h-1), ZnO (3.72 μmolg-1h-1), and ZnO–ZnS (4.15 μmolg-1h-1), respectively. The significantly enhanced reduction of CO2 was imputed to the synergistic effects of the ZnO–ZnS heterostructure and the incorporation of Pt NPs. The synthesized photocatalyst provides a new transfer route through which carriers can migrate to the outer surface as well as pore channels of the mesoporous ZnO–ZnS, therefore significantly minimizing the transfer distance for carriers, inhibiting photoinduced electron-hole recombination, and diminishing the mobility resistance, as determined using photoluminescence, photocurrent response, and electrochemical impedance spectra measurements.

Synergetic Strategy for the Fabrication of Self-Standing Distorted Carbon Nanofibers with Heteroatom Doping for Sodium-Ion Batteries.

Currently, the limited availability of lithium sources is escalating the cost of lithium-ion batteries (LIBs). Considering the fluctuating economics of LIBs, sodium-ion batteries (SIBs) have now drawn attention because sodium is an earth-abundant, low-cost element that exhibits similar chemistry to that of LIBs. Despite developments in different anode materials, there still remain several challenges in SIBs, including lighter cell design for SIBs. The presented work designs a facile strategy to prepare nitrogen-doped free-standing pseudo-graphitic nanofibers via electrospinning. A structural and morphological study implies highly disordered graphitic structured nanofibers having diameters of ∼120–170 nm, with a smooth surface. X-ray photoelectron spectroscopy analysis showed that nitrogen was successfully doped in carbon nanofibers (CNFs). When served as an anode material for SIBs, the resultant material exhibits excellent sodium-ion storage properties in terms of long-term cycling stability and high rate capability. Notably, a binder-free self-standing CNF without a current collector was used as an anode for SIBs that delivered capacities of 210 and 87 mA h g–1 at 20 and 1600 mA g–1, respectively, retaining a capacity of 177 mA h g–1 when retained at 20 mA g–1. The as-synthesized CNFs demonstrate a long cycle life with a relatively high Columbic efficiency of 98.6% for the 900th cycle, with a stable and excellent rate capacity. The sodium storage mechanisms of the CNFs were examined with various nitrogen concentrations and carbonization temperatures. Furthermore, the diffusion coefficients of the sodium ions based on the electrochemical impedance spectra measurement have been calculated in the range of 10–15–10–12 cm2 s–1, revealing excellent diffusion mobility for Na atoms in the CNFs. This study demonstrates that optimum nitrogen doping and carbonization temperature demonstrated a lower Warburg coefficient and a higher Na-ion diffusion coefficient leads to enhanced stable electrochemical performance. Thus, our study shows that the nitrogen-doped CNFs will have potential for SIBs.