Nanocrystalline Electrodes Research Articles

Space charge effects in mixed ionic-electronic conducting electrodes for solid-state batteries.

Mixed ionic-electronic conductors are widely used as electroactive materials in energy applications. The contact of a mixed conductor with another phase plays a crucial role in charge storage and transport in energy devices. However, the interfacial chemistry at the heterojunctions comprising mixed conductors and its interplay with the bulk chemistry remains imperative yet inadequately understood. This study addresses the fundamentals of space charge effects by exploring the equilibrium situations for contacts consisting of mixed conductors. From the perspective of defect chemistry, and by unifying the bulk and interfacial conditions with the electrochemical potential, our treatment allows for predicting the built-in potential at heterojunctions, profiling the space charge distributions, and evaluating the resulting interfacial charge storage and transport. The treatment can be related to experimental characterization, including coulometric titration, conductivity, and capacitance measurements at electrochemical interfaces in all-solid-state batteries. Besides, our treatment also highlights the significance of size and doping effects in nanocrystalline electrodes. This work provides a comprehensive framework for understanding and engineering the heterojunctions in electrochemical devices.

Enhanced Methanol Oxidation in Alkaline Media on Electrodeposited Ni Electrodes by Morphological and Crystallographic Evolution

This research aims at exploiting the electrocatalytic behaviour of nano-crystalline nickel electrodes electrodeposited by different techniques including direct current (DC), pulse current (PC), or pulse reversal current (PRC) for methanol electrooxidation in alkaline solutions. We understand that PC electrodeposition forms pyramidal shaped grains with a preferential Bragg diffraction peak of (111), whereas PRC produced refined spherical grain morphology with a strong (200) diffraction peak. However, DC electrodeposition exhibits an intermediate morphology and crystalline structure. Cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) show that PRC electrodeposition develops Ni electrodes with better electrocatalytic activity for methanol electrooxidation than other two nickel electrodes. Based on the CV curve, the current density for Ni prepared by PRC electrodeposition methods is about 75.26 mA.cm−2, which is higher than those of DC and PC methods. This higher activity of PRC electrodeposited nickel is attributed to the low charge transfer resistance confirmed by Nyquist plots. We attributed this behavior to the (200)-oriented crystallographic texture, spherical grain morphology, and consequently the high electrochemical active surface area of this nickel electrode. This work reveals the importance of surface morphology and crystallography on the electrocatalytic behaviour of nickel electrodes for electrochemical energy devices.

Bismuth oxide-doped graphene-oxide nanocomposite electrode for energy storage application

Nanocrystalline and porous bismuth oxide-doped graphene oxide nanocomposite (Bi2O3-GO NC) electrode material synthesized via microwave irradiation method has been envisaged for supercapacitor application. Surface morphology, elemental configuration, phase purity, surface area, porosity, and binding energy etc., are initially screened and then preferred in electrochemical measurement analysis. The as-obtained Bi2O3–GO NC electrode adduces a better performance with quasi-faradaic redox reactions in electrochemical measurements over pristine GO in 6 M KOH electrolyte solution. The specific capacitance of the Bi2O3–GO NC electrode measured at current densities 5–14 A/g is varied from 1250 to 933 F/g. The as-assembled Bi2O3–GO//Bi2O3–GO symmetric supercapacitor device reveals an exceptional electrochemical performance with 25.83 W-h/kg specific energy at 337 W/kg specific power, and excellent stability of about 80% for 5000 cycles, evidencing usability potential of metal oxide-GO composite electrode materials over solitary carbonaceous electrode. A demonstration of 42 LEDs ignited with full-bright intensity supports our claim as well.

The Development of Highly Fluorescent Hemicyanine and Dicyanoisophorone Dyes for Applications in Dye-Sensitized Solar Cells.

Ruthenium-based metal complex dyes have been employed extensively in dye-sensitized solar cells (DSSCs) as photosensitizers, but the cost and toxicity of metal complexes have promoted the development of metal-free organic dyes. The present investigation deals with the synthesis of hemicyanine and Dicyanoisophorone (DCI) based dyes adopting the D-π-A strategy, and their application on sensitization of nano-crystalline ZnO electrodes by appending the carboxyl (COOH) anchoring group as a pendant on the primary skeleton of dyes. Dyes have been characterized by UV, FTIR, and NMR spectroscopic studies. Absorption maxima (λmax) were found in the region 416-551nm while emission wavelength (λem) was observed in the range 575-685nm. Cyclic voltammetry and DFT calculations were used to estimate redox potential and band gap energies of dyes.

Advanced catalyst for hydrogen evolution reaction by dealloying Al-based nanocrystalline alloys

Developing advanced catalysts for water splitting is the kernel in hydrogen clean-energy technologies. In this paper, an Al82Ni6Co3Mn3Y3Au3 nanocrystalline ribbon is dealloyed as the catalyst in hydrogen evolution reaction (HER), which exhibits outstanding catalytic activity in acidic solution. Its overpotential is about 24 mV @ 10 mA cm−2 and the Tafel slope is about 43 mV dec−1, which are comparable to commercial noble Pt/C electrodes (30 mV @ 10 mA cm−2 and 28 mV dec−1). Such a superior catalytic performance is attributed to the large specific surface area and low charger transferring resistivity of the dealloyed nanocrystalline electrode. The hybrid nanoporous structures composed of nanoporosity and nanoparticles of Au-Ni-Co-Mn-Y alloys and the synergic effects among the multiple components are desirable in catalysts. These results suggest that nanocrystalline multicomponent alloys are promising precursors for fabricating advanced HER catalysts.

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles.

Nanocrystalline anatase TiO2 is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li insertion and electron accumulation, respectively, and these optical signals are used to deconvolute bulk Li insertion from other electrochemical responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO2 systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concentration, and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO2 nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO2 particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochemistry.

Primary photocatalytic water reduction and oxidation at an anatase TiO2 and Pt-TiO2 nanocrystalline electrode revealed by quantitative transient absorption studies

Quantitative assessments of electron and hole transfer dynamics with water on anatase nanocrystalline TiO2 films were conducted by employing a series of transient absorption spectrometers. For water reduction reactions, both conduction band and trapped electrons decay with two different single exponential components with the difference in time scale of 6∼7 orders of magnitudes. The faster reaction occurs in 8–16 ns, while the slower component shows a lifetime of 0.1∼1.4 s. Pt nanoparticle deposition on the TiO2 surface switches the slower single exponential reaction to a stretched exponential with an accelerated half lifetime of 2–7 ms, indicating that this slower reaction is limited by the electron trapping-detrapping movements inside the TiO2. Water oxidation reactions occur multi-exponentially from 100 ns to 200 ms with a half lifetime of 10 μs. Pt on the TiO2 surface catalyses water oxidation, occurring from 25 ns with a half lifetime of 8 μs.

Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study.

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area ∼100 μm in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Synergy of nano-ZnO and 3D-graphene foam electrodes for asymmetric supercapacitor devices.

Two kinds of electrode materials were produced to fabricate asymmetric supercapacitor devices: (i) Highly defective, n-type wide bandgap semiconductor ZnO nanocrystalline electrodes below 50 nm were synthesized with the aid of the high energy ball milling technique. (ii) Flexible 3D-graphene foams were synthesized via the chemical vapor deposition technique. Extensive defect structure analysis was performed via enhanced characterization techniques mainly the spectroscopy ones: electron paramagnetic resonance (EPR), Raman, and photoluminescence (PL). Compared to bulk ZnO electrodes the nanoscale ZnO electrodes revealed a dramatic increase of defect concentration. The surface defect plays a crucial role in the electrochemical performance of supercapacitor devices. Strong decreases in charge transfer resistance were observed for the smallest crystallite size which is 15 nm. This work also shows that synthesis, controlling the defect structures, electronic and electrical characterization and the device production are extremely important to obtain high performance faradaic asymmetric supercapacitors.

Simulation studies of the characteristics of nitrogen-containing additive molecules for solar cells

Dye-sensitized solar cells (DSSC) are of great scientific significance and application prospect and have attracted wide public concern since they were first reported. The charge recombination in the interface of the DSSC nanocrystalline electrode is thought as a critical issue affecting the DSSC’s performance improvement; however, few systematic studies have been carried out to investigate the effect of the electrolyte additives on the charge recombination suppressing. It is reported that the electrolyte additives can enhance the DSSC short-circuit current. In this work, five typical nitrogen-containing six-element heterocyclic additives used in DSSC were researched theoretically at the B3LYP/6-311G** computational level to elucidate the effects of electrolyte additives on DSSC performance. The five additives were pyridine, pyridazine, pyrimidine, pyrazine and 1,3,5-triazine named as A1–A5, respectively. Their molecular properties, some of which are molecule structures, geometrical configurations, dipole moments, charge distributions, vibrational frequencies and frontier orbitals, were calculated and discussed and their effects on DSSC performance were analyzed theoretically. The results showed an enhanced conjugation effect with the increased number of nitrogen atoms in the conjugated rings of A1–A5 molecules and A1–A5 molecules could bond stably onto the electrode surfaces through the coordination bonding of the N atoms. For the electrolyte additives A1–A5-based DSSC devices, open-circuit voltages and light-to-energy conversion efficiencies increased. This work can be helpful to explain the mechanism of charge recombination suppressing of electrolyte additives for DSSC and offers the potential for future development of high-performance DSSC.

Copper-Based Catalytic Anodes To Produce 2,5-Furandicarboxylic Acid, a Biomass-Derived Alternative to Terephthalic Acid

2,5-Furandicarboxylic acid (FDCA) is a key near-market platform chemical that can potentially replace terephthalic acid in various polyesters such as polyethylene terephthalate (PET). FDCA can be obtained from oxidation of 5-hydroxymethylfurfural (HMF), which can be derived from cellulosic biomass through isomerization and dehydration of hexoses. In this study, electrochemical oxidation of HMF to FDCA is demonstrated using Cu, one of the cheapest transition metals, as the catalytic anode. The oxidized Cu surface is not catalytic for water oxidation, which is the major reaction competing with HMF oxidation in aqueous media. Therefore, a wide potential window to oxidize HMF without inducing water oxidation was available, enabling high Faradaic efficiencies for FDCA production. Cu was prepared as nanocrystalline and bulk electrodes by electrodeposition, and key differences in their surface oxidation and electrochemical HMF oxidation were investigated. The oxide and hydroxide layers formed on the nanocrystall...

Local structural changes of nano-crystalline ZnFe2O4 during lithiation and de-lithiation studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy was carried out to investigate local structural changes around Fe and Zn atoms of the nano-crystalline spinel ferrite ZnFe2O4 anode material at various states-of-charge during the 1st and 2nd lithiation/de-lithiation. From the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we propose a possible structure evolution process of the ZnFe2O4 electrode during the 1st discharge and charge cycle. A mixture of metallic iron, ZnO, metallic zinc, LiZn and Li2O phases seem to be formed as the cell is firstly discharged to 0.02V. Instead of the original ZnFe2O4 spinel phase, the metallic iron and zinc particles are re-oxidized to Fe2O3 and ZnO phases during the subsequent de-lithiation. A reversible redox reaction between Fe2O3, ZnO and lithium ions is found in the 2nd cycle. The formation of SEI layer in the initial cycles plays a major role in the irreversible capacity of the electrode. The inactive disordered ZnO formed due to the conversion reaction of ZnFe2O4 during the 1st lithiation is probably the main reason for the poor electrochemical behavior of the nano-crystalline ZnFe2O4 electrode.

High Surface Area Antimony-Doped Tin Oxide Electrodes Templated by Graft Copolymerization. Applications in Electrochemical and Photoelectrochemical Catalysis.

Mesoporous ATO nanocrystalline electrodes of micrometer thicknesses have been prepared from ATO nanocrystals and the grafted copolymer templating agents poly vinyl chloride-g-poly(oxyethylene methacrylate). As-obtained electrodes have high interfacial surface areas, large pore volumes, and rapid intraoxide electron transfer. The resulting high surface area materials are useful substrates for electrochemically catalyzed water oxidation. With thin added shells of TiO2 deposited by atomic layer deposition (ALD) and a surface-bound Ru(II) polypyridyl chromophore, they become photoanodes for hydrogen generation in the presence of a reductive scavenger.

An investigation on electrochemical hydrogen storage performances of Mg-Y-Ni alloys prepared by mechanical milling

The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20–xYxNi10 (x=0, 1, 2, 3 and 4) were fabricated by mechanical milling. Effects of Y content on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. The inspections of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that the substitution of Y for Mg brought on an obvious change in the phase composition of the alloys. The substitution of Y for Mg resulted in the formation of secondary YMgNi4 phases without altering the major phase Mg2Ni when Y content x≤1. But with the further increase of Y content, the major phase of the alloys changed into YMgNi4 phase. In addition, such substitution facilitated the glass forming of the Mg2Ni-type alloy. The discharge capacities of the as-milled alloys had the maximum values with Y content varying, but Y content with which the alloy yielded the biggest discharge capacity was changeable with milling time varying. The substitution of Y for Mg had an insignificant effect on the activation ability of the alloys, but it dramatically improved the cycle stability of the as-milled alloys. The effect of Y content on the electrochemical kinetics of the alloys was related to milling time. When milling time was 10 h, the high rate discharge ability (HRD), diffusion coefficient of hydrogen atom (D) and charge transfer rate all had the maximum value with Y content increasing, but they always decreased in the same condition when milling time increased to 70 h.

Thin Film Nanocrystalline TiO2 Electrodes: Dependence of Flat Band Potential on pH and Anion Adsorption.

Thin nanocrystalline TiO2 films were produced on ITO conductive glass by dip-coating of a sol-gel TiO2 precursor. The transparent films were characterized from the optical and structural point of view with UV-Vis, Spectroscopic Ellipsometry, Raman and X-ray photoelectron spectroscopies, the roughness of the coating by AFM. The changes in the electrochemical properties features of ITO/TiO2 electrodes were evaluated in the presence of different electrolytes (KCI, Na2SO4 and phosphate buffer) with the aim to clarify the role of the ion adsorption on the structure of the electrical double layer. Electrochemical tests (Cyclic Voltammetry, CV, and Impedance Electrochemical Spectroscopy, EIS) showed a strong influence of the electrolyte properties on the semiconductor band edge position in the electrochemical scale and on band bending. The CV profiles recorded can be explained by considering that the interface capacity is due to the charging of surface states (e.g., Ti(IV) surface sites coordinated by oxygen atoms, ≡Ti-OH or Ti-O-Ti). The surface charge is strongly affected also by the density and nature of adsorbed ions and by dissociation of surficial OH. Of interest the fact that for the produced nanocrystalline electrodes the flat band potential, measured from the Mott-Schottky analysis of the space charge layer capacity obtained with EIS, showed a non Nernstian behavior with the pH probably caused by a change in the surface acidity as a consequence of specific anion adsorption. The modulation of flat band potential with adsorbed ions is of interest for many applications, in particular for photocatalysis (change in the redox potential of photogenerated carriers) and for photovoltaic applications like DSSC (change in the photopotentials).

Electrode Materials

This review work was focused on conventional and modern electrodes which play an important role in electrochemical systems. Among many types of existing electrode materials, some of the most prominent materials from the conventional (metals and their alloys, graphite and mixed metal oxides) and the modern (amorphous, modified and composite) electrodes, have been outlined. What is also discussed is the recent intensive usability of nanocrystalline electrodes of better properties than their microcrystalline equivalents, and development trend of electrode materials.

Effect of mechanical grinding on the electrochemical hydrogen storage properties of Mg–Ni–Y alloy

The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20 − xYxNi10 (x = 1 and 4) were fabricated by mechanical milling, and the effects of milling time on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. Test results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) reveal that a nanocrystalline structure can be obtained through mechanical milling, and the amorphization degree of the alloy visibly increases with milling time prolonging. When Y content is x = 1, the substitution of Y for Mg results in the formation of secondary YMgNi4 + YMg3 phases without altering the major phase Mg2Ni, but when Y content is x = 4, the major phase of the alloys changes into YMgNi4 + YMg3 phases. The capacity retaining rate of 20th cycle is reduced from 48.3 to 37.7 % for the x = 1 alloy and from 89.8 to 74.7 % for the x = 4 alloy by increasing milling time from 0 (as-cast is defined as milling time of 0 h) to 70 h. The x = 1 alloy milled for 30 h obtains the maximum discharge capacity of 418.5 mA h g−1, and for x = 4 alloy milled for 10 h, it is 341.5 mA h g−1. The effect of milling time on the electrochemical kinetics of the alloys is related to Y content. When Y content is x = 1, the high rate discharge ability (HRD), diffusion coefficient of hydrogen atom (D), and charge transfer rate all increase with milling time prolonging. However, for x = 4 alloy, the electrochemical kinetics is getting steadily worse with ball milling time increasing.

Voltammetric and impedance behaviours of surface-treated nano-crystalline diamond film electrodes

The electrochemical performances of hydrogen- and oxygen-terminated nano-crystalline diamond film electrodes were investigated by cyclic voltammetry and AC impedance spectroscopy. In addition, the surface morphologies, phase structures, and chemical states of the two diamond films were analysed by scanning probe microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. The results indicated that the potential window is narrower for the hydrogen-terminated nano-crystalline diamond film than for the oxygen-terminated one. The diamond film resistance and capacitance of oxygen-terminated diamond film are much larger than those of the hydrogen-terminated diamond film, and the polarization resistances and double-layer capacitance corresponding to oxygen-terminated diamond film are both one order of magnitude larger than those corresponding to the hydrogen-terminated diamond film. The electrochemical behaviours of the two diamond film electrodes are discussed.

Hierarchically structured nanocrystalline photoanode: Self-assembled bi-functional TiO2 towards enhanced photovoltaic performance

We report a simple strategy towards nanocrystalline photoanodes with structural hierarchies (hn-electrode) and simultaneously enhanced dye-loading capacity and electron conductivity. Totally built up by TiO2 nanocrystallite (5–20nm), the hn-electrode comprised two major parts: the nanocrystalline matrix, and the bi-functional TiO2 mesostructure (BF-TiO2). BF-TiO2 was obtained from common sol–gel, ambient-drying and annealing processes, modified with very high H2O/Ti ratio (48.9:1) and hexamethylenetetramine additive. Two morphologies were found in BF-TiO2: high-surface-area mesoporous nanograin aggregates, and conductive nanosheets composed of both anatase TiO2 and oxygen-deficient Ti6O11, corresponding to its two functions. The preferential capping of amino groups on high-energy facets of TiO2 and the self-assembly process of nanograins along high-energy facets promoted the formation of nanosheets. Hn-electrodes with BF-TiO2 level of 5–40 % were prepared, which exhibited the maximum improvement of 110% in the dye loading capacity, much lower recombination frequency (8.1Hz vs. 31.5Hz), and much longer electron lifetime than the disordered counterpart. The optimal hn-electrode (20% BF-TiO2, ~16μm) with and without the scattering layer exhibited the conversion efficiency of 8.03% and 7.99%, ~29% higher than the control nanocrystalline electrode (6.24%). The work provides a versatile route towards functional-integrating hierarchical materials, and may find vast applications in relevant areas such as lithium batteries, supercapacitors and photocatalysts.

Lithium insertion into TiO2 (anatase): electrochemistry, Raman spectroscopy, and isotope labeling

This article presents a critical discussion of selected structural aspects of electrochemical Li-insertion into TiO2 (anatase). Recent works are reviewed (almost half of the cited works is from 2010+) with a special attention to the crystal-face-specific phenomena, Raman spectroscopy, and single-crystal and nanocrystalline electrodes. The benefits of isotopic labeling are highlighted for the in-depth understanding of Raman spectra and the Raman spectroelectrochemistry of Li-insertion. The persisting open questions and contradictory issues in the field are discussed too.