Double-layer Capacitance Measurements Research Articles

Effects of alkyl chain length and halide anion on hydrogen bonding, electrochemical transport properties and double layer capacitance in eutectic solvents

Eutectic solvents (ESs) have been widely studied due to their tunable solvation and physical properties. The properties of eutectic solvents that are formed from mixtures of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) are governed by a complex hydrogen (H) bonded network among their constituents. However, an understanding of the key factors effecting the H-bonding structure and dynamics, and the resulting physical and electrochemical properties at interfaces is still in its infancy. In this study, the influence of the cation’s alkyl chain length and the anion chemical identity on the characteristics of the H-bonding network in eutectic solvents were examined. In particular, ESs formed from mixtures of tetraalkylammonium salts as HBAs and ethylene glycol as the HBD were studied. Complementary spectroscopic studies revealed the changes in the solvation environment as a function of the ammonium chain length (C1, C2, C3, and C4) and the anion type (Cl−, Br−, I−). With an increase in the alkyl chain length or a decrease in the electronegativity of halide anions (X−), it was found that the O–H bond length shortens and the H···X− bond elongates in the H-bond (O–H···X−) structure, thus resulting in an increase of the solvated ion size and a decrease in the bulk ionic conductivity. Further, double layer capacitance measurements showed that increased capacitance was correlated with shorter H-bonds in anion solvates.

Microstructural Engineering of Flow Battery Electrodes with Non-Solvent Induced Phase Separation

Rising levels of greenhouse gases urge the decarbonization of human activities. Electrification of strategic sectors such as transport, industry, and heating necessitates affordable, safe, and dispatchable large-scale energy storage systems. Redox flow batteries (RFBs) stand as a promising long-duration and large-scale energy storage technology due to their design features[1]. Ongoing research focuses on engineering key cell components (e.g. membranes, electrodes, active materials) to improve cost competitiveness. This work focuses on the fabrication of novel porous carbon electrodes which strongly influence the performance and cost of the technology. The requirements for porous electrodes liquid-phase electrochemical reactors are contradictory: a large surface area is needed to overall exchange current density, but also minimal pressure losses to decrease pumping costs. Additionally, surface properties of the electrode dictate the reaction kinetics whereas its microstructure controls the mass transport of the reactants within the reactor area[2]. However, the current state-of-the-art porous electrodes for RFBs consists of carbon-fiber materials repurposed from other electrochemical technologies (e.g. polymer electrolyte fuel cells) which do not offer a favorable tradeoff between transport properties and surface area. Additionally, their production involves complex manufacturing processes resulting in high cost and limited morphological versatility. Therefore, there is a need to develop alternative synthetic platforms which offer tailorable microstructures and surface properties to match the requirements of emerging electrochemical applications, such as redox flow batteries. In this work, we draw inspiration from the membrane technology and employ non-solvent induced phase separation (NIPS) as a method to synthesize porous carbon electrodes for RFBs[3,4].Here, we present our latest efforts to develop a manufacturing route enabling the control of microstructural features of carbonaceous porous electrodes for RFBs. The microstructure of the carbon scaffold is determined by the nature and composition of the polymer blend used to cast the electrode precursor. We use polyacrylonitrile as scaffold precursor and polyvinylpyrrolidone as a pore forming agent, dissolved in an organic solvent (i.e., dimethylformamide, dimethylsulfoxide or N-methylpyrrolidone). The polymer blends were cast into a mold and immersed in a water coagulation bath prior to being stabilized (270 °C) and carbonized (1050 °C). Various process parameters (e.g. polymer concentration, bath temperature, solvent type) governing the final electrode properties were leveraged to control structural features, opening new opportunities for the fabrication of diverse microstructures (i.e., porosity gradient, isoporosity and with macrovoids). Electrochemical double layer capacitance and pressure drop measurements show an increased design space compared to carbon-fiber electrodes, confirming the versatility of the synthetic route. Microstructural analysis of the prepared NIPS electrodes reveals two porosity domains: large finger-like macrovoids (throat diameter >50 μm) and smaller interconnected microvoids (pore diameter 2-15 μm). Flow cell polarization and electrochemical impedance spectroscopy measurements in two common flow battery electrolytes, an Fe2+/Fe3+ single electrolyte and vanadium full cell, show improvements in performance compared to traditional electrodes. An increase in the polymer concentration of the polymer blend solution produced NIPS electrodes with smaller pores and resulted in better mass transport within the electrode. When the temperature of the coagulation bath was increased to 40 ºC, the electrode has higher electrochemically active surface area, producing a significant power increase in the sluggish vanadium electrolyte. Overall, the NIPS electrodes operate at significantly lower activation and mass transport overpotentials in all-vanadium RFBs, generating higher electrical power at the expense of a slight pressure drop increase. Although preliminary, we demonstrate the potential of NIPS to manufacture electrodes with tailorable microstructures for convection-enhanced electrochemical technologies.[1] T. M. Gür, Energy Environ. Sci. 2018, 11, 2696.[2] A. Forner-Cuenca, E. E. Penn, A. M. Oliveira, F. R. Brushett, J. Electrochem. Soc. 2019, 166, A2230.[3] R. R. Jacquemond, C. T.-C. Wan, Y.-M. Chiang, Z. Borneman, F. R. Brushett, K. Nijmeijer, A. Forner-Cuenca, Cell Reports Physical Science 2022, 3, 100943.[4] C. T.-C. Wan, R. R. Jacquemond, Y.-M. Chiang, K. Nijmeijer, F. R. Brushett, A. Forner-Cuenca, Adv. Mater. 2021, 10. Figure 1

On the Electrochemically Active Surface Area Determination of Electrodeposited Porous Cu 3D Nanostructures

Porous 3D Cu layers with the following average parameters: thickness ~35 µm, pore density ~4.0 × 106 cm−2, and pore sizes ~25 µm were electrodeposited from an acidic sulphate electrolyte, and the suitability of different electrochemically active surface area determination methods for characterising these electrodes was assessed. Structural characterisation of the samples was conducted using SEM and an optical profiler, while electrochemical measurements were performed using cyclic voltammetry and electrochemical impedance spectroscopy. The evaluation of electrochemically active surface area involved the underpotential deposition of Tl and Pb monolayers as well as double-layer capacitance measurements. The results obtained indicate that both methods yield similar results for non-porous Cu electrodes. However, for Cu 3D nanostructures, the evaluation mode significantly influences the results. Double-layer capacitance measurements show significantly higher values for the electrochemically active surface area compared to the underpotential deposition (UPD) technique. The complex spatial structure of the Cu 3D layer hinders the formation of a continuous monolayer during the UPD process, which is the principal reason for the observed differences.

Promoting electrocatalytic CO2 reduction via anchoring quaternary piperidinium cations onto copper electrode

Cation identity in the electrolyte is known to have a critical influence on electrocatalytic CO2 reduction reaction (CO2RR). However, the impact of cation aggregation state at the electrode/electrolyte interface, particularly for organic cations, remains unclear for CO2RR. In this study, we investigated the effect of the organic cation aggregation state, by examining the CO2RR behaviors over a copper (Cu) electrode, before and after the surface modification of a polyelectrolyte (i.e., quaternary ammonia poly (N-methyl-piperidine-co-p-terphenyl) with grafted piperidinium cations), in an aqueous electrolyte with free-moving piperidinium (Pip+) cations. Immobilizing the Pip+ cations on Cu surface was found to effectively promote the CO2RR selectivity. The underlying cause was further explored by in-situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy and electrochemical double-layer capacitance measurements, illustrating that the enhanced CO2RR performance was due to the change in electrode/electrolyte interface properties. These findings would contribute to further optimization of the electrode/electrolyte interface for efficient CO2RR.

Geometric Catalyst Utilization in Zero-Gap CO2 Electrolyzers.

The electrochemical reduction of CO2 (CO2RR) on silver catalysts has been demonstrated under elevated current density, longer reaction times, and intermittent operation. Maintaining performance requires that CO2 can access the entire geometric catalyst area, thus maximizing catalyst utilization. Here we probe the time-dependent factors impacting geometric catalyst utilization for CO2RR in a zero-gap membrane electrode assembly. We use three flow fields (serpentine, parallel, and interdigitated) as tools to disambiguate cell behavior. Cathode pressure drop is found to play the most critical role in maintaining catalyst utilization at all time scales by encouraging in-plane CO2 transport throughout the gas-diffusion layer (GDL) and around salt and water blockages. The serpentine flow channel with the highest pressure drop is then the most failure-resistant, achieving a CO partial current density of 205 mA/cm2 at 2.76 V. These findings are confirmed through selectivity measurements over time, double-layer capacitance measurements to estimate GDL flooding, and transport modeling of the spatial CO2 concentration.

Understanding the Temperature Effects on CO2 Electrolysis Performance at High Current Densities

The CO2 electrolysis driven with renewable sources is a promising alternative to mitigate greenhouse gas emissions by converting CO2 into valuable feedstocks and storing renewable electrical energy 1. Membrane electrode assemblies (MEAs) equipped with gas diffusion electrodes (GDEs) have shown great potential to overcome the current limitation of aqueous-fed systems while bringing this technology to economically-competing levels 2. In the last decade, many studies have been devoted to developing efficient catalyst materials and reactor designs; however, the effect of operating conditions such as temperature has not been thoroughly studied3. Given that the temperature affects CO2 electrolysis in a complex way (simultaneous effects on the CO2 diffusivity, solubility, the ionic conductivity of the membrane, and surface wettability of the GDE4), a systematic investigation is necessary to determine temperature influence on the product distributionIn this study, we investigate the temperature effects on CO2 electrolysis of Cu-based GDEs in an MEA-based approach in a temperature range between 25 and 80˚C, to enhance the selectivity of C2+ products and the energy efficiency while suppressing the hydrogen evolution (HER) and the degradation of the GDE and the membrane. For this investigation, a robust system for controlling and measuring the temperature of all the system components was developed, and we simultaneously set up proper guidelines to perform these electrocatalytic temperature measurements in a consistent and reproducible way.For evaluating the temperature influence on electrocatalytic performance, a series of electrochemical measurements such as linear sweep voltammetry (LSV), double-layer capacitance measurements (DLC), potentiostatic and galvanostatic experiments were performed. The obtained results provide insights into how CO2 diffusion, reaction kinetics, and CO2 mass transport vary with temperature and affect the overall performance. We observed improvement in reaction rates and a drop in cell voltages at higher temperatures due to the enhancement of membranes' ionic conductivity and water management. The experiments focused on selectivity and product crossover revealed a specific trend at temperatures above 60˚C for gas and liquid products, setting up the optimal conditions for a stable operation with higher faradaic efficiencies of carbon-based compounds.1 A. Vasileff, Y. Zheng and S. Z. Qiao, Adv. Energy Mater., 2017, 7, 1–21.2 T. Burdyny and W. A. Smith, Energy Environ. Sci., 2019, 12, 1442–1453.3 B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar and C. Janáky, Prog. Energy Combust. Sci., 2017, 62, 133–154.4 A. Löwe, C. Rieg, T. Hierlemann, N. Salas, D. Kopljar, N. Wagner and E. Klemm, ChemElectroChem, 2019, 6, 4497–4506.

Potential Difference-Modulated Synthesis of Self-Standing Covalent Organic Framework Membranes at Liquid/Liquid Interfaces.

Covalent organic framework (COF) membranes with tailored functionalities hold great promise in diverse applications, but the key to realize their full advantages of highly ordered pore structures is the development of membrane fabrication approaches. In this work, we report a potential difference-modulated biphasic strategy to fabricate large-area, self-standing COF membranes under ambient conditions. The fabrication was conducted at the polarized water/1,2-dichloroethane (water/DCE) interface, where HCl was dissolved in water as a catalyst and monomers (both amine and aldehyde) were added to DCE. The external polarization of the water/DCE interface by cyclic voltammetry can continuously pump H+ from water to DCE to boost the Schiff base reaction of monomers and the growth of COF membranes. Moreover, the growth process can be real-time-monitored by interfacial double-layer capacitance measurement, and the permeability of COF membranes can be in situ-examined by heterogeneous ion transfer voltammetry. Given that the potential difference across the water/DCE interface can be also facilely modulated by dissolving proper electrolyte ions in two phases, the fabrication of large-area COF membranes is made possible in beakers. Using this strategy and different monomers, three types of centimeter-scale, free-standing COF membranes with tunable pore size and surface functionality were prepared, and their defect-free structure was proved by the molecular permeance and ultrafiltration test. We believe that this biphasic strategy offers a controllable and scalable way to fabricate COF membranes and sheds light on development of novel self-supporting membranes with unique functions.

Investigation of Ethylene and Propylene Production from CO2 Reduction over Copper Nanocubes in an MEA-Type Electrolyzer.

Previous work carried out in fully liquid environments and at low current densities has demonstrated that highly uniform faceted copper nanocrystals display different selectivity profiles in CO2 reduction compared to polycrystalline copper. As part of ongoing upscaling efforts, it is a matter of interest to investigate whether the high selectivity toward ethylene of copper nanocubes, which show a preferential (100) orientation, is maintained in gas-fed electrolyzers, thus enabling the energy-efficient production of this valuable commodity chemical at industrially relevant current densities. In this work, we assessed the electrochemical CO2 reduction reaction performance of highly uniform copper nanocubes loaded onto gas diffusion electrodes (GDEs) in a zero-gap device. The copper nanocube-loaded GDEs maintained high Faradaic efficiencies toward ethylene at elevated total current densities, resulting in higher overall partial current densities toward this product compared to benchmark electrodes. Interestingly, CO2 reduction to propylene, albeit with low partial current densities, was also observed. However, double-layer capacitance measurements revealed that the performance observed at high current densities is significantly influenced by electrode flooding. The findings of this study can inform future efforts geared toward optimizing the electrodes with this promising class of catalysts.

Regulating oxygen vacancies in Co3O4 by combining solution reduction and Ni2+ impregnation for oxygen evolution reaction

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 cm−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.

Synergistic Effects of Co and Fe on the Oxygen Evolution Reaction Activity of LaCox Fe1-x O3.

In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCo x Fe1−x O3. Phase pure perovskites were synthesized up to x=0.300 in 0.025/0.050 steps. HAADF STEM and EDX analysis points towards FeO2‐terminated (001)‐facets in LaFeO3, in accordance with the stability diagram obtained from density functional theory calculations with a Hubbard U term (DFT+U). Linear sweep voltammetry conducted in a rotating disk electrode setup shows a reduction of the OER overpotential and a nonmonotonic trend with x, with double layer capacitance measurements indicating an intrinsic nature of activity. This is supported by DFT+U results that show reduced overpotentials for both Fe and Co reaction sites with the latter reaching values of 0.32–0.40 V, ∼0.3 V lower than for Fe. This correlates with a stronger reduction of the binding energy difference of the *O and *OH intermediates towards an optimum value of 1.6 eV for x=0.250 , the OH deprotonation being the potential limiting step in most cases. Significant variations of the magnetic moments of both surface and subsurface Co and Fe during OER demonstrate that the beneficial effect is a result of a concerted action involving many surrounding ions, which extends the concept of the active site.

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Measurement of Electric Double Layer Capacitance Using Dielectrophoresis-Based Particle Manipulation.

An electric double layer (EDL) generally exists at the interface between a conductive electrode and its adjacent liquid electrolyte. Accurate measurement of the capacitance of EDL is requisite but a great challenge due to the complexity of its variation mechanism correlated with the magnitude and frequency of applied signals and the difficulty in measuring the inner layer potentials across the EDL. Herein, a novel dielectrophoresis (DEP)-based approach is proposed to measure the capacitance of an EDL at a microelectrode/electrolyte interface. The measurement is achieved by employing DEP manipulation to micro polystyrene (PS) spheres suspended in a liquid electrolyte and determining the capacitance of EDL on the microelectrodes from the moving velocities of spheres. This method allows measurement of the capacitances of EDL under alternating current (AC) signals with different magnitudes and frequencies, so that the capacitance change with the magnitude and frequency of the applied signal can be characterized. The method avoids the impedance interference from the liquid electrolyte, external measuring systems, and other crosstalks, enabling an accurate measurement of double layer capacitance. In addition, the inner layer potentials across EDL under different magnitudes and frequencies of applied signals are comprehensively investigated, which facilitates an understanding of the ion behavior at the interfacial boundary that governs external observations of electrochemical reactions. The accurate measurement of the capacitance of EDL is of significance to explore the mechanism of interfacial functioning of electrochemical and bioelectrical devices and systems.

Inception of Co3O4 as Microstructural Support to Promote Alkaline Oxygen Evolution Reaction for Co0.85Se/Co9Se8 Network.

Developing electrocatalysts with abundant active sites is a substantial challenge to reduce the overpotential requirement for the alkaline oxygen evolution reaction (OER). In this work, we have aimed to improve the catalytic activity of cobalt selenides by growing them over the self-supported Co3O4 microrods. Initially, Co3O4 microrods were synthesized through annealing of an as-prepared cobalt oxalate precursor. The subsequent selenization of Co3O4 resulted in the formation of a grainy rodlike Co3O4/Co0.85Se/Co9Se8 network. The structural and morphological analysis reveals the presence of Co3O4 even after the selenization treatment where the cobalt selenide nanograins are randomly covered over the Co3O4 support. The resultant electrode shows superior electrocatalytic activity toward OER in alkaline medium by delivering a benchmark current density of 10 mA/cm2geo at an overpotential of 330 mV. As a comparison, we have developed Co0.85Se/Co9Se8 under similar conditions and evaluated its OER activity. This material consumes an overpotential of 360 mV to deliver the benchmark current density, which signifies the role of the Co3O4 support to improve the electrocatalytic activity of Co0.85Se/Co9Se8. Despite having a low TOF value for Co3O4/Co0.85Se/Co9Se8 (0.0076 s-1) compared to Co0.85Se/Co9Se8 (0.0102 s-1), the improved catalytic activity of Co3O4/Co0.85Se/Co9Se8 is attributed to the presence of a higher number of active sites rather than the improved per site activity. This is further supported from the Cdl (double layer capacitance) measurements where Co3O4/Co0.85Se/Co9Se8 and Co0.85Se/Co9Se8 tender Cdl values of about 8.19 and 1.08 mF/cm2, respectively, after electrochemical precondition. As-prepared Co3O4/Co0.85Se/Co9Se8 also manifests rapid kinetics (low Tafel slope ∼ 91 mV/dec), long-term stability, low charge-transfer resistance, and 82% Faradaic efficiency for alkaline electrocatalysis (pH = 14). Furthermore, the proton reaction order (ρRHE) is found to be 0.65, indicating a proton decoupled electron transfer (PDET) mechanism for alkaline OER. Thus, the Co3O4 support helps in the exposure of more catalytic sites of Co0.85Se/Co9Se8 to deliver the improved catalytic activities in alkaline medium.

Cathodic activation of synthesized highly defective monoclinic hydroxyl‐functionalized ZrO 2 nanoparticles for efficient electrochemical production of hydrogen in alkaline media

The high electrochemical stability of Zirconia (ZrO2) at high potentials strongly suggested it as an alternative to carbon supports, which experience reduced efficiency due to some corrosion problems particularly during prolonged electrocatalysis activity. However, the use of ZrO2 was limited by its low electrical conductivity and surface area. In this work, we developed a methodology for synthesizing monoclinic ZrO2 NPs with increased surface area and improved electrical/electrocatalytic characteristics without using any carbon-based co-support material or any metallic nanoparticles. In this context, for the first time, highly defective hydroxyl-functionalized ZrO2 NPs (designated here as ZT NPs) were prepared by a hydrothermal route in the presence of sodium tartrate as a mineralizer. XRD analysis demonstrated that the produced zirconia was semicrystalline microspheres, consisting of monoclinic ZrO2 NPs with high lattice defects. The addition of tartrate ions decreased the crystallite size and increased the defects and microstrain. At the same time, the alkaline hydrogen evolution reaction (HER) catalytic activity of ZrO2 NPs was significantly increased when using sodium tartrate as mineralizer; the overpotential required to obtain 10 mA cm−2 (η10) dropped down from 490 to merely 235 mV, while an exchange current density (jo) increased 12 times to 0.22 mA cm−2. The presence of structural defects (revealed by XRD) and the increased number of active surface sites contending O-H groups (evidenced from ATR-FTIR and XPS) as well as the enhanced electrochemical active surface area (confirmed from double-layer capacitance measurements) were the main reasons behind the high catalytic performance. The ZrO2 NPs catalytic activity increased even further during the long-term stability tests under severe cathodic conditions (ZT*, ZrO2 NPs obtained after the long-term stability, has jo = 0.47 mA cm−2 and η10 = 140 mV), approaching the activity of Pt/C catalyst. This process was assisted by mineralizer removal from the catalyst (testified by XPS). Our studies revealed that ZT* are characterized by larger electroactive surface area and more structure defects compared to ZT, where surface area and microstrains resulting from surface hydroxylation open cavities in zirconia structure.

Applicability of double layer capacitance measurements to monitor local temperature changes at polymer electrolyte membrane fuel cell cathodes

One of the most sensitive parameters to control the performance of automotive-size polymer electrolyte membrane fuel cells (PEMFCs) is the cell temperature. For instance, when starting up the fuel cell, it is critical to know whether the cell temperature is below ~273 K or, approaches the operating conditions. The easiest way to determine this is to perform measurements using some commercially available temperature sensors at the active electrode surface. However, this only applies to single-cells and not for automotive fuel cell stacks with ~400 cells. In this case, the measurement with a standard resistive temperature sensor is practically problematic. In this paper, we demonstrate that the electric double layer capacitance of the PEMFC cathodes (geometric active area size of 43.6 cm2) increases linearly from 268 K up to 333 K; and using this information, it is possible to determine the catalyst temperature accurately within this temperature range. These results should be of particular significance for both modelling the double layer in the PEMFCs as well as for the practical use in automotive applications.

Electrodeposits of nickel with reduced graphene oxide (Ni/rGO) and their enhanced electroactivity towards hydrogen evolution in water electrolysis

Catalytic performance of nickel cathodes for hydrogen production by water splitting can be improved by carbon nanomaterials, in particular graphene and reduced graphene oxide (rGO). In this work, electrodeposits Ni/rGO were examined in 0.1 M Na2SO4 with emphasis on hydrogen evolution reaction (HER) and effect of anodic oxidation. Ni and Ni/rGO were electrodeposited by galvanic square wave method with sonication. In contrast to smooth deposits of Ni, deposits of Ni/rGO had the cumulus/cauliflower type morphology with developed surface. Their increased surface area was confirmed by measurements of double layer capacitance. HER and anodic polarization currents were for Ni/rGO significantly higher than those for Ni. Voltammetric cycling or potentiostatic polarization to and above potentials of stability of Ni oxides (Ni3O4, Ni2O3 or NiO2) caused a rise in HER for both materials, but especially strongly for Ni/rGO. Enhanced HER on Ni/rGO was explained by greater surface area and the formation of larger amounts of electrocatalytic Ni oxides/hydroxides, as a result of the increased anodic reactivity of rGO-containing Ni. Incorporated rGO might also contribute by increasing electronic conductivity.

Template-Free Nanostructured Fluorine-Doped Tin Oxide Scaffolds for Photoelectrochemical Water Splitting.

The synthesis and characterization of highly stable and conductive F:SnO2 (FTO) nanopyramid arrays are investigated, and their use as scaffolds for water splitting is demonstrated. Current densities during the oxygen evolution reaction with a NiFeOx catalyst at 2 V vs reversible hydrogen electrode were increased 5-fold when substituting commercial FTO (TEC 15) by nanostructured FTO scaffolds. In addition, thin α-Fe2O3 films (∼50 nm thick) were employed as a proof of concept to show the effect of our nanostructured scaffolds during photoelectrochemical water splitting. Double-layer capacitance measurements showed a drastic increase of the relative electrochemically active surface area for the nanostructured samples, in agreement with the observed photocurrent enhancement, whereas UV–vis spectroscopy indicates full absorption of visible light at wavelengths below 600 nm.

Oxygen deficient V2O3: A stable and efficient electrocatalyst for HER and high performance EDLCs

The demand of materials for energy conversion and storage is increasing to address the issues related to the depletion of fossil fuels. In the present study, oxygen deficient vanadium oxide (V2O3) has been synthesized in an autoclave at 500 °C for 4 h. The reaction temperature, time and atmosphere affect the pure phase formation of V2O3. The structural, morphological and surface characteristics of pure phase V2O3 has been analysed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The phase transition with respect to temperature has been confirmed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrocatalytic hydrogen evolution reaction (HER) activity has been studied in an acidic medium. The synthesized sample shows higher cyclic stability for 1000 cyclic voltammetry (CV) cycles. It also exhibits a lower Tafel slope of 81.2 mVdec−1. The electrochemical double layer capacitance (EDLC) measurements was done via CV measurements performed at various scan rates. The observed EDLC value of synthesized sample is 242.6 μFcm−2. The study predicts potential applications of V2O3 nano structures for electrochemical applications like HER and supercapacitors.

Dynamic-Stacking of Reduced Graphene Oxide for Active Hydrogen Evolution

Despite remarkable electronic and mechanical properties of graphene, improving catalytic activity of graphene, an atomically-flat, inert and stable carbon network, remains a challenging issue in both fundamental and application studies. In particular, the adsorption of most molecules and ions, including hydrogen (H2 or H+), on graphene is not favorable, underlining the challenge for efficient electrochemical catalytic reaction on graphene electrode. Various defects, edges, and functionalization have been suggested to resolve the issue aiming to have catalytic graphene, but cost-effective, clean and active catalysis with graphene has not been realized yet. Here, we introduce dynamic-stacking of reduced graphene oxide (rGO) via an electrochemical way; conventional hydrogen evolution reaction (HER) was used as a dynamic process, generating hydrogen bubbles during electrochemical deposition of rGO on metallic WTe2, which creates a unique porous rGO structure on the atomically-distorted WTe2 surface with optimized edges and surface acidic groups. The hydrogen bubble template method enables an ideal, stable carbon structure for active HER with a Tafel slope of 39 mV·dec-1 and a double layer capacitance of 12.41 mF·cm-2. As our new dynamic synthesis method improves the catalytic activity of one of the most inert materials, graphene, new strategies for designing novel catalysts could be conceived for promising energy applications. Figure 1. 1a) schematically description of the electrochemical synthesis of the rGO film on WTe2 by hydrogen bubble template method. Fig. 1b) shows the graphite and WTe2 shows poor HER activity (blue and black curve). Meanwhile, a striking HER activity is observed with our hybrid rGO-WTe2 (red curve) with a much smaller onset overpotential (vs reversible hydrogen electrode, RHE) and Tafel slope 39 mV·dec-1 (Fig. 1c). The unique HER performance of our rGO-WTe2 catalyst was investigated in terms of electrochemical surface area via double-layer capacitance (Cdl) measurements in Fig. 1d). The Cdl value was calculated by the slope of current density (at a voltage of 0.135 V vs. RHE) versus scan rate, and the value was 12.41 mF·cm-2, which is 5,000 times higher than that of state-of-the-art 3D graphene networks.[3] Fig. 1e) shows the optical microscopy image of rGO. The as-synthesized rGO on WTe2 substrate exhibits a highly porous structure in the SEM images in Fig. 1 (f-g). The characteristic dimension of the porous structure ranges from 100 nm - 2 μm in diameters with many edges exposed between rGO porous walls; the rGO flakes are stacked in a random way to have many edges as HER active sites. As HR-TEM images shown in Fig. 1 (h-i), the rGO flakes are mixed with crystal and amorphous graphene with an interlayer spacing of 0.43 nm. Keywords: hydrogen evolution reaction, hydrogen bubble template, reduced graphene oxide, tungsten ditelluride (WTe2)

Novel design of hollow g-C3N4 nanofibers decorated with MoS2 and S, N-doped graphene for ternary heterostructures.

Herein, we newly design a ternary structure of 1-dimensional hollow g-C3N4 nanofibers (HGCNF) decorated with molybdenum disulfide (MoS2) and sulfur/nitrogen-doped graphene (SNG) via a one-pot hydrothermal treatment at relatively low temperature. The firstly presented HGCNF are fabricated using electrospinning followed by the thermal sintering method. After that, MoS2 is grown onto HGCNF, while SNG covered the structures during the hydrothermal method. We observed the morphological structures, chemical composition and optical absorbance of this ternary HGCNF/SNG/MoS2 structure. Of the as-prepared catalysts, HGCNF/SNG/MoS2 exhibited a good possibility to produce hydrogen as an electrocatalyst. Furthermore, we evaluated its stability performance using chronoamperometry for 48 hours, as well as by 3000 cycles of cyclic voltammetry. From the double-layer capacitance measurement, HGCNF/SNG/MoS2 proved itself as an electrocatalyst due to the higher value of electrocatalytically active sites to be 6.97 × 10-3 F cm-2 than that of only HGCNF (0.18 × 10-5 F cm-2) and the binary structure of HGCNF/MoS2 (2.54 × 10-3 F cm-2). We believe that our novel 1-dimensional ternary HGCNF/SNG/MoS2 structure has expedited the electron pathways by reducing the resistance at interfaces among HGCNF, SNG and MoS2, to be potentially useful for the hydrogen evolution reaction.