Formation Of Electrical Double Layer Research Articles

Liquid-solid contact electrification and its effect on the formation of electric double layer: An atomic-level investigation

Contact electrification at liquid-solid interface has aroused many research interests. Some recent researches suggest that the formation of electric double layer may be dominated by the charge transfer from liquid-solid contact electrification, which challenges the traditional electric double layer models. However, how the liquid-solid contact electrification affects the electrical double layer is still lacking of atomic-level mechanism details. In this work, an atomic-level investigation on the effect of contact electrification on the liquid-solid electric double layer are performed via first-principles and molecular dynamics simulations. The results suggest that the liquid-solid contact electrification can influence the ions distribution in the solution, thus influencing the formation of electric double layer and can even reversing its polarity under specific surface charge density. The mechanism is that the contact charges on the solid surface can affect the adsorption energy of anion/cation at the liquid-solid interface. The conclusion of this work can provide a more comprehensive understanding of the effect of contact electrification to the electric double layer.

Boosted energy harvesting in droplet electrochemical cell with non-equilibrium electrical double layer

Droplet energy harvesting technologies, such as droplet triboelectric nanogenerators (D-TENGs) and droplet-based electricity generators (DEGs), which are not restricted by geographical location, have been vigorously developed in recent years. However, achieving electricity generation with high performance is still challenging. Here, we demonstrate a new approach for droplet-based energy harvesting device named droplet electrochemical cell (DECC) which could enhance energy generation performance by utilizing the formation of non-equilibrium electrical double layers. The DECC is designed by drawing inspiration from the zinc-copper primary battery system and demonstrated remarkable results when tested with dynamically impinging water droplets. The peak current and transferred charge generated by a 100 μL droplet are found to be 400 μA and 3.2 μC, respectively. The short circuit current is also found to increase by approximately 30 times when the dynamic impinging frequency of the droplets is increased. This study provides valuable insights into the development of energy harvesting devices based on droplets.

High-Performance Supercapacitors: A Comprehensive Review on Paradigm Shift of Conventional Energy Storage Devices

The enormous demand for energy due to rapid technological developments pushes mankind to the limits in the exploration of high-performance energy devices. Among the two major energy storage devices (capacitors and batteries), electrochemical capacitors (known as ‘Supercapacitors’) play a crucial role in the storage and supply of conserved energy from various sustainable sources. The high power density and the ultra-high cyclic stability are the attractive characteristics of supercapacitors. However, the low energy density is a major downside of them, which is also responsible for the extensive research in this field to help the charge storage capabilities thrive to their limits. Discoveries of electrical double-layer formation, pseudocapacitive and intercalation-type (battery-type) behaviors drastically improved the electrochemical performances of supercapacitors. The introduction of nanostructured active materials (carbon-/metal-/redox-active-polymer/metal-organic/covalent-organic framework-based electrode materials), electrolytes (conventional aqueous and unconventional systems) with superior electrochemical stability and unprecedented device architectures further boosted their charge storage characteristics. In addition, the detailed investigations of the various processes at the electrode–electrolyte interfaces enable us to reinforce the present techniques and the approaches toward high-performance and next-generation supercapacitors. In this review, the fundamental concepts of the supercapacitor device in terms of components, assembly, evaluation, charge storage mechanism, and advanced properties are comprehensively discussed with representative examples.

Surface charge induced bioelectricity generation from freshwater macroalgae Pithophora

A new concept of energy harvesting from the interaction of water molecule with macroalgal surface charges has been developed. The formation of electrical double layers on the charged surface of algae was utilized to generate the voltage and current between two aluminum electrodes along the length of the biofilm. In this study, a facile and efficient algal hydrovoltaic device based on the macroalgae Pithophora roettleri was demonstrated which can generate continuous electric power with a current of ∼4.4 μA and voltage ∼0.32 V. Seven series connected devices can glow an LED light, without any external energy. The superior performance of the device is attributed to the intrinsic properties of the Pithophora algal film, including its hydrophilicity, filamentous structure, conductivity and more. This study is providing a new way to research on hydrovoltaic power generation integrated with electrostatic charges present on algal cytoplasmic membrane or surface.

Depression of electro-oxidation of Ti3C2Tx MXene Joule heater by alternating current for Joule heating membrane distillation

Membrane distillation (MD) is a promising technology with great potential to mitigate the crisis of water shortage. However, it still suffers from the low productivity due to that water vaporization on the membrane would reduce the transmembrane temperature and hence the permeate flux. Herein, an electrothermal membrane that can heat the feed solution on the membrane and hence enables the MD operation under a low electrical input was developed. This was achieved by coating a commercial PET-PTFE membrane with MXene nanosheets through a filtration method. The resultant MXene/PET-PTFE composite membrane could heat the surroundings (both air and brine) efficiently, showing a conversion efficiency of 73% at 1 W. Furthermore, it was shown that the composite membrane, while degrades rapidly in highly ionizable brine under a direct current (d.c.) input, could work stably over a long duration at high potential under high-frequency alternating current (a.c.) input. Systematic electrochemical impedance spectroscopic investigation suggested that the reciprocal vibration of ions under a high-frequency a.c. could inhibit the formation of electrical double layer, and thus depresses the electrochemical oxidation of MXene and water splitting. Furthermore, the composite membrane enabled efficient in-situ heating in a Joule heating MD (JHMD) system, delivering a permeate flux of 1.8 kg m−2h−1 at an input power of 3 W, which is 333% higher than that based on the conventional MD process. Theoretical simulation of the temperature distribution by a finite element method (FEM) confirmed a higher transmembrane temperature in the JHMD system than in the conventional MD, which is responsible for the performance enhancement.

Flexible iontronic sensors with high-precision and high-sensitivity detection for pressure and temperature

Flexible sensor with high precision and sensitivity in detecting pressure and temperature is highly desired. Herein, we design a flexible iontronic sensor by sandwiching a sandpaper-like ionogel layer between two silver nanowires-loaded polymeric nanofiber films. The obtained sensor can serve as a flexible pressure sensor with high precision (1.18 Pa), high sensitivity (106.01 kPa−1), wide measuring range (0–77 kPa), fast response/recovery time (16 ms/25 ms), and good stability over 2000 cycles because of the continuous change in the contract between electrode and dielectric layer under pressure and the formation of electric double layer between them. Owing to these excellent merits in pressure sense, the as-prepared sensor can real-time monitor tiny pressure caused by pulse, facial movements, and swallowing. More interestingly, this sensor can also serve as a temperature sensor, which has outstanding integrated performance of high precision of 0.02 °C, high sensitivity of 28.1% °C−1 (20–40 °C), wide operating window from −40 °C to 80 °C, superb repeatability and reliability, attributing to the monotonous relationship between the dielectric constant of TPU/IL ionogel and temperature. The designed sensor has bright prospects in flexible electronics and electronic skins for monitoring human health and human motion.

Hybrid pseudocapacitive sodium titanate/rGO and MXene/rGO nanocomposite electrodes in capacitive deionization

Two hybrid pseudocapacitive materials Na8Ti5O14/rGO (NTO/rGO) and Ti3C2Tx/rGO (MXene/rGO) were successfully synthesized using two-step easy hydrothermal processes, respectively. In this carefully nano-engineered architecture, the dense structure of NTO synthesized firstly were intertwined with the rGO nanosheets framework; whereas the highly organized thoroughly exfoliated MXene nanosheet were deposited with rGO nanosheets. The electrochemical capacitances of both hybrid pseudocapacitive nanocomposites were increased by 99.7 % and 72.7 % compared to the nanocrystals only, the capacitance retention and impendence were also improved. In the CDI experiments, the high salt removal capacities of the hybrid nanocomposite NTO/rGO and MXene/rGO were obtained as 75.6 mg g−1 and 64.3 mg g−1, which exceeded other reported values using similar electrodes. A 100 % regeneration in multiple cycles was achieved. This high desalting performance was attributed to the synergistic effect of efficient ion intercalation along with enhanced electrical conductivity and structure accessibility. These hybrid mechanism of ion intercalation and electrical double layer formation in action opened up the possibility of using hybrid pseudocapacitive nanocomposite electrodes such as Na8Ti5O14/rGO and Ti3C2Tx/rGO for effective capacitive desalination. Their respective selectivity in aqueous ionic solution will be investigated in future studies.

Biocompatible Potato-Starch Electrolyte-Based Coplanar Gate-Type Artificial Synaptic Transistors on Paper Substrates.

In this study, we propose the use of artificial synaptic transistors with coplanar-gate structures fabricated on paper substrates comprising biocompatible and low-cost potato-starch electrolyte and indium-gallium-zinc oxide (IGZO) channels. The electrical double layer (EDL) gating effect of potato-starch electrolytes enabled the emulation of biological synaptic plasticity. Frequency dependence measurements of capacitance using a metal-insulator-metal capacitor configuration showed a 1.27 μF/cm2 at a frequency of 10 Hz. Therefore, strong capacitive coupling was confirmed within the potato-starch electrolyte/IGZO channel interface owing to EDL formation because of internal proton migration. An electrical characteristics evaluation of the potato-starch EDL transistors through transfer and output curve resulted in counterclockwise hysteresis caused by proton migration in the electrolyte; the hysteresis window linearly increased with maximum gate voltage. A synaptic functionality evaluation with single-spike excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), and multi-spike EPSC resulted in mimicking short-term synaptic plasticity and signal transmission in the biological neural network. Further, channel conductance modulation by repetitive presynaptic stimuli, comprising potentiation and depression pulses, enabled stable modulation of synaptic weights, thereby validating the long-term plasticity. Finally, recognition simulations on the Modified National Institute of Standards and Technology (MNIST) handwritten digit database yielded a 92% recognition rate, thereby demonstrating the applicability of the proposed synaptic device to the neuromorphic system.

A flexible and ultrasensitive interfacial iontronic multisensory sensor with an array of unique “cup-shaped” microcolumns for detecting pressure and temperature

Flexible sensors with high-performance pressure and temperature sensing capabilities are highly desirable due to their widespread applications in electronic skins and wearable electronics. Utilizing the transition state between Cassie−Baxter state and Wenzel state during the surface coating on silicon template, we designed a dielectric layer with arrayed “cup-shaped” microcolumns composed of thermoplastic polyurethane (TPU) and ionic liquids (ILs), which was further applied to assemble with two flexible silver nanowires (AgNWs)/polydimethylsiloxane (PDMS)-based electrodes to form a flexible iontronic multisensory sensor with the electric double layer (EDL) generated at the interface between electrode and dielectric layer. As a pressure sensor, a high sensitivity of 87.75 kPa−1, wide detection range up to 170 kPa, high resolution of 0.22 Pa, and fast response/recovery time of 7.52/13.97 ms are achieved due to the formation of interfacial EDL and the unique structural evolution of the arrayed TPU/ILs cup-shaped microcolumns upon pressure. In addition, the as-prepared sensor also exhibits extraordinary temperature sensing performance, including a high resolution of 0.1 °C, high sensitivity of 52.77% °C−1, and wide detection range from − 40 to 90 °C, owing to the dependence of the dielectric constant of TPU/ILs layer on temperature. Attributed to these excellent sensing capabilities, our sensor possesses great potential in soft robotics, prosthetic prosthetics, and medical monitoring systems.

Streaming Electrification of Different Insulating Fluids in Power Transformers

This paper presents a detailed review of the streaming electrification phenomena of different insulating fluids for power transformers. The comparison of different techniques used to assess the charging tendency of fluids is discussed depending on the flow type (planar or centrifugal), volume of oil, and interface material. The charge separation between the insulating fluid and metallic/pressboard interfaces is explained in terms of the electrical double layer formation involving a fixed layer and diffuse layer. Based on the experimental results, the streaming electrification is observed to be a function of various factors such as speed, temperature, electric field, and surface roughness. Depending on the molecular structure of insulating liquids that come into contact with solid insulation at the interface, the streaming current can increase; hence, a suitable additive (benzotriazole, fullerene, Irgamet 39) is selected based on the type of fluid and charge polarity. The degradation of the insulating liquid upon ageing, which increases the streaming current and reclamation of such aged fluids using adsorbents (Fuller’s earth, activated carbon, bentonite, and alumina), is a possible method to suppress the static current through improving its dielectric properties. The nanofluids show a higher streaming current compared to base fluid with no change observed even after the reclamation process. The energization process using alternating current (AC) and direct current (DC) impacts the streaming phenomenon depending on its magnitude and polarity. The diffusion of sulfur compounds in the insulating liquid is another major hazard to transformers because the sulfide ions affect the physio-chemical reaction at the interface material, which is responsible for the formation of streaming current.

N-doped carbon materials produced by CVD with the compounds derived from LDHs

Layered double hydroxides (LDHs) of various compositions, i.e. Mg–Al, Mg–Mn–Al, are applied as the precursors of metal oxides for the preparation of N-doped carbon materials via chemical vapour deposition (CVD) with acetonitrile (as carbon and nitrogen source) at 600 and 700 °C. The use of Mn-containing LDHs for the preparation of the carbon materials is a novelty. The impact of transition metal species, i.e. MnxOy, in a blend of metal oxides derived from LDHs on the amount of carbon deposit and its composition, morphology, textural and capacitive properties is investigated. Mn-containing species occurring in a mixture of metal oxides enhance the quantity of carbonaceous product compared to those derived from Mg–Al LDHs. Thermally heated Mg–Mn–Al LDHs contain structural defects due to manganese oxides, which promote the formation of carbon deposit, especially higher production of amorphous carbons. The addition of Mn into Mg–Al LDHs matrix leads to carbon particles with increased N-doping and enhanced volume of mesopores. Furthermore, graphitic domains occurring in the carbon materials obtained with Mg–Mn–Al LDHs are thicker than those in the corresponding samples obtained with Mg–Al LDHs as Mn-containing species influence the concentration and location of N-containing groups in graphitic array. The specific capacitance of the carbon materials produced by CVD with the compounds derived from Mg–Al LDHs or Mg–Mn–Al LDHs is comparable (20–25 μF cm−2). The formation of electrical double layer at electrode/electrolyte interface is easier for the carbon materials prepared at 700 °C than for the carbon materials prepared at 600 °C. The maximum charge is stored either in the shallow parts of carbon particles for the former, as they contain bottleneck mesopores, or in the deep parts of carbon particles for the latter, as they contain slit-shaped mesopores.Graphical abstract

Water Adsorption on Mica Surfaces with Hydrophilicity Tuned by Counterion Types (Na, K, and Cs) and Structural Fluorination.

The stability of adsorbed water films on mineral surfaces has far-reaching implications in the Earth, environmental, and materials sciences. Here, we use the basal plane of phlogopite mica, an atomically smooth surface of a natural mineral, to investigate water film structure and stability as a function of two features that modulate surface hydrophilicity: the type of adsorbed counterions (Na, K, and Cs) and the substitution of structural OH groups by F atoms. We use molecular dynamics simulations combined with in situ high-resolution X-ray reflectivity to examine surface hydration over a range of water loadings, from the adsorption of isolated water molecules to the formation of clusters and films. We identify four regimes characterized by distinct adsorption energetics and different sensitivities to cation type and mineral fluorination: from 0 to 0.5 monolayer film thickness, the hydration of adsorbed ions; from 0.5 to 1 monolayer, the hydration of uncharged regions of the siloxane surface; from 1 to 1.5 monolayer, the attachment of isolated water molecules on the surface of the first monolayer; and for >1.5 monolayer, the formation of an incipient electrical double layer at the mineral-water interface.

Ion Transport in an Electrochemical Cell: A Theoretical Framework to Couple Dynamics of Double Layers and Redox Reactions for Multicomponent Electrolyte Solutions

Electrochemical devices often consist of multicomponent electrolyte solutions. Two processes influence the overall dynamics of these devices: the formation of electrical double layers and chemical conversion due to redox reactions. However, due to the presence of multiple length and time scales, it is challenging to simulate both processes directly from the Poisson-Nernst-Planck equations. Therefore, common modeling approaches ignore one of the processes, assume the two are independent, or extrapolate the results from reaction-free systems. To overcome these limitations, we formulate and derive an asymptotic model by solving the Poisson-Nernst-Planck equations for an arbitrary number of ions in the thin-double-layer limit. Our analysis reveals that there are two distinct timescales in the system: double-layer charging and bulk diffusion. Our model displays excellent quantitative agreement with direct numerical simulations. Further, our approach is computationally efficient and numerically stable, even for large potentials. We investigate the dynamics of charging for a binary electrolyte and three-ion system, and find that redox reactions impact the double-layer charging process at short times whereas they modify the double-layer capacitance at long times. Overall, the proposed theoretical framework advances our ability to simulate electrochemical devices that contain multiple ions and widens opportunities for future research in the field.

Electroactive Soft Actuators Based on Columnar Ionic Liquid Crystal/Polymer Composite Membrane Electrolytes Forming 3D Continuous Ionic Channels.

Here, we report low-voltage-driven fast-response nanostructured columnar ionic liquid crystal/polymer composite actuators that form three-dimensional continuous ion channels. A three-component self-assembly of a zwitterionic rod-like molecule (49.5 wt %), an ionic liquid (27.5 wt %), and poly(vinyl alcohol) (23.0 wt %) provided a free-standing stretchable membrane electrolyte. The dissociated ions can move through a continuous 3D ionophilic matrix surrounding the hydrophobic columns formed by the hexagonally organized rod-mesogens. Three-layer actuators composed of the electrolyte film sandwiched between two conductive polymer film electrodes of doped polythiophene exhibited a bending motion with 0.32% strain and moved 2 mm within 220 ms under 1 V at 0.1 Hz in 70% relative humidity due to the formation of electric double layers at the soft solid electrolyte/electrode interfaces. The bending strain of the columnar nanostructured actuator is comparable to those of polymer iongel actuators and block polymer actuators containing 25-80 wt % of ionic liquids. It is noteworthy that a small number of ions organized into the 3D nanochannels can generate the large bending deformation, which can contribute to reduce the risk of leakage of ions and the production cost. In addition, we have demonstrated a low-voltage-driven deformable mirror actuator that is expected to be applied to optical devices.

Poly(ionic liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface.

The widespread realization of wearable electronics requires printable active materials capable of operating at low voltages. Polymerized ionic liquid (PIL) block copolymers exhibit a thickness-independent double-layer capacitance that makes them a promising gating medium for the development of organic thin-film transistors (OTFTs) with low operating voltages and high switching speed. PIL block copolymer structure and self-assembly can influence ion conductivity and the resulting OTFT performance. In an OTFT, self-assembly of the PIL gate on the semiconducting polymer may differ from bulk self-assembly, which would directly influence electrical double-layer formation. To this end, we used poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) as a model semiconductor for our OTFTs, on which our PILs exhibited self-assembly. In this study, we explore this critical interface by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) of P(NDI2OD-T2) and a series of poly(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-random-poly(ethylene glycol) methyl ether methacrylate) (poly(S)-b-poly(VBBI+[X]-r-PEGMA)) block copolymers with varying PEGMA/VBBI+ ratios and three different mobile anions (where X = TFSI-, PF6-, or BF4-). We investigate the thin-film self-assembly of block copolymers as a function of device performance. Overall, a mixed orientation at the interface leads to improved device performance, while predominantly hexagonal packing leads to nonfunctional devices, regardless of the anion present. These PIL gated OTFTs were characterized with a threshold voltage below 1 V, making understanding of their structure-property relationships crucial to enabling the further development of high-performance gating materials.

Effect of the gate electrodes/water interface on the performance of ZnO-based water gate field-effect transistors

Field-effect transistors can be gated with water as the dielectric due to the formation of electrical double layers at water interfaces, which results in high specific capacitance and low voltage operation. The incorporation of analytes in aqueous suspension influences the water's electrical properties changing the water-gated field-effect transistors (WGFETs) performance and allowing it to be used as sensor and biosensor platforms. However, the material used as a gate electrode can affect the transistor's performance due to several factors, as the metal work function, its electrochemical range, and the presence of physicochemical reactions. Here, we evaluated the performance of WGFETs using spray-deposited zinc oxide (ZnO) as the active layer and five different gate electrodes: graphite pencil (GP), gold (Au), indium tin oxide (ITO), tungsten (W), and tin (Sn). The threshold voltage (VTH) values found were 0.41, 0.44, 1.12, 1.21 and 1.74 V for the Sn, W, Au, GP and ITO electrodes, respectively. The field-effect mobility was strongly influenced by the gate material, varying in the range from 0.07 to 0.46 cm2/Vs. The WGFETs operating with W and Sn gates showed the highest Ion/Ioff ratio and the lowest VTH when compared to the other electrodes. However, this characteristic may be associated with the formation of a natural oxide layer at its interface with water, which could be undesirable from the stability view. The use of ITO as the gate electrode resulted in a large hysteresis and two orders of magnitude smaller current in the output curve compared to Au and GP. Among the materials used as the gate electrode, GP stands out mainly because it gives the lowest subthreshold swing (SS = 90 mV/dec) and the highest transconductance (gm = 0.40 mS). Also, the GP matches with ZnO as biocompatible material for low-cost, eco-friendly, and metal-free electronics. In summary, the results showed that the gate electrode significantly influences the main parameters of the device and, therefore, should be an analysis factor when aiming to use WGFETs as sensors.

Comprehensive Analysis of Electrostatic Gating in Nanofluidic Systems.

Molecular transport in nanofluidic systems exhibits properties that are unique to the nanoscale. Here, the electrostatic and steric interactions between particle and surfaces become dominant in determining particle transport. At the solid-liquid interface of charged surfaces an electric double layer (EDL) forms due to electrostatic interactions between surfaces and charged particles. In these systems, tunable charge-selective nanochannels can be generated by manipulating electrostatic gating via co-ions exclusion and counterions enrichment of the EDL at the solid-liquid interface. In this context, electrostatic gating has been used to modulate the selectivity of nanofluidic membranes for drug delivery, nanofluidic transistors, and FlowFET, among other applications. While an extensive body of literature investigating nanofluidic systems exists, there is a lack of a comprehensive analysis accounting for all major parameters involved in these systems. Here we performed an all-encompassing modeling investigation corroborated by experimental analysis to assess the influence of nanochannel size, electrolyte properties, surface chemistry, gate voltage, dielectric properties, and molecular charge and size on the exclusion and enrichment of charged analytes in nanochannels. We found that the leakage current in electrostatic gating, often overlooked, plays a dominant role in molecular exclusion. Importantly, by independently considering all ionic species, we found that counterions compete for EDL formation at the surface proximity, resulting in concentration distributions that are nearly impossible to predict with analytical models. Achieving a deeper understanding of these nanofluidic phenomena will help the development of innovative miniaturized systems for both medical and industrial applications.

Architectures of flower-like MoS2 nanosheet coated N-doped carbon sphere electrode materials for enhanced capacitive deionization

The development of promising nanomaterials for capacitive deionization (CDI) is an emerging energy-efficient technology for water recovery and purification. Herein, the nanoarchitecture of deposition of molybdenum disulfide nanosheets onto N-doped carbon sphere (MoS2@NCS) has been successfully developed using hydrothermal processes for CDI applications. The morphological and fine structural analyses clearly indicate that the hexagonal structure of MoS2 nanosheets are homogeneously dispersed onto the 300-nm NCS to form the flower-like MoS2@NCS heterostructure. After calcination for 2 h at 800 °C, the MoS2@NCS exhibits a large specific surface area of 82 m2 g−1 and interconnected meso-macroporous channels for rapid ion and electron transfer, resulting in the superior electrochemical behaviors with the specific capacitance of 340 F g−1 at 5 A g−1 in 1 M Na2SO4 solution. Moreover, the electrosorption performance of MoS2@NCS-800 was investigated under different conditions in terms of initial NaCl concentration and voltage, and the superior specific electrosorption capacity of 59.9 mg g−1 is obtained at 2000 mg L−1 NaCl and applied voltage of 1.4 V. The experimental data is then well-fitted with the modified Donnan model, signifying that the electrosorption performance is highly dependent on the formation of electrical double layers and homogeneous potential distribution throughout the diffuse layer of the porous electrodes. The excellent cyclic durability of MoS2@NCS electrode is also highlighted after 100 electrosorption-desorption cycles. These results elaborate that MoS2@NCS composites are promising electrochemical materials for CDI application, which can open a pathway to design the 2D/3D hybrid nanoarchitectures as a highly potential material for green and sustainable water purification and ion removal in grey and salty waters.

Tuning Ionic Screening To Accelerate Electrochemical CO2 Reduction in Ionic Liquid Electrolytes

Electric double layer formation often governs the rate and selectivity of CO2 electrochemical reduction. Ionic correlations critically define double layer properties that are essential to electrocatalytic performance, including capacitance and localization of potential gradients. However, the influence of ionic correlations on CO2 electroreduction remains unexplored. Here, we use electrochemical conversion of CO2 to CO in ionic liquid-based electrolytes to investigate how the emergence of ionic correlations with increasing ion concentration influences reaction rates and selectivity. Remarkably, we find substantial acceleration of potential-dependent CO2 reduction rates and enhancement of faradaic efficiency to CO at intermediate concentrations of 0.9 M ionic liquid in acetonitrile, a concentration regime that has not been studied previously. We find that onset potentials for CO2 reduction remain relatively unchanged at −2.01 V vs Ag/Ag+ from 0.025 M up to 1.1 M and increase to −2.04 V vs Ag/Ag+ in the limit of neat ionic liquids. Hence, the acceleration of CO2 reduction we observe originates from the amplification of potential-dependent driving forces, as opposed to changes in onset potential. Importantly, our findings are general across cocatalytic and noncatalytic ions. We propose that concentrations of maximum reactivity correspond to conditions where electric double layers exhibit the strongest screening, which would localize electric fields to stabilize polar intermediates. Our study demonstrates that tuning bulk electrostatic screening lengths via modulation of ionic clustering provides a general approach to accelerating both inner-sphere and outer-sphere electrochemical reactions.

Active Pt-Nanocoated Layer with Pt-O-Ce Bonds on a CeO x Nanowire Cathode Formed by Electron Beam Irradiation.

A Pt-nanocoated layer (thickness of approx. 10–20 nm) with Pt–O–Ce bonds was created through the water radiolysis reaction on a CeOx nanowire (NW), which was induced by electron beam irradiation to the mixed suspension of K2PtCl4 aqueous solution and the CeOx NW. In turn, when Pt-nanocoated CeOx NW/C (Pt/C ratio = 0.2) was used in the cathode layer of a membrane electrode assembly (MEA), both an improved fuel cell performance and stability were achieved. The fuel cell performance observed for the MEA using Pt-nanocoated CeOx NW/C with Pt–O–Ce bonds, which was prepared using the electron beam irradiation method, improved and maintained its performance (observed cell potential of approximately 0.8 V at 100 mW cm–2) from 30 to 140 h after the start of operation. In addition, the activation overpotential at 100 mA cm–2 (0.17 V) obtained for MEA using Pt-nanocoated CeOx NW/C was approximately half of the value at 100 mA cm–2 (0.35 V) of MEA using a standard Pt/C cathode. In contrast, the fuel cell performance (0.775 V at 100 mW cm–2 after 80 h of operation) of MEA using a nanosized Pt-loaded CeOx NW (Pt/C = 0.2), which was prepared using the conventional chemical reduction method, was lower than that of MEA using a Pt-nanocoated CeOx/C cathode and showed reduction after 80 h of operation. It is considered why the nanocoated layer having Pt–O–Ce bonds heterogeneously formed on the surface of the CeOx NW and the bare CeO2 surface consisting of Ce4+ cations would become unstable in an acidic atmosphere. Furthermore, when a conventional low-amount Pt/C cathode (Pt/C = 0.04) was used as the cathode layer of the MEA, its stable performance could not be measured after 80 h of operation as a result of flooding caused by a lowering of electrocatalytic activity on the Pt/C cathode in the MEA. In contrast, a low-amount Pt-nanocoated CeOx NW (Pt/C = 0.04) could maintain a low activation overpotential (0.22 V at 100 mA cm–2) of MEA at the same operation time. Our surface first-principles modeling indicates that the high quality and stable performance observed for the Pt-nanocoated CeOx NW cathode of MEA can be attributed to the formation of a homogeneous electric double layer on the sample. Since the MEA performance can be improved by examining a more effective method of electron beam irradiation to all surfaces of the sample, the present work result shows the usefulness of the electron beam irradiation method in preparing active surfaces. In addition, the quantum beam technology such as the electron beam irradiation method was shown to be useful for increasing both performance and stability of fuel cells.