Limiting Diffusion Current Research Articles

Permanent Nanobubbles in Water: Liquefied Hollow Carbon Spheres Break the Limiting Diffusion Current of Oxygen Reduction Reaction.

Porous liquids have traditionally been designed with sterically hindered solvents. Alternatively, recent efforts rely on dispersing microporous frameworks in simpler solvents like water. Here we report a unique strategy to construct macroporous water by selectively incorporating hydrophilicity on the surfaces of hydrophobic hollow carbon spheres (HCS). Specifically, we show that the stable dispersion surface ionized HCS in water while retaining the inherent porosity. The electrocatalytic conversion of small gas molecules in aqueous electrolytes is limited by the concentration and diffusion rates of gas molecules in water. In this case, macroporous water exhibited 6 times gas uptake compared to nonporous (pure) water. By leveraging the high gas capacity and enhanced diffusion kinetics, the limiting diffusion current of oxygen reduction reaction (ORR) in macroporous water is 2 times that in nonporous water, offering promising prospects for sustainable energy conversion technologies.

Dual-channel nano-carbon-liquid/liquid junction electrodes for multi-modal analysis: redox-active (dopamine) and non-redox-active (acetylcholine).

We present here a dual-channel nanoelectrode to detect both redox-active and non-redox-active analytes. The dual-channel nanoelectrode was developed from theta nanopipette. We developed one channel of the theta nanopipette to be a carbon nanoelectrode and the other channel to be a nano interface between two immiscible electrolyte solutions (nanoITIES) electrode, producing a nano-carbon-ITIES platform. The carbon nanoelectrode channel was developed by carbon deposition via pyrolysis followed by focused ion beam milling to measure redox-active analytes. The nanoITIES electrode channel was developed to detect non-redox-active analytes. The nano-carbon-ITIES electrodes were characterized using electrochemistry, scanning electron microscopy and transmission electron microscopy. Dopamine (a redox-active analyte) and acetylcholine (a non-redox-active analyte) were measured on the dual-channel nano-carbon-ITIES platform using the carbon nanoelectrode and the nanoITIES electrode, respectively. Using cyclic voltammetry, the diffusion-limited current of dopamine and acetylcholine detection on the nano-carbon-ITIES electrode increased linearly with increasing their concentrations. Using chronoamperometry (current versus time), we showed that the nano-carbon-ITIES electrode detected acetylcholine and dopamine at the same time. The introduced first-ever dual-functional nano-carbon-ITIES electrodes expand the current literature in multi-channel electrodes for multi-purpose analysis, which is an emerging area of research. Developing the analytical capability for the simultaneous detection of acetylcholine and dopamine is a critical step towards understanding diseases and disorders where both dopamine and acetylcholine are involved.

Activation of ammonium oxalate on dandelion-derived nitrogen-doped carbon for electrocatalytic oxygen reduction reaction

Development of electrocatalysts with high catalytic activity for the oxygen reduction reaction (ORR) plays an important role in sustainable energy technology. Herein, a carbon-based electrocatalyst was prepared by pyrolyzing the dandelion straw, accompanied with the activation of ammonium oxalate (AO) for the first time. Based on the physical characterization, the DAO-1100 exhibits a well-developed pore structure, rich morphologies, high surface area, high graphitization degree, as well as high total content of pyridinic nitrogen and graphitic nitrogen. Therefore, DAO-1100 shows excellent electrocatalytic activity for ORR at the onset potential of 0.991 V, half-wave potential of 0.883 V and limiting diffusion current of 5.30 mA·cm−2 approximately, in comparison to those of the Pt/C catalyst. DAO-1100 catalyzes the ORR through a 4 e- pathway like Pt/C catalyst. In addition, it also possesses the better operational durability and catalytic selectivity than those of Pt/C catalyst. Therefore, ammonium oxalate, serving as a low-cost, efficient activator, can efficiently promote the transformation of abundant dandelion straw to metal-free biochar catalysts.

Effect of Bentonite Properties on Corrosion Behavior of Carbon Steel

Geological disposal is being considered as a disposal method for liquid waste resulting from nuclear power generation. In geological disposal, liquid waste and glass materials are melted together and solidified in an overpack (metal container), and a buffer material is compacted between the overpack and the bedrock. Carbon steel is as a candidate for overpack material and bentonite as a buffer materialin Japan. Overpack materials are required to be safe and reliable over a very long period, and their corrosion behavior and long-term corrosion resistance have been investigated before. However, most of these researches have been conducted in environments where oxygen is fully consumed, and the corrosion behavior and resistance during transient periods have not been fully clarified. In this study, the effects of bentonite properties on the corrosion behavior of carbon steel in compacted bentonite were investigated by electrochemical methods in order to clarify the corrosion behavior of carbon steel during the transient period. An iron wire grinded with sandpapers was used as the sample. The amount of bentonite was weighed to achieve the specified compacted density, and firstly approximately one-half of the amount was filled into a titanium compacted vessel. The iron wire was then set on the bentonite and the remaining bentonite was filled into the vessel using a hydraulic press. The properties of the bentonite were adjusted from 0.9 Mg/m3 to 1.8 Mg/m3 with the compaction density when packed. The titanium vessel was then immersed in a 0.1 mol/L NaCl solution and depressurized at room temperature. The solution was supplied to the cell through a filter on the side of the cell. Polarization and electrochemical impedance measurements were performed in a three-electrode system under open air. The test solution was 0.1 mol/L NaCl solution, a saturated KCl/Ag/AgCl electrode (SSE) was used as the reference electrode, and a platinum wire as the counter electrode. Polarization measurements were conducted by polarizing in the anodic and cathodic directions from the immersion potential at a scanning rate of 0.5 mV/sec. The polarization measurements of an iron wire were also performed in the same solution for comparison. Electrochemical impedance was measured at 24 and 48 hours after immersing the titanium vessel in the test solution. Polarization curve of iron wire in bentonite with a compressibility density of 1.5 Mg/m3 were compared with that in the solution. A clear diffusion-limiting current of oxygen was observed in the polarization curve of iron wire in the solution, whereas it was unclear for iron wire in bentonite. This result is attributed to the influence of water reduction reaction in a cathodic region. On the other hand, in the anodic region, the anodic current of iron in bentonite was about two orders of magnitude lower than that in the solution. This indicates that electrochemical measurements are possible in bentonite as well as in solution, but the influence of compacted bentonite is not small. Electrochemical impedance characteristic of the iron wire in bentonite with a compressive density of 1.6 Mg/m3 showed one time constant after 25 hours of immersion and two time constants after 48 hours, respectively.The diameter of the capacitive semicircles decreased with immersion time. The titanium vessel immersed for 48 hours was opened and the surface of the iron wire was visually observed. As a result, corrosion products were observed. These results suggest that the test solution penetrated into the compacted bentonite after at least 25 hours, and that corrosion progressed to form corrosion products on the electrode surface by 48 hours. Curve fitting was performed on the obtained impedance spectra, assuming the two equivalent circuits. As a result, it was confirmed that the polarization resistance at the electrode surface decreases with time.

The role of cation exchange membrane characteristics in CO2 electrolysis to CO using acid anolyte

Cation exchange membranes are considered a suitable option for zero-gap CO2 electrolysis due to their potential to avoid carbonation and improve carbon efficiency. However, the use of acidic anolytes remains an issue due to high hydrogen production. This study investigates Nafion® membranes (111, 112, 115, 211, and 212) with different thicknesses produced by extrusion or solution-cast processes in a zero-gap cell with an acidic anolyte containing K2SO4. Faradaic efficiencies for CO production (FECO) are higher with thinner membranes, regardless of the manufacturing process, reaching FECO around 75 % at 50 mA cm⁻². Additionally, membranes with similar thicknesses (∼50.8 µm) but produced in different ways displayed flow field carbonation after 3 h of electrolysis at 30 °C and 50 mA cm⁻². Linear sweep voltammetry (LSV) in full and half-cell configurations shows limiting diffusion current (iL) relative to proton transport for all the employed membranes, no matter the thickness. In contrast, the iL for Nafion® 115, the thicker membrane, is suppressed, indicating that proton depletion is fast and the electrode surface alkalinization primarily results from water reduction in this case. A mechanistic analysis was performed to explain the behavior of the limiting currents in the cell with Ar- and CO2-feed, indicating that CO2 reduction aids in the consumption of H+ provided by the membrane, increasing the local pH at less negative potentials. Overall, thinner membranes exhibited higher values of FECO and energy efficiency for CO (EE%CO). However, solution-cast membranes are more prone to provide K+, leading to better performance than those prepared by extrusion.

Boosting Zn-air battery performance: Fe single-atom anchored on F, N co-doped carbon nanosheets for efficient oxygen reduction

Sluggish oxygen reduction reaction (ORR) kinetics limit the development of metal-air batteries and fuel cells, hindering overall energy conversion efficiency. Therefore, significant research has focused on cost-effective, highly active, and exceptionally stable non-precious metal ORR electrocatalysts. This study presents the synthesis of a nanohybrid material called Fe SAs/FN-CNs. It is made up of single iron atoms embedded in ultrathin porous carbon nanosheets that are co-doped with F and N. The synthesis process involves an easy one-step pyrolysis technique without additional post-treatment. The Fe SAs/FN-CNs material is designed to function as an effective zinc-air battery ORR electrocatalyst. Based on their distinctive components and structure, the optimal Fe SAs/FN-CNs exhibit outstanding catalytic efficiency and long-lasting performance in the alkaline ORR. They have an onset potential (Eonset) of 0.95 V, a half-wave potential (E0.5) of 0.85 V, a kinetic current density (JK) of 20.49 mA cm−2 at 0.8 V, and a diffusion-limited current (Jd) of 6.2 mA cm−2. In addition, a Zn-air battery made using homemade Fe SAs/FN-CNs demonstrated a power density of 197 mW cm−2, a specific capacitance of 813.5 mAh g−1, and exceptional stability. It outperformed the commercial Pt/C by operating continuously for over 147 hours at 10 mA/cm² (discharge-charge). The Fe SAs/FN-CNs nanohybrid electrocatalyst shows great potential as an electrocatalyst for various metal-air batteries.

Electrochemical Separation of Pb-Ag-Sb Alloys in the KCl-PbCl2 Melt

Anode dissolution of lead, silver, antimony and liquid lead-bismuth-antimony alloys has been studied by the polarization method depending on the alloy composition in the molten KCl-PbCl2 eutectic. Lead was found to dissolve and to form Pb2+ ions within the whole range of anode current densities in the Pb-Ag-Sb (49.0–28.0-23.0) and Pb-Ag-Sb (11.0-55.0-34.0) melts. The limiting diffusion current of lead dissolution was observed at the anode current density of 0.34 А cm−2 in the Pb-Ag-Sb (5.0-65.0-30.0) alloy. At the anode current densities exceeding these values the antimony dissolution was observed. To define a mechanism of metals electro dissolution, the number of electrons participating in the electrode reactions was calculated. Based on the polarization curves analysis the regime of electrochemical separation of lead alloys was selected. Metallic lead and the remaining Ag-Sb-Pb alloy with the valuable components concentration exceeding 90 wt% were obtained in the laboratory-scale electrolytic cell.

An Ion-Pumping Quasi-Solid Electrolyte Enabled by Electrokinetic Effects for Stable Aqueous Zinc Metal Batteries.

The practical application of aqueous zinc (Zn) metal batteries (ZMBs) is hindered by the complicated hydrogen evolution, passivation reactions, and dendrite growth of Zn metal anodes. Here, an ion-pumping quasi-solid electrolyte (IPQSE) with high Zn2+ transport kinetics enabled by the electrokinetic phenomena to realize high-performance quasi-solid state Zn metal batteries (QSSZMBs) is reported. The IPQSE is prepared through the in situ ring-opening polymerization of tetramethylolmethane-tri-β-aziridinylpropionate in the aqueous electrolyte. The porous polymer framework with high zeta potential provides the IPQSE with an electrokinetic ion-pumping feature enabled by the electrokinetic effects (electro-osmosis and electrokinetic surface conduction), which significantly accelerates the Zn2+ transport, reduces the concentration polarization and overcomes the diffusion-limited current. Moreover, the Zn2+ affinity of the polymer and hydrogen bonding interactions in the IPQSE changes the Zn2+ coordination environment and reduces the amount of free H2O, which lowers the H2O activity and inhibits H2O-induced side reactions. Consequently, the highly reversible and stable Zn metal anodes are achieved. The assembled QSSZMBs based on the IPQSE display excellent cycling stability with high capacity retention and Coulombic efficiency. The high-performance quasi-solid state Zn metal pouch cells are demonstrated, showing great promise for the practical application of the IPQSE.

Effect of pH on the Selectivity of γ-MnO2 Electrocatalysts Towards Oxygen Evolution Reaction in the Presence of Chloride Ions in Alkaline Environment

This paper reports on progress in development of electrocatalysts intended for use in seawater, which optimize the balance of competing reactions between chlorine evolution and oxygen evolution. The oxygen evolution reaction (OER) from H2O is thermodynamically more favorable due to a lower equilibrium potential [1]. However, chlorine evolution successfully competes with the OER due to faster kinetics. One of a few catalysts that demonstrates high selectivity towards the OER in the presence of chloride ions in the alkaline environment (i.e. pH 12) is γ-MnO2 [2].In this presentation, we (1) compare activities and selectivities towards the OER of electrodeposited and chemically synthesized γ-MnO2 and (2) determine how pH affects selectivity in the pH range from 11 to 13.g-MnO2 powder was synthesized using hydrothermal method. The synthesis resulted in fabrication of material with a flower-like morphology, characterized by a crystallite size of 5-15 nm and BET surface area of 75 m2/g. In parallel, MnO2 films were prepared by electrodeposition on a Pt RDE substrate, following the procedure outlined by Tsagareli et al. [3]. The electrocatalytic activity of both γ-MnO2 powder and electrodeposited MnO2 was investigated by the thin-film RDE method. The thin films of γ-MnO2 powder were obtained by drop-casting MnO2/VC ink [1] on glassy carbon substrates.We found that the shape of polarization curves on MnO2/VC films is strongly dependent on the Nafionä loading. At low Nafion loadings, we observe diffusion-limited currents, which we attribute to OH- diffusion through the Nafion. At higher Nafion loadings, neither diffusion-limited currents, nor the currents’ dependence on the rotation rate are observed. Our data indicate that at low Nafion loadings, MnO2/VC films selectively produce O2 in 0.5M NaCl at pH 12 and 13 and current densities up to 15 mA/cm2. Because of the potential effect of Nafion on the permeability of Cl- ions to the catalyst’s active sites, a set of parallel experiments is being carried out on the electrolytically deposited MnO2 films. The results of the latter experiments will be compared to those obtained on MnO2/VC films in the presentation.

Urea-treatment of CoOx carbon nanofibers to improve the electrochemical performance of supercapacitor using aqueous electrolytes

Supercapacitors (SCs) play a crucial role in flexible electronics, necessitating innovative approaches to enhance surface faradaic reactions and minimize faradaic diffusion while using aqueous electrolytes. Thus, the urea treatment of cobalt oxide (CoOx)-decorated carbon nanofibers (CNFs) is proposed in this study to decrease the contribution of faradaic diffusion-limited current. Flexible CoOx/CNF electrodes were obtained by annealing ZIF-67-grafted polyacrylonitrile (PAN) fibers via a wet chemical method. The urea treatment of CoOx/CNFs increased the content of sp2-hybridized carbon and pyridinic nitrogen, as confirmed by X-ray photoelectron spectroscopy, effectively enhancing conductivity and pseudocapacitive charge storage capability via nitrogen doping. Notably, urea-treated CoOx/CNF electrode samples exhibited a capacitance of 750 mF cm−2 at a scan rate of 10 mV s−1, while retaining more than 81 % capacitance at a higher scan rate of 100 mV s-1. The cyclic voltammetry curves during variable bending angle testing (0°, 45°, and 90°) exhibited negligible changes, indicating the excellent flexibility of the SCs. The CoOx/CNFs and urea-treated CoOx/CNFs exhibited 80 % and 91 % capacitance retentions, respectively, after 10,000 galvanostatic charge and discharge cycles. Furthermore, the attained energy densities of 76 and 61 µWh cm−2 at the respective power densities of 2 and 20 mW cm−2 indicated the excellent electrochemical performance of the optimal urea-treated CoOx/CNF electrode.

An Inexpensive and Earth-Abundant Aluminium Based Single Atom Catalyst for Efficacious Oxygen Reduction Reaction

ABSTRACT Oxygen reduction reaction (ORR) is crucial for the commercial success of environmentally benign energy conversion devices such as fuel cells and metal−air batteries [1-2]. Replacement of Pt-based and transition metal-based electrocatalysts with efficient non-precious ORR catalysts is quite challenging so for [3-5]. Single-atom catalysts (SACs) based on first-row transition metals like Fe, Co, Cr have already received significant attention for ORR [6]. In this hunt, we have developed the rarely explored P-block earth abundant aluminium-based efficient and cost-effective ORR single atom catalyst. Nitrogen-doped Aluminium-based single atom catalyst has been synthesised by pyrolyzing aluminium phthalocyanine (AlPc) with dicyandiamide (DCDA) which is the source of carbon and nitrogen at different temperature and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy. For comparison, AlPc has been pyrolyzed at 1000 °C without the addition of DCDA. The catalysts are named Al-N-C/T (T = 700 °C - 1000 °C) depending on the pyrolysis temperature. AlPc/1000 showed poor ORR activity with low diffusion-limited current of 0.76 mA/cm2 (current measured at the potential of 0.3 V vs RHE). The insufficient catalytic activity of AlPc-1000 is due to the strong bonding of aluminium active centers with oxygenated group intermediate. But after the introduction of DCDA, the catalytic activity of Al-N-C/T (T = 700 °C - 1000 °C) has been improved significantly because N-bonded Al atoms have optimal bonding strength with intermediate oxygen species [7]. The coordination environment of Al-N-C plays an essential role in exhibiting excellent catalytic activity towards ORR. The Al-N-C/1000 catalyst exhibits the highest diffusion-limited current (4.49 mA/cm2) as compared to all other Al-N-C/T (T = 700, 800, and 900 °C) based catalysts, along with more positive Eonset (844 mV vs. RHE) and half-wave potential E1/2 (749 mV vs RHE) (Figure 1). Electrochemical calculation further ropes the Al-Nx sites as the origin of ORR via an efficient 4-electron transfer pathway in basic medium. Importantly, negligible reduction in current density after 9000 cycles in alkaline medium is far superior to the durability limit set by the US department of energy and overpasses the state-of-the-art Pt/C catalyst (Figure 1). This methodology can be applied to design a variety of other alkaline earth, P block effective electrocatalysts. The Al-N-C/1000 catalyst exhibits the highest diffusion-limited current as compared to all other Al-N-C/T (T = 700, 800, and 900 °C) based catalysts, along with more positive Eonset and half wave potential. Electrochemical calculation further ropes the Al-Nx sites as the origin of ORR via an efficient 4-electron transfer pathway in basic medium. Importantly, negligible reduction in current density after 9000 cycles in alkaline medium is far superior to the durability limit set by the US department of energy and overpasses the state-of-the-art Pt/C catalyst. This methodology can be applied to design a variety of other alkaline earth, P block effective electrocatalysts. Keywords:ORR; Al-N-C electrocatalyst; SAC; P-block; Excellent stability

Pyrolysis Enzymolysis-Treated Pomelo Peel: Porous Carbon Materials with Fe-Nx Sites for High-Performance Supercapacitor and Efficient Oxygen Reduction Applications.

This paper proposes a different strategy for deriving carbon materials from biomass, abandoning traditional strong corrosive activators and using a top-down approach with a mild green enzyme targeted to degrade the pectin matrix in the inner layer of pomelo peel cotton wool, inducing a large number of nanopores on its surface. Meanwhile, the additional hydrophilic groups produced via an enzymatic treatment can be used to effectively anchor the metallic iron atoms and prepare porous carbon with uniformly dispersed Fe-Nx structures, in this case optimizing sample PPE-FeNPC-900's specific surface area by up to 1435 m2 g-1. PPE-FeNPC-900 is used as the electrode material in a 6 M KOH electrolyte; it manifests a decent specific capacitance of 400 F g-1. The assembled symmetrical supercapacitor exhibits a high energy density of 12.8 Wh kg-1 at a 300 W kg-1 power density and excellent cycle stability. As a catalyst, it also exhibits a half-wave potential of 0.850 V (vs. RHE) and a diffusion-limited current of 5.79 mA cm-2 at 0.3 V (vs. RHE). It has a higher electron transfer number and a lower hydrogen peroxide yield compared to commercial Pt/C catalysts. The green, simple, and efficient strategy designed in this study converts abundant, low-cost waste biomass into high-value multifunctional carbon materials, which are critical for achieving multifunctional applications.

Controlling Electrodeposition Via Electrokinetic Phenomena and Phase Separation

This talk will review the basic physics of dendritic instabilities in electrodeposition and how they may be suppressed, in some cases, by controlling electrokinetic phenomena in the electrolyte or phase separation in the electrode: 1) It is shown both theoretically and experimentally that, if electrodeposition occurs in charged porous media above the diffusion-limited current, then dendritic growth can be suppressed by surface conduction and electro-osmotic flows through the phenomenon of “shock electrodeposition”, as in many examples with copper. 2) It is also shown theoretically and experimentally that dendritic electrodeposition on a multiphase electrode can be avoided by avoiding the separation of a resistive phase, as illustrated by lithium plating on graphite. Examples include composite metal materials manufacturing, metal nanotube growth in templates, rechargeable metal batteries, and conductive-bridge random access memory (CB-RAM). These phenomena may also be leveraged to enhance the selectivity of electrodeposition for metal separations.

Cathodic Polarization Behavior of Steel in Aqueous MgCl2 Solutions Containing Zn2+

So far, we have revealed that the dissolution rate of zinc is accelerated in relative high concentrations of MgCl2 in the corrosion process of galvanic couples consisting of zinc and steel. In this study, we investigated the cathodic polarization behavior of steel in MgCl2 solution containing Zn2+ and the effect of Zn2+ dissolved in the solution on the oxygen reduction reaction and hydrogen evolution reaction.The specimens used in the experiments were carbon steel SS400 (Fe-0.12C-0.60Mn-0.011P-0.019S) machined into a 1.5×1.5×0.3 cm3. One side of the specimen was polished with SiC abrasive paper up to #2000 grit and degreased in acetone before being used for the experiment. The test solution was mixtures of 0.25 M MgCl2 + x mM ZnCl2 (x = 2.5, 5 mM, Mg/Zn molar ratio: 100, 50) at room temperature under a naturally aerated condition. As a comparison solution, 0.5 M NaCl + y mM ZnCl2 (x = 5, 10 mM, molar ratio of Na/Zn: 100, 50) with the same chloride ion concentrations were used. Cathodic polarization curves of steel were measured using a three-electrode method with a platinum wire as the counter electrode and an Ag/AgCl electrode (saturated KCl, SSE) as the reference electrode. The potential sweep rate was 0.5 mV/s. After the polarization test, the specimen surfaces were observed with a scanning electron microscope and analyzed for corrosion products formed during cathodic polarization with XRD and laser Raman spectroscopy.The cathodic polarization behavior of steel in MgCl2 solution without Zn2+ was investigated and compared with that of NaCl solution without Zn2+. Diffusion-limited currents of dissolved oxygen reduction (ORR) were observed at -0.6 V to -0.8 V in MgCl2 and NaCl solutions. Furthermore, cathodic polarization in MgCl2 and NaCl solutions showed an increase in current due to hydrogen evolution reaction (HER), and the cathodic current due to HER on steel was larger in the MgCl2 solution than in the NaCl solution. To investigate the difference in HER in both solutions, the steel surface was observed after constant polarization at -1.0 V for 20 min. Mg(OH)2 deposition was observed on the steel surface in the MgCl2 solution, but not in the NaCl solution. This indicates that the increase in pH due to ORR and HER is mitigated by the formation of Mg(OH)2 during cathodic polarization. Therefore, due to the pH buffering action of Mg2+ associated with the formation of Mg(OH)2, it can be considered that a larger HER current flowed in the MgCl2 solution than in the NaCl solution when compared at the same potential.On the other hand, cathodic polarization curves for steel in MgCl2 and NaCl solution containing Zn2+ were investigated. As a result, ORR and HER were observed at potentials down to -1 V in the MgCl2 and NaCl solutions containing Zn2+; the cathodic current due to ORR and HER in the MgCl2 and NaCl solutions containing Zn2+ was reduced compared to the solutions without Zn2+. The inhibition of ORR and HER can be attributed to the precipitation of Zn corrosion products.These results suggest that in solution environments containing Mg2+ and Zn2+, the promotion of HER by the effect of Mg2+ and the inhibition of HER and ORR by Zn corrosion products occur simultaneously.

Electrochemical properties of Gd(III) ions in LiCl-KCl-GdCl3 at 723–1023 K

Electrochemical behavior of Gd(III) ions in molten LiCl-KCl-GdCl3 was investigated at 723–1023 K via cyclic voltammetry using tungsten as a working electrode, Gd-Bi (mole fraction, xGd = 0.16) as a reference electrode, and Gd-Bi (xGd = 0.02) as a counter electrode. A single reduction–oxidation wave was observed, confirming a single-step, 3-electron transfer Gd(III)/Gd transition. The cathodic peak potential exhibited minimal change (<13 mV) over a wide range of scan rates (0.05–0.30 V s−1), indicating facile charge transfer kinetics (i.e., a reversible electrode process). A nucleation overpotential associated with solid Gd deposition was observable at low temperatures (T < 823 K). The mass transport properties of Gd(III) ions were estimated using the Berzins and Delahay relation based on diffusion-limiting peak current. The diffusivity values were determined to be DGd(III) = 0.5–2.7 × 10−5 cm2 s−1 at 723–1023 K with an associated activation energy of Ea = 33.9 (±1.0) kJ mol−1. The two-phase [liquid + GdBi] Gd-Bi alloy reference electrode experienced less than 0.5 mV of drift over 5 days of repeated electrochemical measurements, indicating high stability.

Halogenate electroreduction from acidic solution at rotating disc electrode. Maximal steady-state convective-diffusion current for comparable concentrations of halogenate ions and protons

Process of electrochemically inactive halogenate-anion (XO3-, X = halogen) electroreduction at rotating disk electrode (RDE) under steady-state conditions via a mediator cycle: XO3- + 5 X- + 6H+→ 3 X2 + 3 H2O in solution (*), X2 + 2 e-⇄ 2 X- on electrode surface in acidic aqueous medium has been analyzed theoretically on the basis of a set of coupled convective-diffusional transport equations for predicting the dependence of its rate on numerous parameters of the system, without application of expensive commercial software for its numerical integration. Approximate solution of these equations for the concentration distributions of all reactive species (halogenate and halide anions, halogen molecules and protons), as well as for the maximal current, as functions of the RDE frequency, bulk-solution concentrations of XO3-, X2 and H+ species, diffusion coefficients of all reactive components, rate constant of comproportionation reaction (*), kinematic viscosity of solution, etc., is given as a simple analytical procedure which provides this solution in the numerical form for any set of values of these parameters.These characteristics of the process are determined primarily by the ratio of the diffusion (for species X2) and kinetic layer thicknesses: xdkC = zdC/ zk, which depends on the RDE frequency and the bulk-solution concentrations of species XO3- (Ao) and H+ (Ho), as well as on other parameters of the system. Weak-current regime takes place for its relatively small values: xdkC < 6, i.e. for sufficiently high RDE frequencies, where the maximal current is proportional to the bulk-solution concentration of X2 molecules, Co, being independent of the Ao and Ho concentrations. Within the opposite range of this parameter: xdkC>6, i.e. for sufficiently low RDE frequencies, the maximal current is comparable to the smaller of the diffusion-limited currents of XO3- and H+ species, jAlim and jHlim, being practically independent of the Co concentration. This strong-current regime which does not exist for the conventional EC-cat (or EC') mechanism is entirely due to the autocatalytic feature of the mediating redox cycle (EC-autocat mechanism) which may lead to enormous accumulation of the X2/X- redox couple components near electrode surface.

Efficient oxygen reduction using a polymorphic tungsten catalyst

The oxygen reduction reaction (ORR) constitutes an important transformation process in fuel cells. Here we report a polymorphic tungsten catalyst with a concerted tungsten single-atom site (W-SAC) and tungsten nitride nanoclusters (WN-NPs) for mediating the ORR processes. The catalyst is prepared via pyrolysis of phthalocyanine-typed material (WPc) with g-C3N4. Synergy of the W-SAC and WN-NP sites over the designed 2D g-C3N4 layer is responsible for the competitive ORR performance of the catalysts. Consequently, the WNPc/gC3N4-700 sample gives a half-wave potential of 0.835 V, a Tafel slope of 40.47mV dec−1, and a mass diffusion-limited current of 0.596 mA/cm2; such results are comparable with and even better than that of 20% Pt/C. Furthermore, WNPc/gC3N4-700 also outperforms 20% Pt/C in terms of stability and durability. Our work may point toward a design strategy for active, stable, non-noble-metal ORR catalysts.

Solid phase synthesis Ni3N and N-CNT synergetic corn-like multifunctional electrocatalyst

Nickel nitride (Ni3N) based catalysts have been studied extensively as oxygen evolution and hydrogen evolution reaction (OER/HER) catalysts because of good stability in alkaline solution. However, relatively poor electrical conductivity of Ni3N seriously restricted electron transfer from the Ni3N nanoparticles surfaces to inner substrates for highly efficient water splitting. In this work, solid phase synthesis strategy is implemented to prepare Ni3N/N-CNT composite. Ni3N/N-CNT shows impressive onset potential (0.81 V) and half wave (0.70 V) in oxygen reduction reaction (ORR), respectively. The higher limiting diffusion current of Ni3N/N-CNT shows the faster ORR kinetics. Ni3N was anchored on N-CNT, which displayed an outstanding OER and HER performance. The low OER overpotential (0.40 V) indicated that the Ni3N nanoparticles on N-CNT has significant enhancement of the OER activity. Significantly, Ni3N/N-CNT displays an excellent HER activity with preferable current density and low overpotential of 259 mV at 10 mA cm−2. The excellent multifunctional catalytic property is due to the electronic interaction between Ni3N and N-CNT which is beneficial for the mass and electron transfer. The conclusion broadens our understanding for exploit multifunctional catalysts.

Electrochemical behavior of lead, silver and bismuth containing alloys in the KCl-PbCl2 melt

The anode dissolution of ternary Pb-Ag-Bi metallic alloys with different components concentration has been studied in the molten KCl-PbCl2 eutectics. A lead dissolution was found to be the main process in the Pb-Ag-Bi (60.0–15.0–25.0), Pb-Ag-Bi (42.9–21.4–35.7), Pb-Ag-Bi (7.0–34.9–58.1) alloys within the whole interval of studied anode current densities. The Pb-Ag-Bi ((4.8–35.7–59.5) and Pb-Ag-Bi ((2.4–36.6–61.0) alloys at the anode current densities of 2.3 A/cm2 and 0.14 A/cm2 have the limiting diffusion current of lead dissolution. At the anode current densities exceeding the aforesaid values a bismuth dissolution takes place. The number of electrons participating in each electrode reaction is determined for each mechanism of metal electrodissolution. A regime of electrochemical separation of lead alloys is chosen based on the analysis of polarization curves. Metallic lead and Ag-Bi-Pb (5.1 wt%) alloy were obtained in the laboratory electrolytic cell.

Direct measurement of oxygen reduction reactions at neurostimulation electrodes

Objective. Electric stimulation delivered by implantable electrodes is a key component of neural engineering. While factors affecting long-term stability, safety, and biocompatibility are a topic of continuous investigation, a widely-accepted principle is that charge injection should be reversible, with no net electrochemical products forming. We want to evaluate oxygen reduction reactions (ORR) occurring at different electrode materials when using established materials and stimulation protocols. Approach. As stimulation electrodes, we have tested platinum, gold, tungsten, nichrome, iridium oxide, titanium, titanium nitride, and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). We use cyclic voltammetry and voltage-step amperometry in oxygenated versus inert conditions to establish at which potentials ORR occurs, and the magnitudes of diffusion-limited ORR currents. We also benchmark the areal capacitance of each electrode material. We use amperometric probes (Clark-type electrodes) to quantify the O2 and H2O2 concentrations in the vicinity of the electrode surface. O2 and H2O2 concentrations are measured while applying DC current, or various biphasic charge-balanced pulses of amplitude in the range 10–30 µC cm−2/phase. To corroborate experimental measurements, we employ finite element modelling to recreate 3D gradients of O2 and H2O2. Main results. All electrode materials support ORR and can create hypoxic conditions near the electrode surface. We find that electrode materials differ significantly in their onset potentials for ORR, and in the extent to which they produce H2O2 as a by-product. A key result is that typical charge-balanced biphasic pulse protocols do lead to irreversible ORR. Some electrodes induce severely hypoxic conditions, others additionally produce an accumulation of hydrogen peroxide into the mM range. Significance. Our findings highlight faradaic ORR as a critical consideration for neural interface devices and show that the established biphasic/charge-balanced approach does not prevent irreversible changes in O2 concentrations. Hypoxia and H2O2 can result in different (electro)physiological consequences.