Low pH Electrolyte Research Articles

Dynamic interactions between adsorbates and catalyst surfaces over long-term OER stability testing in acidic media

The interaction between catalyst surfaces and adsorbed oxygen intermediates is critical to catalytic performance for electrochemical water oxidation to oxygen. However, the relationship between adsorption energetics and electrocatalytic activity is primarily assessed for pristine catalyst materials, which leaves much unknown about the dynamics of these properties in relationship to catalyst performance during long-term operation. In this work, we experimentally assess OH and O adsorption on Ca2IrO4 nanoparticles and monitor their evolution during extensive chronoamperometry tests at highly oxidizing potentials in a range of low pH electrolytes. In situ x-ray absorption spectroscopy reveals changes for surface adsorbate energetics and local iridium structures with applied potentials. Increasingly unfavorable adsorption of OH and formation of O intermediates after long-term operation is correlated with severe metal dissolution, distorted [IrO6] octahedral linkages, and a decreased average Ir valence. This work establishes connections between surface adsorption energetics, Ir structure, OER kinetics, and material stability outcomes.

Electrochemical implications of modulating the solvation shell around redox active organic species in aqueous organic redox flow batteries

Organic and organometallic reactants in aqueous electrolytes, being composed of earth-abundant elements, are promising redox active candidates for cost-effective organic redox flow batteries (ORFBs). Various compounds of ferrocene and methyl viologen have been examined as promising redox actives for this application. Herein, we examined the influence of the electrolyte pH and the salt anion on model redox active organic cations, bis((3-trimethylammonio) propyl)- ferrocene dichloride (BTMAP-Fc) and bis(3-trimethylammonio) propyl viologen tetrachloride (BTMAP-Vi), which have exhibited excellent cycling stability and capacity retention at ≥1.00 M concentration [E. S. Beh, et al. ACS Energy Lett. 2, 639-644 (2017)]. We examined the solvation shell around BTMAP-Fc and BTMAP-Vi at acidic and neutral pH with SO42-, Cl-, and CH3SO3- counterions and elucidated their impact on cation diffusion coefficient, first electron transfer rate constant, and thereby the electrochemical Thiele modulus. The electrochemical Thiele modulus was found to be exponentially correlated with the solvent reorganizational energy (λ) in both neutral and acidic pH. Thus, λ is proposed as a universal descriptor and selection criteria for organic redox flow battery electrolyte compositions. In the specific case of the BTMAP-Fc/BTMAP-Vi ORFB, low pH electrolytes with methanesulfonate or chloride counterions were identified as offering the best balance of transport and kinetic requirements.

Seedless Cu Electroplating on Co-W Thin Films in Low pH Electrolyte: Early Stages of Formation.

The use of Ta/TaN barrier bilayer systems in electronic applications has been ubiquitous over the last decade. Alternative materials such as Co-W or Ru-W alloys have gathered interest as possible replacements due to their conjugation of favourable electrical properties and barrier layer efficiency at reduced thicknesses while enabling seedless Cu electroplating. The microstructure, morphology, and electrical properties of Cu films directly electrodeposited onto Co-W or Ru-W are important to assess, concomitant with their ability to withstand the electroplating baths/conditions. This work investigates the effects of the current application method and pH value of the electroplating solution on the electrocrystallisation behaviour of Cu deposited onto a Co-W barrier layer. The film structure, morphology, and chemical composition were studied by X-ray diffraction, scanning electron microscopy and atomic force microscopy, as well as photoelectron spectroscopy. The results show that the electrolyte solution at pH 1.8 is incapable of creating a compact Cu film over the Co-W layer in either pulsed or direct-current modes. At higher pH, a continuous film is formed. A mechanism is proposed for the nucleation and growth of Cu on Co-W, where a balance between Cu nucleation, growth, and preferential Co dissolution dictates the substrate area coverage and compactness of the electrodeposited films.

The Effect of Chloride Anions on Charge Transfer in Dye-Sensitized Photoanodes for Water Splitting.

The photoelectrochemical behavior of dye-sensitized photoelectrochemical cells based on a TiO2 layer sensitized with ruthenium components, including an absorber, ruthenium(II)bis(2,2′-bipyridine)([2,2′-bipyridine]-4,4′-diylbis(phosphonic acid)) dibromide (RuP), and a catalyst, ruthenium(II) tris(4-methylpyridine)(4-(4-(2,6-bis((l1-oxidanyl)carbonyl)pyridin-4-yl)phenyl) pyridine-2,6-dicarboxylic acid) (RuOEC), was investigated in the following water-based electrolyte configurations: KCl (pH ≈ 5), HCl (pH ≈ 3), ethylphoshonic acid (pH ≈ 3) with a different KCl concentration, and a standard phosphate buffer (pH ≈ 7). The rate of charge transfer on the photoanode’s surface was found to increase in line with the increase in the concentration of chloride anions (Cl−) in the low pH electrolyte. This effect is discussed in the context of pH influence, ionic strength, and specific interaction, studied by cyclic voltammetry (CV) in dark conditions and upon illumination of the photoanodes. The correlations between photocurrent decay traces and CV studies were also observed.

A cationic β-cyclodextrin as a dynamic coating for the separation of proteins in capillary electrophoresis.

A cationic cyclodextrin was used as dynamic coating for the capillary electrophoresis of a model mixture of proteins (i.e., ubiquitin, α-lactoglobulin, cytochrome-c, and myoglobin) as positively charged species in a fused silica capillary. An interesting feature of the coating is that by simple adjustment of the concentration of cyclodextrin added into the background electrolyte, a neutral or positively charged surface, which was beneficial in preventing protein adsorption at the inner capillary wall surface, was obtained. This is the first demonstration of a dynamic coating that yielded a neutral surface for protein separations in capillary electrophoresis. Based on electro-osmotic flow measurements, addition of 0.05 to 0.10mg/mL quaternary β-cyclodextrin in a low pH electrolyte resulted in a neutral or positive surface (undetectable to very slow anodic electro-osmotic flow). The coating approach afforded the electrophoretic separation of the mixture of proteins at positive polarity with good repeatability and separation performance.

PH-Induced versus Oxygen-Induced Surface Enrichment and Segregation Effects in Pt–Ni Alloy Nanoparticle Fuel Cell Catalysts

We present a voltammetric, spectroscopic, and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt–Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5 nm depth, yet vastly different ORR reactivities. On the basis of our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline ele...

Investigating the Role of Electrolyte Composition and Oxide Structure in Galvanic Corrosion Susceptibility Under Atmospheric Electrolytes

The goal of this study is to develop a better mechanistic understanding of how electrolyte composition and surface oxides can influence corrosion kinetics in the electrolytes formed from atmospheric processes. It has been observed that galvanic couples between aluminum alloys and titanium or stainless steel fasteners exposed to electrolytes formed from atmospheric processes lead to very different corrosion rates (Z. Feng, G. Frankel, Corrosion, 70(2014) pgs 95-106). In order to collect the data necessary to accomplish this goal, sixteen 2”x2” panels of AA2024, AA7075, CRES 13-8, CRES 17-4, Ti-6Al-4V, and Custom 465 were cut from panel or bar stock and polished to 600 grit SiC. The samples were then mounted in 300 mL Gamry Paracells and exposed for 18 hours to the following electrolytes: 0.6M NaCl at ambient aeration, 6M NaCl at ambient aeration, deoxygenated 0.6M NaCl saturated with nitrogen,0.6M NaCl + 0.01M HCl, 0.6M NaCl + 0.01M H2SO4, and 0.6M NaCl + 0.005M HNO3 + 0.005M H2SO4. The aluminum alloys were then anodically polarized at 0.167 mV/sec from -0.02V below EOC to + 1.5VSCE. The other alloys were polarized cathodically at 0.167 mV/sec from +0.02V above EOC to -1.4VSCE. Results for the cathodic polarization scans of the CRES 13-8PH stainless steel and the Ti-6Al-4V titanium alloy are shown in Figure 1. The differences in cathodic current capacity for materials exposed to the same electrolyte at the same aeration level reinforces the fact that oxide microstructure plays a significant role in catalyzing reduction reactions, though the kinetics of the reaction rates may not be predicted from a simple galvanic series. Of more interest are the changes in reaction rates from acidification of the electrolyte. This suggests that the electrolyte is changing the the oxide microstructure or that new reduction reactions are supported. This is significant because overlays of the anodic polarizations of the aluminum alloys indicate that material loss rates can be orders of magnitude higher for acidified atmospheric electrolytes. These effects are being investigated using microelectrode experiments on droplets of the various electrolytes along with supporting experiments using Scanning Kelvin Probe techniques. Acknowledgements The authors gratefully acknowledge the financial support of the Office of Naval Research Figure 1: Cathodic polarization curves for the titanium alloy Ti-6Al-4V and the stainless steel CRES 13-8PH in 0.6M NaCl and 0.6M NaCl + 0.005M H2SO4 + 0.005M HNO3, showing that materials with different surface oxides and in near-neutral to low-pH electrolytes can support very different cathodic reaction rates with potentially very deleterious consequences for galvanic couples with more active materials.

Micelle to solvent stacking of organic cations in capillary zone electrophoresis with electrospray ionization mass spectrometry

The direction of the effective electrophoretic mobility of small organic cations in micellar electrokinetic chromatography using sodium dodecyl sulphate in a low-pH electrolyte can be reversed in the presence of organic solvent. This effective electrophoretic mobility change is presented here as a new dimension for on-line sample preconcentration of cations in capillary zone electrophoresis (CZE) using a background solution (BGS) modified by an organic solvent. The sample is prepared in a micellar solution without organic solvent. The focusing effect relies on the reversal in the effective electrophoretic mobility at the boundary zone between the micellar matrix and the BGS modified with organic solvent. This on-line sample preconcentration technique, called micelle to solvent stacking (MSS) afforded more than an order of magnitude improvement in concentration sensitivity compared to typical CZE-UV or CZE-electrospray ionization (ESI) MS analysis. The calculated limit of detection (S/N = 3) for pindolol and metoprolol analysed by MSS-CZE-ESI-MS was found to be 0.03 and 0.01 μg/mL, respectively.

Selective extraction and elution of weak bases by in-line solid-phase extraction capillary electrophoresis using a pH step gradient and a weak cation-exchange monolith

A polymer monolith bearing weak cation-exchange functionality was prepared for the purpose of demonstrating pH-selective extraction and elution in in-line solid-phase extraction-capillary electrophoresis (SPE-CE) utilising a model set of cationic analytes, namely imidazole, lutidine and 3-phenylpropanamine. Optimization of the electrolyte conditions for efficient elution of the adsorbed analytes using a moving pH boundary required that the capillary and monolith be filled with 44 mM sodium acetate at high pH (pH 6) and a low pH electrolyte of 3 mM sodium acetate pH 3 was placed in the electrolyte vials. This combination allowed the adsorbed analytes to be simultaneously eluted and focused into narrow bands, with peak widths of the eluted analytes having a baseline width of 1.2 s immediately after the monolith. Using these optimum elution conditions, the versatility of the SPE-CE approach was demonstrated by removing unwanted adsorbed components after extraction with a wash at a different pH and also by selecting a pH at which only some of the model weak bases were ionised. The analytical performance of the approach was evaluated and the relative standard deviation for peak heights, peak area and migration times were in the ranges of 1.4-5.3, 1.2-3.3 and 0.4-1.2% respectively. Analytes exhibited linear calibrations with r(2) values ranging from 0.996 to 0.999 over two orders of magnitude. Analyte pre-concentration provided excellent sensitivity, and limits of detection for the analyte used in this study were in the range 8.0-30 ng ml(-1), which was an enhancement of 63 when compared to normal hydrodynamic injection occupying 1.3% of the capillary of these bases in water.

Mechanism of the coupled 24-electron reduction and transformations among the “blues”, the “browns” and the “reds” of ammonium metatungstate

The complete mechanistic pathway for the 24-electron redox behavior of the metatungstate anion has been determined through the simulation of cyclic voltammetry data in low pH electrolytes. After the electrochemical reduction by up to four electrons, forming the “tungsten-blues”, coupled chemical and electrochemical reactions occur with increasing complexity upon further reduction of the metatungstate anion. Quantitative analysis of the data accounts for the formation of stable reduced “tungsten-browns” from the 6- and 12-electron reduced forms, and “tungsten-reds” from the 18- and 24-electron reduced forms.

Hybrid Aqueous Energy Storage Cells Using Activated Carbon and Lithium-Ion Intercalated Compounds: III. Capacity Fading Mechanism of at Different pH Electrolyte Solutions

The electrochemical stability of in a -containing aqueous electrolyte solution is critically dependent on the pH value of the electrolyte solution. It shows the capacity fading upon cycling in the electrolyte solution below pH 11. The mechanism responsible for the capacity fading has been extensively investigated by means of cyclic voltage, ac impedance, charge/discharge, ex situ X-ray diffraction, and chemical analysis. It was found that and its partly charged product is chemically stable in the low pH electrolyte solution, while it is not electrochemically stable during the discharge process in which the proton cointercalation parallel to the lithium ion intercalation occurred, but the intercalated proton cannot be reversibly extracted during the charge process. The intercalated proton limits the intercalation of lithium ion, thus resulting in capacity fading.

Rapid speciation of Se(IV) and Se(VI) by flow injection-capillary electrophoresis system with contactless conductivity detection.

A flow injection-capillary electrophoresis system with contactless conductivity detection and hydrostatic-pressure-generated flow was used for the fast and sensitive speciation of Se(IV) and Se(VI). The sample throughput was 25 samples per hour using a background electrolyte solution containing 8.75 mM L-histidine (His) adjusted to pH 4.00 with acetic acid. The repeatability of peak areas (n=8) was better than 1.41% and the limits of detection were 190 microg L(-1) and 7.5 microg L(-1) for Se(IV) and Se(VI), respectively. The interference from carbonate, typically present in water samples, was eliminated by using a low-pH electrolyte in which carbonate is uncharged and migrates at the EOF front. The method was applied to the analysis of Se(IV) and Se(VI) in soil samples that were spiked with both selenium species and the results for recovery of both selenium species were in good agreement with their introduced concentrations.

On-line concentration of acidic compounds by anion-selective exhaustive injection-sweeping-micellar electrokinetic chromatography.

An easy, simple, and highly efficient on-line preconcentration method for acidic compounds in capillary electrophoresis was investigated. It combined two on-line concentration techniques, field-amplified sample injection (FASI) and sweeping. A low-pH (2.5) background electrolyte was used to suppress the electroosmotic flow (EOF), obviating the need of a coated capillary, as well as to neutralize the weakly acidic analytes. After injection of a plug of water inside the separation capillary, negative voltage was applied to initialize FASI for a much longer time than usual. The anions experienced a high electric field and moved quickly to the boundary of the water and the low-pH nonmicellar electrolyte. When the anions encountered the low-pH electrolyte, they were neutralized and a focused sample zone was formed. Then both inlet and outlet vials were changed to those containing the low-pH micellar background electrolyte. As negative voltage was applied, the anionic micelles moved into the capillary, and sweeping and separation began. The novelty in the present procedure is that a low-pH buffer is used to suppress the EOF and also the ionization of the analytes, without need of any other additives or use of a coated capillary. This method afforded 100,000-fold improvement in peak heights for some phenoxy acidic herbicides. The detection limits for these compounds could be low as 100 pg/mL

Protonation reactions of anthraquinone-2,7-disulphonic acid in solution and within monolayers

Abstract The pH dependent electrochemical properties of anthraquinone-2,7-disulphonic acid, 2,7-AQDS, have been investigated both in solution and as adsorbed monolayers. In low pH electrolyte, the voltammetric responses observed for the conversion of the quinone, Q, to hydroquinone, H2Q, are close to ideal for both diffusive and adsorbed reactants and redox switching is controlled by electron rather than proton transfer. In unbuffered solution, reduction of the quinone causes significant polarisation of the interfacial proton concentration causing a discontinuity in plots of E° versus pH for both solution phase and monolayer species. The pKa values for the H2Q/HQ− and HQ−/Q2− couples increase significantly from 7.6±0.2 and 10.6±0.2 in solution to 10.6±0.2 and 12.0±0.2 when 2,7-AQDS is immobilised within dense monolayers. Measurements of the differential capacitance reveal that this difference arises predominantly because the effective dielectric constant within the monolayer, efilm, (8.0+0.2) is significantly lower than that of bulk water. At low pH, efilm is insensitive to the oxidation state of the monolayer being 8±0.5. However, at pH 11.2, the effective dielectric constants are larger and depend on the oxidation state being approximately 13±0.7 and 16±0.5 for oxidised and reduced forms, respectively. The pH dependence of the rate of heterogeneous electron transfer to both diffusive and adsorbed 2,7-AQDS has been measured from the scan rate dependence of the peak-to-peak separation, ΔEp. The apparent standard heterogeneous electron transfer rate constant for the solution phase reactant, k°Soln depends on the electrolyte pH decreasing from 4.0±0.3×10−3 to 8.1±0.5×10−4 cm s−1 as the electrolyte pH is increased from 1.4 to 4.2. For monolayers, the corresponding rate constants are 1.4±0.1×103 and 3.4±0.2×102 s−1. Significantly, the normalised rate constant for the monolayers appears to be more than an order of magnitude smaller than those found for the solution phase reactants. This result suggests that adsorption is accompanied by either a significant increase in the activation barrier for electron transfer or that the adsorbate is weakly electronically coupled to the electrode surface.

Potential dependent adsorption of anthraquinone-2,7-disulfonate on mercury†

Monolayers of anthraquinone-2,7-disulfonic acid, 2,7-AQDS, have been formed by equilibrium adsorption onto mercury electrodes. In low pH electrolytes, cyclic voltammetry is nearly ideal with a peak-to-peak splitting of 8 ± 2 mV, and a full width at half maximum of 57 ± 1 mV, being observed for scan rates less than 3 V s–1. The dependence of the surface coverage of 2,7-AQDS as measured using voltammetry on its bulk concentration is described by the Langmuir isotherm over the concentration range 0.06–9 µM. A saturation surface coverage, Γs, of (8.7 ± 0.5) × 10–11 mol cm–2 and an adsorption coefficient, β, of (5.6 ± 0.5) × 105 M–1 are observed where the supporting electrolyte is 1.0 M HClO4. Consistent with the formation of an organic film, the double layer capacitance, Cdl, as measured at –0.700 V, decreases from approximately 40 µF cm–2 in the absence of dissolved 2,7-AQDS to 10 µF cm–2 when the bulk concentration exceeds 10 µM. By measuring the potential dependence of Cdl as the bulk concentration of 2,7-AQDS is varied systematically, an insight into the potential dependence of the free energy of adsorption, ΔG‡ads, has been obtained. These data reveal that ΔG‡ads is similar for both fully oxidised and fully reduced monolayers. However, these quinonoid monolayers are least strongly bound (ΔG‡ads = –28.5 kJ mol–1) when the film exists in the quinhydrone form.

The Electrolytic Codeposition of Silica and Titania Modified Silica with Zinc

This paper describes the electrolytic codeposition of pure and titania modified spherical silica particles with zinc from zinc sulfate electrolytes on a rotating disk electrode. The adsorption of zinc ions onto silica and the morphology of composite zinc‐silica coatings as a function of the pH of the plating solution were investigated. It was found that the adsorption of zinc ions onto silica, the codeposition of silica, and the formation of hydroxides increase with pH. Titania modified silica particles were found to codeposit at low pH. Their incorporation in zinc from low pH electrolytes is explained based on an electrode blocking effect by the adsorbed particles leading to a local pH increase in the vicinity of the cathode. This leads to the formation of zinc hydroxides which causes silica particles to be codeposited with zinc even from plating solutions of low bulk pH.