Local pH Increase Research Articles

Activated Migration of Hydroxide Ion in Anolyte for Enhancing Performance of Scaled-up Non-Noble Catalysts in Pure Water-Fed Anion Exchange Membrane Water Electrolysis

The continuous advancement in water electrolysis technologies has illuminated the potential of sustainable energy sources, particularly in the realm of green hydrogen production. Anion exchange membrane water electrolysis (AEMWE) stands out as a promising technology capable of theoretically yielding high-purity hydrogen. However, the commercialization of AEMWE faces challenges arising from the inherent discord between high performance and the elevated costs associated with noble metal catalysts, such as platinum and iridium. Moreover, prevalent studies often focus on a limited electrode area of less than 4 cm2, thereby restricting the exploitation of the high purity hydrogen production capabilities inherent in AEMWE. This research prioritizes the enhancement of performance through a meticulously crafted membrane-electrode assembly (MEA) employing a 25 cm2 Ni-frame caged NiMoO4 catalyst for cathode synthesis. The observed performance improvement is attributed to the augmented active site of the catalyst, consisting of NiMoO4 entrapped in the Ni substrate during the compression process of the electrode. Additionally, the increase in local pH at the interfacial between electrode and bulk electrolyte is facilitated by the capillary phenomenon resulting from the space between the Ni substrate and the NiMoO4 catalyst layer, leading to enhanced migration of the electrolyte containing hydroxide ions. Acknowledgements This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040590).

(Invited) Graphene Oxide As an Additive for Ni-Fe Electrodeposition

In contrast to graphene, graphene oxide (GO) is readily disperse in aqueous electrolytes and can be reduced along with nickel and iron metal ions. While others had incorporated graphene in Ni-Fe deposits, no previous studies have addressed the role of GO particles on metal deposition. Ni-Fe is characterized by the anomalous codeposition behavior where there is a tendency of more Fe to be deposited than Ni even though there is an excess of Ni ions in the electrolyte. This behavior has been modeled assuming adsorbed intermediates. Here, the influence of GO on the deposit composition, structure and anomalous codeposition effect is investigated. The deposits were galvanostatically electrodeposited from a boric acid-sulfamate electrolyte at pH 2 containing commercial graphene oxide platelets. Two different cell configurations were examined:1) a rotating cylinder electrode (RCE) and 2) a copper mesh. The rotating cylinder as a working electrode was used in order to investigate mass transport. The copper mesh working electrode was placed in close contact to a flat bottom pH probe in order to examine the local pH change. The addition of GO to the electrolyte had an effect on the deposit composition and the local pH change (Fig 1). The deposit composition had either more Ni or remained the same compared to an electrolyte without GO on the RCE electrode at electrode rotation rates of 372 and 1000 rpm (Fig 1 (a) and (b)). The larger Ni content in the deposit was due to an enhanced Ni partial current density in the presence of GO. The local pH during the deposition on the mesh electrode with vigorous electrolyte stirring increased the local pH (Fig 1(c)). The composition behavior is similar to the results from the RCEs, with higher Ni content in the presence of GO. The local pH increase when GO was present is considered a factor that drives the enhanced Ni content, that may be a consequence of changes in adsorbed hydrogen. GO was not incorporated into the deposit; however, it had a dramatic effect on the morphology. There was less micro-cracks when GO was present in the electrolyte suggesting less adsorbed hydrogen. Thus, GO influenced the adsorbed intermediates making the electrodeposition less anomalous and having less cracks. Acknowledgement: This work was supported in part by the AESF Foundation. Figure 1. The influence of GO in the electrolyte during Ni-Fe electrodeposition (a) composition on a RCE, rotating rate (a) 372, (b)1000 rpm and (c) local pH measurements on a mesh electrode under vigorous stirring, 30 mA/cm2. Figure 1

Electrochemically Induced Templated Sol-gel Deposition of Mesoporous Silica and Nanocomposites: The New Kid on the ECiD Block

A relatively new member of the group of electrochemically induced deposition is that of templated sol-gel deposition. Uniform and smooth mesoporous silica thin films were first fabricated by an ECiD process nicknamed electrochemically assisted self-assembly or EASA, introduced by Walcarius et al. The addition of cetyltrimethylammonium bromide or CTAB as organic template molecules to the tetraethyl orthosilicate (TEOS) sol-gel precursor solution resulted in silica thin films with highly ordered, hexagonal arrays of vertical mesopores of 2–3 nm in diameter. These films have been proposed for applications in electroanalysis and sensors where effects such as pre-concentration or perm selectivity have been exploited. Their synthesis combines the well-established field of templated sol-gel synthesis with an electrochemically induced deposition process. To prevent spontaneous gel formation, the silica sol-gel precursor solution has a mildly acidic pH which kinetically inhibits the gelation reaction. Only when a cathodic potential/current is applied to electrogenerate OH- ions from the pro-base (water mostly), the gelation reaction is catalyzed by the local increase in pH, resulting in the growth of silica gel films on the electrode. We will first review the principles of sol-gel reactions, before proceeding to details of ECiD of sol-gel films.

Evaluation of pH variation in cathodic protection conditions by FEM simulation

AbstractCathodic protection is an electrochemical technique used to control the corrosion of metals. It does this by sending a proper amount of current equal to the current required by the cathodic processes at the protection potential. One of the chemical effects of cathodic protection is a local pH increase at the surface, which is an important factor in the reduction of corrosion rate, as carbon steel may work in a passive condition. In the present study, a tertiary current distribution finite element method model is introduced. A dynamic boundary condition is considered, by defining a Fe anodic Tafel slope as a pH function, and oxygen reduction reaction limiting current density as a function of available oxygen at the cathode surface. The model has been validated by direct pH measurement during a short‐term polarization test, and subsequently used to predict the pH and the available oxygen at the cathode surface for long‐term condition.

In Situ Insonation of Alkaline Buffer Containing Liposomes Leads to a Net Improvement of the Therapeutic Outcome in a Triple Negative Breast Cancer Murine Model.

Breast cancer is characterized by an acidic micro-environment. Acidic extracellular pH gives cancer cells an evolutionary advantage, hence, neutralization of the extracellular pH has been considered as a potential therapeutic strategy. To address the issue of systemic pH alteration, an approach based on the targeted delivery of the buffering solution to the tumor region is investigated. The method relies on the use of low frequency ultrasound and sono-sensitive liposomes loaded with buffers at alkaline pH (LipHUS). After the i.v. injection of LipHUS, the application of ultrasound (US) at the sites of the pathology induces a local increase of pH that results highly effective in i) inhibiting primary tumor growth, ii) reducing tumor recurrence after surgery, and iii) suppressing metastases' formation. The experiments are carried out on a triple negative breast cancer mouse model. The results obtained demonstrate that localized and triggered release of bicarbonate or PBS buffer from sonosensitive liposomes represents an efficient therapeutic tool for treating triple-negative breast cancer. This approach holds promise for potential clinical translation.

Long-Term Stability of Seawater Acidification and Its Effect on the Formation of Mg(OH)2 Films with a Hierarchical Porous Structure in Bipolar Membrane-Based Direct Seawater Electrolysis

This is the first report discussing the long-term stability (1000 h) of direct seawater electrolysis (DSWE) in relation to seawater acidification and inorganic precipitation. Unlike general DSWE, in which inorganic precipitates are formed at the cathode surface due to a local pH increase caused by the hydrogen evolution reaction (HER), bipolar membrane-based DSWE acidified natural seawater to pH 3, suppressing the formation of inorganic precipitates and inducing a positive shift in the equilibrium potential for the HER. Because the acidified seawater became alkaline after 300 h, periodic seawater replacement was suggested as a method of maintaining seawater acidification for 1000 h, during which a Mg(OH)2 film with a thickness of 1 mm or more was formed at the cathode surface. Using Brunauer–Emmett–Teller (BET) analysis, water vapor sorption, and electrochemical impedance analysis, it was confirmed that the Mg(OH)2 film has a hierarchical mesoporous structure and high affinity for water, which maintained mass transport. The unique properties of the Mg(OH)2 film under seawater acidification contributed to a lower rate of increase in the cathodic potential than that under seawater alkalization, where very thin inorganic deposits were formed.

Two-dimensional modeling of CO2 mineral trapping through the oxalate‑carbonate pathway: Influence of the root system model

Two-dimensional reactive transport models, one with a simplified root system and the other accounting for dynamically evolving root architecture, were constructed to examine the influence of model complexity on capturing the effect of soil-root dynamics relating to the Oxalate Carbonate Pathway (OCP) of the Iroko tree over 170 years. Oxidation of oxalate from fallen tree tissue by soil bacteria enables local soil pH increase, leading to the sequestration of atmospheric carbon in carbonate minerals (calcite) in the shallow soil surrounding the tree. Simulations of both root models corroborate previous one-dimensional models of the OCP focused on Ca and C mass balance, where high weathering rates of Ca-containing silicate minerals in bedrock, along with contributions from groundwater, provided sufficient Ca for precipitation of observed quantities of calcite. Both simulations demonstrate the development of a distinct high pH zone where oxalate is oxidized, Ca accumulates, and calcite precipitates (OCP zone); and a low pH zone where roots collect Ca, later returned to the top soil as calcium oxalate (Total Root Extent/TRE zone) via litterfall. While the extent of OCP zone development near the ground surface was very similar between simulations, differences in localized root water uptake between the two approaches resulted in variation in water and solute transport and influenced the geometry of the OCP zone at depth, with implications for calcite precipitation in the soil. Trends in CO2 and O2 partial pressures in the OCP zone were mirrored in the TRE zone, suggesting linkage between the two zones with regard to gas transport. Near the end of the tree's lifespan, results indicate that soil permeability decreases due to calcite precipitation may limit O2 ingress and availability in the shallow soil, while trapping CO2 released from the oxidation of organics in the shallow soil, with implications for the long-term sustainability of the OCP itself.

Imaging the Footprint of Nanoscale Electrochemical Reactions for Assessing Synergistic Hydrogen Evolution.

This work proposes a novel method for measuring the intrinsic activity of single metal-based nanoparticles towards water reduction in neutral media at industrially relevant current densities. Instead of using gas nanobubbles as proxy, the method uses optical microscopy to track the local footprint of the reaction through the precipitation of metal hydroxide, which is associated to the local pH increase during electrocatalysis. The results show the electrocatalytic activities of different types of metal nanoparticles and bifunctionnal core-shell nanostructures made of Ni and Pt, and demonstrate the importance of metal hydroxide nano-shells in enhancing electrocatalysis. This method should be generalizable to any electrocatalytic reaction involving pH changes such as nitrate or CO2 reduction.

Imaging the Footprint of Nanoscale Electrochemical Reactions for Assessing Synergistic Hydrogen Evolution

AbstractThis work proposes a novel method for measuring the intrinsic activity of single metal‐based nanoparticles towards water reduction in neutral media at industrially relevant current densities. Instead of using gas nanobubbles as proxy, the method uses optical microscopy to track the local footprint of the reaction through the precipitation of metal hydroxide, which is associated to the local pH increase during electrocatalysis. The results show the electrocatalytic activities of different types of metal nanoparticles and bifunctionnal core‐shell nanostructures made of Ni and Pt, and demonstrate the importance of metal hydroxide nano‐shells in enhancing electrocatalysis. This method should be generalizable to any electrocatalytic reaction involving pH changes such as nitrate or CO2 reduction.

Initial Electrodeposition Behavior of Chromium from Hydrate-Melt Based Trivalent Chromium Baths

Trivalent chromium electrodeposition is expected to substitute the conventional hard chromium electroplating that requires harmful hexavalent chromium. Recently, we revealed that crystalline chromium, which is effective for hard chromium properties, can be electrodeposited from trivalent chromium baths using chloride-based hydrate-melts. Herein, we investigated the initial behavior of the trivalent chromium electrodeposition by in situ analyses using electrochemical quartz crystal microbalance (EQCM) and ex situ characterization of resulting electrodeposits. In the very initial stage of electrolysis, proton reduction proceeds preferentially, resulting in chromium hydroxide precipitation on the electrode due to the local pH increase. Chromium reduction was found to require a few seconds of induction time to start. The transient was interpreted by the Sand equation which also indicated proton depletion near the cathode. In the hydrate-melts, due to the depletion of free water, the high proton mobility due to Grotthuss mechanism is lost, resulting in the suppression of hydrogen evolution after the induction time. This explains why chromium electrodeposits are obtained at extremely high current efficiencies of 60%–80%. Additionally, the proton reduction of the initial electrolysis stage may lead to negative effects, for example, impairing adhesion of chromium electrodeposits.

Study on ultrasonic depolymerization of Si-Ge precipitation in zinc oxide dust leaching process

Under the background of the increasing demand for germanium in high-tech, this paper finds out the mechanism of silicon germanium polymerization precipitation by simulating the leaching process of zinc oxide dust containing germanium. By comparing and analyzing the simulation experiments under ultrasonic and conventional conditions, the mechanism of efficient dispersion and depolymerization of germanium silicon precipitation induced by ultrasonic was found out. The results show that in the leaching process of germanium-containing zinc oxide dust, due to the increase of local pH, some silicon ions in the solution will hydrolyze and polymerize to form silicic acid colloid to adsorb germanium in the solution, causing the loss of germanium 14.81%, resulting in low recovery rate of germanium industry. Ultrasonic wave was introduced in the leaching process, and the shock wave and micro-jet released by ultrasonic cavitation continuously impacted the surface of polysilicate colloid. The germanium adsorbed on the surface of the silicic acid colloid is desorbed, and the large particles of silica gel are depolymerized into small particles of silicic acid, which reduces the adsorption of silica gel on germanium. The loss of germanium under ultrasonic conditions can be reduced by 59.35%. This study can effectively realize the efficient recovery of germanium in zinc oxide dust, and provide process and theoretical basis for the sustainable development of germanium industry.

Engineering of Conformal Electrode Coatings by Atomic Layer Deposition for Aqueous Na-ion Battery Electrodes

The application of atomic layer deposition on active material particles or as conformal layers directly on electrodes is an effective and viable approach for protecting the battery materials from degradation. Al2O3, TiO2, and HfO2 coatings are applied on NaTi2(PO4)3, which is among the most studied negative electrode materials for aqueous Na-ion batteries. The coated electrodes are characterized in terms of electrochemical kinetics, charge capacity retention, and electrochemical impedance spectra. Al2O3, a widely used protective coating in non-aqueous batteries, is shown to be insufficient to suppress parasitic processes and is eventually dissolved by reaction with hydroxide during extended cycling in aqueous Na2SO4. However, this process provides a local buffering effect making the protective action of this coating mainly of chemical nature. TiO2 is found to be very resistant to increase in pH and remains almost intact during electrochemical cycling. However, we provide strong evidence that TiO2 itself is electrochemically active in aqueous electrolytes at negative potentials. The protonation of TiO2 leads to an additional increase in local pH which is detrimental to NaTi2(PO4)3 and results in even faster capacity loss than in uncoated electrodes. Only HfO2 is found to be sufficiently stable and electrochemically inert ALD coating for negative NaTi2(PO4)3 electrodes operating in aqueous electrolytes.

Propagation of mineral dissolution waves driven by cation exchange in low salinity waterflooding

Low salinity waterflooding (LSWF) is an emerging improved/enhanced oil recovery (IOR/EOR) technique with an applicability in both sandstone and carbonate reservoirs. The underlying recovery mechanism of LSWF is still not completely understood due to the multiscale complex interactions occurring in crude oil/brine/rock (COBR) systems. There are several proposed mechanisms, where mineral dissolution is the most prominent among them. The objective of this paper is to study the brine-rock interactions, and understand the propagation of mineral dissolution waves in LSWF processes through theoretical analysis and geochemical modeling. A reactive transport geochemical modeling using PHREEQC (pH-REdox-EQuilibrium in C programing language) simulator has been carried out to understand the interplay of different geochemical effects such as multivalent cation exchange, and mineral dissolution during flow and transport in LSWF. The presence of clays and carbonate cements is most common in sandstone reservoirs, while some carbonate reservoirs also contain clay minerals. Therefore, a core scale geochemical model containing clay and carbonate minerals as active minerals for brine-rock interactions is chosen as a representative analog for most sandstone reservoirs and some of the carbonate reservoirs in this study. The results show that LSWF disturbs the geochemical equilibrium between the formation brine and reservoir rock minerals thereby causing the carbonate mineral dissolution. It is also inferred that without the cation exchange effect of clay minerals, carbonate mineral dissolution effects are local and they occur only close to the inlet of a core plug sample during LSWF. The new equilibrium between brine and rock minerals is then established rapidly. In the presence of clay minerals and cation exchange effect, the clay surfaces were found to preferentially adsorb divalent cations over monovalent cations during LSWF. Such cation exchange process softens the injected low salinity brine to maintain the brine under saturation with respect to carbonate minerals, and subsequently propagates carbonate mineral dissolution waves across the full length of the core sample. The novelty of this work is that it proposes and demonstrates a new geochemical based mechanism for LSWF, i.e., the propagation of mineral dissolution waves driven by the cation exchange process in the reservoir. The identified mechanism has several practical implications in the field: (1) carbonate mineral dissolution and the associated local pH increase can extend from local to reservoir scale, (2) the resulting mineral dissolution effectively releases the oil attached to the mineral surfaces, and (3) increasing the pH modifies the surface charges of both the rock and crude oil to be more negatively charged. The resultant electrostatic repulsion between the two interfaces would increase the water-wetness of rock surfaces to achieve higher improved oil recovery in LSWF.

Ultra-Fast Zinc Phosphate Coatings Obtained By Pulsed Electrochemical Deposition

Phosphating is one of the most common conversion coatings for steels and other ferrous alloys. Owing it to the process simplicity and unique morphology of the coatings, this treatment has cemented itself in a wide variety of applications, ranging from painting to corrosion resistance and cold working operations. In recent years, the need for more sustainable technologies has however driven the research for new alternatives to phosphating. One of the main drawbacks of this process is in fact the formation of a sludge byproduct composed mainly of phosphates of heavy metals, which must be continuously filtered off from the deposition tanks and disposed properly. These operations take a big toll on the environmental burden of the process and are the reason why an alternative will eventually become a necessity.One possible solution to this problem is to study conversion coatings with a different chemical nature. Formulations based on vanadium [1] and zirconium [2] have already been proposed as an alternative to phosphating, showing good results in terms of corrosion protection and coating adhesion; however it’s still unclear whether their application at the industrial scale would have a lower environmental impact than phosphating.A completely opposite solution is instead to keep the same, well-known chemistry of phosphating baths and change the method of application. In electrolytic zinc phosphating the coating process is carried out in an electrochemical cell, where the conditions required for phosphate precipitation are promoted by the application of a cathodic potential. In this scenario, substrate dissolution is prevented thanks to the cathodic polarization applied and the formation of the sludge byproduct is completely avoided. [3]By applying a cathodic polarization, hydrogen evolution is promoted at the substrate-electrolyte interface, leading to a local increase in pH, which triggers zinc phosphate precipitation. Therefore, the deposition rate in electrolytic phosphating is related to the speed at which the ideal pH conditions are created. In principle, it’s therefore possible to increase the current density to enhance the deposition rate, similarly to common electroplating systems. In practice, a current density too high, would lead to accumulation of hydrogen bubbles on the substrate surface and growth of dendritic metallic zinc, which would drastically reduce the quality of the final coating. [4]To overcome these limitations, in this work we demonstrate the use of pulsed current deposition as a mean to achieve higher coating weights in electrolytic zinc phosphating on mild steel substrates. The high current density applied during the impulse promotes the formation of the proper pH conditions for phosphate precipitation to occur, while the off-time allows for hydrogen bubbles to leave the surface, improving the overall coating quality. To highlight the effects of the impulse parameters on the final coatings, surface characterization is carried out with multiple techniques, such as SEM imaging, EDS mapping, XRD and GDOES. Moreover, corrosion resistance of the zinc phosphate coatings is evaluated with both destructive and non-destructive techniques in a NaCl 3.5% environment. Bibliography [1] M. Motamedi and M. M. Attar, “Nanostructured vanadium-based conversion treatment of mild steel substrate: Formation process: Via noise measurement, surface analysis and anti-corrosion behavior,” RSC Adv., vol. 6, no. 50, pp. 44732–44741, May 2016.[2] A. Ghanbari and M. M. Attar, “Surface free energy characterization and adhesion performance of mild steel treated based on zirconium conversion coating: A comparative study,” Surf. Coatings Technol., vol. 246, pp. 26–33, May 2014.[3] S. Jegannathan, T. S. N. Sankara Narayanan, K. Ravichandran, and S. Rajeswari, “Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests,” Electrochim. Acta, vol. 51, no. 2, pp. 247–256, Oct. 2005.[4] C. Kavitha, T. S. N. N. Sankara Narayanan, K. Ravichandran, I. S. Park, and M. H. Lee, “Deposition of zinc–zinc phosphate composite coatings on steel by cathodic electrochemical treatment,” J. Coatings Technol. Res., vol. 11, no. 3, pp. 431–442, May 2014.

Systematic variation of inorganic additives and their impact on interfacial delamination processes of automotive coating systems

Inorganic fillers incorporated in adhesives for the automotive industry enable both functional and mechanical tuning of adhesive properties. While integral effects of additives are well understood, the individual effects of single components and how they may lead to synergies as well as antagonistic effects remain unclear. Here, we systematically vary the concentration of CaO, CaCO3 and hydrophobic fumed SiO2 to understand individual and combined effects. For this purpose, the inorganic additives were mixed into a model formulation without inorganic additives and compared with a close to real application one-component epoxy adhesive, which also contained these fillers. The water uptake of the filled adhesives was measured with EIS and the results were compared with findings from cataplasma tests under atmospheric conditions. These results indicated that hydrophobic SiO2 can result in accelerated water uptake, although the opposite would be expected due to the more hydrophobic surface coating. In addition, blistering was observed on all galvanized steel (ZE) surfaces, while filiform corrosion was observed on all hot-dip galvanized Zn-Al-Mg steels (ZM). Cataplasma tests in oxygen-depleted and oxygen-rich atmospheres allowed to identify interfacial delamination processes. Our results showed that CaO can cause osmotic blistering and accelerate the diffusion of water into the adhesive due to a local increase in pH near the hydrated CaO particles, while CaCO3 reduces water diffusion. However, coating defects may occur due to the addition of CaCO3, leading to increased cathodic blistering. A strategy for improving beneficial and synergistic effects is discussed.

Amelioration of injury-induced tissue acidosis by a nonsteroidal analgesic attenuates antinociceptive effects of the pH-dependent opioid agonist NFEPP

Opioid agonists are powerful drugs for managing pain. However, their central side effects are limiting their use and drugs with similar potency, but a lower risk profile are needed. (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenylpropionamide (NFEPP) is a novel opioid agonist that preferentially activates opioid receptors at acidic extracellular pH. NFEPP was designed to activate peripheral opioid receptors in injured tissue, therefore precluding side effects elicited at normal pH in brain or intestinal wall. Considering the common combination of opioids and nonsteroidal anti-inflammatory drugs (NSAIDs) in multimodal analgesia, we investigated the interaction between NFEPP and a widely prescribed prototypical NSAID, diclofenac (DCF), in a rat model of unilateral hindpaw inflammation induced by complete Freund’s adjuvant. We evaluated the effects of systemically applied DCF on the paw tissue pH, on the expression of inflammatory mediators in immune cells from inflamed paws and on the expression of opioid receptors in dorsal root ganglia. Additionally, we investigated the antinociceptive efficacy of NFEPP injected into the inflamed paws after DCF treatment. We found that DCF reduced inflammation-induced nociceptive responses and tissue acidosis, but did not change the mRNA expression of IL-1β, TNF-α, IL-6, IL-4, NGF, or of mu-, delta-, or kappa-opioid receptors. The treatment with DCF moderately reduced the antinociceptive efficacy of NFEPP, suggesting a correlation between an increase in local tissue pH and the decreased antinociceptive effect of this pH-sensitive opioid agonist.

Erosion-Driven Enamel Crystallite Growth Phenomenon at the Tooth Surface In Vitro.

The erosion of tooth enamel is a common oral disease. The erosion pattern and location and the effects of nanoscale chemical composition on the erosion susceptibility of enamel have been well documented. However, the enamel remineralization accompanied by erosion and its underlying physicochemical mechanisms still remain poorly understood. Here, using rat molars selected for its good relevancy to human teeth, we investigated the remineralization behavior of the outermost enamel surface at the nanoscale level during erosion in diluted hydrochloric acid solutions. While particles on the outermost enamel surface that represent the termination of crystallites protruding to the surface from the near-surface core eroded by acid-attack, the lateral-growth of the particles (i.e., the main remineralization picture of the surface enamel) occurred concurrently. Ionic analyses indicate that the particle growth is driven by the local increase in pH near the eroding enamel surface as a result of the combination of the PO43- and CO32- released from the enamel surface with H+. As the pH increases eventually to the equilibrium pH level (∼5.5), a local supersaturation of solute ions is induced, resulting in particle growth. A simple growth model based on the experimental results together with an assumption that the particle growth is a diffusional process suggests that the particle growth rate is controlled by the degree of supersaturation and accommodation site for solute ions, which are affected by the pH of solution eroding the enamel surface. The remineralization mechanism presented by our study can explain how the enamel on being acid-exposed or tooth decay progress by beverage or food can naturally remineralize in the oral cavity and how remineralization can foster different surface topology at the nanoscale, depending on the pH value of etchant before the dental filling material is applied.

Novel long-acting smart anticorrosion coating based on pH-controlled release polyaniline hollow microspheres encapsulating inhibitor

• Long lasting polyaniline microspheres with pH-controlled release of MBT were prepared. • A smart self-healing coating based on the microspheres was fabricated. • The scratched smart coating exhibited excellent self-healing ability during 70 d. • The 180-d EIS proved the long-term anticorrosion effect of the intact smart coating. For the first time, the pH-sensitive polyaniline hollow microspheres encapsulating corrosion inhibitor 2-mercaptobenzothiazole (MBT@PANI) which could provide reliable controlled release of MBT for several months were incorporated with water-borne epoxy coating as smart filler. The EIS, SVET, XPS and salt spray test proved that the scratched MBT@PANI coating had relevant self-healing ability due to the responsive release of MBT triggered by the local pH increase caused by the corrosion. The 180-d EIS study confirmed that the intact coating also had expected long-term corrosion protection effect on aluminum alloy.

Indirect electrodeposition of a NiMo@Ni(OH)2MoOx composite catalyst for superior hydrogen production in acidic and alkaline electrolytes

Hydrogen production from renewable energy urgently needs application-oriented catalysts. In this work, a coupled procedure of indirect electrodeposition and in situ electrochemical activation is presented to synthesize a NiMo@Ni(OH)2MoOx composite coating on a graphite substrate. A mixed oxide coating of Ni(OH)2 and MoO3 is indirectly electrodeposited from a weak acidic citrate plating bath by means of local pH increase at the electrode interface caused by the hydrogen evolution reaction (HER). Without the need of special treatment, low valence of Ni0, Mo0 and MoO2 are naturally formed and dispersed in the oxide matrix at the initial stage of the HER process in 1.0 M H2SO4 or 1.0 M KOH, leading to a significant activation of the catalyst coating. The final catalyst can achieve a HER activity comparable or even superior to Pt-based electrocatalysts in both acidic and alkaline solutions. Density functional theory calculations support a bimetallic catalytic mechanism involving NiMo active centers, which need a Gibbs free energy of atomic hydrogen absorption smaller than that on the Pt (111) surface. This work provides a facile and unique procedure for preparing highly active and industrially attractive electrocatalysts.

Local pH induced electrochemical CO2 reduction on nanostructured Ag for adjustable syngas composition

Carbon monoxide is an industrially significant chemical because it is an essential precursor for the Fisher-Tropsch process. However, solid fuels such as coal yield syngas with H2:CO ratio of ∼0.7, which is unideal for hydrocarbon chain growth. Herein, we present electrochemical CO2 reduction to adjustable H2/CO syngas ratio by controlling the thickness of silver nanostructure. By looking into the local species generated during the electrolysis using the in-situ ATR-FTIR, the increased amount of anions such as OH− and CO32− trapped within the porous structure, which increases with the thickness of the nanostructure, leads to an increase in local pH near the electrode surface, and thus at the expense of the HER, the CO production is promoted. As a result, syngas composition can be adjusted simply by controlling the thickness of the nanostructure.