Specific Adsorption Of Anions Research Articles

Effect of Chlorine on Copper Electrodissolution

A computational study on the role of adsorbed Cl ions on copper dissolution is presented. Extensive grand canonical Monte Carlo (GCMC) calculations using reactive force-field (ReaxFF) are used to check the formation chlorine (Cl) adlayer structures on Cu (100) surface as a function of the chemical potential (µ). From experimental studies conducted by other researchers it has been observed that Cl adlattice on Cu in c(2X2) at maximum coverage. We observe that at low µ, Cl adlayer is disordered and moves to ordered c(2X2) structure at high µ. The binding energy of Cl, oxygen (O), hydroxide (OH-) and H2O on Cu at different sites is calculated and compared with Density-functional theory (DFT) values. Metal surface is often highly uneven and rough during electrodissolution process. Therefore, we also discuss the adsorption isotherm, disorder-order transformation, and preferential adsorption sites for rough surfaces. Specific anion adsorption (SAA) of Cl involves partial charge transfer to Cu substrate and these effect the type of bond and bond strength which is also briefly discussed. The electrodissolution of Cu in the presence of Cl is modelled using kinetic Monte Carlo simulation to obtain the polarization curve.Keywords: Adsorbed chlorine, electrodissolution, grand canonical monte carlo, molecular dynamics, ReaxFF, kinetic monte carlo.

Effect of Electrolyte Anions on the Activity of Iridium Oxide Catalysts for Water Electrolysis

Developing environmentally friendly and renewable energy sources is essential due to the environmental and energy security issues caused by traditional combustion of fossil fuel-based energy sources. Therefore, as a clean and sustainable energy carrier, hydrogen has attracted great attention in recent years. In this regard, water electrolysis utilizing renewable energy sources to produce hydrogen is considered one of the most promising technologies. It is believed elucidation of the reaction mechanism of highly active iridium oxides is promising for the further development of cost-efficient PEM water electrolysis catalysts. The properties of the MEA could be affected by many parameters such as the catalysts, operating temperature, and pressure1,2. Another important factor is the cation impurities that can be found in the circulating water, which could severely degrade the MEA performance during the operation of a PEM water electrolyze. However, there are less reports on anion impurities such as SO4 2- and PO4 3-. Therefore, in this study, the relationship between electrolyte anions and OER activity was investigated for IrOx catalysts with different crystal structure. By using operando X-ray absorption spectroscopy (XAS), we investigated the electronic state of Ir and O under operation potential.IrOx with different degree of crystallinity (SA3.5 and SA58) were provided by Tanaka Kikinzoku Kogyo K.K., Chemical & Refining Company and named after their BET surface areas. These two samples were then tested for electrochemical properties in different concentrations of phosphoric acid and perchloric acid, respectively. The electrochemical measurements were performed using a standard three-electrode cell connected to an MPG-205-NUC system (Bio-Logic). The cyclic voltammograms (CV) were scanned from 0.1 V to 1.2 V vs. RHE at 50 mV s-1. The electrochemical activities of the catalysts were subsequently investigated using linear sweep voltammetry (LSV) in the range of 1.1 to 1.7 V vs RHE at 10 mV s−1. operando XAS were collected using a home-made flow-type cell, where Pt wire and RHE worked as counter and reference electrodes, respectively. The catalyst ink was coated onto an Nafion membrane to prepare both the X-ray window and working electrode. The electrolyte was circulated at the flow rate of 100 mL min-1.The catalytic performance of IrOx was investigated at different types and content of electrolyte under H3PO4 and HClO4. Figure 1 shows the CVs of SA3.5 and SA58 under different types and content of electrolyte. In the presence of phosphate anions, SA3.5 showed a tendency to decrease Ir redox between 0.6 and 1.1 V with increasing electrolyte concentration. On the other hand, there were no obvious changes in SA58. It was found that IrOx catalysts have an great effect of specific adsorption of anions and low symmetry of monoclinic phase catalysts (SA 3.5) are more susceptible to anion poisoning. Figure 2 shows the result of examining the poisoning mechanism using soft X-ray XAS. O K-edge results present the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process3. The peak intensity at 528.7 eV of the amorphous samples was in the same order as the OER activities. According to potential dependence change of the pre-edge peak, under the presence of phosphate anions, the hole formation to Ir-O hybrid orbitals was suppressed in SA3.5. This result indicates that the anion adsorbs to the active site, poisoning the catalyst surface and inhibiting the reaction. Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Zhang, Y.; Zhu, X.; Zhang, G.; Shi, P.; Wang, A.-L.; et al. J. Mater. Chem. A: 2021, 9, 5890-5914.Chen, Z.; Duan, X.; Wei, W.; Wang, S.; Ni, B.-J.; et al. Nano Energy: 2020, 78, 105270.Nong, H. N.; Falling, L. J.; Bergmann, A.; et al. Nature 2020, 587 (7834), 408-413. Figure 1

Deconvolution of Charge-Transfer, Mass Transfer, and Ohmic Resistances of Phosphonic Acid-Sulfonic Acid Ionomer Binders in Electrochemical Hydrogen Pumps

Ion-pair high-temperature polymer electrolyte membranes (HT-PEMs) paired with phosphonic acid ionomer electrode binders have substantially improved the performance of HT-PEM electrochemical hydrogen pumps (EHPs)1, 2 and fuel cells3. Recently, blending poly(pentafluorstyrene-co-tetrafluorostyrene phosphonic acid) (PTFSPA) with NafionTM improved ionomer conductivity under anhydrous conditions in the temperature range of 100 °C to 250 °C. Using the said polymer blend as an electrode binder resulted in a 2 W.cm-2 peak power density of fuel cells4 at 240 °C (a HT-PEM fuel cell record). However, much is still unknown about how phosphonic acid ionomers blended with perfluorosulfonic acid (PFSA) materials affect electrode kinetics and gas transport in porous electrodes. In this work, we studied the ionic conductivity, electrode kinetics, and gas transport resistance of 3 types of phosphonic acid ionomers, poly(vinyl phosphonic acid), poly(vinyl benzyl phosphonic acid) by themselves and when blended with Aquivion®. These studies were performed in the context of an EHP platform – both membrane electrode assemblies (MEAs) and interdigitated electrode arrays (IDAs) decorated with nanoscale platinum electrocatalysts with thin film ionomer electrolytes. For all phosphoric acid ionomer types, the addition of Aquivion® promoted ionic conductivity, hydrogen oxidation/evolution reaction kinetics (HOR/HER), and hydrogen gas permeability. Solid-state 31P NMR revealed that the addition of Aquivion® significantly reduced phosphate ester formation in phosphoric acid ionomers and this played a vital role in enhancing ionomer blend conductivity. Using the best blend variant, PTFSPA-Aquivion®, a remarkable EHP performance of 5.1 A.cm-2 at 0.4 V at T = 200 °C was attained. Density functional theory (DFT) simulations identified that phosphonic acids with electron-withdrawing moieties reduced the propensity of the phosphonic acid to adsorb to platinum electrocatalyst surfaces. All phosphonic acid ionomers are anticipated to have a greater affinity for adsorption on platinum when compared to sulphonic acid ionomers (PFSAs). The relative adsorption affiliation of the various phosphoric acid ionomers from DFT is consistent with an explanation that stronger anion-specific adsorption has a detrimental impact and is commensurate with experimentally obtained MEA charge-transfer kinetics. A machine learning-aided compositional model5 revealed that the addition of Aquivion® reduced activation and concentration overpotentials in EHP MEAs and improved exchange current density and diffusivity in EHP IDAs. Future work involves employing the EHPs in a real-world application for separating and compressing hydrogen from the natural gas mixture.References Venugopalan, G.; Bhattacharya, D.; Andrews, E.; Briceno-Mena, L.; Romagnoli, J.; Flake, J.; Arges, C. G., Electrochemical Pumping for Challenging Hydrogen Separations. ACS Energy Letters 2022, 7 (4), 1322-1329.Venugopalan, G.; Bhattacharya, D.; Kole, S.; Ysidron, C.; Angelopoulou, P. P.; Sakellariou, G.; Arges, C. G., Correlating high temperature thin film ionomer electrode binder properties to hydrogen pump polarization. Materials Advances 2021, 2 (13), 4228-4234.Atanasov, V.; Lee, A. S.; Park, E. J.; Maurya, S.; Baca, E. D.; Fujimoto, C.; Hibbs, M.; Matanovic, I.; Kerres, J.; Kim, Y. S., Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells. Nat Mater 2021, 20 (3), 370-377.Lim, K. H.; Lee, A. S.; Atanasov, V.; Kerres, J.; Park, E. J.; Adhikari, S.; Maurya, S.; Manriquez, L. D.; Jung, J.; Fujimoto, C.; Matanovic, I.; Jankovic, J.; Hu, Z.; Jia, H.; Kim, Y. S., Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells. Nature Energy 2022, 7 (3), 248-259.Briceno-Mena, L. A.; Venugopalan, G.; Romagnoli, J. A.; Arges, C. G., Machine learning for guiding high-temperature PEM fuel cells with greater power density. Patterns 2021, 2 (2), 100187. Figure 1

Anion-Specific Adsorption of Carboxymethyl Cellulose on Cellulose.

Integration of fiber modification step with a modern pulp mill is a resource efficient way to produce functional fibers. Motivated by the need to integrate polymer adsorption with the current pulping system, anion-specific effects in carboxymethylcellulose (CMC) adsorption have been studied. The QCM-D adsorption experiments revealed that CMC adsorption to the cellulose model surface is prone to anion-specific effects. A correlation was observed between the adsorbed CMC and the degree of hydration of the co-ions present in the magnesium salts. The presence of a chaotropic co-ion such as nitrate increased the adsorption of CMC on cellulose compared to the presence of the kosmotropic sulfate co-ion. However, anion-specificity was not significant in the case of salts containing zinc cations. The hydration of anions determines the distribution of the ions at the interface. Chaotropic ions, such as nitrates, are likely to be distributed near the chaotropic cellulose surface, causing changes in the ordering of water molecules and resulting in greater entropy gain once released from the surface, thus increasing CMC adsorption.

Reveal the Electrical Double Layer (EDL) in Zinc Battery Electrolytes with Electrocapillarity

The electrified interface between electrodes and electrolytes, aka, electrical double layer (EDL), resides at the heart of electrochemistry and exists universally in all electrochemical devices. It governs the kinetics and reversibility of all electrochemical processes, and predicts the interphase chemistries in advanced batteries. However, despite such importance, its understanding remains insufficient. While many theoretical models have been developed to predict EDL structure and to correlate key properties to the structures, so far there have been very rare direct experimental characterization, due to the limitations of existing techniques and the extreme elusiveness and sensitivity of the electrified interfaces.In this talk, I will discuss a modernized high-resolution electrocapillarity method, developed in my group. Applying this reborn electrocapillarity on a bivalent electrolyte of ZnCl2 and ZnSO4, we reveal the interfacial structure of Zn ion battery electrolytes in the absence and presence of anion specific adsorption and established the abnormal potential distribution across the interface due to anion adsorption and its effect on the kinetics of Zn ion deposition chemistry.

Effect of trace impurities in perchloric acid on blank voltammetry of Pt(111)

Trace impurities of nitrate and sulphate species have been detected as the cause for the charge quench of existing voltammetric peaks and the appearance of new voltammetric peaks in the blank voltammetry of Pt(111) in high-purity perchloric acid. Cyclic voltammetry of Pt(111) in high concentrations of HClO4 (1 M) electrolyte display anomalous reduction peaks at 0.5 and 0.32 VRHE. The reduction of nitrates to nitric oxide, resulting in the reduction peak at 0.32 VRHE, has been detected using in situ Fourier transform infrared spectroscopy. The peak at 0.5 VRHE, previously assigned in the literature to specific adsorption of perchlorate anions catalytically dissociating to chloride anions, is consistent with sulphate desorption as elucidated by spiking HClO4 electrolyte with trace quantities (10 ppm) of sulphuric acid. The presence of kosmotropic sulphate anions was further confirmed by the positive onset potential shift of the sharp OHads butterfly peak at ∼0.8 VRHE with increasing concentrations of HClO4 (0.01–1 M). Although commonly overlooked, we show here the marked deleterious impact these trace impurities have on the blank voltammetry of Pt(111), and provide recommendations for minimising this impact.

Operando X-Ray Absorption Spectroscopic Study on Influence of Specific Adsorption of Sulfo Group in Perfluorosulfonic Acid Ionomer Towards ORR Activity of Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are clean energy devices with polymer electrolytes and are expected to play an important role in the development of a global renewable energy system and a pure hydrogen energy society in the future. PEFCs have already been applied for small-scale energy systems such as fuel cell vehicles (FCVs). Pt-based catalysts are basically used due to the relatively inefficient reaction kinetics and catalyst degradation under strong acidic conditions during the oxygen reduction reaction (ORR). However, there are still many difficulties in realizing acceptable cost-effective FCVs. It is known that the ORR activity of Pt-based catalysts is reduced by specific adsorption of anionic species. Perfluorosulfonic acid ionomers such as Nafion® are commonly included in the catalyst layers of PEFCs to provide protonic pathways for promoting the ORR1. However, direct observation of anion adsorption on practical Pt nanoparticle/C catalysts for perfluorosulfonic acid ionomers using X-ray absorption spectroscopy (XAS) still has less report. In this study, to achieve specific adsorption over a higher potential range that is at least equal to the ORR active potential (around 0.90 V vs. RHE), operando XAS measurements were performed; the electronic status and local structural change of Pt atoms induced by the specific adsorption of anions at any potential were observed2.Pt/C (29.1 wt.%; TEC10V30E) catalyst supported was purchased from TKK. Nafion® solution was employed to prepare Pt/C catalyst ink with I/C ranging from 0.0 to 1.0. And the amount of Pt/C was adjusted to form a 20 µgcarbon cm-2 catalyst layer after dropping 10 μL on a glassy carbon RDE. The electrochemical cell was constructed using a model electrode fabricated in the working electrode, a Pt mesh for the counter electrode, a reversible hydrogen electrode for the reference electrode, and 0.1 M HClO4 aq for the electrolyte. operando XAS measurement of Pt L-edge was carried out in SPring-8 (Japan).The specific activity decreased as the I/C ratio increased from 0.0 to 0.20. We utilized many methods to evaluate the adsorption species separately, including the measurements of ECSA and oxygen coverage, CO stripping voltammetry, operando X-ray absorption fine structure, and analysis of 5d orbital vacancy. The ECSA and oxygen coverage did not change with increasing ionomer content, indicating that the Pt/C catalyst activity was affected by other adsorption species. A comparison of the CO displacement charge and 5d orbital vacancies of the Pt/C catalysts with I/C = 0.0 and 1.0 suggests that the ionomer-specific adsorption increases when the I/C ratio of the Pt/C catalyst is 1.0; active sites on the surface of the Pt/C catalyst are occupied, resulting in lower catalyst activity. Acknowledgment This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

(Invited) Tailored Electrochemical Interfaces for the Production of Renewable Fuels

Tailoring the structure of the electrochemical interface at the atomic level is key to design new electrocatalysts for renewable energy conversion and storage. This talk will focus on novel catalyst materials and engineered interfaces for electrochemical energy conversion. In particular, I will discuss structure sensitivity and electrolyte effects for oxygen and carbon dioxide/carbon monoxide electrocatalysis.First, I will present our work toward understanding the structure-activity-stability relations on high surface area nanostructured Ir-based materials for oxygen evolution. Then, I will highlight the importance of carrying out model studies on Cu-based surfaces to understand the structure-properties relations for CO2 and CO reduction. We have investigated the effect of pH, specific anion adsorption and potential dependence for CO reduction on Cu single-crystalline electrodes. We show how well defined studies are essential to design efficient electrocatalysts for the production of renewable fuels.

Modifying the Electrocatalytic Selectivity of Oxidation Reactions with Ionic Liquids.

The “solid catalyst with ionic liquid layer” (SCILL) is an extremely successful new concept in heterogeneous catalysis. The idea is to boost the selectivity of a catalyst by its modification with an ionic liquid (IL). Here, we show that it is possible to use the same concept in electrocatalysis for the selective transformation of organic compounds. We scrutinize the electrooxidation of 2,3‐butanediol, a reaction which yields two products, singly oxidized acetoin and doubly oxidized diacetyl. When adding the IL (1‐ethyl‐3‐methyl‐imidazolium trifluormethanesulfonate, [C2C1Im][OTf]), the selectivity for acetoin increases drastically. By in situ spectroscopy, we analyze the underlying mechanism: Specific adsorption of the IL anions suppresses the activation of water for the second oxidation step and, thus, enhances the selectivity for acetoin. Our study demonstrates the great potential of this approach for selective transformation of organic compounds.

Equilibrium thickness of foam films and adsorption of ions at surfaces: Aqueous solutions of sodium hydrogencarbonate and sodium carbonate

Hypothesis.The origin of the negative charge at water/air interface was proved to be not only specific adsorption of OH– ions but that of HCO3– and/or CO32– ions in the previous study [1]. To determine which anionic species is primarily responsible for the surface charge, the surface density of ions in the Stern layer is numerically evaluated from foam-film thickness of aqueous solutions of NaHCO3 and Na2CO3.Experiments.Equilibrium thickness (equivalent thickness at equilibrium) of the foam films formed from aqueous solutions of NaHCO3 and Na2CO3 was measured as a function of electrolyte concentration at 298.15 K.Findings.Applying a modified Poisson-Boltzmann (PB) equation developed for various kinds of electrolytes to the equilibrium thickness gave the surface density of ions in the Stern layer for NaHCO3 and Na2CO3 systems. From the concentration dependence of the surface density together with that for NaCl and NaOH in the previous study [1], the negative surface charges for water and very dilute solutions were found to be due to specific adsorption of HCO3– ions. The surface charge at high electrolyte concentration is determined by the specific adsorption of electrolyte anions. The specific-adsorption ability of anion increases in the order CO32–≫ HCO3–> OH–≫ Cl-.

Electrolyte Effects on the Stabilization of Prussian Blue Analogue Electrodes in Aqueous Sodium-Ion Batteries.

Aqueous sodium-ion batteries based on Prussian Blue Analogues (PBA) are considered as promising and scalable candidates for stationary energy storage systems, where longevity and cycling stability are assigned utmost importance to maintain economic viability. Although degradation due to active material dissolution is a common issue of battery electrodes, it is hardly observable directly due to a lack of in operando techniques, making it challenging to optimize the performance of electrodes. By operating Na2Ni[Fe(CN)6] and Na2Co[Fe(CN)6] model electrodes in a flow-cell setup connected to an inductively coupled plasma mass spectrometer, in this work, the dynamics of constituent transition-metal dissolution during the charge-discharge cycles was monitored in real time. At neutral pHs, the extraction of nickel and cobalt was found to drive the degradation process during charge-discharge cycles. It was also found that the nature of anions present in the electrolytes has a significant impact on the degradation rate, determining the order ClO4- > NO3- > Cl- > SO42- with decreasing stability from the perchlorate to sulfate electrolytes. It is proposed that the dissolution process is initiated by detrimental specific adsorption of anions during the electrode oxidation, therefore scaling with their respective chemisorption affinity. This study involves an entire comparison of the effectiveness of common stabilization strategies for PBAs under very fast (dis)charging conditions at 300C, emphasizing the superiority of highly concentrated NaClO4 with almost no capacity loss after 10 000 cycles for Na2Ni[Fe(CN)6].

EFFECT OF ELECTROLYTES ON ELECTROSURFACE PROPERTIES OF MICROCRYSTALLINE CELLU-LOSE PARTICLES

The electrical surface properties (specific surface charge of particles - σ0, point of zero charge – pHPZC, electrokinetic potential - ζ and position of isoelectric point - pHIEP) of dispersions of microcrystalline cellulose (MCC) in aqueous solutions of H2SO4, NaOH, Na2SO4 and TiOSO4 were investigated. Significant effects of specific adsorption of anions and cations on pHPZC and pHIEP have been demonstrated. The isoelectric point of the MCC particles in the H2SO4 solution is at pH 2.0, while the zero charge point in 5·10-4 M solution Na2SO4 at pH 5.6. An explanation of the observed effects is given. Possible schemes of the structure of the electrical double layer at different pH values, taking into account specific adsorption of ions in the Stern layer, are given. The introduction of TiOSO4 (10-5 mol/dm3) shifts the position of IEP to a less acidic region (pHIEP 4.5) due to the specific adsorption of positively charged TiOSO4 hydrolysis products particles.

On the Differential Capacitance and Potential of Zero Charge of Au(111) in Some Aprotic Solvents

AbstractVoltammetric and Gouy‐Chapman capacitance minimum measurements were conducted on Au(111) and roughened Au(111) electrodes in aprotic electrolytes in the absence and presence of specifically adsorbed ions for concentrations ranging from 0.001 to 0.5 M. Negative of the point of zero charge (pzc), the capacitance maximum increases in the order Ca2+<Li+<K+, but the pzc value is independent of cation nature. The capacitance values have a slight dependence on cation type at a more negative potential. The pzc depends on the solvent and is influenced by the dielectric constant, metal‐solvent interactions and donor number; it is shifted to more positive values with increasing water content. Specific anion adsorption shifts the pzc to more negative values. The pzc value is more negative for the roughened electrode compared to the single crystal electrode. Propylene carbonate (PC) adsorption in the presence of perchlorate and iodide was examined on gold film by surface‐enhanced infrared absorption spectroscopy (SEIRAS). The potential dependent intensity is explained by a stronger electric field and thus a better alignment of the PC molecules when the absolute potential difference to the pzc is large.The capacitance curves were theoretically modelled. From the capacitance maxima on both sides of the pzc the ionic volume and thus the solvation numbers were estimated. The width of the capacitance minimum is believed to be related to the layer of adsorbed solvent molecules.

Adsorption of bromide and iodide anions on a renewable liquid (Cd–Ga) electrode from acetonitrile solutions. Effect of (Cd–Ga)–solvent chemisorption interaction on the anion adsorption energy

The specific adsorption of bromide and iodide anions was studied at a (Cd–Ga)/acetonitrile (AN) interface from mixed solutions of [0.1 m M LiBr + 0.1 (1–m) M LiBF4] and [0.1 m M LiI + 0.1 (1–m) M LiBF4] at the following values of molar fraction m of the surface–active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The main experimental method was measurement of differential capacitance on a renewable liquid (Cd–Ga) electrode. The obtained experimental results can be described quantitatively by the Frumkin isotherm in a wide range of charge densities (σ). The adsorption parameters of halide anions (Hal−) were determined using regression analysis of two-dimensional pressure isotherms. It is shown that the adsorption energies (ΔGads) of Br− and I− at the (Cd–Ga)/AN interface practically coincide. It follows that the difference in the interaction energies of Br− and I− with (Cd–Ga) is equal to the difference of the solvation energies of these anions: (ΔG(Cd‐Ga)‐Br‐ − ΔG(Cd‐Ga)‐I‐)≈(ΔGAN‐Br‐ − ΔGAN‐I‐) and therefore ΔG(Cd-Ga)-I− < ΔG(Cd-Ga)-Br−. The adsorption parameters of Hal− obtained on the (Cd–Ga)/AN interface are compared with the similar data for the (Cd–Ga)/DMF interface. The adsorption energy of Br− and I− on a (Cd–Ga) electrode increases significantly in AN as compared to DMF. This result can be unambiguously explained by a significant decrease in the energy of chemisorption interaction between a (Cd–Ga) electrode and solvent molecules in the Helmholtz layer upon transition from DMF to AN. The ΔGads,σ-dependence obtained at the (Cd–Ga)/AN interface in the first approximation characterizes the change of the free energy of (Cd–Ga)–Hal− interaction with σ. It is shown that the slope of the ΔGads, σ-dependence decreases upon transition from the (Cd–Ga)/AN interface to the (Cd–Ga)/DMF interface due to a significant increase in the energy of the chemisorption interaction of (Cd–Ga) with DMF molecules in the Helmholtz layer during the transition from negative to positive values of charge density.

Why the activity of the hydrogen oxidation reaction on platinum decreases as pH increases

Platinum is a very effective electrode for the hydrogen evolution and oxidation reactions (HER/HOR) in acidic media. However, the activity for the HOR on platinum falls two orders of magnitude from acidic to alkaline media, which has not been completely understood yet. Here, we provide an explanation for that. Both the HER and the HOR were investigated on the three basal planes of platinum in a pH range near neutral pH conditions in buffered solutions in the absence of anion specific adsorption for guaranteeing the protons availability. Whereas changes in the pH from acid to neutral values produced negligible effects on the HER, the HOR was found to be pH sensitive, even under near neutral pH conditions. From these results, it can be consistently reasoned that the drastic fall in the activity of the HOR on platinum from acidic to alkaline media is an effect of the charge on the electrode, which is more negative as the pH increases. With the aid of density functional theory calculations, kinetic arguments explaining the unfavorable effect that negative charge on the electrode has on the HOR are provided.

Potential-dependent reconstruction kinetics probed by HER on Au(111) electrodes

The electrocatalytic hydrogen evolution reaction (HER) is employed to monitor the kinetics of potential-induced surface reconstruction of Au(111) in contact with either 0.1 M H2SO4 or 0.1 M HClO4 in the temperature range between 17 and 25 °C. An apparent activation energy varying between 0.6 and 0.7 eV is found for the reconstruction process at 20 °C in the potential range between −0.1 and −0.7 V vs. saturated mercury-mercurous sulfate electrode (MSE). The rate constant of the first-order formation kinetics shows an exponential dependence on electrode potential and decreases with more negative potentials. The potential-dependence is associated with an increase in bond strength for Au surface atoms with increasing negative excess charge. Specific adsorption of anions hinders surface reconstruction at potentials close to the potential of zero charge, while the presence of carbon monoxide causes a significant enhancement of the rate for potential-induced Au(111) reconstruction.

Adstructures of platinum-complex precursors and platinum nanoparticles formed on low-index single-crystal Au surfaces for oxygen reduction reaction

Abstract We investigated the adlayer structures of Pt complexes, PtCl42− and PtBr42−, by using in situ scanning tunneling microscopy (STM) and the electrochemical behavior on the Pt nanoparticles deposited from the PtCl42− and PtBr42− precursors on Au(100), Au(111), and Au(110) surfaces in 0.1 M HClO4. The STM investigation revealed that the PtCl42− and the PtBr42− adlayers had very similar structures on Au(111) and Au(110) surfaces: the (√7 × √7)R19.1° structure on Au(111), and the c(2 × 4) structure on Au(110). On Au(100), PtCl42− formed a (√5 × √5)R26.7° structure, whereas PtBr42− was adsorbed randomly in a flat configuration. High-resolution STM images also revealed pinwheel features of the Pt complexes, resulting from the flat-lying adsorption configurations. The electrochemical reduction of the PtBr42− adlayer proceeded at a more negative potential than the PtCl42− adlayer to form metallic Pt nanoparticles. The average size of the Pt particles varied on the different Au surfaces. The cyclic voltammograms and the catalytic activities for the oxygen reduction reaction were different on the Pt complexes and the three Pt-modified Au(hkl) electrodes. The structures of deposited Pt nanoparticles depended on the substrate Au electrodes. The order of the ORR activity of Pt nanoparticles on Au substrates, Pt/Au(111) > Pt/Au(100) ≥ Pt/Au(110), corresponded to the relative ease of specific adsorption of anions on Pt single-crystal electrodes, Pt(111) > Pt(110) > Pt(100).

Adsorption processes on a Pd monolayer-modified Pt(111) electrode.

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt(111) single crystal electrode in perchloric acid solution. The “hydrogen region” of this PdMLPt(111) surface exhibits two voltammetric peaks: the first “hydrogen peak” at 0.246 VRHE actually involves the replacement of hydrogen by hydroxyl, and the second “hydrogen peak” HII at 0.306 VRHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the PdMLPt(111) surface than to Pt(111).

Adsorption of anions on bismuth and cadmium single crystal plane electrodes from various solvents and ionic liquid mixtures

Electrical double layer characteristics and specific adsorption of anions at Bi and Cd single crystal planes from aprotic and other non-aqueous electrolytes, as well as mixtures of surface-inactive and surface-active ionic liquids have been studied and analysed. Strong influence of sp-metal electronic characteristics (concentration of charge carriers, effective mass of electrons) on the potential of zero charge and differential capacitance values have been demonstrated. Influence of solvent chemical composition and dielectric characteristics on the surface activity and Gibbs adsorption energy values of anions at sp-metal single crystal planes has been demonstrated. Influence of ionic liquid anion composition on the specific adsorption characteristic time constant at Bi(hkl) and Cd(0001) single crystal electrodes in comparison with aprotic and non-aqueous solvent based electrolytes demonstrates the role of the solvent properties on the adsorption process characteristics. The non-aqueous solvent (acetonitrile, ethylene carbonate, propylene carbonate and gamma-butyrolactone) based electrolytes, ionic liquids and ionic liquid mixtures discussed are important electrolytes applicable in electrical double layer capacitors, secondary-ion batteries and other electrochemical devices. Appreciable increase of specific energy and power densities has been demonstrated for supercapacitors with ionic liquid electrolytes containing iodide and bromide anions, development of which was based on the adsorption data collected and discussed.

Synchronous phosphate and fluoride removal from water by 3D rice-like lanthanum-doped La@MgAl nanocomposites

The frequent occurrence of algae boom and skeletal fluorosis has drawn global attention to developing feasible technologies for phosphate and fluoride removal from water bodies. This study reported the synthesis of novel three-dimensional rice-like La@MgAl nanocomposites by anchoring La species into Mg-Al LDH matrix to achieve effective and synchronous phosphate and fluoride capture. Results from multiple detection methods including XRD, SEM, FTIR, and XPS revealed the introduced La cations could serve as a structural inducer to regulate the crystallinity of the La@MgAl composites and rice-like nanocomposite La@MgAl-1 was fabricated under optimal La loading amount. La@MgAl-1 possessed high adsorption capacities for both phosphate and fluoride (101.59 mg/g and 51.03 mg/g, respectively). The effective adsorption performance also sustained even after five adsorption-desorption cycles, indicating favorable reusability. The adsorption mechanism analysis revealed that the presence of unique hydrotalcite-like structure triggered memory effect to exert strong adsorption of P and F anions into the interlayered gallery. Besides, an optimum amount of La species organized on the composite wielded specific anion adsorption to phosphate and fluoride through inner-sphere complexation and ligand exchange. These encouraging results highlight the positive consequence of combining the strategic La species and LDH matrix to render a nanostructured Mg/Al/La ternary metal assembly in which various components are united in a controllable way to exert collective properties for accumulative anion adsorption properties. Besides, this study offered promising candidates for real water purification against phosphate and fluoride pollution.