Interfacial Proton Transfer Research Articles

Mechanistic Insights into Surfactant-Modulated Electrode-Electrolyte Interface for Steering H2O2 Electrosynthesis.

Electrocatalytic reactions taking place at the electrified electrode-electrolyte interface involve processes of proton-coupled electron transfer. Interfacial protons are delivered to the electrode surface via a H2O-dominated hydrogen-bond network. Less efforts are made to regulate the interfacial proton transfer from the perspective of interfacial hydrogen-bond network. Here, we present quaternary ammonium salt cationic surfactants as electrolyte additives for enhancing the H2O2 selectivity of the oxygen reduction reaction (ORR). Through in situ vibrational spectroscopy and molecular dynamics calculation, it is revealed that the surfactants are irreversibly adsorbed on the electrode surface in response to a given bias potential range, leading to the weakening of the interfacial hydrogen-bond network. This decreases interfacial proton transfer kinetics, particularly at high bias potentials, thus suppressing the 4-electron ORR pathway and achieving a highly selective 2-electron pathway toward H2O2. These results highlight the opportunity for steering H2O-involved electrochemical reactions via modulating the interfacial hydrogen-bond network.

(Invited) Oxide-Water Interfaces from Ab Initio Based Deep Neural Network Simulations

Photo-electrocatalysis involving complex oxide-water interfaces is a highly promising technology for the sustainable production of fuels. However, probing these complex interfaces and gaining atomistic insights is still very challenging for current experimental methods, and is often only possible through accurate computational simulations. While first principles-based simulations provide the best accuracy, they are also limited to small system sizes and time-scales due to their high computational cost. Machine learned potentials that reproduce the accuracy of first-principles methods, while at the same time allowing the exploration of larger systems and timescales, are a promising approach to circumvent this issue. In this talk, I will discuss recent applications of deep neural network based molecular dynamics simulations to understand the structure and dynamics of interfacial water on photoelectrochemically relevant oxide surfaces. Focusing on TiO2 and IrO2 surfaces, we examine interfacial proton transfer and discuss how the water dissociation fraction and hydroxyl lifetimes depend on the surface atomic and electronic structures.

Enhanced Performance of Protonic Solid Oxide Steam Electrolysis Cell of Zr-Rich Side BaZr0.6Ce0.2Y0.2O3-δ Electrolyte with an Anode Functional Layer.

Proton-conducting solid oxide electrolysis cells (H-SOEC) containing a 15-μm-thick BaZr0.6Ce0.2Y0.2O3−δ (BZCY622) electrolyte thin film, porous cathode cermet support, and La0.6Sr0.4Co0.2Fe0.8O3−δ anodes were fabricated using a reactive cofiring process at approximately 1400 °C. Steam electrolysis was conducted by supplying wet air to the anode at a water partial pressure of 20 kPa. The performance was evaluated using electrochemical measurements and gas chromatography. At 600 °C, the cells generated an electrolysis current of 0.47 A cm–2 at a 1.3 V bias while the Faradaic efficiency reached 56% using 400 mA cm–2. The electrolysis performance was efficiently improved by introducing a 40-nm-thick La0.5Sr0.5CoO3−δ (LSC) nanolayer as an anode functional layer (AFL). The cells with LSC AFL produced an electrolysis current of 0.87 A cm–2 at a 1.3 V bias at 600 °C, and the Faradaic efficiency reached 65% under 400 mA cm–2. Impedance analysis showed that the introduction of the AFL decreased the ohmic resistances and improved interfacial proton transfer across the anode/electrolyte interface and polarization resistances related to the anode reaction. These results demonstrate opportunities for future research on AFL to improve the performance of H-SOECs with Zr-rich BaZrxCe1–x–yYyO3−δ electrolytes.

Accelerated interfacial proton transfer for promoting electrocatalytic activity.

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pK a of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H+ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.

The role of proton dynamics on the catalyst-electrolyte interface in the oxygen evolution reaction

The development of non-precious metal catalysts that facilitate the oxygen evolution reaction (OER) is important for the widespread application of hydrogen production by water splitting. Various perovskite oxides have been employed as active OER catalysts, however, the underlying mechanism that occurs at the catalyst-electrolyte interface is still not well understood, prohibiting the design and preparation of advanced OER catalysts. Here, we report a systematic investigation into the effect of proton dynamics on the catalyst-electrolyte interfaces of four perovskite catalysts: La 0.5 Sr 0.5 CoO 3-δ (LSCO), LaCoO 3 , LaFeO 3 , and LaNiO 3 . The pH-dependent OER activities, H/D kinetic isotope effect, and surface functionalization with phosphate anion groups were investigated to elucidate the role of proton dynamics in the rate-limiting steps of the OER. For oxides with small charge-transfer energies, such as LSCO and LaNiO 3 , non-concerted proton-coupled electron transfer steps are involved in the OER, and the activity is strongly controlled by the proton dynamics on the catalyst surface. The results demonstrate the important role of interfacial proton transfer in the OER mechanism, and suggest that proton dynamics at the interface should carefully be considered in the design of future high-performance catalysts. A systematical study was conducted to probe the role of proton dynamics in the multiple PCET steps during OER on the catalyst-electrolyte interfaces using perovskite oxides as model catalysts.

Interfacial Proton Transfer for Hydrogen Evolution at the Sub-Nanometric Platinum/Electrolyte Interface.

Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm2 and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)-O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.

Spontaneous Electric Fields Play a Key Role in Thermochemical Catalysis at Metal-Liquid Interfaces.

Large oriented electric fields spontaneously arise at all solid–liquid interfaces via the exchange of ions and/or electrons with the solution. Although intrinsic electric fields are known to play an important role in molecular and biological catalysis, the role of spontaneous polarization in heterogeneous thermocatalysis remains unclear because the catalysts employed are typically disconnected from an external circuit, which makes it difficult to monitor or control the degree of electrical polarization of the surface. Here, we address this knowledge gap by developing general methods for wirelessly monitoring and controlling spontaneous electrical polarization at conductive catalysts dispersed in liquid media. By combining electrochemical and spectroscopic measurements, we demonstrate that proton and electron transfer from solution controllably, spontaneously, and wirelessly polarize Pt surfaces during thermochemical catalysis. We employ liquid-phase ethylene hydrogenation on a Pt/C catalyst as a thermochemical probe reaction and observe that the rate of this nonpolar hydrogenation reaction is significantly influenced by spontaneous electric fields generated by both interfacial proton transfer in water and interfacial electron transfer from organometallic redox buffers in a polar aprotic ortho-difluorobenzene solvent. Across these vastly disparate reaction media, we observe quantitatively similar scaling of ethylene hydrogenation rates with the Pt open-circuit electrochemical potential (EOCP). These results isolate the role of interfacial electrostatic effects from medium-specific chemical interactions and establish that spontaneous interfacial electric fields play a critical role in liquid-phase heterogeneous catalysis. Consequently, EOCP—a generally overlooked parameter in heterogeneous catalysis—warrants consideration in mechanistic studies of thermochemical reactions at solid–liquid interfaces, alongside chemical factors such as temperature, reactant activities, and catalyst structure. Indeed, this work establishes the experimental and conceptual foundation for harnessing electric fields to both elucidate surface chemistry and manipulate preparative thermochemical catalysis.

Realizing Ultrafast Oxygen Evolution by Introducing Proton Acceptor into Perovskites

AbstractThe oxygen evolution reaction (OER) is of prime importance in multiple energy storage devices. Perovskite oxides involving lattice‐oxygen oxidation are generally regarded as highly active OER catalysts, but the deprotonation of surface‐bound intermediates limit the further activity improvement. Here, it is shown that this kinetic limitation can be removed by introducing Sr3B2O6 (SB) which activates a proton‐acceptor functionality to boost OER activity. As a proof‐of‐concept example, an experimental validation is conducted on the extraordinary OER performance of a Sr(Co0.8Fe0.2)0.7B0.3O3−δ (SCFB‐0.3) hybrid catalyst, made using Sr0.8Co0.8Fe0.2O3−δ as active component and SB as a proton acceptor. This smart hybrid exhibits an exceptionally ultrahigh OER activity with an extremely low overpotential of 340 mV in 0.1 m KOH and 240 mV in 1 m KOH required for 10 mA cm−2 which is the top‐level catalytic activity among metal oxides reported so far, while maintaining excellent durability. The correlation of pH and activity study reveals that this enhanced activity mainly originates from the improved interfacial proton transfer. Such a strategy further demonstrated to be universal, which can be applied to enhance the OER activity of other high covalent oxides with close O 2p‐band centers relative to Fermi energy.

Platinum Nanoparticle Impacts at a Liquid|Liquid Interface

AbstractSingle nanoparticle (NP) electrochemistry detection at a micro liquid|liquid interface (LLI) is exploited using the catalyzed oxygen reduction reaction (ORR). In this way, current spikes reminiscent of nanoimpacts were recorded, which corresponded to electrocatalytic enhancement of the ORR by Pt NPs. The nature of the LLI allows exploration of new phenomena in single NP electrochemistry. The recorded impacts result from a bipolar reaction occurring at the Pt NP straddling the LLI. O2 reduction takes place in the aqueous phase, while ferrocene hydride (Fc‐H+; a complex generated upon facilitated interfacial proton transfer by Fc) is oxidized in the organic phase. Ultimately, the role of reactant partitioning, NP bouncing, or the ability of NPs to induce Marangoni effects, is demonstrated.

Platinum Nanoparticle Impacts at a Liquid|Liquid Interface.

Single nanoparticle (NP) electrochemistry detection at a micro liquid|liquid interface (LLI) is exploited using the catalyzed oxygen reduction reaction (ORR). In this way, current spikes reminiscent of nanoimpacts were recorded, which corresponded to electrocatalytic enhancement of the ORR by Pt NPs. The nature of the LLI allows exploration of new phenomena in single NP electrochemistry. The recorded impacts result from a bipolar reaction occurring at the Pt NP straddling the LLI. O2 reduction takes place in the aqueous phase, while ferrocene hydride (Fc-H+ ; a complex generated upon facilitated interfacial proton transfer by Fc) is oxidized in the organic phase. Ultimately, the role of reactant partitioning, NP bouncing, or the ability of NPs to induce Marangoni effects, is demonstrated.

Phosphate Ion Functionalization of Perovskite Surfaces for Enhanced Oxygen Evolution Reaction.

Recent findings revealed that surface oxygen can participate in the oxygen evolution reaction (OER) for the most active catalysts, which eventually triggers a new mechanism for which the deprotonation of surface intermediates limits the OER activity. We propose in this work a "dual strategy" in which tuning the electronic properties of the oxide, such as La1-xSrxCoO3-δ, can be dissociated from the use of surface functionalization with phosphate ion groups (Pi) that enhances the interfacial proton transfer. Results show that the Pi functionalized La0.5Sr0.5CoO3-δ gives rise to a significant enhancement of the OER activity when compared to La0.5Sr0.5CoO3-δ and LaCoO3. We further demonstrate that the Pi surface functionalization selectivity enhances the activity when the OER kinetics is limited by the proton transfer. Finally, this work suggests that tuning the catalytic activity by such a "dual approach" may be a new and largely unexplored avenue for the design of novel high-performance catalysts.

Molecular electrocatalysis of oxygen reduction by iron(II) phthalocyanine at the liquid/liquid interface

Liquid/liquid interface electrochemistry has manifested itself as a good approach to study oxygen reduction reaction (ORR) and ORR catalyzed by various catalysts. In this work, we investigated the ORR catalyzed by iron(II) phthalocyanine (FePc), which has a structure similarity to the heme group of O2-binding proteins and reducing enzymes, at the polarized water/1,2-dichloroethane interface. Using the four-electrode cyclic voltammetry and the shake-flask biphasic reaction under chemically controlled polarization, it was demonstrated that FePc could catalyze the molecular oxygen reduction by lipophilic electron donors, such as 1,1′-dimethylferrocene (DFc) or tetrathiafulvalene (TTF), at the heterogeneous phase boundary. The overall process essentially can be equivalent to an interfacial proton transfer coupled ORR, which proceeds preferentially via a four electron reduction pathway to produce mainly H2O with only minority of H2O2 (less than 5%). The catalytic activity of FePc was compared with all previously studied porphyrins and phthalocyanines. The reaction mechanism was also analyzed, in which a hydroperoxo intermediate was probably involved.

Understanding "on-water" catalysis of organic reactions. Effects of H+ and Li+ ions in the aqueous phase and nonreacting competitor H-bond acceptors in the organic phase: on H2O versus on D2O for Huisgen cycloadditions.

For a typical Huisgen cycloaddition, carried out on water, the behavior of water molecules at the oil-water interface depended on the properties of the reactants. With weakly basic reactants, a small quantity of added H(+) (HClO4, 0.0001-0.01 M) present in the aqueous phase had negligible effects, but larger quantities of H(+) (HClO4, 0.1-3.0 M) increased the catalytic effect and caused protons to cross the water-organic interface and affect the products. Added Li(+) ions (LiClO4, 0.1-3.0 M) had no effect for on-water reactions but enhanced the rates and endo products for in-water reactions. For these cycloaddition reactions, the product endo:exo ratios, when compared to those in organic solvents, can be used to distinguish between the on-water and in-water modes. Comparisons of organic reactions on H2O and on D2O indicate that on-water catalysis ranges from weak to strong trans-phase H-bonding for reactants with basic pK(a) < ca. -6 and to interfacial proton transfer for reactants with higher basic pK(a) > ca. 2 (pKa of conjugate acid). Water shows a chameleon-type response to organic molecules at hydrophobic surfaces.

Analytical Expressions for Proton Transfer Voltammetry: Analogy to Surface Redox Voltammetry with Frumkin Interactions

Theory for interfacial proton transfer voltammetry of a molecular film containing any acid/base loading has been developed under equilibrium conditions. Diagnostic criteria to disentangle the interplay between diffuse layer and ionization effects are outlined. Easy-to-use analytical expressions for the voltammetric features are derived for the particular case of an invariant diffuse layer effect, which turn out to be entirely analogous to those for a surface redox conversion with Frumkin interactions. It is demonstrated that, regardless of the electrolyte concentration, significant ionization of the external acid groups located nearby the diffuse layer is sufficient for the fulfillment of this relevant particular case. A strategy is outlined to determine the amount, the intrinsic pKa, and the burial depth of the voltammetrically active groups from the surface concentration dependence of the main voltammetric features. Self-assembled monolayers of 11-mercaptoundecanoic acid deposited on Au(111), containing higher amounts of buried carboxylic groups than previously reported, have been studied to assess more critically the influence of electrostatic effects on the ionization process. Preliminary evidence suggests that the protonation/deprotonation voltammetric wave involves physisorbed rather than chemisorbed thiol molecules. Application of the present theoretical approach to this system reveals that the voltammetrically active carboxylic groups are located close to the electrode surface and become more acidic upon increasing their surface concentration.

Adsorptions of Formic and Acetic Acids on Ice Surface: Surface Binding Configurations and a Possibility of Interfacial Proton Transfer

Adsorptions of formic (FA) and acetic acids (AA) on Ih ice surface were studied using quantum mechanical/effective fragment potential (QM/EFP) scheme. Contrasting to the earlier studies in which trans-conformers were found as major surface configurations, our QM/EFP models found various cis-and trans-conformers on ice surfaces with the cis-conformers being more stable. The surface binding energies and configurations were largely dependent on surface heterogeneity yielding the various surface conformers. In addition, the overall binding energies of acetic acid are slightly higher as compared to formic acid, implying the stabilization effect of methyl group. Our study also found a feasible deprotonation route of adsorbed trans-formic acid. In contrast, acetic acid prefers molecular form due to the unfavorable hydrophobic methyl group. Therefore it is interesting to note that the additional methyl group of acetic acid enhances surface binding energies. But at the same time it reduces the chance of its deprotonation. Our ice model clearly demonstrated the significant effects of intrinsic surface heterogeneity on the distributions of surface binding energies and configurations, which cannot be represented by small water clusters.

Anions dramatically enhance proton transfer through aqueous interfaces

Proton transfer (PT) through and across aqueous interfaces is a fundamental process in chemistry and biology. Notwithstanding its importance, it is not generally realized that interfacial PT is quite different from conventional PT in bulk water. Here we show that, in contrast with the behavior of strong nitric acid in aqueous solution, gas-phase HNO(3) does not dissociate upon collision with the surface of water unless a few ions (> 1 per 10(6) H(2)O) are present. By applying online electrospray ionization mass spectrometry to monitor in situ the surface of aqueous jets exposed to HNO(3(g)) beams we found that NO(3)(-) production increases dramatically on > 30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that the sizable barrier hindering HNO(3) dissociation on the surface of small water clusters is drastically lowered in the presence of anions. Anions electrostatically assist in drawing the proton away from NO(3)(-) lingering outside the cluster, whose incorporation is hampered by the energetic cost of opening a cavity therein. Present results provide both direct experimental evidence and mechanistic insights on the counterintuitive slowness of PT at water-hydrophobe boundaries and its remarkable sensitivity to electrostatic effects.

Quantum chemical insights into the dissociation of nitric acid on the surface of aqueous electrolytes

AbstractRecent experiments in our laboratory have shown that the probability of gaseous HNO3 deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO3 molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO3 always binds to the outermost water molecules both via donating and accepting hydrogen‐bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl−. As the dissociation of HNO3 embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO3−. Our findings suggest that nearby anions catalyze HNO3 dissociation by preorganizing interfacial water and drawing the proton—away from the incipient [H+‐‐‐NO3−] close ion‐pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl− range (1 Cl− in ∼5.5 × 104 water molecules) covered by our experiments if weakly adsorbed HNO3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water. © 2012 Wiley Periodicals, Inc.

Faradaic impedance titration and control of electron transfer of 1-(12-mercaptododecyl)imidazole monolayer on a gold electrode

In this work, we studied interfacial proton transfer of the self-assembled monolayer (SAM) of 1-(12-mercaptododecyl)imidazole on a gold electrode by faradaic impedance titration method with Fe(CN) 6 3− as an anionic redox probe molecule. The surface p K 1/2 was found to be 7.3, which was nearly the same as that of 1-alkylimidazole in solution. We also investigated the electrochemical properties of the SAM-modified electrode by cyclic voltammetry. Cyclic voltammetry was performed (1) in the solution containing Fe(CN) 6 3− with repeated alternation of pH values to investigate the electrostatic interaction of the protonated or deprotonated imidazole with Fe(CN) 6 3− and (2) in the acidic or basic electrolyte containing Ru(NH 3) 6 3+ as a cationic redox probe to verify the effect of the polarity of a redox probe. We observed the reversible adsorption/desorption of Fe(CN) 6 3− and concluded that the adsorbed Fe(CN) 6 3− catalyzed the electron transfer of both Fe(CN) 6 3− itself and cationic Ru(NH 3) 6 3+.

Study of electrode potential effect on acid-base behavior of ?-functionalized self-assembled monolayers using Fourier transform surface-enhanced Raman scattering spectroscopy

Interfacial proton transfer reactions of 2-aminoethanethiol (2AT) self-assembled monolayers (SAMs) at different fixed potentials were studied using the Fourier transform surface-enhanced Raman scattering (FT-SERS) titration method. ClO4− was used as a counter anion of the protonated ω-amino group of the monolayers to probe the extent of protonation with change of solution pH. At aqueous buffer/SAMs-modified gold interfaces, the electrochemical in situ FT-SERS spectra were measured at different electrode potentials. Under potentials of 0, 0.1 and 0.2 V vs Ag|AgCl (saturated KCl), the surface pKa values determined for the 2AT SAMs modified on gold were 5.0 ± 0.2, 4.2 ± 0.2 and 3.4 ± 0.2, respectively, at 0.25 M ionic strength. The surface pKa shift direction follows the polarity of the applied potential. This indicates that the surface pKa is controllable within this potential range by the electrode potential. Concerning the origin of the surface pKa shift to the acidic side with positively increased potential, a preliminary explanation based on analysis with local pH at the interface and a discussion about the effect of the double layer on the surface pKa shift of the 2AT SAMs by the electrode potential are given. Copyright © 2005 John Wiley & Sons, Ltd.

Faradaic impedance titration of pure 3-mercaptopropionic acid and ethanethiol mixed monolayers on gold

Interfacial proton transfer reactions of pure 3-mercaptopropionic acid (MPA) and ethanethiol (EtSH) mixed self-assembled monolayers (SAMs) have been studied using the faradaic impedance titration method. The Fe(CN) 6 3− is used to probe the charge development of the terminal COOH group of the monolayers. The charge-transfer resistance ( R ct) is measured with the monolayer composition, adsorption coverage, and the ionic strength of pH solution. The surface p K determined for the pure MPA is 6.0 at a monolayer coverage and 0.1 M ionic strength. The in-plane electrostatic force effect, which causes a broadening of the titration curves, on the surface p K for the MPA SAM is not significant at an ionic strength over 0.1 M. The p K value for the pure MPA SAM shifts negatively as the surface coverage decreases, indicating that the in-plane interactions between acids and the hydrophobicity surrounding acids decrease at the same time. When the pure MPA SAM is compared with the EtSH mixed SAM, the surface p K of the mixed SAM is larger than that of the pure SAM. Such positive p K shifts are more pronounced at the pure MPA SAM with a lower coverage. This implies that the p K shifts of sparsely adsorbed acidthiol SAM are more sensitive to the introduction of hydrophobicity than to the decrease of the in-plane interactions.