Local Proton Concentration Research Articles

Insights of the Proton Transport Efficiency of a Membrane Electrode Assembly by Operando Monitoring of the Local Proton Concentration during Water Oxidation

Insights of the Proton Transport Efficiency of a Membrane Electrode Assembly by Operando Monitoring of the Local Proton Concentration during Water Oxidation

Quantification of Distributions of Local Proton Concentrations in Heterogeneous Soft Matter and Non-Anfinsen Biomacromolecules.

A new method to quantitatively analyze heterogeneous distributions of local proton densities around paramagnetic centers in unstructured and weakly structured biomacromolecules and soft matter is introduced, and its feasibility is demonstrated on aqueous solutions of stochastically spin-labeled polysaccharides. This method is based on the pulse EPR experiment ih-RIDME (intermolecular hyperfine relaxation-induced dipolar modulation enhancement). Global analysis of a series of RIDME traces allows for a mathematically stable transformation of the time-domain data to the distribution of local proton concentrations. Two pulse sequences are proposed and tested, which combine the ih-RIDME block and the double-electron-electron resonance (DEER) experiment. Such experiments can be potentially used to correlate the local proton concentration with the macromolecular chain conformation. We anticipate an application of this approach in studies of intrinsically disordered proteins, biomolecular aggregates, and biomolecular condensates.

Surface charge modulation boosts electrocatalytic water splitting over iridium catalysts on a polyimide support.

Developing high-performance iridium (Ir)-based catalysts with minimal precious Ir metal is a significant but challenging step towards the acidic oxygen evolution reaction (OER). Here, we report a high-performance OER catalyst with Ir nanoparticles on a polyimide support, where the polyimide support can effectively modulate the electronic structures of the Ir active sites for decreased thermodynamic barriers, but also enrich the local proton concentration near the Ir active sites, enhancing the OER rates.

First-principles molecular dynamics simulation study on Ti/Mn ions in aqueous sulfuric acid for use in redox flow battery

The effect of the Ti4+ ion on the structural and chemical features of the Mn3+ ion in aqueous sulfuric acid was investigated using first-principles molecular dynamics (FPMD) simulation and second-order perturbation theory (CASPT2) with a complete active space self-consistent field (CASSCF) reference function. The focus was on the role of the Ti ion in the MnO2 formation reaction from the Mn3+ ion. The simulations showed that Mn3+ forms two types of complexes, [Mn(H2O)4(SO4)2]1− and [Mn(H2O)6]3+ in aqueous sulfuric acid with Ti ions where the population of the former was found to be higher than that of the latter. We found that when the HSO4− ion was coordinated to a Ti4+ ion, it released one proton, which increases proton concentration in the solution. Detailed quantum chemical calculations on the isolated Mn complexes show that if the Mn complexes interact with a hydronium ion, the stability of Mn3+ ion is increased to a large extent. Such a result implies that the increased local proton concentration due to Ti4+ ions hinders formation of MnO2 particle.

Effect of Polypeptide Complex Coacervate Microenvironment on Protonation of a Guest Molecule.

Complex coacervate droplets formed by the liquid-liquid phase separation of polyelectrolyte solutions capture several important features of membraneless organelles including their ability to accumulate guest molecules and to provide distinct microenvironments. Here, we examine how polyions in complex coacervates can influence localized guest molecules, leading to a shifted protonation state of the guest molecule in response to its electrostatic environment. A fluorescent ratiometric pH indicator dye was used as a model guest molecule able to report its protonation state in the coacervate phase. Experimentally observed differences in dye-reported local apparent pH inside versus outside of coacervate droplets were largest for polyion pairs having lower salt stabilities and/or larger polyion length mismatch, which we attribute to the relative concentration of open sites on polyions within the coacervates based on theoretical calculations. Using the transfer matrix method, we confirmed that theoretical phase diagrams and critical salt stabilities generated for each polyion pair were consistent with experimental turbidity measurements and estimated the amount of available binding sites on polyions for guest molecules. We conclude that dye molecules likely experience an effective pKa shift due to interactions with coacervate polyions rather than reporting directly on local proton concentrations. Such a local pKa shift can also be anticipated for other guest molecules having protonatable groups, including, for example, many metabolites, ligands, and/or drug molecules that partition into coacervates or membraneless organelles based on ion pairing interactions.

Conjugated Coordination Polymer as a New Platform for Efficient and Selective Electroreduction of Nitrate into Ammonia.

Electroreduction of nitrate into ammonia (NRA) provides a sustainable route to convert the widespread nitrate pollutants into high-value-added products under ambient conditions, which unfortunately suffers from unsatisfactory selectivity due to the competitive hydrogen evolution reaction (HER). Previous strategies of modifying the metal sites of catalysts often met a dilemma for simultaneously promoting activity and selectivity toward NRA. Here, a general strategy is reported to enable an efficient and selective NRA process through coordination modulation of single-atom catalysts to tailor the local proton concentration at the catalyst surface. By contrast, two analogous Ni-single-atom enriched conjugated coordination polymers (NiO4 -CCP and NiN4 -CCP) with different coordination motifs are investigated for the proof-of-concept study. The NiO4 -CCP catalyst exhibits an ammonia yield rate as high as 1.83mmol h-1 mg-1 with a Faradaic efficiency of 94.7% under a current density of 125mA cm-2 , outperforming the NiN4 -CCP catalyst. These experimental and theoretical studies both suggest that the strategy of coordination modulation can not only accelerate the NRAby adjusting the adsorption energies of NRA intermediates on the metal sites but also inhibit the HER through regulating the proton migration with contributions from the metal-hydrated cations adsorbed at the catalyst surface, thus achieving simultaneous enhancement of NRA selectivity and activity.

Towards a realistic prediction of catalyst durability from liquid half-cell tests.

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Identification of Photoacidic Behavior Using AC and Open-Circuit Photoelectrochemical Techniques

Photoacids are molecules whose acidity increases through absorption of light. When the excited-state lifetime of a photoacid is sufficiently long, proton transfer from its thermally equilibrated electronic excited state results in a transient change in pH and/or pOH, which is commonly detected using spectroscopic techniques. Herein we expand this measurement toolkit by introducing alternating AC and open-circuit photoelectrochemical techniques that characterize photoacidic behavior from a model photoacid, the sodium salt of 8-hydroxypyrene-1,3,6-trisulfonate, dissolved in aqueous solutions in a thin-pathlength two-electrode cell. Continuous illumination of protonated photoacids in their electronic ground state results in significant and reproducible changes in low-frequency impedance and open-circuit potential. When these molecules are made to be non-acidic, via deprotonation using more alkaline pH conditions or methoxylation via synthesis, electrochemical data measured in the dark and under illumination are nearly identical. Best fits of AC electrochemical data to a simplified equivalent circuit support that photoelectrochemical responses are likely due to changes in local proton concentration at the electrode∣electrolyte interface, and not changes in proton flux due to mass transfer, as previously suggested. Collectively, our results provide further insight into the utility of these photoelectrochemical techniques to probe photoacidic behavior.

Proton-Coupled Electron Transfer of Coenzyme Q in Unbuffered Solution by Pore Confined In Situ Liquid ToF-SIMS

Proton-coupled electron transfer (PCET) process of coenzyme Q in buffered solutions, which is a well-defined overall 2 e−, 2 H+ process, has been systematically studied, while that in unbuffered aqueous solutions is still too complicated to be fully understood, primarily due to the uncontrolled local proton concentration at the electrode-electrolyte interface. Herein, time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with a microfluidic electrochemical reactor, namely pore confined in situ liquid ToF-SIMS analysis, was adopted to monitor the PCET process of coenzyme Q0 (CoQ0) at the electrode-electrolyte interface in unbuffered aqueous solution. Evolutions of CoQ0 and related intermediates during the electrochemical reaction were measured in real-time, which provided direct molecular evidences for the PCET process. The direct observation of CoQ0H2 and hydrated CoQ0 dianion implied that the reduction of CoQ0 in unbuffered electrolyte was not a simple overall 2 e−, 2 H+ procedure. Moreover, the identification of the CoQ0H2 dimer and the quinone-hydroquinone complex demonstrated the further transformation of CoQ0 and CoQ0H2 by hydrogen bonding interaction or π-interaction. These results provided a full picture of the mechanism for the PCET process of CoQ0 in unbuffered aqueous solution, which could contribute to the comprehensive understanding of the electrochemical reactions of coenzyme Q.

Unravelling unsaturated edge S in amorphous NiSx for boosting photocatalytic H2 evolution of metastable phase CdS confined inside hydrophilic beads

The unsaturated edge S in the cocatalyst plays an important role in the enrichment of both protons and electrons during the photocatalytic hydrogen evolution process. However, a mild and simple strategy for effectively exposing edge S is still a significant challenge. Herein, amorphous nickel sulfide (NiSx) as an efficient cocatalyst was decorated on the cubic phase CdS surface, which were confined growth inside hydrophilic chitosan hydrogel beads (CHB@CdS-NiSx) for efficient and durable photocatalytic H2 evolution. The optimal CHB@CdS-0.02NiSx achieved a photocatalytic activity of 11.88 mmol/h/g and displayed ultra-long stability of 60 h. NiSx could effectively accelerate the electron migration from CdS to NiSx in composite photocatalyst and provide abundant unsaturated edge active S as H2-evolution sites. Further density functional theory (DFT) calculation revealed that NiSx lowered activation energy for H2O dissociation to boost the local proton concentration, thereby facilitating the activity of H2 evolution. This study highlights the significance of amorphization strategy for consciously modulating edge sulfur of transition metal sulfide cocatalysts to improve the photocatalytic performance.

A Nanometric Probe of the Local Proton Concentration in Microtubule-Based Biophysical Systems.

We show a double-functional fluorescence sensing paradigm that can retrieve nanometric pH information on biological structures. We use this method to measure the extent of protonic condensation around microtubules, which are protein polymers that play many roles crucial to cell function. While microtubules are believed to have a profound impact on the local cytoplasmic pH, this has been hard to show experimentally due to the limitations of conventional sensing techniques. We show that subtle changes in the local electrochemical surroundings cause a double-functional sensor to transform its spectrum, thus allowing a direct measurement of the protonic concentration at the microtubule surface. Microtubules concentrate protons by as much as one unit on the pH scale, indicating a charge storage role within the cell via the localized ionic condensation. These results confirm the bioelectrical significance of microtubules and reveal a sensing concept that can deliver localized biochemical information on intracellular structures.

Engineered Nanoconfinement Accelerating Spontaneous Manganese-Catalyzed Degradation of Organic Contaminants.

Manganese(III/IV) oxide minerals are known to spontaneously degrade organic pollutants in nature. However, the kinetics are too slow to be useful for engineered water treatment processes. Herein, we demonstrate that nanoscale Mn3O4 particles under nanoscale spatial confinement (down to 3-5 nm) can significantly accelerate the kinetics of pollutant degradation, nearly 3 orders of magnitude faster compared to the same reaction in the unconfined bulk phase. We first employed an anodized aluminum oxide scaffold with uniform channel dimensions for experimental and computational studies. We found that the observed kinetic enhancement resulted from the increased surface area of catalysts exposed to the reaction, as well as the increased local proton concentration at the Mn3O4 surface and subsequent acceleration of acid-catalyzed reactions even at neutral pH in bulk. We further demonstrate that a reactive Mn3O4-functionalized ceramic ultrafiltration membrane, a more suitable scaffold for realistic water treatment, achieved nearly complete removal of various phenolic and aniline pollutants, operated under a common ultrafiltration water flux. Our findings mark an important advance toward the development of catalytic membranes that can degrade pollutants in addition to their intrinsic function as a physical separation barrier, especially since they are based on accelerating natural catalytic pathways that do not require any chemical addition.

The multiple biological roles of the cholinesterases

It is tacitly assumed that the biological role of acetylcholinesterase is termination of synaptic transmission at cholinergic synapses. However, together with its structural homolog, butyrylcholinesterase, it is widely distributed both within and outside the nervous system, and, in many cases, the role of both enzymes remains obscure. The transient appearance of the cholinesterases in embryonic tissues is especially enigmatic. The two enzymes’ extra-synaptic roles, which are known as ‘non-classical’ roles, are the topic of this review. Strong evidence has been presented that AChE and BChE play morphogenetic roles in a variety of eukaryotic systems, and they do so either by acting as adhesion proteins, or as trophic factors. As trophic factors, one mode of action is to directly regulate morphogenesis, such as neurite outgrowth, by poorly understood mechanisms. The other mode is by regulating levels of acetylcholine, which acts as the direct trophic factor. Alternate substrates have been sought for the cholinesterases. Quite recently, it was shown that levels of the aggression hormone, ghrelin, which also controls appetite, are regulated by butyrylcholinesterase. The rapid hydrolysis of acetylcholine by acetylcholinesterase generates high local proton concentrations. The possible biophysical and biological consequences of this effect are discussed. The biological significance of the acetylcholinesterases secreted by parasitic nematodes is reviewed, and, finally, the involvement of acetylcholinesterase in apoptosis is considered.

Fluorescent sensors for measuring local proton concentrations in single cells: from classical methods to new approaches

Fluorescent sensors for measuring local proton concentrations in single cells: from classical methods to new approaches

Modifying Catalyst-Electrolyte Interactions for Enhanced Electrochemical CO2 Reduction

Electrochemical CO2 reduction (CO2R) could potentially be used with electricity from renewable sources to convert CO2 into various products such as CO, formic acid (HCOOH), methane (CH4), ethylene (C2H6), etc. The overall efficiency of the CO2R process is largely linked to the catalyst on which CO2 is being reduced. However, the local environment surrounding the catalyst is generally subjected to reaction-driven changes during CO2R which play a significant role in affecting the overall efficiency, product selectivity, and reaction rate. Therefore, the understanding of catalyst-electrolyte interactions during CO2R could be effective in further improving the performance of the catalyst. For example, the potential buffering effects of different alkali cations at the catalyst surface due to variations in cation hydrolysis can influence the local proton concentration and thus CO2R.1 The surface tethering of catalyst surface with functional additives such as glycine2, poly(acrylamine)3, thiol group4, etc. have been shown to improve product selectivity by affecting the binding strength with CO2R intermediates. In our recent work5, we uncovered that the interactions between a polycrystalline Ag foil and reline solution including (i) in-situ nano-structuring of Ag foil by electrodeposition of dissolute native Ag oxide layer, (ii) HER suppression by specifically adsorbed choline ions which restrict the protons availability at the interface, and (iii) stabilization of CO2R intermediates by hydrogen bonding with amino group of urea led to a remarkable CO selectivity of (96±8)% at - 0.884 V vs. RHE.Anions of the electrolyte especially halide ions (Cl-, Br-, and I-) have been reported to affect both the selectivity and activity of CO2R.6, 7 Specifically adsorbed halide ions can modulate the coverage of adsorbed CO on the catalyst surface by stabilizing the intermediates.8 Moreover, halide ions can induce morphological changes of the catalyst surface during CO2R which have demonstrated to enhance the selectivity of CO over Ag catalysts.9 Inspired by the effect of halide ions, we are investigating the role of different halide ions such as Cl-, Br- and I- in choline based electrolytes over Ag foil for CO2R. Our initial experiments have shown that at less negative potentials (between – 1.0 to – 0.7 V vs RHE), I- exhibited the lowest selectivity for CO, and we attribute this result to I- having the highest specific adsorption at the Ag surface and adsorbed I- may restrict the active sites for CO2R. However, at larger negative potentials the electrostatic repulsion between the Ag surface and specifically halide ions increases causing the interactions of the catalyst-halide ions to be weakened and thus improving the CO selectivity. SEM characterization of the Ag foil after CO2R has confirmed the change in morphology of the Ag surface (more-rougher). Moreover, we performed several flow-cell CO2R experiments over Ag-based gas diffusion electrodes (GDEs) and could achieve a CO selectivity over 90% at 150 mA·cm-2 in all three electrolytes. Therefore, altering the interactions between the Ag catalyst and choline-based halide ions during CO2R could be a potential approach to enhance the catalytic activity of Ag-metal or Ag nanoparticles.

Effect of Electrolyte on the Proton Transport through Graphane in the Electrochemical Cell: A First-Principles Study.

The proton transport behaviors through graphane in the electrochemical environment are not only determined by the film but also correlated with the properties of the electrolytes. Here, the effect of electrolytes is studied for this transport process. The step of proton transfer from electrolyte to graphane is the rate-determining step of the whole transport process in most of the studied cases and is indeed influenced much by the electrolytes, while the following steps are affected little. Its energy barrier increases significantly with the number of water molecules but only fluctuates with the number of mimicked Nafion molecules until the bulk case. This barrier could be further affected by the hydration number of Nafion and be reduced by increasing local proton concentrations. The dynamical effect of the environment and the nuclear quantum effect are found to further reduce the energy barrier of the transport process but by a relatively small amount.

Selective ligand modification of cobalt porphyrins for carbon dioxide electrolysis: Generation of a renewable H2/CO feedstock for downstream catalytic hydrogenation

Catalytic hydrogenation is an attractive approach to produce green fuels and chemicals. The building blocks for these processes may be effectively produced from renewable power via direct electrochemical reduction of carbon dioxide in an aqueous media. For the first time, the impact of increasing the local proton concentration of cobalt porphyrin was examined by synthesizing new cobalt porphyrins 2, Co(o-OCH3)TPP and cobalt porphyrin 3, Co(o-OH)TPP. Cobalt porphyrins coated on carbon paper converted carbon dioxide and water into a mixture of hydrogen and carbon monoxide in an aqueous electrolyte at near neutral pH. Increasing the local proton availability of the commercial cobalt porphyrin 1, accelerates hydrogen generation under heterogeneous conditions across the range of potentials tested (−0.85 to −1.5 V vs. Ag/AgCl) and demonstrates high Faradaic efficiencies (ca. 90%) at low over-potentials (ca. 540 mV). The culmination of this work can help identify key parameters that facilitate generation of sustainable reagents for catalytic hydrogenation under practical and scalable conditions.

A Bi-functional Second Coordination Sphere for Electrocatalytic CO2 Reduction: The Concerted Improvement by a Local Proton Source and Local Coulombic Interactions

A homogeneous rhenium(I) electrocatalyst possessing hydroxy and ammonio groups in the second coordination sphere is reported for the reduction of carbon dioxide (CO2) to carbon monoxide, wherein the former and latter groups induce a high local concentration of protons and local coulombic interactions with a metallocarboxylate reaction intermediate, respectively. In a comparison with precedent catalysts possessing single functional groups in the spheres, we found that the catalyst exhibits a concerted improvement over these single functional groups in the efficiencies of catalysis, based on experimental standard electrode potentials for the reduction in aqueous N,N-dimethylformamide or acetonitrile solutions and the equilibrium potentials depending on the catalytic systems.

Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions.

Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe3+, Co2+, and Ni2+) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H2) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni2+ used here). That is, the nickel precursor of Ni(OAc)2 changes to Ni(H2O)62+ in neutral H2O and eventually transforms to Ni(OH)2 nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)2 to a defective structure [Nix0/Ni1-x(OH)2], which acts as the real catalytic species of H2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (VNi) on the Ni(OH)2 nanosheets enhance the adsorption and dissociation of H2O molecules to enhance the local proton concentration, while the Ni0 clusters behave as H2-evolution sites, thereby synergistically promoting the activity of H2 photogeneration in alkaline media.

Fluorocarbon-Based Ionomers with Single Acid and Multiacid Side Chains at Nanothin Interfaces

Unraveling ion conduction limitations in nanothin ionomer films is crucial for designing efficient ionomer–catalyst interfaces and improving redox efficiency in electrochemical devices. This work took a multifaceted approach to understand local proton conduction environments in sub-μm thick films of three fluorocarbon-based ionomers, Nafion, 3M PFSA and 3M PFIA with IEC ∼ 0.91, 1.21, and 1.61 mequiv/g, respectively. After incorporating fluorescent photoacid probe pyranine (HPTS) into films, the extent of proton conduction (Id/Ip), local proton concentration, pH, and ionic domain size (did) were predicted by monitoring the ratio of fluorescence intensity of deprotonated (Id) to that of the protonated (Ip) state of HPTS. Id/Ip decreased with film thickness and followed the trend: 3M PFIA > 3M PFSA > Nafion. A higher water uptake did not necessarily lead to higher Id/Ip indicating that other factors than water uptake control proton conduction under confinement. Size of the ionic domains (did), measured independently using in-plane reflection small-angle X-ray scattering and fluorescence spectroscopy, followed the same trend as Id/Ip. As the RH and film thickness decreased, did became smaller. The close match of did obtained from both techniques supported the reliability of information confered by fluorescence spectroscopy about key controlling parameters of the local proton conduction environment. The highest Id/Ip of 3M PFIA films was attributed to its flexible, multiacidic side chain that helped to form larger ionic domains with better phase segregation. Conversely, smaller, extremely acidic and poorly phase segregated ionic domains with highly confined water molecules led to lower Id/Ip in Nafion films, despite high water uptake.