Hard Soft Acid Base Theory Research Articles

Fundamental Aspects of the Electron Transfer Processes in Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) have garnered increasing attention for energy storage applications with non-aqueous RFBs offering several advantages such as larger voltage windows and the prospect of high energy densities. To increase the efficacy of non-aqueous RFBs, much effort has gone into the design of redox moieties that are highly soluble, cheaper, and robust, with desired electrochemical properties. To evaluate the capability of these redox moieties for RFB applications, the kinetics and thermodynamics of electron transfer events need to be understood. These insights could help towards rational design of redox moieties and to predict RFB performance. Techniques such as cyclic voltammetry and rotating disk electrode are commonly used to study the fundamental parameters of electron transfer process and thermodynamic properties of the redox moieties in both the neutral and charged states.In this work, we investigate the effects of supporting electrolytes on the thermodynamics and kinetics of the redox moieties. Vanadium (III) acetylacetonate and iron complexes are the bipolar model redox moieties used as both negative and positive electrolyte in non-aqueous RFB applications.1 While many redox moieties are stable in the neutral state, it is during cycling that degradation of the complexes typically occurs. As shown previously, the supporting electrolyte composition plays a key role in the performance and stability of the redox species in RFBs.2 We found that ion pairing between the charged redox moieties and the supporting electrolyte resulted in lower diffusion coefficients, it also has a negative impact on the kinetics of charge transfer process. We propose that the observed ion pairing can explain the stabilizing effects of supporting electrolytes on redox moieties. The magnitude of this effect can be explained by hard soft acid base theory (HSAB) which can be further used to predict the extent of ionic interactions of the charged species in the RFB electrolytes. Acknowledgements: Los Alamos National Laboratory is operated by Triad, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (Contract No. 89233218NCA000001). Authors would like to thank Dr. Imre Gyuk and financial support from the U.S. Department of Energy’s Office of Electricity for the publication of this work is gratefully acknowledged. Sharma, G.A. Andrade, S. Maurya, I.A. Popov, E.R. Batista, B.L. Davis, R. Mukundan, N.C. Smythe, A.M. Tondreau, P. Yang, J.C. Gordon, Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications, Energy Storage Mater., 37 (2021), pp. 576-586Wei X, Xu W, Huang J, Zhang L, Walter E, Lawrence C, Vijayakumar M, Henderson WA, Liu T, Cosimbescu L, Li B, Sprenkle V, Wang W. “Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery” Angew Chem Int Ed Engl. 2015, 54 (30): 8684-7.

Two Robust Isoreticular Metal-Organic Frameworks with Different Interpenetration Degrees Exhibiting Disparate Breathing Behaviors.

Herein, two robust isoreticular metal-organic frameworks (MOFs), ([Bi(CPTTA)]·[Me2NH2]·2DMF) (JLU-MOF120, H4CPTTA = 5'-(4-carboxyphenyl)-[1,1':3',1″-terphenyl]-3,4″,5-tricarboxylic acid, DMF = N, N- dimethylformamide) and ([In(CPTTA)]·[MeNH3]·2.5H2O·1.5NMF) (JLU-MOF121, NMF = N- methylformamide), with different interpenetration degrees were successfully constructed. According to the hard-soft acid-base (HSAB) theory, high-valent metal ions and carboxylate-based ligands were selected and formed twofold interpenetrated structures with saturated coordinated mononuclear second building units ([M(COO)4], M = Bi, In). Owing to the features of the frameworks, JLU-MOF120 and JLU-MOF121 exhibited excellent stability, which could retain their integrity in water for at least 14 days and aqueous solutions with a pH range of 3-11 for at least 24 h. According to the structural regulation strategy, by changing the torsion angles of the ligand, the degrees of interpenetration for JLU-MOF120 and JLU-MOF121 were different, leading to various gate-opening pressures in CO2 at 195 K. Furthermore, JLU-MOF120 exhibits the scarce potential of C2H2/CO2 separation among Bi-MOF materials at 298 K under 101 kPa, JLU-MOF121 shows high CO2/CH4 selectivity under ambient conditions (11.7 for gas mixtures of 50 and 50% and 16.1 for gas mixtures of 5 and 95%). Moreover, owing to the flexibility of the structure, JLU-MOF121 possesses disparate breathing behaviors for C2H2 and C2H6 at 273 and 298 K, with the differences in uptakes among C2 hydrocarbons resulting in the potentiality of C2H4 purification. Overall, such HSAB theory and the structural regulation strategy could provide a valid method for constructing stable and flexible structures for the application in gas separation.

Homogeneous solution assembled Turing structures with near zero strain semi-coherence interface

Turing structures typically emerge in reaction-diffusion processes far from thermodynamic equilibrium, involving at least two chemicals with different diffusion coefficients (inhibitors and activators) in the classic Turing systems. Constructing a Turing structure in homogeneous solutions is a large challenge because of the similar diffusion coefficients of most small molecule weight species. In this work, we show that Turing structure with near zero strain semi-coherence interfaces is constructed in homogeneous solutions subject to the diffusion kinetics. Experimental results combined with molecular dynamics and numerical simulations confirm the Turing structure in the spinel ferrite films. Furthermore, using the hard-soft acid-base theory, the design of coordination binding can improve the diffusion motion of molecules in homogeneous solutions, increasing the library of Turing structure designs, which provides a greater potential to develop advanced materials.

Heterometallic Clusters with Uranium-Metal Bonds Supported by Double-Layer Nitrogen-Phosphorus Ligands.

ConspectusHeterometallic clusters with M-M bonds have significantly interested chemists because of their attractive structures and synergistic effects in small-molecule activation and catalysis. However, reports of the isolation of heterometallic clusters with uranium-transition metal (U-TM) bonds remain very limited. In this Account, we describe our research in the construction of heterometallic molecular clusters with multiple U-TM single or multiple bonds supported by novel double-layer N-P ligands. Multimetallic synergistic catalysis and small-molecule activation with these species are also summarized.First, according to the hard-soft acid-base theory, we employed a three-armed N-P ligand, which can be used to construct heterometallic clusters with four or six U-Ni bonds. This strategy was also effective in the construction of complexes with direct rare earth metal-TM bonding. The similar two-armed N-P ligands also are effective platforms for the synthesis of heterometallic complexes with U-Ni, U-Pd, and U-Pt bonds.Second, a set of heterometallic clusters featuring U≡Rh, U≡Co, and U≡Fe triple bonds were constructed under routine experimental conditions. X-ray diffraction analysis of these clusters exhibits the shortest U-TM bond distance (1.9693(4) Å for the U≡Fe triple bond) in these complexes. Theoretical studies reveal that the nature of the triple bond is one covalent σ bond and two TM → U dative π bonds. A large Wiberg bond index (WBI) of 2.93 and a significant degree of covalency for the U≡TM triple bonds were also found in these complexes.Third, these uranium complexes supported by the double-layer N-P ligands exhibit great potential in small-molecule activation. For instance, N2 cleavage without an external reducing agent was achieved by a U(III)-P(III) synergistic six-electron reduction. The synergism between U(III) and P(III) enables the activation of other small molecules, such as O2, P4, and As0(nano), and highlights the importance of the P atom in the double-layer N-P ligand for the activation of small molecules. A heterometallic cluster with U-Rh bonds can break the strong N≡N triple bond in N2 in the presence of potassium graphite, suggesting a synergistic effect between U and Rh. This multimetallic synergistic effect was also observed in catalytic processes. A heterometallic cluster with U≡Co triple bonds shows excellent selectivity and activity in the hydroboration of a series of alkynes under mild conditions. These results lead to effective methods for the construction of heterometallic molecular clusters with U-TM single or multiple bonds and could promote the application of heterometallic clusters with U-TM bonds in catalysis and the activation of small molecules.

A foundational theoreticalAl12E12(E = N, P) adsorption and quinolone docking study: cage–quinolone pairs, optics and possible therapeutic and diagnostic applications

This combined Al12 E 12 (E = N, P) surface adsorption and docking study describes the new possibility of prospective potential probing(photophysical/optical) and therapy(medicinal/biochemical) with these adsorbent conjugates. DFT investigations were undertaken herein to help generate geometrical models and better understand the possible favorable adsorption energetics. We attempt to explain their adsorption behaviors and docking involving SARS–CoV–2 viruses (PDB)to assess their possible pharmaceutical potential against the pandemic virus (COVID–19). The adsorption behavior of 8–hydroxy–2–methylquinoline (MQ) and its halogenated derivatives, 5,7–diiodo–8–hydroxy–2–methylquinoline (MQI), 5,7–dichloro–8–hydroxy–2–methylquinoline (MQCl), and 5,7–dibromo–8–hydroxy–2–methylquinoline (MQBr), with aluminum–nitrogen (AlN), and aluminum–phosphorous (AlP) fullerene–like nanocages is reported. A decrease in the hardness of the nanoclusters when adsorbed with drug molecules resulted in an incrementally improved chemical softness (see e.g., Hard–Soft Acid Base theory) indicating that reactivity of the drug molecule in the resulting complex increases upon cluster chemical adsorption. The energy gap is found to be maximized for AlN–MQ and minimized for AlP–MQI; the reduced density gradient (RDG) iso–surfaces and AIM studies also corroborated this. Therefore, these two were found, respectively, to be the least and most electrically conductive of the species under study. We selected a simple medicinal building block (chelator)in addition to selecting the cluster based on previous literature reports. Important parameters such as gap energies and global indices were determined. We assessed NLO properties. The SARS–CoV–2 virus PDB docking data for 6VW1, 6VYO, 6WKQ, 7AD1, 7AOL, 7B3C, were enlisted as ligand targets for studies of docking (PatchDock Server) using the requisite PDB geometries (For the structure of 6VW1, kindly see reference, 2020; For the structure of 6VYO kindly see reference, 2020; For the structure of 6WKQ kindly see reference, 2020; For the structure of 7AD1 kindly see reference, 2021; For the structure of 7AOL kindly see reference, 2021; For the structure of 7B3C kindly see reference, 2021). Such findings indicate that the AlN–drug conjugation have inhibitory effect against these selected receptors.

Hard-soft chemistry guides the adaptable charge transport in lysine-doped heptapeptide junctions.

Counterions always coexist with charged peptides in charge transport processes, which are excellent candidate components for tunable molecular electronic devices. Here, we introduced hard-soft acid base theory to analyze the counterion-modulated peptide charge transport. We demonstrate that the peptide charge transport is improved by enhanced peptide-counterion interactions.

Target-directed design of dual-functional Z-scheme AgIn5S8/SnS2 heterojunction for Pb(II) capture and photocatalytic reduction of Cr(VI): Performance and mechanism insight

• Dual-functional Z-scheme AgIn 5 S 8 /SnS 2 heterojunction was target-directed designed. • AgIn 5 S 8 /SnS 2 has high adsorption capacity for Pb(II) capture. • AgIn 5 S 8 /SnS 2 shows superior photocatalytic activity for Cr(VI) reduction. • Pb(II) adsorption was attributed to S-Pb coordination and electrostatic attraction. • Z-scheme photocatalytic mechanism was based on internal electric field. Target-directed design and synthesis of dual-functional materials with superior adsorption for Pb(II) and photocatalytic activity for Cr(VI) reduction are challenging but of vital importance for heavy metal-laden wastewater remediation. In this work, dual-functional Z-scheme AgIn 5 S 8 /SnS 2 heterojunctions were designed according to hard-soft acid-base theory and band structure matching, and were synthesized by simple hydrothermal reaction. The AgIn 5 S 8 /SnS 2 has high adsorptive capacity for Pb(II) with almost 100% Pb(II) removal, and the adsorptive capacity of AgIn 5 S 8 /SnS 2 for Pb(II) is about 5 and 3.3 times that of AgIn 5 S 8 and SnS 2 , respectively. Moreover, AgIn 5 S 8 /SnS 2 shows outstanding visible-light photocatalytic performance for Cr(VI) reduction with 96% reduction efficiency. The XPS analysis, Zeta potential, DFT calculation reveal that S atoms are dominant adsorption sites, and the adsorption of Pb(II) is more favorable on AgIn 5 S 8 (3 1 1)/SnS 2 (1 0 1) than SnS 2 (1 0 1) or AgIn 5 S 8 (3 1 1) due to the remarkable coordination of S atoms with Pb(II) and strong electrostatic attraction. Furthermore, the photocatalytic mechanism of Z-scheme AgIn 5 S 8 /SnS 2 heterojunction for Cr(VI) reduction was identified and uncovered by experimental studies.

Recovery of Gold in Au/Cu/Mg System from SH/Fe3O4@SiO2 as a Magnetically Separable and Reusable Adsorbent

The recovery of Au(III) in the Au/Cu/Mg system from mercapto-silica hybrid coated magnetite (SH/Fe3O4@SiO2) adsorbent has been investigated. This adsorbent characterized using FT-IR to determine functional groups, crystallinity study using XRD, surface morphology using SEM, material compositions with XPS, surface area using nitrogen adsorption, and TGA to study thermal stability. Adsorption of metal ions carried out with batch system for 30 minutes at a pH of 3. In the Au/Cu/Mg multi-metal system, Au(III) ions were easily desorbed (approximately 85%) by SH/Fe3O4@SiO2 adsorbent based on HSAB (Hard Soft Acid Base) theory that Au(III) ion is a softer metal than Cu(II) and Mg(II) where Au(III)>Cu(II)>Mg(II). The recovery of Au(III) ions was easily desorbed using thiourea 7% in 0,1 M HCl solution with the percentage of 79%. The process of SH/Fe3O4@SiO2 adsorbent separation after adsorption and recovery was very easy. The adsorbent could perfectly separate in 5 minutes using an external magnet. The SH/Fe3O4@SiO2 adsorbent can be reused on the adsorption-desorption process of Au(III) in the Au/Cu/Mg system approximately four times of cycle reactions.

Recovery of Gold in Au/Cu/Mg System from SH/Fe3O4@SiO2 as a Magnetically Separable and Reusable Adsorbent

The recovery of Au(III) in the Au/Cu/Mg system from mercapto-silica hybrid coated magnetite (SH/Fe3O4@SiO2) adsorbent has been investigated. This adsorbent characterized using FT-IR to determine functional groups, crystallinity study using XRD, surface morphology using SEM, material compositions with XPS, surface area using nitrogen adsorption, and TGA to study thermal stability. Adsorption of metal ions carried out with batch system for 30 minutes at a pH of 3. In the Au/Cu/Mg multi-metal system, Au(III) ions were easily desorbed (approximately 85%) by SH/Fe3O4@SiO2 adsorbent based on HSAB (Hard Soft Acid Base) theory that Au(III) ion is a softer metal than Cu(II) and Mg(II) where Au(III)>Cu(II)>Mg(II). The recovery of Au(III) ions was easily desorbed using thiourea 7% in 0,1 M HCl solution with the percentage of 79%. The process of SH/Fe3O4@SiO2 adsorbent separation after adsorption and recovery was very easy. The adsorbent could perfectly separate in 5 minutes using an external magnet. The SH/Fe3O4@SiO2 adsorbent can be reused on the adsorption-desorption process of Au(III) in the Au/Cu/Mg system approximately four times of cycle reactions.

Interpret the elimination behaviors of lead and vanadium from the water by employing functionalized biochars in diverse environmental conditions

Wide-ranging researches have been executed to treat groundwater from different mining areas, although complex behaviors of diverse metal ion species in the groundwater have not been illustrated clearly. This research study explored the mechanisms through which Pb(II) and V(V) are eliminated in single and binary-metal removal processes by oxygen, nitrogen, and sulfur-doped biochars also considering the kinetic and characterization techniques. The adsorption efficiency of V (V) was enhanced by oxygen-doped biochar at pH 4 with an adsorption capacity of ~70 mg/g. However, Pb (II) was rapidly removed at pH 6 with a higher adsorption capacity of ~180 mg/g by the nitrogen and sulfur-doped biochar forming PbCO3 and V(CO)6 crystals along the single-metal removal process. These results could be explained by the Hard Soft Acid Base theory. The hard Lewis acid vanadium was attracted by the hard Lewis base oxygen, and the intermediate Lewis acid lead was attracted by the intermediate and soft Lewis base nitrogen and sulfur. Besides, the removal ability of Pb(II) and V(V) in the binary-metal removal process showed a similar phenomenon for all types of biochars at pH 4 with the adsorption capacity of ~400 mg/g for Pb(II) and 175 mg/g for V(V), but the composition of vanadium species remains unclear on the surface of the biochars. Initially, H3V2O7−, H2VO4−, and HVO42− species were electrostatically attracted by the oxygen-based functionalities, then V(V) species was partially reduced to VO2+ by the oxygen, nitrogen, and sulfur functionalities in different ratios. Finally, H3V2O7−, H2VO4−, and HVO42− species produced Pb5(VO4)3Cl and Pb2V2O7 which co-precipitate with Pb(II), but VO2+ does not generate any form of precipitates. The above-explained technique supports the treatment of vanadium mining groundwater with valuable vanadinite (Pb5(VO4)3Cl) mineral.

A tuned Lewis acidic catalyst guided by hard–soft acid–base theory to promote N2 electroreduction

A paradigm of catalyst design for enhancing N2 activation is presented based on hard–soft acid–base theory using a MoSx model catalyst. The optimized MoSx delivers a NRR Faradaic efficiency of 21.60 % and NH3 yield rate of 40.4 μg h–1 mgcat.–1.

Reaction Mechanism Dominated by the Hard–Soft Acid–Base Theory for the Oxidation of CH2Cl2 and CH3Br over a Titanium Oxide-Supported Ru Catalyst

In this study, the divergent behaviors were elucidated for CH2Cl2 and CH3Br oxidation over TiO2 and Ru/TiO2. Possible reaction mechanisms were analyzed. It was concluded that the adsorption and act...

Manganese-Mediated Growth of ZnS Shell on KMnF3:Yb,Er Cores toward Enhanced Up/Downconversion Luminescence.

Epitaxially growing a semiconductor shell on the surface of upconversion nanocrystals to form a core/shell structure is believed to be a promising strategy to improve the luminescent efficiency of lanthanide ions doped in particle cores and, meanwhile, enriches the optical properties of the resulting nanocrystals. However, liquid-phase synthesis of such core/shell-structured nanocrystals comprised of a lanthanide ion-doped core and semiconductor shell remains challenging because of the chemical incompatibilities between lanthanides and the most intermediate gap semiconductors. In this context, the successful growth of ZnS shell on a KMnF3 core codoped with Yb3+/Er3+ ions is reported to enhance the upconversion luminescence of Er3+ ions. The underlying core/shell formation mechanism is elucidated in detail combining the hard-soft acid-base theory with structural analysis of the resulting nanocrystals. Quite unexpectedly, Mn2+ diffusion across the core/shell interface occurs during ZnS shell growth, giving rise to Mn2+ emission from the ZnS shell. Thus, the resulting core/shell particles exhibited unique up/downconversion luminescence from doped lanthanide metal ions and transition-metal ions, respectively. By manipulating the ion diffusion and shell growth kinetics, the upconversion and downconversion luminescent performance of KMnF3:Yb,Er@ZnS nanocrystals are further optimized and the related mechanisms are discussed. Further, temperature-dependent upconversion and downconversion photoluminescence properties of KMnF3:Yb,Er@ZnS nanocrystals show potential for ratiometric luminescence temperature sensing.

Transistor-Based Work-Function Measurement of Metal-Organic Frameworks for Ultra-Low-Power, Rationally Designed Chemical Sensors.

A classic challenge in chemical sensing is selectivity. Metal-organic frameworks (MOFs) are an exciting class of materials because they can be tuned for selective chemical adsorption. Adsorption events trigger work-function shifts, which can be detected with a chemical-sensitive field-effect transistor (power ≈microwatts). In this work, several case studies were used towards generalizing the sensing mechanism, ultimately towards our metal-centric hypothesis. HKUST-1 was used as a proof-of-principle humidity sensor. The response is thickness independent, meaning the response is surface localized. ZIF-8 is demonstrated to be an NO2 -sensing material, and the response is dominated by adsorption at metal sites. Finally, MFM-300(In) shows how standard hard-soft acid-base theory can be used to qualitatively predict sensor responses. This paper sets the groundwork for using the tunability of metal-organic frameworks for chemical sensing with distributed, scalable devices.

Effect of Cation Size on Solvation and Association with Superoxide Anion in Aprotic Solvents.

Influence of cation size on solvation strength, diffusion, and kinetics of the association reaction with anions O2- in aprotic solvents, such as acetonitrile and dimethyl sulfoxide, has been investigated by means of molecular dynamics simulations. The work is motivated by the need to understand the molecular nature of the solvent-induced changes in capacity of Li-air batteries. We have shown that the dependence of the solvation shell stability on the cation size has a maximum at a particular ion radius that corresponds to a solvent coordination number of 4. The shell stability maximum coincides with the diffusion coefficient minimum. The variation of the cation shell stability has a crucial impact on the kinetics of the cation-O2- association. We have demonstrated that profound inhibition of the association reaction for Li+ in dimethyl sulfoxide is a result of the lock-and-key effect that cannot be described in the framework of Hard Soft Acid Base theory.

Unprecedented Selectivity and Rapid Uptake of CuS Nanostructures toward Hg(II) Ions.

Fast, selective, and effective enrichment is critical for onsite detection and online monitoring of extremely low-concentration toxic heavy metal ions in complex environmental samples. In the current work, varied CuS nanostructures (hollow nanospheres, nanoflowers, nanoparticles) were prepared and applied to the enrichment of Hg(II) ions. Surprisingly, the as-prepared CuS nanostructures exhibited unprecedented ultrahigh selectivity and rapid uptake toward Hg(II) ions in the presence of other seven metal ions, suggesting specificity of mercury enrichment by the CuS nanostructures. Upon treating a 100 mL aqueous sample containing 8 different metal ions with only 10 mg of CuS hollow nanospheres, over 99.5% of Hg(II) ions could be removed within just 1 min, achieving a final Hg(II) ion level down to 0.1 ppb. This excellent selectivity was well accounted for by the Hard Soft Acid Base theory and especially the solubility product constant, where the solubility product constant of CuS is higher than that of HgS but lower than that of sulfides of other interfering metal ions. The current results are striking and would open a new avenue to the search for highly selective and efficient absorptive nanomaterials toward varied heavy metal ions.

Eu3+/TFA Functionalized MOF as Luminescent Enhancement Platform: A Ratiometric Luminescent Sensor for Hydrogen Sulfide in Aqueous Solution

In this work, a facile and fast luminescence sensing platform for the ratiometric detection of hydrogen sulfide (H2S/HS−) has been constructed by encapsulating the Eu3+/β-diketone chromophore into the host framework of MIL-140 (Eu3+/TFA@MIL-140C, TFA = 1,1,1-Trifluoro-2,4-pentanedione). It is well known that the energy transfer from the host framework or/and TFA to the Eu3+ ions can be disturbed easily by some metal ions, such as Cu2+ ions. Therefore, the ratiometric sensor was constituted with the Eu3+/β-diketone moiety as the response signal and the host framework moiety as the reference signal. Furthermore, on the basis of the hard–soft acid–base theory, the Cu2+ ions could be rapidly caught by HS−, profiting from the low solubility of CuS, realizing the real-time detection by luminescence color change. This sensor shows remarkable property on the detection of H2S (HS− in water solution) with low limit of detection (LOD = 7.43 μM) and short response time (< 0.5 min). The good linearity between luminescent intensity and concentration makes the luminescent ratio meter feasible.

Preparation of cysteamine-modified cellulose nanocrystal adsorbent for removal of mercury ions from aqueous solutions

According to the concept of sustainable development, it is important to develop a biosorbent for the selective and efficient removal mercury ions. A novel biosorbent, cysteamine-modified cellulose nanocrystals (Cys-CNCs), was synthesized by a mild periodate oxidation of cellulose nanocrystals, followed by grafting with cysteamine and ultimately used for adsorption of mercury ions from aqueous solutions. Cysteamine was grafted onto cellulose nanocrystals to improve its adsorption of mercury ions, based on the Hard–Soft Acid–Base theory. The effect of pH, contact time, and mercury ions initial concentration was thoroughly investigated to optimize the adsorption process. The pseudo-second order model could accurately describe the adsorption kinetics. The adsorption isotherm study of Hg(II) followed the Langmuir model of monolayer adsorption and the maximum adsorption capacity was 849 mg g−1. Cys-CNC4.05 can rapidly remove mercury ions with 99% removal within 10 min from a 51 mg L−1 solution. Furthermore, Cys-CNC4.05 showed a good regeneration performance after four adsorption/desorption cycles.

Unifying the Hydrogen Evolution and Oxidation Reactions Kinetics in Base by Identifying the Catalytic Roles of Hydroxyl-Water-Cation Adducts.

Despite the fundamental and practical significance of the hydrogen evolution and oxidation reactions (HER/HOR), their kinetics in base remain unclear. Herein, we show that the alkaline HER/HOR kinetics can be unified by the catalytic roles of the adsorbed hydroxyl (OHad)-water-alkali metal cation (AM+) adducts, on the basis of the observations that enriching the OHad abundance via surface Ni benefits the HER/HOR; increasing the AM+ concentration only promotes the HER, while varying the identity of AM+ affects both HER/HOR. The presence of OHad-(H2O) x-AM+ in the double-layer region facilitates the OHad removal into the bulk, forming OH--(H2O) x-AM+ as per the hard-soft acid-base theory, thereby selectively promoting the HER. It can be detrimental to the HOR as per the bifunctional mechanism, as the AM+ destabilizes the OHad, which is further supported by the CO oxidation results. This new notion may be important for alkaline electrochemistry.

Correlating Ionic Conductivity, Oxygen Transport and ORR with Structure of Dialkylacetamide-Based Electrolytes for Lithium-Air Batteries

Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF6−, CF3SO3−, ClO4− and NO3−), solvation occurs when the Li+ bonds with the solvent's carbonyl group forming Li+[O=C(CH3)N(CH3)2]nX− ion pairs. Infrared and 13C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF6−, ClO4− or NO3−, and a contact ion pair in the case of CF3SO3−. Chemical interactions between TBA+ and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA+ and Li+ electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF6/DMAc, the O2 solubility and diffusion coefficient (3.09 × 10−6 mol/cm−3 and 5.09 × 10−5 cm2s−1, respectively) measured using microelectrode technique are typical of values measured in several TBA+ solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.