Evolution Of H2O Research Articles

Unveiling Transport Mechanisms of Cesium and Water in Operando Zero-Gap CO2 Electrolyzers

In zero-gap CO2 electrolyzers, maintaining the balance of water and cations is crucial. Excessive accumulation at the cathode causes performance degradation, leading to flooding and salt precipitation. Using operando wide-angle X-ray scattering and X-ray fluorescence techniques, we observed the dynamic evolution of H2O and Cs+ inside a membrane electrode assembly. While no motion of Cs+ and H2O was observed during open circuit voltage (OCV), our findings indicate that Cs+ movement across the membrane from the anode to the cathode is governed by migration and drags H2O via electroosmosis when a potential was applied. H2O diffusion then allows Cs+ diffusion further within the gas diffusion electrode. When decreasing the applied voltage, the concentration gradient causes Cs+ to quickly diffuse back to the anode. The H2O content in the macro-porous layer remains at the same level, thus showcasing an origin of gas diffusion electrode (GDE) flooding, which is one of the detrimental degradation mechanisms to these systems and result in an enhanced H2 production. By regulating the electrolyte concentration, we deconvolute the correlation of water and cations for selectivity changes. Our work underscores the significance of water/cation management strategies in zero-gap electrolyzers.

Mo Migration‐Induced Crystalline to Amorphous Conversion and Formation of RuMo/NiMoO4 Heterogeneous Nanoarray for Hydrazine‐Assisted Water Splitting at Large Current Density

AbstractManipulating the atomic structure of the catalyst and tailoring the dissociative water‐hydrogen bonding network at the catalyst‐electrolyte interface is essential for propelling alkaline hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR), but remains a great challenge. Herein, we constructed an advanced a‐RuMo/NiMoO4/NF heterogeneous electrocatalyst with amorphous RuMo alloy nanoclusters anchored to amorphous NiMoO4 skeletons on Ni foam by a heteroatom implantation strategy. Theoretical calculations and in situ Raman tests show that the amorphous and alloying structure of a‐RuMo/NiMoO4/NF not only induces the directional evolution of interfacial H2O, but also lowers the d‐band center (from −0.43 to −2.22 eV) of a‐RuMo/NiMoO4/NF, the Gibbs free energy of hydrogen adsorption (ΔGH*, from −1.29 to −0.06 eV), and the energy barrier of HzOR (ΔGN2(g)=1.50 eV to ΔGN2*=0.47 eV). Profiting from these favorable factors, the a‐RuMo/NiMoO4/NF exhibits excellent electrocatalytic performances, especially at large current densities, with an overpotential of 13 and 129 mV to reach 10 and 1000 mA cm−2 for HER. While for HzOR, it needs only −91 and 276 mV to deliver 10 and 500 mA cm−2, respectively. Further, the constructed a‐RuMo/NiMoO4/NF||a‐RuMo/NiMoO4/NF electrolyzer demands only 7 and 420 mV to afford 10 and 500 mA cm−2.

Mo Migration-Induced Crystalline to Amorphous Conversion and Formation of RuMo/NiMoO4 Heterogeneous Nanoarray for Hydrazine-Assisted Water Splitting at Large Current Density.

Manipulating the atomic structure of the catalyst and tailoring the dissociative water-hydrogen bonding network at the catalyst-electrolyte interface is essential for propelling alkaline hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR), but remains a great challenge. Herein, we constructed an advanced a-RuMo/NiMoO4/NF heterogeneous electrocatalyst with amorphous RuMo alloy nanoclusters anchored to amorphous NiMoO4 skeletons on Ni foam by a heteroatom implantation strategy. Theoretical calculations and in situ Raman tests show that the amorphous and alloying structure of a-RuMo/NiMoO4/NF not only induces the directional evolution of interfacial H2O, but also lowers the d-band center (from -0.43 to -2.22 eV) of a-RuMo/NiMoO4/NF, the Gibbs free energy of hydrogen adsorption (ΔGH*, from -1.29 to -0.06 eV), and the energy barrier of HzOR (ΔGN2(g)=1.50 eV to ΔGN2*=0.47 eV). Profiting from these favorable factors, the a-RuMo/NiMoO4/NF exhibits excellent electrocatalytic performances, especially at large current densities, with an overpotential of 13 and 129 mV to reach 10 and 1000 mA cm-2 for HER. While for HzOR, it needs only -91 and 276 mV to deliver 10 and 500 mA cm-2, respectively. Further, the constructed a-RuMo/NiMoO4/NF||a-RuMo/NiMoO4/NF electrolyzer demands only 7 and 420 mV to afford 10 and 500 mA cm-2.

Synergistic promotion strategy of “dual-site” and “dual-path” to enhance the C–C coupling between CO2 and HCHO driven by photoelectrocatalysis

Photoelectrocatalytic coupling CO2 and volatile organic compounds (VOCs) is a promising green strategy for the synergistic conversion of the two carbon-containing resources to C2 products. The catalytic efficiency is always at the mercy of chemical inertness of CO2 and the competitive hydrogen evolution of H2O. Herein, a modified g-C3N4/ZnAl-LDH Z-scheme heterojunction catalyst with dual reaction site was rationally designed and precisely constructed. The Faraday efficiency of ethanol reached 68.67% with a corresponding formation rate of 227.3 μmol g−1 h−1. As revealed by in-situ characterizations and density functional theory calculations, CO2 and HCHO were absorbed at Zn site and N site, respectively. Then, *CO generated from CO2 and HCHO was converted to *CH3O and *CHO on the dual-active-site heterojunction. The detailed reaction mechanism experiments indicated that C–C coupling only occurred between *CO and *CH3O in electrocatalysis process. Apart from the “*CO + *CH3O” path, another “*CO + *CHO” coupling path was also detected in photoelectrocatalytic process. The selectivity of ethanol was significantly enhanced due to the synthesis of dual-site catalyst and the dual-path coupling path between CO2 and HCHO simultaneously driven by light and electricity.

Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial-Level Current Densities Promoted by Alkali Metal Cations.

Acidic H2O2 synthesis through electrocatalytic 2e- oxygen reduction presents a sustainable alternative to the energy-intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e- oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial-level currents, resulting in an effective current densities of 50-421 mA cm-2 with 84-100 % Faradaic efficiency and a production rate of 856-7842 μmol cm-2 h-1 that far exceeds the performance observed in pure acidic electrolytes or low-current electrolysis. Finite-element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e- oxygen reduction through interacting with coordinated H2O.

Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial‐Level Current Densities Promoted by Alkali Metal Cations

AbstractAcidic H2O2 synthesis through electrocatalytic 2e− oxygen reduction presents a sustainable alternative to the energy‐intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e− oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial‐level currents, resulting in an effective current densities of 50–421 mA cm−2 with 84–100 % Faradaic efficiency and a production rate of 856–7842 μmol cm−2 h−1 that far exceeds the performance observed in pure acidic electrolytes or low‐current electrolysis. Finite‐element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e− oxygen reduction through interacting with coordinated H2O.

Unveiling the Descriptor of Parasitic Reactions of Zinc Anode: A Comparative Study of Trace Pyridinesulfonic Acid‐Based Additives in Aqueous Electrolyte

AbstractUnderstanding and controlling parasitic reactions on the Zn metal anode (ZMA) surface is essential to enhance the energy capabilities of aqueous zinc‐ion batteries (ZIBs). However, the accurate regulation scheme is often obscured due to the lack of fundamental understanding concerning the ZMA/electrolyte interface. Herein, the descriptor of interfacial parasitic reactions is revealed through a systematic comparative study of three model trace adsorption‐type pyridinesulfonic acid‐based additives with structural variations. Using in situ spectroscopies coupled with density functional theory calculations, direct spectroscopic evidence of interfacial H2O evolution during Zn2+ deposition process is obtained. It is proposed that, beyond the traditional cognitions, the distance between solvated Zn(H2O)62+ and ZMA surface highly dictates the stability of ZMAs. Consequently, the trace 3‐Pyridinesulfonic acid with most effective capacity to drive solvated Zn(H2O)62+ away from the ZMA surface, enables a robust cycle life over 420 h for the Zn||Zn symmetric cell at 10 mA cm−2/10 mAh cm−2 (depth of discharge of 45%), a high Coulombic efficiency of 99.78% and an extended cycling life of 1500 cycles for the Zn//NH4V4O10 full battery. The work sheds light on the underlying mechanism of parasitic reactions on ZMA surface and provides fundamental insights into the design of trace additives for better ZIBs.

Influence of atmospheric moisture on the gas evolution tolerance of halide solid electrolytes

Much attention has been paid on research and development on solid electrolytes for all-solid-state Li batteries. Although halide solid electrolytes such as Li3YCl6 and Li3InCl6 are promising due to fast Li ion conductivity and oxidation-resistant against positive electrode, a better understanding of their reactivity with atmospheric H2O is required for commercialization. In this study, the gas evolution tolerances of Li3YCl6 and Li3InCl6 were investigated. Temperature-programmed desorption mass spectrometry (TPD-MS) experiments at dew points below − 60 °C and gas detector tube experiments at dew points of − 30 °C both revealed significant differences in the H2O and HCl evolution behavior of Li3YCl6 and Li3InCl6. In TPD-MS, the onset temperature of HCl evolution for Li3YCl6 (~ 100 °C) was significantly lower than that for Li3InCl6 (~ 220 °C), indicating that Li3InCl6 solid electrolytes have superior gas evolution tolerance. This difference may be attributable to differences in the retention of H2O derived from the material synthesis stage and from contact with the atmosphere during the measurements. In particular, based on first-principles calculations, the low-temperature HCl evolution observed in Li3YCl6 was ascribed to the partial replacement of Cl− ions by OH− ions upon contamination with trace H2O. Because the heating and drying of solid electrolytes (including slurries) are inevitable processes during battery manufacturing, these findings can aid in the rational design of halide solid electrolytes for all-solid-state batteries.

Dynamical evolution of CO2 and H2O on garnet electrolyte elucidated by ambient pressure X-ray spectroscopies

Garnet-type Li6.5La3Zr1.5Ta0.5O12 (LLZO) is considered a promising solid electrolyte, but the surface degradation in air hinders its application for all-solid-state battery. Recent studies have mainly focused on the final products of the LLZO surface reactions due to lacking of powerful in situ characterization methods. Here, we use ambient pressure X-ray spectroscopies to in situ investigate the dynamical evolution of LLZO surface in different gas environments. The newly developed ambient pressure mapping of resonant Auger spectroscopy clearly distinguishes the lithium containing species, including LiOH, Li2O, Li2CO3 and lattice oxygen. The reaction of CO2 with LLZO to form Li2CO3 is found to be a thermodynamically favored self-limiting reaction. On the contrary, the reaction of H2O with LLZO lags behind that of CO2, but intensifies at high pressure. More interestingly, the results provide direct spectroscopic evidence for the existence of Li+/H+ exchange and reveal the importance of the initial layer formed on clean electrolyte surface in determining their air stability. This work demonstrates that the newly developed in situ technologies pave a new way to investigate the oxygen evolution and surface degradation mechanism in energy materials.

Inorganic intumescing alkali silicate particles

New generation intumescent coatings apply self-intumescing particles to induce expansion in a controlled manner. Various studies have investigated alkali silicate-based coatings due to their inherent expansion with the release of H2O. The presented work investigated the self-intumescing concept of alkali silicates as particles. A thorough study of inorganic alkali silicate particles as potential intumescent ingredients in passive fire protection systems was carried out by applying HSDM, TGA, FTIR, XRD, and SEM. Herein, particles of lithium-, sodium-, and potassium silicates were synthesized with varying silica-to-alkali oxide molar ratios and curing humidity. Subsequently, the hydration chemistry and thermal behavior were examined. The results revealed increased expansion potential with decreasing SiO2/M2O molar ratio (M = Na, Li, or K) in the range 3.1–7.4 and increasing curing humidity in the range 35, 50, and 90% RH. Previous studies have emphasized the effect of solid-bound H2O to induce intumescence. In this study, the influence of the softening of the silicate matrix and H2O release is highlighted to allow increased expansion potential. Here, the coupling of thermal analysis with microscopy suggests an intumescence mechanism in the order of 1) initial shrinkage due to loss of free and physical bound H2O, 2) sphere formation to minimize surface energy, 3) expansion as a result of H2O evolution and viscoelastic melt formation, and 4) second shrinkage due to sintering before 5) melting. The sodium alkali silicate particles with a SiO2/M2O molar ratio of 3.4 exhibited a timely softening of the silicate matrix from about 155 °C to provide a distinct spherical particle followed by ionic H2O evolution. As a result, hereof, hollow, spherical particles with relative solid expansions of up to 550% were obtained and maintained until a second shrinkage at 650 °C with a subsequent melting at 950 °C. In comparison, the imbalance in viscoelastic melt formation and H2O release of lithium- and potassium silicate particles resulted in less expansion. Considering the expansion potential and thermal behavior of the alkali silicate particles, it is suggested as a possible, sustainable intumescence ingredient.

Mars Hot Oxygen Density and Effective Temperature Derived From the MAVEN IUVS Observations

AbstractThe escape of hot oxygen atoms from Mars is important for the evolution of the planet's H2O and CO2 inventories. Owing to the lack of data constraints prior to the Mars Atmosphere and Volatile Evolution (MAVEN) mission, previous understanding of this key loss channel of the Mars atmosphere relied mostly on physics‐based models. Using the optical observations from the MAVEN Imaging Ultraviolet Spectrograph (IUVS) during 2014–2018, we perform the first inversion analysis of the coronal scans at 130.4 nm to quantify the density and effective temperature of the Mars hot oxygen corona. Our results indicate that the exobase densities of the hot oxygen atoms are ∼6.3–8.0 × 103 and ∼1.2 × 104 cm−3 during low and moderate solar activities, respectively, which agree well with recent empirical estimates using MAVEN's in situ measurements and, however, are about a factor of two higher than previous model predictions. The effective temperature varies only slightly between ∼4100–4500 K. The temperature and density variations with solar, seasonal, and dust conditions are consistent with previous models. Comparison with inversion results from IUVS limb scans indicates that the cold and the hot oxygen populations dominate below ∼200 and above ∼600 km, respectively, between which the kinetic distribution depends on both populations. Our results demonstrate for the first time the feasibility of extracting information about the Mars hot oxygen corona directly from optical observations, opening up a powerful means for monitoring the Mars oxygen escape on a global scale in future missions.

New thermal decomposition pathway for TATB

Understanding the thermal decomposition behavior of TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is a major focus in energetic materials research because of safety issues. Previous research and modelling efforts have suggested benzo-monofurazan condensation producing H2O is the initiating decomposition step. However, early evolving CO2 (m/z 44) along with H2O (m/z 18) evolution have been observed by mass spectrometric monitoring of head-space gases in both constant heating rate and isothermal decomposition studies. The source of the CO2 has not been explained, until now. With the recent successful synthesis of 13C6-TATB (13C incorporated into the benzene ring), the same experiments have been used to show the source of the CO2 is the early breakdown of the TATB ring, not adventitious C from impurities and/or adsorbed CO2. A shift in mass m/z 44 (CO2) to m/z 45 is observed throughout the decomposition process indicating the isotopically labeled 13C ring breakdown occurs at the onset of thermal decomposition along with furazan formation. Partially labeled (N18O2)3-TATB confirms at least some of the oxygen comes from the nitro-groups. This finding has a significant bearing on decomposition computational models for prediction of energy release and deflagration to detonation transitions, with respect to conditions which currently do not recognize this oxidation step.

Assessing the importance of H2O content in the tectono‐metamorphic evolution of shear zones: A case study from the Dora‐Maira Massif (Western Alps)

AbstractMetamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re‐equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H2O in such processes, forward thermodynamic modelling calculations commonly assume either H2O‐saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH2O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora‐Maira Massif (Western Alps, Italy). After the eclogite‐facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite‐facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH2O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H2O content at the transition from eclogite‐ to amphibolite‐facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H2O‐undersaturated conditions. This work thus underlines the necessity of investigating the H2O evolution within shear zones, as the H2O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.

The crucial role of oxygen in NO heterogeneous reduction with NH3 at high temperature

The high-temperature ammonia-injected technology is acknowledged as a promising denitrification method with great NOx reduction potential. In this work, char reduction experiments and density functional theory (DFT) calculations were conducted to investigate the heterogeneous reduction mechanism of NO by ammonia (NH3) synergism with char, focusing on the role of O2. Experimental results indicate that O2 has an inhibition effect on the NO reduction efficiency, whose detailed mechanisms can be inferred from a molecular perspective. Theoretical results reveal that both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms exist in the NH3-injected denitrification reactions, presenting significant differences in N2, N2O, and H2O evolution. By comparison, the E-R mechanism governs the NH3–NO reactions, which reduces barriers by up to 14.5 %. The presence of O2 can lead to a significant increase in the energy barriers with a concomitant decrease in reaction rates within a certain temperature range, which however does not change the dominant mechanism governing the NH3 and NO interactions. Although some controversies are found in the kinetic analysis when the temperature increases, the results here shed light on the NO heterogeneous reduction mechanisms with NH3 at high temperature, which also guides the coal/NH3 co-firing investigations.

Complexities in the structural evolution with pressure of water-ammonia mixtures.

The structural evolution with pressure of icy mixtures of simple molecules is a poorly explored field despite the fundamental role they play in setting the properties of the crustal icy layer of the outer planets and of their satellites. Water and ammonia are the two major components of these mixtures, and the crystal properties of the two pure systems and of their compounds have been studied at high pressures in a certain detail. On the contrary, the study of their heterogeneous crystalline mixtures whose properties, due to the strong N-H⋯O and O-H⋯N hydrogen bonds, can be substantially altered with respect to the individual species has so far been overlooked. In this work, we performed a comparative Raman study with a high spatial resolution of the lattice phonon spectrum of both pure ammonia and water-ammonia mixtures in a pressure range of great interest for modeling the properties of icy planets' interiors. Lattice phonon spectra represent the spectroscopic signature of the molecular crystals' structure. The activation of a phonon mode in plastic NH3-III attests to a progressive reduction in the orientational disorder, which corresponds to a site symmetry reduction. This spectroscopic hallmark allowed us to solve the pressure evolution of H2O-NH3-AHH (ammonia hemihydrate) solid mixtures, which present a remarkably different behavior from the pure crystals likely to be ascribed to the role of the strong H-bonds between water and ammonia molecules characterizing the crystallites' surface.

Redox Dynamics of Pt and Cu Nanoparticles on TiO2 during the Photocatalytic Oxidation of Methanol under Aerobic and Anaerobic Conditions Studied by In Situ Modulated Excitation X-ray Absorption Spectroscopy

In situ modulated excitation X-ray absorption spectroscopy (ME-XAS) at the Pt L3- and Cu K-edges has been successfully used in this work to shed light on the controversial role that Pt and Cu nanoparticles (NPs) on TiO2 have in photocatalysis. TiO2 photocatalysts, either bare or modified by Pt and Cu NPs, synthesized in a single step by flame spray pyrolysis, were tested in the gas-phase photocatalytic oxidation of methanol to CO2 under both aerobic (CH3OH + 3/2O2 → CO2 + 2H2O) and anaerobic (CH3OH + H2O → CO2 + 3H2) conditions. Both Pt- and Cu-containing TiO2 showed lower activity than bare TiO2 under aerobic conditions, while higher H2 and CO2 production rates were attained under anaerobic conditions in the presence of the Pt co-catalyst, with copper-containing TiO2 having in this case an activity similar to that of bare titania. ME-XAS, coupled with phase-sensitive detection, proved in fact that Pt NPs efficiently capture the electrons photopromoted in the conduction band of TiO2 and are the active sites for H2 or H2O evolution under anaerobic or aerobic conditions, respectively. Much more complex is the role of Cu-NPs on titania, with ME-XAS revealing that copper is simultaneously present in different oxidations states, depending on the reaction conditions. Cu NPs are not the reaction active sites under anaerobic conditions, while a Cu(II)/Cu(I) redox switching occurs under chopped irradiation under aerobic conditions.

Study on the thermal decomposition mechanism of Mg(NO3)2·6H2O from the perspective of resource utilization of magnesium slag

ABSTRACT The magnesium slag (magnesium nitrate hydrate Mg(NO3)2·6H2O) produced in the nitric acid leaching process of laterite nickel ore can be effectively recycled by thermal decomposition. To this end, this study placed great emphasis on disclosing the thermal decomposition mechanism of Mg(NO3)2·6H2O. Firstly, thermal decomposition paths of Mg(NO3)2·6H2O were revealed through Thermogravimetry-Mass Spectrometry, Differential Scanning Calorimetry and powder X-ray diffraction. It was found that the thermal decomposition of Mg(NO3)2·6H2O was a multistep endothermic reaction involving two dehydration stages and one denitration stage. The two dehydration stages were characterized by the evolution of H2O, with the formation of magnesium nitrate dihydrate and anhydrous magnesium nitrate. The denitration stage was characterized by the simultaneous evolution of O2 and NO2, with the formation of MgO. The conventional kinetic analysis was not suitable for describing such complex multistep reaction behaviour. Thus, the kinetic rate data (dα/dt-T) for the overall reaction were separated into those for three contributing stages by mathematical peak deconvolution. Then, the complete kinetic interpretations of the separated reaction stages for Mg(NO3)2·6H2O pyrolysis were achieved by the Friedman method and the master plots method. Finally, the original experimental α-T curves were successfully simulated using the resulting kinetic triplets.

Stress Regulation of the Oxygen Reduction Reaction on a Pt (100) Surface Using First Principles Calculations

The oxygen reduction reaction (ORR) on a strained Pt (100) surface has been studied using periodic density functional theory (DFT) calculations. The results showed a linear relationship between strain and the d-band center. The adsorption energy displayed greater sensitivity to compressive strain and the adsorption energy results indicated that the bridge site was more stable after the adsorption of OH, O and OOH. Concomitantly, the calculated free energy diagram results confirmed that compression deformation could reduce the over potential of the ORR reaction and increase the catalyst activity on the Pt (100) surface. The evolution of H2O exhibited the largest free energy difference, suggesting that it was a potentially rate limiting step. Notably, the limiting potentials for the individual steps were fitted by the OOH vs. OH or O scaling relationship and the maximum overpotential value for the ORR reaction was 0.820 V when the compressive strain reached −10%. This study provided mechanistic and quantitative guidance for the rational selection of stress regulation strategies to fabricate Pt catalysts with desirable electrocatalytic activity in emerging energy applications.

A Method for Deconvoluting and Quantifying the Real‐Time Species Fluxes and Ionic Currents Using In Situ Electrochemical Quartz Crystal Microbalance

AbstractElectrochemical quartz crystal microbalance (EQCM) is a powerful tool to screen the gravimetric response of electrochemically active electrodes. In this study, a method is proposed to deconvolute and quantify the real‐time fluxes and ionic currents of different species based on the EQCM measurement results. This work creatively conceptualizes the flux cyclic voltammograms (CVs) and ionic current CVs, and applys them to analyze the real‐time molecules and ion evolution. As a proof of concept, Ti3C2Tx MXene, the most studied 2D metal carbide, is investigated as a supercapacitor electrode in 1 m H2SO4 electrolyte. The real‐time H2O and H+ fluxes are obtained using the proposed approach and corresponding flux CVs are constructed. The potential‐dependent evolution of H2O and H+ fluxes suggest that the insertion of hydrated H+ contributes significantly to the double‐layer capacitance. The H+ CV calculated from the H+ flux overlaps with the experimental measured CV, confirming that H+ is the only interactive charge carrier for screening the Ti3C2Tx electrode charge.

Influence of Al2O3 Coatings on HF Induced Transition Metal Dissolution from Lithium-Ion Cathodes

Transition metal (TM) dissolution from oxide cathode materials is a major challenge limiting the performance of modern Li-ion batteries. Coating the cathode materials with thin protective layers has proved to be a successful strategy to prolong their lifetime. Yet, there is a lack of fundamental understanding of the working mechanisms of the coating. Herein, the effect of the most commonly employed coating material, Al2O3, on suppressing hydrofluoric acid(HF)-induced TM dissolution from two state-of-the-art cathode materials, LiMn2O4 and LiNi0.8Mn0.1Co0.1O2, is investigated. Karl Fischer titration, fluoride selective probe and inductively coupled plasma optical emission spectrometry are coupled to determine the evolution of H2O, HF and TM concentrations, respectively, when the active materials come in contact with the aged electrolyte. The coating reduces the extent of TM dissolution, in part due to the ability of Al2O3 to scavenge HF and reduce the acidity of the electrolyte. Delithiation of the cathode materials, however, increases the extent of TM dissolution, likely because of the higher vulnerability of surface TMs in +IV oxidation state towards HF attack. In conclusion, the current study evidences the important role of acid-base reactions in governing TM dissolution in Li-ion batteries and shows that coatings enhance the chemical integrity of the cathode towards an acidic electrolyte.