Fundamental Reaction Mechanisms Research Articles

Revealing Asymmetric Phase Transformation of the BiVO4 Anodes for Potassium-Ion Batteries by In Situ Transmission Electron Microscopy.

Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), thanks to the cost-effectiveness of potassium resources and a favorable redox potential of approximately -2.936 V. The monoclinic BiVO4, known for its layered structure, shows noteworthy electrochemical properties when utilized as an anode material for both LIBs and sodium-ion batteries. However, the fundamental electrochemical reaction mechanisms of the BiVO4 anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO4 anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO4 during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO4 nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO4 material is transformed into an amorphous KxBiVO4 phase after the first cycle. More interestingly, the anode materials near and far from the potassium source exhibit opposite volume-changing trends under the same voltage potential. Also, this phenomenon should be attributed to the mass flow of the unstable K-Bi alloy under the electric field. Our findings provide significant insights into the electrochemical mechanism of BiVO4 nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.

Enhancing oil production via radical reactions during hydrothermal coliquefaction of biomass model compounds and plastics: A molecular dynamic simulation study

Recently, hydrothermal coliquefaction of biomass and plastic waste has attracted considerable research interest. However, there is a notable gap in understanding the fundamental reaction mechanisms between biomass and plastics during coliquefaction. This study focused on the coliquefaction of biomass model compounds and plastic polymers using ReaxFF molecular dynamics simulations under both subcritical and supercritical water conditions. Molecular-level tracking and probing of the reaction mechanisms between biomass model compounds and plastics were conducted to purposefully enhance oil production. The study observed related radical reactions between by-product molecules, with detailed mechanisms primarily involving (1) ▪OH radicals released by aqueous phase molecules from biomolecules, transferring as H2O molecules and facilitating plastic depolymerization, and (2) C1–C4 radicals in the gaseous phase, emitted from biomolecule and plastic, colliding and subsequently recombining to form oil molecules. Moreover, the yield of multiple products from various mixtures were evaluated by considering the key reaction parameters including reaction temperature and feedstock blended ratio. An exploration into the effect of coliquefaction on oil yield was conducted to precisely identify the optimal coliquefaction conditions. The positive effect of coliquefaction was more pronounced between biomass model compounds and aromatic polymers compared to aliphatic polymers. Analysis of reaction mechanisms and product outcomes has shown that hydrothermal coliquefaction is a viable approach to improving oil production from multi-source organic solid waste.

Physics-Based Continuum Modeling for an Aqueous Rechargeable Zn/MnO2 Battery

While Lithium-ion batteries dominate high-energy-density applications, their reliance on scarce and expensive materials poses a challenge for large-scale renewable energy storage. Aqueous Zinc/Manganese Oxide (Zn/MnO2) batteries, utilizing abundant and cost-effective materials, emerge as a viable alternative. However, the lack of understanding of their fundamental reaction mechanisms hampers their optimization and commercialization. In this study, we present a physics-based model to aid in elucidating the reaction mechanisms governing Zn/MnO2 batteries. When coupled with statistical parameter estimation, the model accurately replicates the discharge behavior across various C-rates and offers predictive insights into the underlying phenomena. Our findings inform the design of future targeted experiments and lay the groundwork for in-depth mechanistic studies. Moreover, the model serves as a tool for in-silico optimization, accelerating the path to commercial viability.

(Invited) Unveiling Fundamental Reaction Mechanisms at the Electrochemical Interface by Ab Initio Simulations

Direct experimental exploration of atomistic mechanisms governing electrocatalysis or wet corrosion at the solid-liquid interface is notoriously difficult. While fully parameter-free ab initio calculations have the potential to accurately elucidate such mechanisms, they encounter substantial conceptual challenges. Recent methodological breakthroughs, including the advent of efficient computational electrodes and the development of effective thermopotentiostats, overcame many of these limitations thus providing new research opportunities. This presentation outlines the fundamental concepts underlying these innovative methodologies and demonstrates their application in performing fully explicit calculations of structure and chemical reactions at electrified solid-water interfaces. The power of this approach in discovering new mechanisms will be demonstrated for fundamental electrochemical reactions such as metal dissolution and the hydrogen evolution reaction (HER). The identified mechanisms challenge existing understandings and empfasize the active involvement of water molecules in electrochemical reactions, challenging the perception that they are passive bystanders.

Pyrolysis mechanistic study on sulfated polysaccharide from marine algal biomass with density functional theory method

Sulfated polysaccharide (SPS) derived from marine algae represents a promising renewable carbon biomass source. Through pyrolysis, SPS can be converted into C5 platform chemicals, including furfural (FF) and 5-methylfurura (5-MF). Understanding the fundamental reaction mechanisms during pyrolysis is essential for the advancing pyrolysis techniques. This work presents a detailed reaction mechanism that reveal new pathways leading to the most important products and intermediates. Simulations focusing on the initial steps in SPS pyrolysis showed that the unimolecular degradation of SPS is dominated by SO3 elimination, with a barrier height of 157.2 kJ/mol; ring-opening and glycosidic bond fission were identified as important competitive reactions, both of which have a barrier height of 207.6 kJ/mol and 260.1 kJ/mol, respectively. The 4,5-unsaturated acyclic rhamnose derived from the glycosidic bond fission of acyclic reducing end group can transform into 5-MF with high selectivity. In addition, the acyclic reducing end group is readily dehydrated to form a 2,3-unsaturated acyclic reducing end group, a reactive unsaturated intermediate. Its decomposition involves an 8-membered ring transition state that requires a low barrier height and results in the formation of a C5 fragment, which can be cyclized directly and dehydrated to form 5-MF. The majority of CO2 is mostly formed by the synergistic breakage of C-Ccarboxyl bonds and glycosidic bonds, with a barrier height of 197.9 kJ/mol. Moreover, a mechanism is uncovered for the formation of formic acid (FA) from a 4,5-unsaturated no-reducing end group. The major pyrolysis channel is initiated by a retro Diels-Alder reaction and followed by a retro-ene reaction to produce a 3,4-unsaturated non-reducing end group, which further decomposes into FA and a new 4,5-unsaturated non-reducing end group.

Coherence mapping to identify the intermediates of multi-channel dissociative ionization

Identifying the short-lived intermediates and reaction mechanisms of multi-channel radical cation fragmentation processes remains a current and important challenge to understanding and predicting mass spectra. We find that coherent oscillations in the femtosecond time-dependent yields of several product ions following ultrafast strong-field ionization represent spectroscopic signatures that elucidate their mechanism of formation and identify the intermediate(s) they originate from. Experiments on endo-dicyclopentadiene show that vibrational frequencies from various intermediates are mapped onto their resulting products. Aided by ab initio methods, we identify the vibrational modes of both the cleaved and intact molecular ion intermediates. These results confirm stepwise and concerted fragmentation pathways of the dicyclopentadiene ion. This study highlights the power of tracking the femtosecond dynamics of all product ions simultaneously and sheds further light onto one of the fundamental reaction mechanisms in mass spectrometry, the retro-Diels Alder reaction.

Recent progress in single atomic catalysts for electrochemical N2 fixation

Ammonia (NH3), as an important chemical product, is industrially produced using the traditional energy-intensive Haber-Bosch method at high temperature and pressure. Electrochemical nitrogen reduction reaction (NRR) to synthesize NH3 at ambient conditions has been considered as a promising candidate for replacing Haber-Bosch process. However, major obstacles, such as poor catalytic activity and selectivity and extensive competitive hydrogen evolution reaction, remain in NRR, which urgently need to be addressed. Single atom electrocatalysts (SACs) have attracted wide attention in view of their nearly 100% atomic utilization and outstanding catalytic performance. In this review, recent theoretical and experimental advances on novel atomically dispersed electrocatalysts for NRR are summarized and highlighted. We start with the fundamental reaction mechanism of NRR. Then, different preparation methods and the strategies of boosting catalytic performance of SACs from the aspects of coordination environment, coordination number, metal-support interaction and spatial microenvironment regulation are presented and analyzed in detail. Following this, the extensive applications of SACs in terms of noble metal based-SACs and transition metal-based SACs in NRR are discussed. Finally, we provide a perspective of the challenges of SACs for NRR, aiming to guide the rational design of advanced NRR catalysts.

Interplay between Catalyst Corrosion and Homogeneous Reactive Oxygen Species in Electrochemical Ozone Production.

Electrochemical ozone production (EOP), a six-electron water oxidation reaction, offers promising avenues for creating value-added oxidants and disinfectants. However, progress in this field is slowed by a dearth of understanding of fundamental reaction mechanisms. In this work, we combine experimental electrochemistry, spectroscopic detection of reactive oxygen species (ROS), oxygen-anion chemical ionization mass spectrometry, and computational quantum chemistry calculations to determine a plausible reaction mechanism on nickel- and antimony-doped tin oxide (Ni/Sb-SnO2, NATO), one of the most selective EOP catalysts. Antimony doping is shown to increase the conductivity of the catalyst, leading to improved electrochemical performance. Spectroscopic analysis and electrochemical experiments combined with quantum chemistry predictions reveal that hydrogen peroxide (H2O2) is a critical reaction intermediate. We propose that leached Ni4+ cations catalyze hydrogen peroxide into solution phase hydroperoxyl radicals (•OOH); these radicals are subsequently oxidized to ozone. Isotopic product analysis shows that ozone is generated catalytically from water and corrosively from the catalyst oxide lattice without regeneration of lattice oxygens. Further quantum chemistry calculations and thermodynamic analysis suggest that the electrochemical corrosion of tin oxide itself might generate hydrogen peroxide, which is then catalyzed to ozone. The proposed pathways explain both the roles of dopants in NATO and its lack of stability. Our study interrogates the possibility that instability and electrochemical activity are intrinsically linked through the formation of ROS. In doing so, we provide the first mechanism for EOP that is consistent with computational and experimental results and highlight the central challenge of instability as a target for future research efforts.

Harvesting Thermal Energy through Pyroelectric and Thermoelectric Nanomaterials for Catalytic Applications

The current scenario sees over 60% of primary energy being dissipated as waste heat directly into the environment, contributing significantly to energy loss and global warming. Therefore, low-grade waste heat harvesting has been long considered a critical issue. Pyroelectric (PE) materials utilize temperature oscillation to generate electricity, while thermoelectric (TE) materials convert temperature differences into electrical energy. Nanostructured PE and TE materials have recently gained prominence as promising catalysts for converting thermal energy directly into chemical energy in a green manner. This short review provides a summary and comparison of catalytic processes initiated by PE and TE effects driven by waste thermal energy. The discussion covers fundamental principles and reaction mechanisms, followed by the introduction of representative examples of PE and TE nanomaterials in various catalytic fields, including water splitting, organic synthesis, air purification, and biomedical applications. Finally, the review addresses challenges and outlines future prospects in this emerging field.

Ambient Electrochemical Ammonia Synthesis: From Theoretical Guidance to Catalyst Design.

Ammonia, a vital component in the synthesis of fertilizers, plastics, and explosives, is traditionally produced via the energy-intensive and environmentally detrimental Haber-Bosch process. Given its considerable energy consumption and significant greenhouse gas emissions, there is a growing shift toward electrocatalytic ammonia synthesis as an eco-friendly alternative. However, developing efficient electrocatalysts capable of achieving high selectivity, Faraday efficiency, and yield under ambient conditions remains a significant challenge. This review delves into the decades-long research into electrocatalytic ammonia synthesis, highlighting the evolution of fundamental principles, theoretical descriptors, and reaction mechanisms. An in-depth analysis of the nitrogen reduction reaction (NRR) and nitrate reduction reaction (NitRR) is provided, with a focus on their electrocatalysts. Additionally, the theories behind electrocatalyst design for ammonia synthesis are examined, including the Gibbs free energy approach, Sabatier principle, d-band center theory, and orbital spin states. The review culminates in a comprehensive overview of the current challenges and prospective future directions in electrocatalyst development for NRR and NitRR, paving the way for more sustainable methods of ammonia production.

A DFT study for in-situ CO2 utilization realized by calcium-looping dry reforming of methane based on Ni/CaCO3

The calcium-looping dry reforming of methane (CaL-DRM) process couples CO2 capture and dry reforming of methane process using Ni/CaO dual-functional material. This process in-situ converts CO2 captured by CaO through reacting with CH4, catalyzed by Ni, to produce syngas. However, researches on reaction mechanism for CaL-DRM are rare. In this work, the fundamental reaction mechanism of calcium-looping dry reforming of methane is elucidated by density functional theory (DFT) analysis. The energy barriers for elementary reactions involved in various potential pathways were investigated to determine the primary reaction pathway. The DFT analysis shows two possible reaction pathways for the CaL-DRM, with the CH* assisted CaCO3 dissociation pathway being the more favorable one. Along this pathway, CH4 undergoes three dehydrogenation steps to form CH*. Then, CH* reacts with the CaCO3* to produce CHO* and CaCO2*, accompanying the cleavage of one C-O bond in the carbonate. Afterwards, CHO* dissociates into CO* and H*. Finally, another C-O bond in CaCO2* breaks, generating CO* and CaO. Notably, CH4 reduces the energy barrier for CaCO3 dissociation from 3.47 to 2.733 eV. Moreover, the perturbation of electron clouds around O in CaCO3* in the presence of CH or C, as evidenced in electron density differential results, highlights the effective activation of C-O bond by CH or C, thereby promoting CaCO3 decomposition. This work provides further support for the potential mechanism for in-situ CO2 utilization achieved through CaL-DRM.

Determination of local pH in CO2 electroreduction.

The electrocatalytic conversion of CO2 and H2O into fuels and valuable chemicals has gained significant interest as a prospective method for the storage of renewable energy and the utilization of captured CO2. In the process of electroreduction of CO2, pH near the surface of the electrocatalysts plays an important role in the catalytic selectivity and activity. However, to elucidate the local pH effect on the fundamental reaction mechanism and modify the catalytic CO2 reduction performance, the localized pH determination method is highly desirable. In this minireview, we present the recent advances in the strategies of the local pH probe for CO2 electrolysis in both H-type cell reactors and GDE-type flow electrolyzers, followed with a better understanding of the local reaction environment in CO2 reduction. Additionally, pertinent advantages and drawbacks of the different localized pH probe techniques are discussed, and perspectives on future research efforts are also provided in this minireview.

On the nature of Cu-carbon interaction through N-modification for enhanced ethanol synthesis from syngas and methanol.

Direct conversion of syngas into ethanol is an attractive process because of its short route and high-added value, but remains an enormous challenge due to the low selectivity caused by unclear active sites. Here, the Cu(111) supported N-modified graphene fragments C13-mNm/Cu(111) (m = 0-2) are demonstrated to be an efficient catalyst for fabricating ethanol from syngas and methanol. Our results suggest that the Cu-carbon interaction not only facilitates CO activation, but also significantly affects the adsorption stability of C2 intermediates and finally changes the fundamental reaction mechanism. The impeded hydrogenation performance of C13/Cu(111) due to the introduced Cu-carbon interaction is dramatically improved by N-doping. Multiple analyses reveal that the promoted electron transfer and the enhanced electron endowing ability of C13-mNm/Cu(111) (m = 1-2) to the co-adsorbed CH3CHxOH (x = 0-1) and H are deemed to be mainly responsible for the remarkable enhancement in hydrogenation ability. From the standpoint of the frontier molecular orbital, the decreased HOMO-LUMO gap and the increased overlap extent of HOMO and LUMO with the doping of N atoms also further verify the more facile hydrogenation reactions. Clearly, the Cu-carbon interaction through N-modification is of critical importance in ethanol formation. The final hydrogenation reaction during ethanol formation is deemed to be the rate-controlling step. The insights gained here could shed new light on the nature of Cu-carbon interaction in carbon material modified Cu-based catalysts for ethanol synthesis, which could be extended to design and modify other metal-carbon catalysts.

Recent advances in electrosynthesis of H2O2via two-electron oxygen reduction reaction.

The electrosynthesis of hydrogen peroxide (H2O2) via a selective two-electron oxygen reduction reaction (2e- ORR) presents a green and low-energy-consumption alternative to the traditional, energy-intensive anthraquinone process. This review encapsulates the principles of designing relational electrocatalysts for 2e- ORR and explores remaining setups for large-scale H2O2 production. Initially, the review delineates the fundamental reaction mechanisms of H2O2 production via 2e- ORR and assesses performance. Subsequently, it methodically explores the pivotal influence of microstructures, heteroatom doping, and metal hybridization along with setup configurations in achieving a high-performance catalyst and efficient reactor for H2O2 production. Thereafter, the review introduces a forward-looking methodology that leverages the synergistic integration of catalysts and reactors, aiming to harmonize the complementary characteristics of both components. Finally, it outlines the extant challenges and the promising avenues for the efficient electrochemical production of H2O2, setting the stage for future research endeavors.

Deciphering Unexplored Reversible Potassium Storage and Small Volume Change in a CaV4O9 Anode with In Situ Transmission Electron Microscopy

AbstractVanadium‐based compounds are explored as promising electrode materials in lithium/sodium‐ion batteries exhibiting superior energy storage properties. However, similar attempts are rarely reported for potassium‐ion batteries (PIBs), in which the fundamental reaction mechanisms remain inexplicit. Herein, porous CaV4O9 nanobelts (NBs) are selected as a PIB anode to systematically investigate potassium storage mechanisms through in situ transmission electron microscopy. In situ measurements track overall electrochemical potassiation reactions of CaV4O9 and identify a polyphase state of V4O7, CaO, and K2O phases after potassiation. Unexpectedly, the potassiated products can be partially converted back to the original CaV4O9 phase with residual VO2 and CaO phases, which is different from the irreversible phase transformations in lithium/sodium storages of CaV4O9. Impressively, the cavities in NBs alternately disappear and appear with (de)potassiation, avoiding the drastic volume change and structural degradation of anodes. The reversible potassium storage and stable cycling are evaluated by electrochemical measurements and in situ X‐ray diffraction analysis. This work provides a paradigm by revisiting the existing anode materials in lithium/sodium‐ion batteries to seek out viable anodes for next‐generation PIBs.

Urea Electrosynthesis Accelerated by Theoretical Simulations

AbstractUrea is not only a primary fertilizer in modern agriculture but also a crucial raw material for the chemical industry. In the past hundred years, the prevailing industrial synthesis of urea heavily relies on the Bosch–Meiser process to couple NH3 and CO2 under harsh conditions, resulting in high carbon emissions and energy consumption. The conversion of carbon‐ and nitrogen‐containing species into urea through electrochemical reactions under ambient conditions represents a sustainable strategy. Despite the increasing reports on urea electrosynthesis, a comprehensive review that delves into a profound, atomic‐level comprehension of the fundamental reaction mechanisms is currently absent. In this Perspective, recent advancements in electrochemical urea synthesis from CO2/CO and various nitrogenous species (i.e., N2, NOx−, and NO) under ambient conditions are presented, with special emphasis on theoretical understanding of the C─N coupling reaction mechanisms. Several key strategies to facilitate the C─N coupling are then pinpointed, which not only enhance their applicability in practical experiments but also highlight the significant progress achieved in this field. At the end, the major obstacles and potential opportunities in advancing urea electrosynthesis accelerated by theoretical simulations and in situ techniques are discussed. This review is hoped to act as a roadmap to ignite fresh insights and inspiration for the development of electrocatalytic urea synthesis.

Nanosheet architecture of MnO2/carbon with improved reaction kinetics toward advanced zinc energy storage

Aqueous zinc ion batteries (AZIBs) hold substantial promise for large-scale energy storage system by virtue of their good safety and cost-effectiveness. However, the typical cathode material of MnO2 is practically hampered by poor rate/cycling performance owing to its low electrical conductivity and structural fragility. Herein, a nanosheet architecture of MnO2/carbon is constructed via in situ rooting MnO2 nanoarrays on carbon substrate to achieve high-rate and long-durable zinc energy storage. The unique nano-architecture significantly promotes the reaction kinetics by supplying plentiful active sites, short diffusion pathways, and improved charge transfer conductivity. The advantageous features endow the MnO2/carbon with remarkable rate capability (175 mAh/g at 3.0 A/g) and excellent cycling lifespan (87.8 % capacity retention after 1000 cycles at 1.0 A/g). In addition, the kinetic enhancement and fundamental reaction mechanism are systematically investigated. The encouraging results will shed new insight on designing MnO2-based materials to advance the development of AZIBs.

Experimental and Modeling Study on the Ignition Kinetics of Nitromethane behind Reflected Shock Waves.

Nitromethane (NM) is the simplest nitroalkane fuel and has demonstrated potential usage as propellant and fuel additive. Thus, understanding the combustion characteristics and chemistry of NM is critical to the development of hierarchical detailed kinetic models of nitro-containing energetic materials. Herein, to further investigate the ignition kinetics of NM and supplement the experimental database for kinetic mechanism development, an experimental and kinetic modeling analysis of the ignition delay times (IDTs) of NM behind reflected shock waves at high fuel concentrations is reported against previous studies. Specifically, the IDTs of NM are measured via a high-pressure shock tube within the temperature from 900 to 1150 K at pressures of 5 and 10 bar and equivalence ratios of 0.5, 1.0, and 2.0. Brute-force sensitivity analysis and chemical explosive mode analysis in combination with reaction path analysis are employed to reveal the fundamental ignition kinetics of NM. Finally, a skeletal mechanism for NM is derived via the combination of directed relation graph-based methods, which demonstrates good prediction accuracy of NM ignition and flame speeds. The present work should be valuable for understanding the combustion chemistry of NM and the development of the fundamental reaction mechanism of nitroalkane fuels.

Reaction kinetics of basaltic elements in cementitious matrices: theoretical considerations

Basalt fibers, the frequently mentioned alternative to those made of steel, possess very good mechanical properties and temperature resistance. The alkaline environment of cement matrix makes it vulnerable due to partial fiber decomposition by the effects of OH- ions. This paper aims at computational modelling of such reactions in order to approximate the course of degradation or to predict it lately. The isothermal reaction models are discussed to reveal their strong/weak points by means of fundamental reaction mechanisms analysis. The shape factor and diffusion-based deceleration of the reactions are mentioned as the most significant ones in that respect. The model accuracy is quantified based on fitting the modelling outputs to reference experimental data. The effect of discussion was found to be the most significant factor as the model fitting reached the lowest RMSE (0.0047). Further application of a diffusion model is therefore recommended. The geometrical models need to have reaction rate reduction explicitly incorporated in the reaction constant, otherwise inapplicable data is produced (RMSE = 0.0193).

Micellar Solubilization for High‐Energy‐Density Aqueous Organic Redox Flow Batteries

AbstractHigh solubility of active materials is crucial for achieving a high‐energy‐density catholyte/anolyte in redox flow batteries. However, solubility largely depends on the compatibility with electrolyte, limiting the types of redox‐active materials that can be used in aqueous electrolytes. Herein, a universal strategy is introduced to attain a high solubility of active materials regardless of the compatibility with aqueous electrolytes while preserving their intrinsic redox activity via micellar solubilization. Leveraging the amphiphilic nature of surfactant molecules, insoluble redox‐active materials are encapsulated by surfactants to be dissolvable with significant solubility. As a demonstration, it is showed that an order‐of‐magnitude solubility enhancement can be achieved for (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) in aqueous catholyte (≈0.8 m). Consequently, the catholyte performance of TEMPO is fully harnessed, leading to an energy density enhancement of more than ten times compared to that in bare electrolyte. It is also observed that micellar solubilization unexpectedly improves the cycle stability, attributed to the mitigation of intermolecular side reactions and reduced crossover. Finally, the fundamental electrochemical reaction mechanism of micelle‐encapsulated TEMPO is discussed. This strategy offers a new insight regarding the solubility and stability of the catholyte/anolyte, and is expected to be applicable to other redox‐active molecules, opening up an unexplored micellar chemistry in redox flow batteries.