Electrochemical Reaction Mechanism Research Articles

Study on the electrochemical reaction mechanism of non-corrosive and long-life rechargeable aluminum battery

Study on the electrochemical reaction mechanism of non-corrosive and long-life rechargeable aluminum battery

Recent Advances of Flexible MXene and its Composites for Supercapacitors.

MXenes have unique properties such as high electrical conductivity, excellent mechanical properties, rich surface chemistry, and convenient processability. These characteristics make them ideal for producing flexible materials with tunable microstructures. This paper reviews the laboratory research progress of flexible MXene and its composite materials for supercapacitors. And introduces the general synthesis method of MXene, as well as the preparation and properties of flexible MXene. By analyzing the current research status, the electrochemical reaction mechanism of MXene was explained from the perspectives of electrolyte and surface terminating groups. This review particularly emphasizes the composite methods of freestanding flexible MXene composite materials. The review points out that the biggest problem with flexible MXene electrodes is severe self-stacking, which reduces the number of chemically active sites, weakens ion accessibility, and ultimately lowers electrochemical performance. Therefore, it is necessary to composite MXene with other electrode materials and design a good microstructure. This review affirms the enormous potential of flexible MXene and its composite materials in the field of supercapacitors. In addition, the challenges and possible improvements faced by MXene based materials in practical applications were also discussed.

Stable 2,5‐Dihydroxy‐1,4‐benzoquinone Based Organic Cathode Enabled by Coordination Polymer Formation and Binder Optimization

AbstractOrganic electrode materials are gaining increased attention in energy storage applications, but often encounter challenges such as low capacity and poor cycling stability. This paper explores the utilization of a 1‐D coordination polymer, copper(II)‐2,5‐dihydroxy‐1,4‐benzoquinone (Cu‐DHBQ), as a cathode material for Li‐ion batteries for the first time. Cu‐DHBQ is air‐stable, devoid of coordinated water molecules, and possesses a high theoretical capacity of 266 mAh g−1. With an optimal sodium alginate (SA) binder content of 25 wt%, the Cu‐DHBQ cathode demonstrates a notable initial capacity of 214 mAh g−1 and outstanding cycling stability, retaining 210 mAh g−1 (98% capacity retention) after 200 cycles at a current rate of 100 mA g−1. A comprehensive investigation into the capacity fading mechanisms, the bonding capability of the SA binder, and the interactions between Cu‐DHBQ and SA is conducted using scanning electron microscope (SEM), peel tests, and Fourier transform infrared spectroscopy (FTIR), respectively. Additionally, the electrochemical reaction mechanisms of Cu‐DHBQ are examined using ex situ X‐ray photoelectron spectroscopy (XPS) and FTIR. These findings offer valuable insights into understanding the electrochemical properties of coordination polymers and metal‐organic frameworks based on DHBQ, shedding light on their potential as materials for battery electrodes.

Electrochemical reaction mechanism of milled surface of NiTi shape memory alloy

NiTi shape memory alloy is widely used in medical field because of its excellent properties. Milling machining has become one of the main processing methods in the medical field due to its good shape adaptability. However, the change of the shape and properties of milled surface affects its biocompatibility and corrosion resistance. Electrochemical machining is used as the follow-up process of milling machining. The polarization curve method, cyclic voltammetry method and constant potential electrolysis are used in this paper to study the electrochemical reaction mechanism of the NiTi alloy milled surface. The results show that the plastic deformation layer appears compared with the original material, making electrode reactions and electron transfer relatively difficult. The surface eventually forms TiO2, without the formation of Ni-related oxide. The electrochemical reaction mechanism of the NiTi alloy milled surface in the HClO4-CH3COOH system provide important theoretical guidance for the production and manufacturing of medical products.

Advanced Polymers in Cathodes and Electrolytes for Lithium-Sulfur Batteries: Progress and Prospects.

Lithium-sulfur (Li-S) batteries, which store energy through reversible redox reactions with multiple electron transfers, are seen as one of the promising energy storage systems of the future due to their outstanding advantages. However, the shuttle effect, volume expansion, low conductivity of sulfur cathodes, and uncontrollable dendrite phenomenon of the lithium anodes have hindered the further application of Li-S batteries. In order to solve the problems and clarify the electrochemical reaction mechanism, various types of materials, such as metal compounds and carbon materials, are used in Li-S batteries. Polymers, as a class of inexpensive, lightweight, and electrochemically stable materials, enable the construction of low-cost, high-specific capacity Li-S batteries. Moreover, polymers can be multifunctionalized by obtaining rich structures through molecular design, allowing them to be applied not only in cathodes, but also in binders and solid-state electrolytes to optimize electrochemical performance from multiple perspectives. The most widely used areas related to polymer applications in Li-S batteries, including cathodes and electrolytes, are selected for a comprehensive overview, and the relevant mechanisms of polymer action in different components are discussed. Finally, the prospects for the practical application of polymers in Li-S batteries are presented in terms of advanced characterization and mechanistic analysis.

Niobium-Based Oxide for Anode Materials for Lithium-Ion Batteries.

Recently, it has become imperative to develop high energy density as well as high safety lithium-ion batteries (LIBS) to meet the growing energy demand. Among the anode materials used in LIBs, the currently used commercial graphite has low capacity and is a safety hazard due to the formation of lithium dendrites during the reaction. Among the transition metal oxide (TMO) anode materials, TMO based on the intercalation reaction mechanism has a more stable structure and is less prone to volume expansion than TMO based on the conversion reaction mechanism, especially the niobium-based oxide in it has attracted much attention. Niobium-based oxides have a high operating potential to inhibit the formation of lithium dendrites and lithium deposits to ensure safety, and have stable and fast lithium ion transport channels with excellent multiplicative performance. This review summarizes the recent developments of niobium-based oxides as anode materials for lithium-ion batteries, discusses the special structure and electrochemical reaction mechanism of the materials, the synthesis methods and morphology of nanostructures, deficiencies and improvement strategies, and looks into the future developments and challenges of niobium-based oxide anode materials.

Ultrasensitive Plasmon-Enhanced Infrared Spectroelectrochemistry.

IR spectroelectrochemistry (EC-IR) is a cutting-edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC-IR platforms due to the lack of high-enhancement substrates. Here, we propose an innovative plasmon-enhanced infrared spectroelectrochemistry (EC-PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6 ]3- /[Fe(CN)6 ]4- redox reaction and electrochemical CO2 reduction reaction (CO2 RR) are investigated on the EC-PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2 RR mechanism. This study presents a promising technique for boosting the in-depth understanding of interfacial events across diverse applications.

Ultrasensitive Plasmon‐Enhanced Infrared Spectroelectrochemistry

AbstractIR spectroelectrochemistry (EC‐IR) is a cutting‐edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC‐IR platforms due to the lack of high‐enhancement substrates. Here, we propose an innovative plasmon‐enhanced infrared spectroelectrochemistry (EC‐PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6]3−/[Fe(CN)6]4− redox reaction and electrochemical CO2 reduction reaction (CO2RR) are investigated on the EC‐PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2RR mechanism. This study presents a promising technique for boosting the in‐depth understanding of interfacial events across diverse applications.

Metal ion and organic ligand disubstituted bimetallic metal-organic framework nanosheets for high-performance alkaline zinc-based batteries.

Herein, a disubstitution strategy of metal ions and organic ligands is employed to synthesize metal-organic framework (MOF) nanosheets with a three-dimensional (3D) porous structure and a highly active metal-sulfur (M-S, M = Co and Ni) region. The obtained MOFs//Zn batteries exhibit excellent electrochemical properties and the electrochemical reaction mechanism was elucidated through a series of ex situ characterizations.

The CO2 electrolysing mechanism in single-phase mixed-conducting cathode of solid oxide cell.

In the field of solid oxide cells (SOC), unveiling the electrochemical reaction and transfer mechanisms in mixed ionic and electronic conducting (MIEC) electrodes is of great importance. Due to the chemical capacitance effects of MIEC materials, SOC often shows large capacitance current during electrochemical tests, which might interfere with the polarization behaviors. This work presents a numerical multiphysical model based on the transport of oxygen species, which accurately and concisely replicates the current-voltage curves of a solid oxide electrolysis cell (SOEC) with MIEC electrodes under various scanning rates. The scanning IV and electrochemical impedance spectra measurement under different SOEC working conditions are combined to enable the separation of Faradic and charging currents. Thus, both the bulk diffusion and surface gaseous diffusion of the oxygen species are encompassed, which explains how the current being generated due to intertwined chemical capacitance effects and chemical reactions in the MIEC electrodes.

Surface charge-induced electrospray for high-throughput analysis of complex samples and electrochemical reaction intermediates using mass spectrometry.

Electrospray-related ion sources are promising for direct mass spectrometric analysis of complex samples, but current protocols suffer from complicated components and low analytical sensitivity. Here, we propose a surface charge-induced electrospray ionization (SCIESI) inspired by flashover on an insulator surface under high voltage. This protocol not only effectively avoids contact between the sample solution and metal electrode, but also allows completion of the entire analytical process in less than 40 seconds and limits of detection in the pictogram per milliliter range. SCIESI coupled to mass spectrometry can also be used to monitor electro-chemical processes, and a number of oxidation and reduction reactions have been studied, demonstrating that it is a powerful tool for understanding electrochemical reaction mechanisms.

Bioinspired graphene-based metal oxide nanocomposites for photocatalytic and electrochemical performances: an updated review.

Considering the rapidly increasing population, the development of new resources, skills, and devices that can provide safe potable water and clean energy remains one of the vital research topics for the scientific community. Owing to this, scientific community discovered such material for tackle this issue of environment benign, the new materials with graphene functionalized derivatives show significant advantages for application in multifunctional catalysis and energy storage systems. Herein, we highlight the recent methods reported for the preparation of graphene-based materials by focusing on the following aspects: (i) transformation of graphite/graphite oxide into graphene/graphene oxide via exfoliation and reduction; (ii) bioinspired fabrication or modification of graphene with various metal oxides and its applications in photocatalysis and storage systems. The kinetics of photocatalysis and the effects of different parameters (such as photocatalyst dose and charge-carrier scavengers) for the optimization of the degradation efficiency of organic dyes, phenol compounds, antibiotics, and pharmaceutical drugs are discussed. Further, we present a brief introduction on different graphene-based metal oxides and a systematic survey of the recently published research literature on electrode materials for lithium-ion batteries (LIBs), supercapacitors, and fuel cells. Subsequently, the power density, stability, pseudocapacitance charge/discharge process, capacity and electrochemical reaction mechanisms of intercalation, and conversion- and alloying-type anode materials are summarized in detail. Furthermore, we thoroughly distinguish the intrinsic differences among underpotential deposition, intercalation, and conventional pseudocapacitance of electrode materials. This review offers a meaningful reference for the construction and fabrication of graphene-based metal oxides as effective photocatalysts for photodegradation study and high-performance optimization of anode materials for LIBs, supercapacitors, and fuel cells.

Study of Suregada Multiflora Leaves Extract as a Green Corrosion Inhibitor on Iron in HCl Medium

Suregada multiflora leaves extract (SMLE) was prepared and used as a new eco-friendly green inhibitor for acid induced corrosion of mild steel(MS) in 1M HCl solution. The aqueous extract of leaves of Suregada multiflora was tested by dipping MS coupons in 1M HCl with and without SMLE solution at 303 k, 313 k, 333k and 343k using gravimetric method. The data obtained by weight loss measurement revealed that the SMLE has good inhibiton effect and mitigates the rate of corrosion. The inhibition efficiency (IE) increases with an increase in inhibitor concentration and with the exposure time. The adsorption study showed that the use of SMLE obeyed the Langmuir isotherm with regression co-efficient value nearly equal to unity. A part from this, Flory-Huggins isotherm and Langmuir-Freundlich isotherm were also used to study the interaction between the inhibitors and the mild steel surface, and mechanism of electrochemical reaction. At last the free energy of adsorption and activation energy were calculated and discussed.

Exploring in electrochemical behaviors and anions storage characteristics of carbonaceous materials in molten salts Ni/NiCl2-graphite battery

Carbonaceous materials that possess reversible anions storage capacity and can worked as the cathode for Ni/NiCl2-Graphite battery. However, differences in storage capacity of carbonaceous cathodes and factors affecting anions storage capacity have not been thoroughly investigated. Here, we systematically evaluated the electrochemical properties and intercalation reaction mechanism of carbonaceous materials cathodes firstly. Expanded graphite and graphite paper cathodes display the highest specific capacity of 72 mAh/g and diffusion-controlled capacity ratios (∼98 %). X-ray powder diffraction and Raman spectroscopy analysis on the graphite intermetallic compounds confirmed the presence of stage-5 intercalation/de-intercalation reactions. Density functional theory calculation and ionic diffusion model analysis showed that the intercalation and diffusion process of AlCl4- in graphite interlayers play a crucial role in anions storage reaction. Rich-in-edged outer graphite-layers are more conducive to form intercalation reaction active sites and store anions effectively. The expanded graphite electrode exhibits the highest intercalation reaction ratio (φR-GP = 1.40), confirming sufficient intercalation reaction active sites in each unit mass. And the multiple wrinkles and curled graphite sheets in expanded graphite provide adequate ionic transport/diffusion channels during electrochemical intercalation/de-intercalation reactions. Our findings provide significant support for understanding anions storage behaviors and designing cathodes with high anions storage capacity in the Ni/NiCl2-Graphite battery.

Heterointerface-Induced Fast Na+ Transport of Sn Anode Using a Yolk-Shell Structure for Fast-Charging and Stable Sodium-Ion Batteries

Sodium-ion battery (SIB) is a promising next-generation secondary battery system that uses inexpensive sodium resources, which can lower battery production costs. Moreover, based on similar chemical and electrochemical reaction mechanisms as conventional lithium-ion battery (LIB), it is expected that technology development can be rapidly achieved by utilizing existing battery manufacturing processes. However, because of the larger ionic radius (1.02 Å) and weight of sodium compared to those of lithium (0.76 Å), SIBs inevitably have lower energy and power densities compared to LIB. Therefore, to address these challenges, developing suitable host material with superior electrochemical performance (high capacity/high power density/long cyclability) is urgently required.In this regard, metallic Sn has attracted attention as a promising anode material with a high theoretical capacity of 870 mAh/g through the reversible electrochemical (de)alloying reaction with sodium, which could overcome the low energy density limit of hard carbon anode. Nevertheless, due to the interfacial instability of Sn particles upon cycling and the slow alloying reaction rate during the sodiation process, the Sn anode exhibits poor rate performance and insufficient long-term cycling stability. Additionally, the structural degradation caused by significant volume variations further limits its practical implementation in SIBs.Here, to solve the above issues, we propose a hierarchically designed composite anode consisting of Sn yolk and multifunctional C/SiOC shell (denoted as Sn@C/SiOC) via a simple pyrolysis method using the suspension of the Sn precursor in silicone oil. We conducted various physical, chemical, and electrochemical characterization, and we further carried out an in-depth investigation including post-mortem and in-situ EIS analyses to reveal the heterointerface-induced improved sodium-ion storage performance. We expect that the heterointerface modification strategy will contribute to the realization of advanced electrode materials in SIBs and other next-generation energy storage devices.

Electrochemical Analysis on Transition Metal Nitride Catalysts for Nitrogen Reduction Reaction

A versatile and sustainable process for ammonia production is needed to ensure a sustainable future for the growing global population, both with regards to fertilizer production as well as energy carrier. Theoretical simulations of transition metal nitride catalysts have demonstrated the possibilityof a nitrogen reduction reaction catalysis cycle on four transition metal nitrides, CrN, VN, ZrN and NbN.[1,2] Reproducible and reliable research and development approaches in the direction of electrochemical reduction of nitrogen towards ammonia are therefore of importance. An operando ammonia quantification methodology for electrochemical catalysis studies has been developed in-house and recently published [3]. Allowing for automatic, quantitative detection of aqueouos ammonia at the ppb level, within minutes from the sample preparation. We have now analyzed all four theoretically proposed TMNs, as well as rutile Niobium oxynitride (NbON) of several stochiometries as thin films on a conductive electrode substrate in ambient environment and aqueouos electrochemical system [3,4].Electrochemical analysis and ammonia quantification suggest NbON of specific stochiometry and ZrN to be promising catalyst candidates, while lacking stability in the environment of choice in these studies.Literature:[1] Y. Abghoui, A.L. Garden, J.G. Howalt, T. Vegge and E. Skúlason, Electroreduction of N2 to ammonia at ambient conditions on mononitrides of Zr, Nb, Cr, and V: A DFT guide for experiments. ACS Catalysis, 2016.6(2): p. 635.[2] Y. Abghoui, A.L. Garden, V.F. Hlynsson, S. Björgvinsdóttir, H. Ólafsdóttir and E.Skúlason, Enabling electrochemical reduction of nitrogen to ammonia at ambient conditions through rational catalyst design. Physical Chemistry Chemical Physics,2015.17(7): p. 4909.[3] F. Hanifpour, C.P. Canales, E.G. Fridriksson, A. Sveinbjörnsson, T.K. Tryggvason,E. Lewin, . H.D. Flosadóttir, Investigation into the mechanism of electrochemical nitrogen reduction reaction to ammonia using niobium oxynitride thin-film catalysts. Electrochimica Acta, 2022.403: p. 139551.[4] F. Hanifpour, C.P. Canales, E.G. Friðriksson, A. Sveinbjörnsson, T. Tryggvason, Y.Abghoui, . E. Skúlason, Operando Quanitification of ammonia produced from computationally derived transition metal nitride electro catalysts. Journal of Catalysis, 2022. 413: p.956

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

2D Metal Covalent Organic Frameworks Towards Electrochemical Catalysis

In the past century, innovative electrochemical reactions with low-consumption and environmental-friend have been a key substance for worldwide chemical industry.[1] Nowadays, the majority of the electrochemical reactions rely on low-cost and high-efficient catalysts, and there is an ever-growing need for the development of advanced catalysts to secure a sustainable future for our society.[2] The pursuit for sustainable energy stimulates the innovative concepts of advanced catalytic materials for electrochemical reactions, such as metal-air battery, nitrogen fixation and carbon neutrality etc.[3] The electrocatalysts play a vital role to promote the renewable energy conversion efficiencies and system performance.[4] However, the electrocatalytic activities of most inorganic catalysts arises from the unsaturated metal sites exposed on the surfaces or edges, and the internal metal atoms in bulk phase are totally inert, resulting in the low utilization and efficiency of metal atoms for electrocatalysis.[5] Exploring novel electrocatalysts with high metal utilization is essential for electrochemical reactions. Recently, single-atom catalysts (SACs) strategy has been proposed to promote the efficiency and utilization of metal atoms in electrocatalysts. In particular, SACs could provide an ideal approach to maximize the efficiency of active metal atom utilization (in principle, up to 100%) and lower the cost of the electrochemical reactions.[6] Inspired by SACs strategy, metal covalent-organic-frameworks (MCOFs) can be emerged as efficient catalysts, due to the numerous single-atom sites, ordered in-plane porous structure and outstanding stable framework. We propose a novel two-dimensional MCOFs, constructed by porphyrin subgroup (Figure 1), for achieving high efficiency of electrochemical reactions. Porphyrinsubgroup is a type of heterocyclic molecules with conjugated structure, which has strong ability to coordinate almost all transition metal ions to form high active single-metal sites. Thus, MCOFs with porphyrin could be a promising next-generation electrocatalyst, and an ideal catalytic model to reveal the mechanism of electrochemical reactions. Acknowledgments: The authors gratefully acknowledge the financial support from H2020 Marie Skłodowska-Curie Actions (101024758) and National Natural Science Foundation of China (22002083).

In Situ Real-Time Simultaneous NMR Analyses of Anode and Cathode Exhausts of Direct Ethanol and Methanol Fuel Cells

Understanding the electrochemical reaction mechanisms of energy conversion and storage systems is essential for improving their performance. Nuclear magnetic resonance (NMR) spectroscopy is excellent for probing electrochemical reactions due to its capability to provide quantitative and qualitative information. For the in situ simultaneous acquisition of the anode and cathode exhaust spectra of direct alcohol fuel cells such as a direct methanol fuel cell (DMFC) or direct ethanol fuel cell (DEFC), real-time flow-NMR spectroscopy using a toroid cavity detector was developed in our laboratory. In the cathode exhaust of DMFC, the CD3OH crossed over from the anode as well as HOD was observed. Ex situ NMR spectroscopy cannot detect gas products because they are lost during sample preparation. On the other hand, the amount of CO2 gas detected in the anode exhaust using our in situ detection method was proportional to the HOD amount. It was also proportional to the cell current generated. This explains differences in the fuel cell performance by identifying generated and consumed chemicals and their pathways in the cells at respective conditions. Analyses of DEFC exhausts showed various reaction products such as acetic acid and acetaldehyde but CO2 gas detected was negligible. Our results show that this in situ real-time analysis could identify and quantify the exhaust components, including the gaseous products. Therefore, our results demonstrate that this in situ real-time analysis is appropriate to study the reaction mechanisms of diverse other liquid-flowing electrochemical systems in addition to fuel cells. Furthermore, it may be applicable to designing advanced materials for the systems.

Surface reconstruction of pyrite-type transition metal sulfides during oxygen evolution reaction

Reconstruction universally occurs over non-layered transition metal sulfides (TMSs) during oxygen evolution reaction (OER), leading to the formation of active species metal (oxy)hydroxide and thus significantly influences the OER performance. However, the reconstruction process and underlying mechanism quantitatively remain largely unexplored. Herein, we proposed an electrochemical reaction mechanism, namely sulfide oxidation reaction (SOR), to elucidate the reconstruction process of pyrite-type TMSs. Based on this mechanism, we evaluated the reconstruction capability of NiS2 doped with transition metals V, Cr, Mn, Fe, Co, Cu, Mo, Ru, Rh, and Ir within different doped systems. Two key descriptors were thus proposed to describe the reconstruction abilities of TMSs: USOR (the theoretical electric potential of SOR) and ΔU (the difference between the theoretical electric potential of SOR and OER), representing the initiation electric potential of reconstruction and the intrinsic reconstruction abilities of TMSs, respectively. Our finding shows that a lower USOR readily initiate reconstruction at a lower potential and a larger ΔU indicating a poorer reconstruction ability of the catalyst during OER. Furthermore, Fe-doped CoS2 was used to validate the rationality of our proposed descriptors, being consistent with the experiment findings. Our work provides a new perspective on understanding the reconstruction mechanism and quantifying the reconstruction of TMSs.