Thermodynamic Limit Research Articles

Effects of MgF2 anti-reflection coating on optical losses in metal halide perovskite solar cells

The importance of light management for perovskite solar cells (PSCs) has recently been emphasized because their power conversion efficiency approaches their theoretical thermodynamic limits. Among optical strategies, anti-reflection (AR) coating is the most widely used method to reduce reflectance loss and thus increase light-harvesting efficiency. Monolayer MgF2 is a well-known AR material because of its optimal refractive index, simple fabrication process, and physical and chemical durabilities. Nevertheless, quantitative estimates of the improvement achieved by the MgF2 AR layer are lacking. In this study, we conducted theoretical and experimental evaluations to assess the AR effect of MgF2 on the performance of formamidinium lead-triiodide PSCs. A sinusoidal tendency to enhance the short-circuit current density (J SC) was observed depending on the thickness, which was attributed to the interference of the incident light. A transfer matrix method-based simulation was conducted to calculate the optical losses, demonstrating the critical impact of reflectance loss on the J SC improvement. The predicted J SCs values, depending on the perovskite thickness and the incident angle, are also presented. The combined use of experimental and theoretical approaches offers notable advantages, including accurate interpretation of photocurrent generation, detailed optical analysis of the experimental results, and device performance predictions under unexplored conditions.

(De)carboxylation mechanisms of heteroaromatic substrates catalyzed by prenylated FMN-dependent UbiD decarboxylases: An in-silico study

The UbiD enzymes are proposed to catalyze reversible (de)carboxylation reaction of unsaturated carboxylic acids using prenylated flavin mononucleotide (prFMN) as a cofactor. This positions UbiD enzymes as promising candidates for converting CO2 into valuable chemicals. However, their industrial-scale biotransformation is currently constrained by low conversion rates attributed to thermodynamic limitations. To enhance the carboxylation activity of UbiD enzymes, a molecular-level understanding of the (de)carboxylation mechanisms is necessary. In this study, we investigated the reaction mechanisms of heteroaromatic substrates catalyzed by PtHmfF, PaHudA, and AnlnD enzymes using molecular dynamics (MD) simulations and free energy calculations. Our extensive mechanistic study elucidates the mechanisms involved in the formation of the initial prFMN-substrate intermediate. Specifically, we observed nucleophilic attack during decarboxylation, while carboxylation reactions involving furoic acid, pyrrole, and indole tend to favor a 1,3-dipolar cycloaddition mechanism. Furthermore, we identified proton transfer as the rate-limiting step in the carboxylation reaction. In addition, we considered the perspectives of reaction energies and electron transfer to understand the distinct mechanisms underlying decarboxylation and carboxylation. Our calculated free energies are consistent with available experimental kinetics data. Finally, we explored how different rotamers of catalytic residues influence the efficiency of the initial intermediate formation.

On De Gennes narrowing of fluids confined at the molecular scale in nanoporous materials.

Beyond well-documented confinement and surface effects arising from the large internal surface and severely confining porosity of nanoporous hosts, the transport of nanoconfined fluids remains puzzling in many aspects. With striking examples such as memory, i.e., non-viscous effects, intermittent dynamics, and surface barriers, the dynamics of fluids in nanoconfinement challenge classical formalisms (e.g., random walk, viscous/advective transport)-especially for molecular pore sizes. In this context, while molecular frameworks such as intermittent Brownian motion, free volume theory, and surface diffusion are available to describe the self-diffusion of a molecularly confined fluid, a microscopic theory for collective diffusion (i.e., permeability), which characterizes the flow induced by a thermodynamic gradient, is lacking. Here, to fill this knowledge gap, we invoke the concept of "De Gennes narrowing," which relates the wavevector-dependent collective diffusivity D0(q) to the fluid structure factor S(q). First, using molecular simulation for a simple yet representative fluid confined in a prototypical solid (zeolite), we unravel an essential coupling between the wavevector-dependent collective diffusivity and the structural ordering imposed on the fluid by the crystalline nanoporous host. Second, despite this complex interplay with marked Bragg peaks in the fluid structure, the fluid collective dynamics is shown to be accurately described through De Gennes narrowing. Moreover, in contrast to the bulk fluid, the departure from De Gennes narrowing for the confined fluid in the macroscopic limit remains small as the fluid/solid interactions in severe confinement screen collective effects and, hence, weaken the wavevector dependence of collective transport.

Dense random packing of disks with a power-law size distribution in thermodynamic limit.

The correlation properties of a random system of densely packed disks, obeying a power-law size distribution, are analyzed in reciprocal space in the thermodynamic limit. This limit assumes that the total number of disks increases infinitely, while the mean density of the disk centers and the range of the size distribution are kept constant. We investigate the structure factor dependence on momentum transfer across various number of disks and extrapolate these findings to the thermodynamic limit. The fractal power-law decay of the structure factor is recovered in reciprocal space within the fractal range, which corresponds to the range of the size distribution in real space. The fractal exponent coincides with the exponent of the power-law size distribution as was shown previously by the authors of the work of Cherny et al. [J. Chem. Phys. 158(4), 044114 (2023)]. The dependence of the structure factor on density is examined. As is found, the power-law exponent remains unchanged but the fractal range shrinks when the packing fraction decreases. Additionally, the finite-size effects are studied at extremely low momenta of the order of the inverse system size. We show that the structure factor is parabolic in this region and calculate the prefactor analytically. The obtained results reveal fractal-like properties of the packing and can be used to analyze small-angle scattering from such systems.

Thermodynamic limit of spin systems on random graphs

We utilize the graphon—a continuous mathematical object which represents the limit of convergent sequences of dense graphs—to formulate a general, continuous description of quantum spin systems in thermal equilibrium when the average coordination number grows extensively in the system size. Specifically, we derive a closed set of coupled nonlinear Fredholm integral equations which governs the properties of the system. The graphon forms the kernel of these equations, and their solution yields exact expressions for the macroscopic observables in the system in the thermodynamic limit. We analyze these equations for both quantum and classical spin systems, recovering known results and providing analytical solutions for a range of more complex cases. We supplement this with controlled, finite-size numerical calculations using Monte Carlo and tensor network methods, showing their convergence towards our analytical results with increasing system size. Published by the American Physical Society 2024

System Size Dependence of the Diffusion Coefficients in MD Simulations: A Simple Correction Formula for Pure Dense Fluids.

A practical correction formula relating the self-diffusion coefficient of dense liquids from molecular dynamics (MD) simulations with periodic boundary conditions to the self-diffusion coefficient in the thermodynamic limit is discussed. This formula applies to pure dense fluids and has a very simple form D = D0(1 - γN-1/3), where D0 is the self-diffusion coefficient in the thermodynamic limit and N is the number of particles in the simulation. The numerical factor γ is dependent on the geometry of the simulation cell. Remarkably, γ ≃ 1.0 is the most popular cubic geometry. The success of this formula is supported by results from MD simulations, including very recent simulations with a "magic" simulation geometry.

Process optimization of osmotic membrane distillation for the extraction of valuable resources from water streams

The rising demand for sustainable wastewater management and high-value resource recovery is pressing industries involved in, e.g., textiles, metals, and food production, to adopt energy-efficient and flexible liquid separation methods. The current techniques often fall short in achieving zero liquid discharge and enhancing socio-economic growth sustainably. Osmotic membrane distillation (OMD) has emerged as a low-temperature separation process designed to concentrate valuable elements and substances in dilute feed streams. The efficacy of OMD hinges on the solvent’s migration from the feed to the draw stream through a hydrophobic membrane, driven by the vapor pressure difference induced by both temperature and concentration gradients. However, the intricate interplay of heat and mass processes steering this mechanism is not yet fully comprehended or accurately modeled. In this research, we conducted a combined theoretical and experimental study to explore the capabilities and thermodynamic limitations of OMD. Under diverse operating conditions, the experimental campaign aimed to corroborate our theoretical assertions. We derived a novel equation to govern water flux based on foundational principles and introduced a streamlined version for more straightforward application. Our findings spotlight complex transport-limiting and self-adjusting mechanisms linked with temperature and concentration polarization phenomena. Compared with traditional methods like membrane distillation and osmotic dilution, which are driven by solely temperature or concentration gradients, OMD may provide improved and flexible performance in target applications. For instance, we show that OMD—if properly optimized—can achieve water vapor fluxes 50% higher than osmotic dilution. Notably, OMD operation at reduced feed temperatures can lead to energy savings ranging between 5 and 95%, owing to the use of highly concentrated draw solutions. This study underscores the potential of OMD in real-world applications, such as concentrating lithium in wastewater streams. By enhancing our fundamental understanding of OMD’s potential and constraints, we aim to broaden its adoption as a pivotal liquid separation tool, with focus on sustainable resource recovery.

Gaussian processes for finite size extrapolation of many-body simulations.

Key to being able to accurately model the properties of realistic materials is being able to predict their properties in the thermodynamic limit. Nevertheless, because most many-body electronic structure methods scale as a high-order polynomial, or even exponentially, with system size, directly simulating large systems in their thermodynamic limit rapidly becomes computationally intractable. As a result, researchers typically estimate the properties of large systems that approach the thermodynamic limit by extrapolating the properties of smaller, computationally-accessible systems based on relatively simple scaling expressions. In this work, we employ Gaussian processes to more accurately and efficiently extrapolate many-body simulations to their thermodynamic limit. We train our Gaussian processes on Smooth Overlap of Atomic Positions (SOAP) descriptors to extrapolate the energies of one-dimensional hydrogen chains obtained using two high-accuracy many-body methods: coupled cluster theory and Auxiliary Field Quantum Monte Carlo (AFQMC). In so doing, we show that Gaussian processes trained on relatively short 10-30-atom chains can predict the energies of both homogeneous and inhomogeneous hydrogen chains in their thermodynamic limit with sub-milliHartree accuracy. Unlike standard scaling expressions, our GPR-based approach is highly generalizable given representative training data and is not dependent on systems' geometries or dimensionality. This work highlights the potential for machine learning to correct for the finite size effects that routinely complicate the interpretation of finite size many-body simulations.

Gibbs equilibrium conditions and the thermodynamic limit

In the present work we critically examine a structural model of a heterophase body, whose equilibrium is described by the equations of Gibbs equilibrium conditions. We demonstrate that these equilibrium conditions describe the behaviour of the heterophase body in a thermodynamic limit. Additionally, these equations can be viewed as the phase transition conditions from any homophase system that make up the heterophase body. In light of the proposed interpretation of Gibbs equilibrium conditions, we offer a new way to define phase transition temperature.

The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges.

Green-hydrogen is considered a "key player" in the energy market for the upcoming decades. Among currently available hydrogen (H2) production processes, photoelectrochemical (PEC) water splitting has one of the lowest environmental impacts. However, it still presents prohibitively high production costs compared to more mature technologies, such as steam methane reforming. Therefore, the competitiveness of PEC water splitting must rely on its environmental and functional advantages, which are strongly linked to the reactor design, to the intrinsic properties of its components, and to their successful upscaling. This review gives special attention to the engineering aspects and categorizes PEC devices into four main types, according to the configuration of electrodes and strategies for gas separation: wired back-to-back, wireless back-to-back, wired side-by-side, and wired separated electrode membrane-free. Independently of the device architecture, the use of concentrated sunlight was found to be mandatory for achieving competitive green-H2 production. Additionally, feasible strategies for upscaling the key components of PEC devices, especially photoelectrodes, are urgently needed. In a pragmatic context, the way to move forward is to accept that PEC devices will operate close to their thermodynamic limits at large-scale, which requires a solid convergence between academics and industry. Research efforts must be redirected to: (i) build and demonstrate modular devices with a low-cost and highly recyclable embodiment; (ii) optimize thermal and power management; (iii) reduce ohmic losses; (iv) enhance the chemical stability towards a thousand hours; (v) couple solar concentrators with PEC devices; (vi) boost PEC-H2 production through the use of organic compounds; and (vii) reach consensual standardized methods for evaluating PEC devices, at both environmental and techno-economic levels. If these targets are not met in the next few years, the feasibility of PEC-H2 production and its acceptance by industry and by the general public will be seriously compromised.

The Sharp Interface Limit of an Ising Game

The Ising model of statistical physics has served as a keystone example of phase transitions, thermodynamic limits, scaling laws, and many other phenomena and mathematical methods. We introduce and explore an Ising game, a variant of the Ising model that features competing agents influencing the behavior of the spins. With long-range interactions, we consider a mean-field limit resulting in a nonlocal potential game at the mesoscopic scale. This game exhibits a phase transition and multiple constant Nash-equilibria in the supercritical regime. Our analysis focuses on a sharp interface limit for which potential minimizing solutions to the Ising game concentrate on two of the constant Nash-equilibria. We show that the mesoscopic problem can be recast as a mixed local/nonlocal space-time Allen-Cahn type minimization problem. We prove, using a Γ-convergence argument, that the limiting interface minimizes a space-time anisotropic perimeter type energy functional. This macroscopic scale problem could also be viewed as a problem of optimal control of interface motion. Sharp interface limits of Allen-Cahn type functionals have been well studied. We build on that literature with new techniques to handle a mixture of local derivative terms and nonlocal interactions. The boundary conditions imposed by the game theoretic considerations also appear as novel terms and require special treatment.

Thermodynamic limits of the depolymerization of poly(olefin)s using mechanochemistry.

Mechanochemistry is a promising approach for chemical recycling of commodity plastics, and in some cases depolymerization to the monomer(s) has been reported. However, while poly(olefin)s comprise the largest share of global commodity plastics, mechanochemical depolymerization of these polymers in standard laboratory-scale ball mill reactors suffers from slow rates. In this work, the observed reactivities of poly(styrene), poly(ethylene) and poly(propylene) are rationalized on the basis of thermodynamic limitations of their depolymerization by depropagation of free radical intermediates. In addition, subsequent phase partitioning equilibria for the removal of monomers from the reactor via a purge gas stream are discussed for these polymers. For poly(styrene), a typical vibratory ball mill supplies just enough energy for its depolymerization to be driven by either thermal hotspots or adiabatic compression of the impact site, but the same energy supply is far from sufficient for poly(propylene) and poly(ethylene). Meanwhile, removal of styrene from the reactor is thermodynamically hindered by its lower volatility, but this is not an issue for either propylene or ethylene. The implications of these thermodynamic limitations for mechanochemical reactor design and potential for mechanocatalytic processes are highlighted.

Numerical studies on the propagation of iron dust flames in confinement

Iron powder is a promising alternative fuel owing to its high energy density, non-volatile combustion, and recyclability using green hydrogen. For the safe storage, transportation, and use of iron powder as a reactive energy source, the physical mechanisms behind pressurising dust flames in explosions must be understood. Here, we study the propagation of one-dimensional iron dust flames in confinement using an Euler–Lagrange framework with reflective adiabatic boundary conditions. The gas phase is described by the compressible Navier–Stokes equations, and heterogeneously burning iron particles follow the parabolic rate law for solid-phase oxidation, and diffusion-limited combustion during liquid-phase combustion. We demonstrate that the pressurisation in a closed vessel leads to an increase in unburnt gas density, decelerating flame propagation via a decrease in thermal diffusivity. Flames through dust suspensions at concentrations above the thermodynamic limit are quenched, while for concentrations near the quenching limit, flames are quenched before re-ignition. The trajectories of particles and gas parcels demonstrate that particles are strongly entrained in the gas streams, causing fluctuations in local dust concentrations. The transient evolution of flame speeds in confinement is used to inform a pressure-rise model, which predicts the time to peak pressure in a 20-L vessel with good agreement to experimental measurements. Our insights provide mechanistic understandings about dust flame propagation under pressurising conditions, particularly relevant to the informed design of explosion protection measures.

Widening of the fundamental gap in cluster GW for metal-molecular interfaces.

The GW approximation is very promising for an accurate first-principles description of charged excitations in single-molecule-metal interfaces. In the cluster approach for electronic transport across molecules, the infinite metal (with an adsorbed molecule) is replaced by a finite cluster whose volume should be incrementally increased to test the approach to the thermodynamic limit. Here we show that in GW, the approach to the thermodynamic limit will be much slower than in Kohn-Sham density-functional theory (DFT) because of the Coulomb interaction. To demonstrate this statement, we investigate spectral gaps in an ensemble of disordered sodium clusters in Kohn-Sham DFT, quasiparticle eigenvalue-self-consistent GW and Hartree-Fock. The fundamental gaps (i.e. difference between the lowest unoccupied and highest occupied level) in GW scale as N-1/3 on average, where N is the number of atoms. We demonstrate that this slow decrease artificially depletes the density of states at the Fermi level when the cluster is used to simulate a semi-infinite electrode. Therefore, the GW method cannot be taken as an out-of-the-box improvement of the DFT in cluster geometries, unless careful convergence checks are performed.

The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern.

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.

Measurement-based quantum computation in finite one-dimensional systems: string order implies computational power

We present a new framework for assessing the power of measurement-based quantum computation (MBQC) on short-range entangled symmetric resource states, in spatial dimension one. It requires fewer assumptions than previously known. The formalism can handle finitely extended systems (as opposed to the thermodynamic limit), and does not require translation-invariance. Further, we strengthen the connection between MBQC computational power and string order. Namely, we establish that whenever a suitable set of string order parameters is non-zero, a corresponding set of unitary gates can be realized with fidelity arbitrarily close to unity.

Revealing the Rate-Limiting Electrode of Lithium Batteries at High Rates and Mass Loadings

Lithium-ion batteries with superior capacities and rate performance are needed due to the soaring demands for even higher energy and power device requirements. [1-2] However, the main hurdle on achieving this predominately results from poor rate performance of the electrode, related to thermodynamic limitations and slow kinetics. To determine the rate-limiting electrochemical processes in a NMC622 vs graphite cell, a methodology based upon the galvanic intermittent titration technique, for investigating the diffusion coefficient and rate kinetics from the observed overpotential at each electrode has been developed. Variable current densities have been used to simultaneously extract the thermodynamic and kinetic properties of each electrode with increasing mass loading. Graphite is observed to reach its thermodynamic limits quicker than NMC, due to the flat plateaus and overpotentials observed from the charge transfer kinetics and mass transport. At high rates and high mass loadings, the graphite electrode is responsible for limiting both lithium-ion diffusion and reaction rates in the full cell. Slow diffusion kinetics are caused by the transport of the electrolyte in the porous electrode, which limits the availability of lithium for reaction at the surface of the graphite. This current interrupt testing methodology is proposed as a fast single technique for comprehensively parameterizing the rate limitations observed in a full cell configuration [3].

Interface Engineering for Highly Efficient Perovskite Photovoltaics

In the past decade, major progress has taken place in the field of metal halide perovskite photovoltaics (PV), as witnessed by the recently achieved conversion efficiency of 25.7% [1] and the concrete opportunity to go beyond the thermodynamic limit of single junction PV by means of perovskite/ crystalline silicon tandems, with the very recent record of 33.2% [2]. Among the most recent efforts towards further development of perovskite PV and subsequent commercialization, strategies ranging from perovskite surface passivation and compositional engineering to thin film encapsulation, contribute to suppress the perovskite absorber and device instability to moisture ingress. In parallel, research efforts presently investigate the role of inorganic charge transport layers (CTLs) next to the one of the most commonly used organic CTLs, such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or C60. The inorganic layers are acknowledged to impart thermal and mechanical stability to the device, as well as to improve device efficiency yield.In this contribution, our recent research on atomic layer deposited (ALD) selective CTLs for metal halide perovskite PV [3] is reviewed by addressing two case studies, namely, SnO2 thin films, selective towards electron transport, and NiO thin films, selective towards hole transport. ALD is selected as deposition technology because of its merit of ultimate control over film thickness and conformality.ALD SnO2 [4]. based on cycles of tetrakis(dimethylamido)-tin and H2O as precursor and co-reactant, respectively, is the state-of-the art in both p-i-n perovskite single junction as well as tandem perovskite/crystalline silicon and perovskite/CIGS PV. Its presence is key to the device thermal and mechanical stability, to prevent humidity ingress in the device and suppress damage to the fullerene (PCBM or C60) during the sputtering of the transparent front contact. Moreover, SnO2 plays a key role in perovskite/perovskite tandem PV, where it serves as solvent barrier to prevent damage of the wide-band gap perovskite sub-cell during solution processing of the narrow-band gap perovskite top cell. In parallel, our in situ studies by means of IR spectroscopy during ALD of SnO2 on perovskite/fullerene disclose the reason behind the difference in device conversion efficiency when SnO2 is processed either on PCBM or C60, i.e. the devices based on PCBM underperform those based on C60 by 3% absolute difference. Specifically, the Sn-precursor is responsible for the modification of the ester group in PCBM thereby affecting the PCBM/perovskite interface and device efficiency.ALD NiO thin (7 nm) films [5], based on cycles of bis(methylcyclopentadienyl)nickel and H2O as precursor and co-reactant, respectively, are found to impart stability to the perovskite device under acceleration test at 85°C, with 80% retention of the initial conversion efficiency after 300 hours at 1 sun illumination. At the same time, the presence of the phosphonic acid-based self-assembled monolayer (SAM), is essential to engineer an almost lossless (i.e., in terms of suppression of charge recombination) NiO/SAM/perovskite interface. Also, the SAM homogeneity and surface coverage on NiO improve with respect to direct SAM processing on ITO (see figure), as witnessed by transmission electron microscopy (TEM) and conductive atomic force microscopy. This result is explained in terms of chemisorption reactions between SAM phosphonic acid groups and NiO hydroxyl groups. The SAM homogeneity on NiO leads to higher shunt resistance in the device with respect to the one with SAM directly processed on ITO. Finally, the combination of NiO and SAM results in a narrower distribution of device performance reaching more than 20% efficient champion device. In parallel, two-terminal perovskite/crystalline silicon tandem devices with an ITO/NiO/SAM tunnel recombination junction (TRJ) exhibit a better device yield with respect to tandem devices with an ITO/SAM TRJ, i.e. the standard deviation decreases from 4.6% with ITO/SAM to 2.0% with ITO/NiO/SAM.[1] Green et al., Prog. Photovolt. Res. Appl. 30, 687 (2022)[2] https://www.kaust.edu.sa/news/kaust-team-sets-world-record-for-tandem-solar-cell-efficiency[3] Zardetto et al., Sustainable Energy & Fuels 1, 30 (2017)[4] Bracesco et al., J. Vac. Sci. Technol. A, 38(6), 063206-1 (2020)[5] Phung et al., ACS Applied Materials and Interfaces. 14(1), 2166 (2022)[6] Phung et al., submitted to Sol. Mat. (2023) Figure 1

Effect of Parasitic Gas Evolution Reactions on the All-Vanadium Redox Flow Battery Performance

All-vanadium redox flow batteries (VRFB) as the most promising candidates for large-scale energy storage can overcome the inherent defect of natural energy, such as intermittent and fluctuating, et al. During the operation of VRFB, the parasitic gas evolution reactions (GERs) are unavoidable due to thermodynamic limitation, which yet will cause a series of issues, such as imbalance of state of charge (SOC), graphite electrode degradation, security risk, etc. To have a deeper understanding of the parasitic GERs in VRFB, the present work developed a transient, multidimensional, multiphysics VRFB model coupling the multiphase species transport and electrochemical reactions. The model was validated with in-house experimental data and published literature, which showed great consistency. Following, the temporal-spatial characteristics of GERs and their competition with main cell reactions were analyzed in detail under different charging current densities and electrolyte flowrate. The results showed that the rate of hydrogen evolution reaction (HER) at the negative electrode is several magnitudes higher than that of the oxygen evolution reaction (OER) at the positive electrode. Besides, the HER happens during the whole charge-discharge process since the activation overpotential remains negative while the OER occurs only at the end of the charging process under high charging current density. It is notable that the HER mainly occurs in the region near the bipolar plate and increases with increasing charging current density. Since HER is favored thermodynamically over the reduction reaction of trivalent vanadium ions, the activation overpotential of the HER remains much higher than that of the main cell reaction during the whole charge-discharge process, albeit the volumetric current density for hydrogen evolution accounts for only part of the overall current due to the limitation of the reaction kinetics. The HER increases with the SOC and causes a SOC imbalance of 0.54% within 10 charge-discharge cycles, thereby accounting for an 11.0% degradation in the VRFB’s capacity. A high current density decreases the amount of hydrogen generated during a charge-discharge cycle yet also causes hydrogen accumulation in the porous electrode due to faster hydrogen generation. The present work takes a comprehensive and detailed discussion about the characteristics of parasitic gas evolution reactions and their effect on the VRFB performance, which gives a deeper understanding of the energy loss mechanism of VRFB for its development and optimization. Figure 1

Influence of Salt Concentration on Electrochemical Stability Window in Aqueous Electrolytes

The thermodynamic limit of water electrolysis restricts the output voltage of aqueous metal ion batteries (MIBs) due to the low electrochemical stability window of water (1.23 V) in dilute electrolytes. It occurs due to the electrolysis of free water molecules into O2 or H2. Such value is unfortunately too low for many applications. For effective utilization of this technology, the ESW needs to be expanded to at least 2 V to achieve high energy density in metal batteries. Interestingly, water electrolysis can be suppressed by binding free water content with high concentrations of salts in highly concentrated water-in-salt electrolytes.In this work, low-cost non-flammable aqueous Zn(ClO4)2 electrolytes have been studied for their electrochemical behaviour at different concentrations. The electrolytes have been evaluated via Raman spectroscopic analysis to examine the modification in local water structures associated with increasing salt concentration. We also used linear sweep voltammetry on a glassy carbon electrode to determine the electrochemical stability window of the electrolytes. Electrochemical results indicate the existence of two distinct regions in the concentration behavior of the onset potential for oxygen evolution reaction (OER) at a cut-off current density of 0.1 mA cm-2. In dilute electrolytes, the overpotential in OER increases with a low slope with increasing concentration, while there is a sharper overpotential increase at higher salt concentrations in the water-in-salt electrolytes. Raman spectroscopic analysis provides evidence of this electrochemical stability expansion as being a result of the disruption of the hydrogen bonding network present in pure water. A more compact and strongly coordinated ion hydration structure occurs in the water-in-salt electrolytes. In this way, a significant increase in oxidative stability and a decline in the Zn oxidation overpotential has been achieved in the highly concentrated water-in-salt electrolytes. Our results indicate that the Zn(ClO4)2 water-in-salt electrolytes offer more efficient Zn redox reactions and a higher electrochemical stability window than the diluted aqueous electrolytes, which is particularly beneficial for the development of high-voltage aqueous Zn ion batteries [1].