Although microbial fuel cells (MFC) could be a promising energy source, their implementation is largely limited by low performance. There are several approaches to overcome this issue. For example, MFC performance can be enhanced using redox mediators (RM) capable of transferring electrons between microorganisms and MFC electrodes. The other, quite novel approach is to use zero-gap electrochemical cells, which minimize the distance between MFC electrodes and, therefore, its internal resistance. This work aims to investigate the compatibility of these approaches. First, a template electropolymerization of polypyrrole (PPy) on carbon felt was carried out in the presence of 2,7-anthraquinone disulfonate (AQDS) acting as an RM. These materials were then used as the anode of a zero-gap double chamber MFC inoculated with sediment from a natural water body and continuously fed with artificial wastewater. On the scales of 45 and 64 days, such cells exhibited power density of up to 900 mW m-2, while unmodified cells demonstrated values tens of times lower, indicating that RM appears to extensively incorporate weak electricigens from the inoculant in the MFC operation. PPy/AQDS electrodes retain electroactive properties during long-term tests, resulting in a theoretical turnover rate of AQDS molecules up to 590.

Here, we analyzed the performance of a hydrogen/bromate fuel cell (HBFC) operating in batch recirculation mode. We adjusted the catholyte composition and operating conditions to ensure stable cell discharging and prevent the formation of liquid bromine. To achieve this, we analyzed the evolution of bromate catholytes with different acid content using thermodynamic calculations, which considered equilibria between bromine compounds and material balance for protons. We used the results of these calculations and data on catholyte chemical stability to select compositions appropriate for HBFC testing. To obtain data on catholyte evolution, we employed novel spectrophotometric and electrochemical in situ/operando techniques. Our study revealed that the composition of the catholyte and its average oxidation state, as well as the interplay between chemical and electrochemical reactions, significantly impact the power output of the HBFC. The main practical finding of our study is that under optimized conditions, the HBFC demonstrated both high energy density and reasonable performance. For instance, with a catholyte composition of 1 M LiBrO3 3 and 0.3 M H2SO4, 2 SO 4 , energy density is 116 Wh L- 1 (theoretical 1270 Wh L- 1 for 5.5 M LiBrO3) 3 ) the cell operated with 98.7% capacity utilization and power of 194 mW cm- 2 at 250 mA cm-- 2 .

An enormous amount of energy is wasted annually in the form of low-grade heat with a temperature below 100 degrees C. Recently, studies on heat harvesting have focused on semiconductor-based devices, but nowadays, electrochemical devices based on a thermally regenerative cycle (TREC) are growing in importance due to their superior thermoelectric power. Among them, TRECs based on flow batteries (TREC-FB) are especially attractive since they offer more flexibility for heat harvesting and an opportunity for continuous heat-to-power (HTP) conversion. However, this technology struggles due to the high cost of the flow batteries they are based on, particularly the costly electrolyte solutions. We offer a novel approach for continuous heat harvesting based on the emerging technology of the neutralization flow battery (NFB) with low-cost and highly soluble electrolytes, which consists of two hydrogen electrodes immersed in acid and base solutions. Since values of both electrodes' temperature coefficient differ, NFB can be used for heat harvesting: the cell temperature coefficient varies in the range of 0.64-1.1 mV K 1. Here, we present both intermittent and continuous TREC based on NFB, with a focus on the latter due to its convenience. Our study on continuous heat harvesting shows that for a temperature difference between the heat source and heat sink of 25 degrees C, one can achieve power of 10-24 mu W cm 2, and efficiency of 1.4-2.9 % even without any recuperation. Alternatively, increasing the temperature difference to 55 degrees C results in a 6-fold increase in power at 1.6-3.5 % efficiency. We found that due to the low freezing point of the electrolytes, one can obtain even higher HTP performance if the heat sink temperature is about 0 degrees C. Additionally, we examined NFB performance under various temperatures and proposed low-grade heat harnessing using intermittent NFB heating/cooling to enhance its cycling performance. We believe that the proposed approaches facilitate further development of high-performance TREC-FB and thereby extend the possibilities for heat harvesting.

The gradual capacity decrease of vanadium redox flow battery (VRFB) over long-term charge-discharge cycling is determined by electrolyte degradation. While it was initially believed that this degradation was solely caused by crossover, recent research suggests that oxidative imbalance induced by hydrogen evolution reaction (HER) also plays a significant role. In this work by using vanadium pentoxides with different impurities content, we prepared three grades of vanadium electrolyte. By measuring electrochemical properties on carbon felt electrode in three-electrode cell and VRFB membrane-electrode assembly we evaluate the influence of impurity content on battery polarization and rate of side reactions which is indicated by the increase of average oxidation state (AOS) during charge-discharge tests and varies from 0.061 to 0.027 day-1 for electrolytes made from 99.1 and 99.9 wt % V2O5. We found that increase of AOS correlates with the increase of open-circuit voltage of VRFB in the discharged state ranging from 9.6 to 14.9 mV day-1 for highest and lowest electrolyte purity levels, respectively. While AOS increase is significant, it does not solely determine capacity fade. It is demonstrated that the presence of vanadium crossover decreases capacity fade, i. e. levels the contribution of side reactions on capacity drop.

In this work, we aimed to reveal two main contributors to the capacity fade of a vanadium redox flow battery (VRFB). These contributors are the oxidative imbalance caused by the hydrogen evolution reaction (HER) competing with V3+ reduction during charging, and crossover, particularly the net transfer of vanadium ions from negolyte to posolyte. To investigate this, we performed VRFB cycling under various operation conditions with sequential monitoring of the electrolytes composition. We discovered that shortly after cycling starts, the crossover makes the negolyte capacity-limiting. This leads to high polarization of the negative electrode inducing HER and increasing average oxidation state (AOS) of the electrolyte. Our examination shows that the magnitude of the oxidative imbalance correlates with that of the crossover, both being dependent on the state-of-charge (SoC). Therefore, by changing operation conditions, we can slightly influence the crossover, while dramatically affect the oxidative imbalance. We also found that the resulting magnitude of the capacity fade is a trade-off between these two side-processes. This necessitates careful optimization of the battery and electrolyte composition, along with its cycling regime. From the data obtained, we conclude that the only proper approach to achieve lower capacity fade is by ensuring that the negolyte is capacity-limiting over long-term cycling. We hope that the data on the capacity fade mechanisms presented here will facilitate development of more stable and cost-effective VRFBs.

Having a long lifespan and being capable of scaling capacity and power independently, redox flow batteries (RFB) offer great opportunities for energy storage. However, the challenge lies in finding an ideal electrolyte. The most mature version of RFB utilizes vanadium solutions and suffers from rising and highly volatile prices of this metal. To address this, organic electrolytes are gaining attention, as they can be obtained from abundant feedstocks. Among those, Anthraquinone-2,7-disulfonic acid (2,7-AQDS) solutions are particularly prominent, demonstrating reversible and fast redox kinetics coupled with reasonable solubility. This paper explores the possibility of synthesizing 2,7-AQDS together with other electroactive compounds (2,6-AQDS, 2-AQS) through the reaction of anthraquinone sulfonation. It shows that obtained mixtures act as electrolytes without any purification or separation, while synthesis conditions can adjust mixture composition and hence their redox behavior. Although the performance of anthraquinone-bromine RFB utilizing these mixtures exhibits a trade-off between power and stability, the best of them are comparable or even superior to 2,7-AQDS. For instance, RFB with a mixture free of 2-AQS demonstrates an energy efficiency of 76.4 % and a capacity fade rate of 0.04 %/cycle at a current density of 75 mA cm- 2. The specific capacity of such mixtures can reach 70 Ah L- 1, which makes them promising and affordable RFB negolyte.

The 6-electron electrochemical reduction of IO3− to I− represents a breakthrough for the development of next-generation redox flow batteries, offering substantially higher energy densities for oxidizer storage. Our study reveals that on a glassy carbon (GC) electrode in acidic electrolytes, HIO3 undergoes an autocatalyzed electrochemical reduction to I−. This process is mediated by the formation of a thin iodine layer on the electrode, acting as an intermediate and a catalyst. Under steady-state conditions, the iodine layer forms via a comproportionation reaction (HIO3 + I− + 5H+ = I2 (s) + 3H2O). Initially, the iodine layer is generated through the slow direct electrochemical reduction of HIO3 on pristine GC. Once established, this layer significantly enhances the rate of iodate reduction. On voltammetry curves, it is clearly observable as a step-wise current surge to reach a plateau. The limiting current density on the GC seemingly aligns with the Levich equation, varying with the RDE rotation rate. Earlier, we demonstrated the electrochemical oxidation of I− back to HIO3 using an H2/HIO3 flow cell, showcasing a full cycle that underpins the feasibility of this approach for energy storage. This study advances the understanding of iodate electroreduction and underscores its role in enhancing the capacity of next-generation energy storage systems.

Developing the mixed metal oxide (MMO) electrodes with high electrocatalytic activity, low cost of materials and long service life is an important and relevant task for various applications including the vanadium electrolyte rebalancing. In this work, the cost-effective electrodes based on mixed iridium-zirconium oxides were fabricated using the Pechini method followed by thermal decomposition. The Raman and XPS analysis of mixed IrO2-ZrO2 electrodes showed a partially crystalline (IrO2 and ZrO2) and partially amorphous (IrOx and ZrOx) structure. Conducted electrochemical tests demonstrated the high electrocatalytic performance of the electrodes with 15-30 mol% of Zr addition comparable to Ir-based electrodes toward the OER. Successful proceeding of the electrochemical reduction in the rebalancing cell was proved using operando absorbance spectra analysis of the reducing vanadium electrolyte. The results of a prolonged vanadium electrolyte electroreduction showed more effective performance of the cell with 70Ir-30Zr electrode than the same cell with 70Ir electrode under a high current density of 250 mA cm-2. This approach allows reducing the electrode cost with reducing the Ir loading without sacrificing the electrocatalytic properties with Zr adding.

It is necessary for vanadium redox flow battery (VRFB) to become more cost-effective due to long-term stable operation with minimal life-cycle maintenance for its further development. Despite the absence of self-discharge in idle mode, the available capacity of VRFB gradually decreases during the battery cycling. The capacity decrease is attributed to side reactions appearance, which increase the average oxidation state (AOS) of vana-dium electrolyte. This negative effect can be minimized by periodic electrolyte capacity recovery. However, it is crucial to have reliable information on the electrolytes composition to conduct rebalancing efficiently and timely. Various methods for vanadium electrolyte assessment have been previously proposed, however, none of them can be used coupled with industrial VRFBs. In this work, we present an operando coulometric analysis of an electrolyte sample in a VRFB cell and its subsequent return to the bulk solution. The proposed approach out-performs other methods due to sensitivity of the signal to the overall vanadium content and AOS and stable performance under imbalance conditions. The analysis of AOS was verified by three methods - coulomb -counting, ex-situ spectrophotometry, and coulometry. Therefore, this method can be successfully used not only in lab-scale VRFB studies, but can also be coupled with industrial VRFB stacks.

Due to its high solubility and fast kinetics of redox reactions, anthraquinone-2,7-disulfonic acid is a promising electroactive molecule for redox-flow-battery electrolytes and other energy applications. However, its widespread use is currently limited, primarily due to its tendency to chemical side-reactions and the formation of quinhydrone complexes between the molecule’s different redox-forms. The possibility of overcoming these shortcomings by using a simple anthraquinone-2,7-disulfonic acid functionalization with the poly(diallyldimethylammonium) polycation is studied. The ionic complexes are shown to be formed in this mixture, which leads to the suppression of the quinhydrone compound formation. At the same time, the poly(diallyldimethylammonium)/anthraquinone-2,7-disulfonic acid mixtures retain their redox activity and can be used as a negolyte in anthraquinone–bromine redox flow batteries, while all key characteristics of such a battery are comparable with those of anthraquinone–bromine redox flow batteries which used anthraquinone-2,7-disulfonic acid without any additives. The poly(diallyldimethylammonium)/anthraquinone- 2,7‑disulfonic acid-based battery (0.1 M anthraquinone-2,7-disulfonic acid) has the power density of 105 and 65 mW/cm 2 for the battery state-of-charge values 100% and 50%, respectively; the energy efficiency for five charging–discharging cycles, 57.4%. In the future, the composition of the poly(diallyldimethylammonium)/anthraquinone-2,7-disulfonic acid ionic complexes can be optimized, in order to maintain good kinetics and solubility of anthraquinone-2,7-disulfonic acid and at the same time reduce the intensity of chemical side-reactions, including quinhydrone-complexes formation.

A new technology of neutralization flow batteries was recently proposed as a promising alternative to conven-tional redox flow batteries due to the low cost of employed electrolytes - acid and base solutions. Despite the advantages of this concept, their performance metrics are still quite modest for industrial applications. To fulfill this gap, in previous work, the authors presented a new two-membrane neutralization flow battery that is meant to overcome issues of reverse electrodialysis cells with bipolar membrane. This concept demonstrates promising results, but the battery discharge performance was limited by alkaline anode where the hydrogen oxidation reaction occurs. In this study we offer the approach to advance the battery performance by implementation of gas diffusion electrodes with gas-fed supply mode. The aforementioned modification allows us to achieve record power (40-87 mW cm-2) and energy (8-48 Wh L-1) densities along with a significant enhancement of the cycling performance.

One of the major challenges in vanadium redox flow batteries (VRFB) is a gradual decrease of available capacity over operation time. The VRFB capacity fade is a complex issue that affects volume, total content, and average valence of vanadium ions in posolyte and negolyte. Imbalances that occur due to crossover of vanadium ions, osmosis and electroosmosis of water can be dealt with by intermittent mixing of posolyte and negolyte, thereby prolonging VRFB operation at high capacity rates. However, the change in the average oxidation state (AOS) should be considered for stable VRFB operation. The imbalance of vanadium valence arises due to side reactions during charge–discharge processes. This work proposes a novel operando method for restoration of the initial VRFB capacity. Partial posolyte reduction on the cathode and oxygen evolution reaction on the RuO2/Ti electrode of the electrolysis cell, following which electrolytes mix and recharge, allow to recover initial performance parameters. Optimizing electrolysis conditions ensures high energy efficiency of the rebalancing procedure. Coulometric sensors data provide the charge required for posolyte reduction. The proposed approach for VRFB capacity recovery allows for complete restoration of performance parameters regardless of the magnitude of a capacity drop.

Monitoring the state of charge (SoC) is one of the most important challenges of vanadium redox flow battery (VRFB) technology to solve. Among other methods, optical spectroscopy seems promising, however, existing approaches have their problems, primarily because of limited applicability for high vanadium concentrations, uncertainty in accounting for electrolyte imbalance during VRFB operation, and applicability limitations for mixed acid electrolytes. In this work, a method for determining SoC of VRFB based on the deconvolution of electrolyte absorption spectra is proposed. This method was successfully implemented for 0.5 M and 1 M vana-dium electrolytes with 0.05 M phosphoric acid additive and demonstrated good accuracy for determining vana-dium concentration and SoC in both half-cells, including analysis during VRFB galvanostatic cycling. Besides, this method allows us to obtain complex spectra: V2O33+ in posolyte and a previously never demonstrated com-plex in negolyte that presumably corresponds to the V3+ complex.

This review aims to highlight the current advances in hybrid redox flow battery (HRFB) technology, encompassing one of the best combinations of efficiency, cost and flexibility due to its module construction, which offers independent scaling of power density and energy capacity. This work emphasizes the interest of the scientific community both in (i) studying the properties and principles of HRFB operation in order to improve commonly proposed systems, and in (ii) the development of energy storage devices with new reagent types or RFB concepts. The data provided enhances the reader to conclude whether novel concepts in halogen oxidizers utilization could help to overcome the problem of insufficient power and energy densities of common RFB.

Gradual capacity fade of vanadium redox flow batteries (VRFB) is one of the greatest challenges for further distribution of this technology, since capacity stability is crucial for industrial energy storages. It is suggested that the main reason for this issue is vanadium ions crossover, however recent studies show that appearance of chemical and electrochemical side reactions leading to increase of the electrolyte average oxidation state (AOS) should be considered as a greater contributor to capacity fade. It should be noted that increase of overall AOS is more challenging to eliminate compared to vanadium ions crossover. To date, a great variety of VRFB capacity fade mitigation methods have been proposed, but very little attention has been paid to development of methods for the battery capacity recovery. Despite several methods for AOS decreasing via partial posolyte reduction have been demonstrated, none of them were implemented for operando capacity recovery due to a number of ongoing issues. In this work, a novel method for operando VRFB capacity is proposed. It consists of partial electro-reduction of posolyte in electrolysis cell utilizing chemically regenerative electrolyte. Regeneration of VO2+ by reducing agents is cost-effective, and separation of this process from the battery using electrolysis cell allows proceeding VRFB capacity recovery operando. This study is devoted to investigation of both chemical and electrochemical stages of the proposed method, and its optimization for rebalancing under cycling conditions. It is suggested that integration of the VRFB stack with the proposed rebalancing system allows to demonstrate high battery performance while maintaining nominal capacity indefinitely.

Renewable energy in recent years plays an increasingly important role in the energy industry. Therefore, the problem of inventing efficient and affordable energy storage devices is of current importance. Vanadium redox flow battery stands out between a wide range of various chemical sources of electric energy. Discovering the way for optimizing the price of energy storage is one of the most important questions of the mentioned battery. New graphite-polymer composite materials study shows that they may become a replacement for commonly used brittle graphite. The current article presents the study of graphite foil (or flexible graphite) filling with a copolymer of tetrafluoroethylene and vinylidene fluoride F-42 properties. The article uses an implementation approach of putting the fluoropolymer into the pores of graphite foil, which shows a slight decrease in electrical conductivity, while a significant increase in its electrochemical stability in comparison with untreated graphite foil. Testing results of such material as current collectors (or bipolar plates) in a single membrane-electrode assembly of vanadium redox flow battery cell show peak value of discharge power density of 843 mW/cm(2), energy efficiency of 78%-79% during cycling at 200 mA/cm(2) allowing us to assume the possibility of its successful application in redox flow battery stacks.

The manuscript deals with the fundamental problem of platinum hydrogen oxidation catalyst poisoning of the hybrid chemical power source based on bromate electroreduction and hydrogen electro-oxidation reactions. The poisoning is caused by the crossover of bromine-containing species through the proton exchange membrane separating compartments of the flow cell. Poisoning results in a drastic decrease in the flow cell performance. This paper describes the results of the direct measurement of bromine-containing species’ crossover through perfluorosulfonic acid membranes of popular vendors in a hydrogen−bromate flow cell and proposes corresponding scenarios for the flow battery charge−discharge operation based on the electrolyte’s control of the pH value. The rate of the crossover of the bromine-containing species through the membrane is found to be inversely proportional to the membrane thickness.

Shunting currents are among the main problems of all-vanadium redox flow battery stacks since, in addition to capacity losses, they cause negative effects associated with the local destruction of electrodes and bipolar plates. The values of both the shunting currents and their destructive effects on materials can be reduced at the battery development stage by adjusting the resistance of the electrolyte supply channels. The solution to this problem can be found using a calculation model for current distribution based on the current balance in the nodes as well as voltage drops and electromotive force in internal circuits according to Kirchhoff’s laws. This paper presents the verification of the model of current distribution in an all-vanadium redox flow battery stack of an original design that allows for the determination of membrane-electrode assembly resistances and electrolyte supply channels via direct measurements. Based on a comparison of the calculated and experimental values of the coulombic efficiency of charge–discharge cycles, the capacity fade associated with the crossover of vanadium compounds through the membrane has been determined.

Anthraquinone-2,7-disulfonic acid (2,7-AQDS) is a promising organic compound, which is considered as a negolyte for redox flow batteries as well as for other applications. In this work we carried out a well-known reaction of anthraquinone sulfonation to synthesize 2,7-AQDS in mixture with other sulfo-derivatives, namely 2,6-AQDS and 2-AQS. Redox behavior of this mixture was evaluated with cyclic voltammetry and was almost identical to 2,7-AQDS. Mixture was then assessed as a potential negolyte of anthraquinone-bromine redox flow battery. After adjusting membrane-electrode assembly composition (membrane material and flow field)), the cell demonstrated peak power density of 335 mW cm−2 (at SOC 90%) and capacity utilization, capacity retention and energy efficiency of 87.9, 99.6 and 64.2%, respectively. These values are almost identical or even higher than similar values for flow battery with 2,7-AQDS as a negolyte, while the price of mixture is significantly lower. Therefore, this work unveils the promising possibility of using a mixture of crude sulfonated anthraquinone derivatives mixture as an inexpensive negolyte of RFB.

The proposed anthraquinone-bromate cell combines the advantages of anthraquinone-bromine redox flow batteries and novel hybrid hydrogen-bromate flow batteries. The anthraquinone-2,7-disulfonic acid is of interest as a promising organic negolyte due its high solubility, rapid kinetics of electrode reactions and suitable redox potentials combined with a high chemical stability during redox reactions. Lithium or sodium bromates as posolytes provide an anomalously high discharge current density of order ~A cm−2 due to a novel autocatalytic mechanism. Combining these two systems, we developed a single cell of novel anthraquinone-bromate flow battery, which showed a power density of 1.08 W cm−2, energy density of 16.1 W h L−1 and energy efficiency of 72% after 10 charge–discharge cycles.