Redox flow batteries are a reliable option for the storage of energy from renewable resources and the all vanadium cell chemistry features the highest level of commercialization. The storage market demands further cost reductions. A tubular cell design can lower power specific costs by featuring reduced material needs and enabling the use of cost efficient extrusion processes of cell components. The feasibility of tubular all vanadium cells has already been shown in previous studies of the authors. In this study, we report on the progress of a recent cell generation and the integration of single cells into stacks. The performance of the current cell and stack generation has reached the region of planar cells with discharge current densities of i(dch )> 300 mA cm(-2) at SOC approximate to 0.5 and U-cell = 0.8V as well as maximum power densities of P / Area > 300 mW cm(-2) with respect to the membrane area and of P / Volume > 750 kW m(-3) with respect to the cell body volume. The single cell performance turns out to be reproduceable with negligible losses at stack level in a 5p setup. In this way, the feasibility of tubular stacks is demonstrated. We claim the tubular approach to be transferrable to other cell chemistries.

Proton Exchange Membrane Water Electrolyzers (PEMEL) are promising for the production of green hydrogen, but costs have to be reduced and lifetime has to be improved. In this study we investigate PEMEL with low catalyst loading achieved by Atomic Layer Deposition (ALD) and 3D printing of the electrodes. Degradation processes were characterized and allocated using different electrochemical and physicochemical methods. The cell setup was optimized using Distribution of Relaxation Times Analysis (DRT) of the Electrochemical Impedance Spectroscopy (EIS) measurements and the long-term performance of the optimized cell was investigated using different electrolytes (sulfuric acid and deionized water).

The increasing importance of recycling end-of-life photovoltaic modules is demonstrated by the rising quantity of discarded crystalline silicon solar cells that contain valuable metals. Despite advanced recycling methods, the surplus of broken Si wafers poses challenges for reintegration into new module manufacturing. The present study introduces a novel recycling process that addresses this issue and promotes sustainable waste processing, focusing on the untapped resources of Si wafer breakage and environmentally harmful red mud. The proposed method uses these two critical waste materials to enable a silicothermal reduction, yielding ferrosilicon-based alloys. To comprehensively analyze the influence of the iron oxide source on alloy composition, a readily available iron oxide pigment (Bayferrox 110) is implemented as a reference material. Fe–Si-based alloys containing 15 to 65 wt % Si are produced by the silicothermal reduction with soda ash as a flux, at a temperature of 1600 °C. The use of Bayferrox as an iron oxide source facilitates the production of Fe–Si alloys that are free from additional impurities. Moreover, the use of red mud as the source of iron oxide leads to the production of Fe–Si–Ti alloys, containing up to 8.6 wt % of Ti. The inclusion of Ti in the ferrosilicon-based alloy elevates the market value of the resulting products, emphasizing the commercial viability of the suggested recycling process. By simultaneously utilizing two critical waste materials, namely, red mud and Si wafer breakage, this novel recycling strategy demonstrates significant potential, especially in view of a circular and holistic waste management.

Ferrate (Fe(VI)) is of great interest in energy storage solutions, organic synthesis, and wastewater treatment due to its decent oxidation potential and non-toxic end-product formation, making it a green oxidizer. The electrochemical generation of ferrate in NaOH at current densities of j >= 100 mA cm-2 is presented using low-cost sacrificial iron anodes, mild steel, and spheroidal graphite cast iron (ductile iron). Under optimized reaction parameters with 40 wt.% (14 m) NaOH and a ZrO2-based diaphragm, spheroidal graphite cast iron shows no signs of passivation in 5 h experiments even at j = 150 mA cm-2. The results are used in a novel electrolysis cell with a combined geometric anode surface area of 230 cm2, incorporated in a mini-plant suitable for continuous synthesis. This setup produces a peak ferrate concentration of 10.1 g L-1 (84 mm) after 5 h in 1.6 L anolyte volume, resulting in a total ferrate mass of 16.2 g. Optimal electrolysis temperatures are between 35 and 50 degrees C. The highest current efficiency is 63.0%, and the lowest specific energy consumption is 9.2 kWh kg-1 ferrate. The presented work is an essential step toward the continuous electrochemical synthesis of ferrate using sacrificial anodes under basic conditions.

Co-extrusion of both half-cells in tubular PEM water electrolyzers can lower the costs for hydrogen production, since the number of components is reduced and the production process is simplified. However, after co-extrusion of the inner half-cell and the ion exchange membrane, the membrane is in its fluoride sulfonyl form and must be hydrolyzed to achieve the proton conductive sulfonic acid to be ready for use. Common practice is the hydrolysis using concentrated alkaline solutions, which causes a corrosion of the laminated anode electrode. We developed a less corrosive method using triethylsilanol as reactant. Tubular membranes hydrolyzed with this new procedure were characterized and tested in an electrolyzer laboratory test setup.

Polymer electrolyte membrane electrolysis (PEMEL) is a technology with a major role in linking the hydrogen production to renewable energy resources with a volatile behaviour such as wind and solar. High amounts of precious metals and a labour intensive production also make it a cost intensive technology. A tubular cell design has the potential to reduce production costs by co-extrusion of cells which feature a reduced sealing length. For the inner half cell, additive manufacturing (AM) of titanium offers a high degree of freedom for the electrode design to reach a high electric conductivity and active surface area. In combination with atomic layer deposition (ALD) of iridium catalyst a porous transport electrode (PTE) can be fabricated. Using planar test cell results and model based PTE design, this study demonstrates the feasibility of a tubular PEMEL cell consisting of an additively manufactured, iridium coated anode PTE in the inner half cell, an extruded membrane and a platinum coated graphite felt cathode PTE in the outer half cell. The outer titanium current collector can be replaced by an extruded graphite polymer compound current collector to reduce the amount of titanium without performance losses. The cell is operated at 60 degrees C in 1 mol L-1 sulphuric acid and experimentally characterized by polarization curves and electrochemical impedance spectroscopy (EIS). At 2.0 V cell potential a current density of approximate to 450mAcm-2 was reached corresponding to an iridium mass specific current density >1500Ag-1 which is significant larger than literature values.(c) 2023 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license.

Reduction of catalyst loading on Proton Exchange Membrane (PEM) Electrolysis Cells (EC) is needed because of scarcity and high price of the catalyst. Monitoring the degradation of these cells is important to be able to allocate the different processes taking place. Electrochemical Impedance Spectroscopy (EIS) might be suitable for this purpose, but in its usual implementation, the system and the contribution of its components to the total resistance must be already known to build a suitable equivalent circuit model (ECM). Furthermore, the overlap of different processes in the Nyquist and Bode plots hinders the identification of single components of the system. The Distribution of Relaxation Times (DRT) analysis converts the EIS data into a distribution of time constants of individual processes. The contribution of each component to the full cell resistance can be identified by varying different operation parameters. In this work, a PEMEC with low iridium loading was analyzed by DRT and the faradaic processes were identified. A long-term test was carried out and the degradation of the cell was investigated by DRT analysis, to determine the components and processes which limit the cell performance.

Redox flow batteries are a promising technology to enable the middle term storage of fluctuating renewable electricity production. The membrane is a key component in the battery system and to further develop and improve the battery systems, detailed understanding of the membrane aging and degradation mechanisms are required. This review gives a comprehensive overview about the various membrane degradation mechanisms in the most relevant redox flow battery systems. We discuss different testing approaches for membranes and compare the influence of different battery chemistries, testing protocols and degradation mechanisms. Based on the current state of the art, an outlook on the greatest challenges for developing novel and more stable membrane materials is given.

Data presented in the article with the same title

Electro-Fenton (EF) represents an eco-friendly and cost-effective advanced oxidation process that can remove highly persistent and hazardous pharmaceuticals, e.g., contrast media agents, from water bodies. However, up to date, EF modules incorporate a planar carbonaceous gas diffusion electrode (GDE) cathode containing fluorinated compounds as polymeric binders. Here, we introduce a novel flow-through module that deploys freestanding carbon microtubes (CMT) as microtubular GDEs, omitting any risks of secondary pollution by highly persistent fluorinated compounds (e.g., Nafion). The flow-through module was characterized for electrochemical hydrogen peroxide (H2O2) generation and micropollutant removal via EF. H2O2 electro-generation experiments illustrated high production rates (1.1 & PLUSMN; 0.1-2.7 & PLUSMN; 0.1 mg cm-2 h-1) at an applied cathodic potential of 0.6 V vs. SHE, depending on the porosity of CMTs. Diatrizoate (DTZ), as the model pollutant, with a high initial concentration of 100 mg L-1 was successfully oxidized (95-100 %), reaching mineralization (TOC-total organic carbon removal) efficiencies up to 69 %. Additionally, Electro-adsorption experiments demonstrated the capability of positively charged CMTs to remove negatively charged DTZ with a capacity of 11 mg g-1 from a 10 mg L-1 DTZ solution. These results reveal the potential of the as-designed module to serve as an oxidation unit coupled with other separation techniques, e.g., electro-adsorption or membrane processes.

In the struggle against climate change, hydrogen has been considered as a key player, since it can be used as a fuel in electrochemical devices, but also as feed in the chemical and metallurgical industry. However, only if this hydrogen is produced by CO2-free process, the net zero-emission goals will be achieved. Green hydrogen can be obtained by water electrolysis using renewable electricity as power source. While alkali (AEL) and acid (PEMEL) electrolyzers are the most advanced technologies, further improvements especially in terms of catalyst stability are still needed to increase their lifetime and economical profitability for megawatt and gigawatt scaling-up. In this work, we report on electrochemical degradation tests of commercially available AEL and PEMEL single cells with 5 cm2 active, geometrical surface. The AEL anode and cathode consist of Raney-Ni electrodes with PTFE binder supported on a Ni grid, separated by a Zirfon diaphragm. 30% KOH at 80°C was used as electrolyte in both chambers. The commercial catalyst coated membrane (CCM) for PEMEL consists of 2 mgIr/cm2 anode and 1 mgPt/cm2 cathode coated on Nafion N117. Titanium felt and carbon paper were used as porous transport layers in the anode and cathode side, respectively. Ultrapure water at 60 °C was used as electrolyte in both sides. For long-term electrochemical tests, current density of 0.5 A/cm2 and 2 A/cm2 was applied to the AEL and PEMEL cells, respectively, for 1000 h. Since at nominal current density, low degradation rate was expected, accelerated degradation tests (ADT) were performed at two identical cells by using triangular polarization profile aiming to reflect dynamic electrolyzer operation in case of e.g., direct coupling with wind turbine. Thereby, the current density was varied between 0.05 - 1 A/cm2 for AEL and 0.05-3 A/cm2 for PEMEL for at least 1000 h. After each 100 h period during both constant and triangular polarization experiments, the cell was electrochemically characterized by impedance spectroscopy and current/voltage (UI) profiles and anolyte and catholyte were collected for ICP-OES measurements in order to evaluate dissolved species. After test, the cells were disassembled and the cell components were analyzed by SEM/EDX, XRD and XPS technique. By correlating the physicochemical with the electrochemical changes in the components of the cell, main degradation mechanisms will be exposed.

For an efficient flow battery operation, knowledge of the state of charge of the battery is essential. Monitoring the state of charge of both half cells is advantageous concerning a timely detection of crossover processes. We present the first results for amperometric and electrochemical quartz crystal microbalance measurements in a vanadium flow battery test setup. By validating with half cell potential measurements as well as ex situ titration we investigate the applicability of both electrochemical methods for an in situ half cell state of charge monitoring.

The increasing number of photovoltaic (PV) panels installed worldwide requires solutions for their disposal at their end-of-life. PV modules contain several valuable metals like copper and silver as well as toxic ones like lead used in solder. Another valuable component are silicon wafers, because of their high purity, which is obtained in energy and resource intensive production processes. Many recycling procedures, however, are downcycling processes that do not recover these components, but rather are based on shredding and landfilling the modules or obtaining only one of those materials. Often, nitric acid is used as leaching agent for the recovery of silver and copper resulting in harmful NOx emissions that necessitate additional gas treatment. We present an alternative electrochemical process by which leaching reagents are generated on boron-doped diamond electrodes as anode material, which are ideal for this task due to their high overpotential for the oxygen evolution reaction. By evaluating different electrolytes, sulfuric acid was found to be suitable by generating peroxydisulfate (S₂O₈²⁻). With its standard redox potential of +2.0 V, it is possible to oxidize rather noble metals like silver. To evaluate the efficiency of the process S₂O₈²⁻ was generated from sulfuric acid in an H-cell setup and the leaching rate of copper, silver and tin was investigated and optimized by varying acid concentration and current density. Efficient leaching of all metals was achieved in 5 M sulfuric acid at 200 mA/cm². The feasibility of the process for real waste streams was demonstrated by disassembling PV modules and leaching busbars and silver coatings from wafers. The complete removal of the metal coatings from the residual wafers was confirmed by EDX. A significant advantage of the process with S₂O₈²⁻ as leaching agent is, that the reaction with metals forms sulfate that can be oxidized again within a cyclic process driven by electricity as displayed in figure 1. Additionally, the metals can be recovered by electrowinning from the etching solution within the same cell. This was demonstrated by recovering Ag, Cu and Sn from sulfuric acid solution. By using potentiostatic conditions it was additionally possible to separate the metals by consecutively plating them in the order of their respective redox potentials. Leached metals from PV waste were recovered electrochemically with 99 % and 88 % yield for Cu and Ag, respectively. Sn could not be plated due to the high amounts of persulfate present in solution, however recovery by precipitation is possible. Pb is obtained as undissolved residue during leaching, due to the extremely low solubility of PbSO4 and PbO2. Aside from the H-cell experiments the generation of S₂O₈²⁻ and electrowinning was successfully investigated in a flow cell setup. This system benefits from a significantly decreased cell voltage making the process more energy efficient and allows easier upscaling towards industrial applications. The presented process is energy efficient as both electrode reactions are utilized and furthermore does not require the continuous addition of chemicals, therefore, making it an eco-friendly and sustainable alternative to conventional recycling methods. Figure 1: Scheme for the cyclic process of peroxydisulfate generation, leaching and electrowinning during the electrochemical recycling of PV waste. Figure 1

Polymer electrolyte membrane (PEM) water electrolysis is already widely used for hydrogen production but still needs further cost reductions. While tubular cell designs might reduce production costs by extrusion production of cell components and small sealing lengths, catalyst coating methods like atomic layer deposition (ALD) might reduce catalyst costs significantly. This study demonstrates the feasibility of a tubular PEM electrolyzer membrane electrode assembly (MEA) for the oxygen half cell with 5.0 mm diameter. An extruded perfluorosulfonic acid (PFSA) cation exchange membrane is combined with a porous transport electrode (PTE) consisting of a titanium felt with a low iridium catalyst loading obtained by ALD. The performance is experimentally characterized in a complete tubular cell setup by polarization curve and ohmic resistance measurements. Operation in sulphuric acid at a cell voltage of 1.7 V and a cell temperature of 60 degrees C results in an overall current density of 55 mA cm(-1) and an iridium mass activity > 680 A g(-1) which is up to 3 times larger than literature values. The high frequency ohmic resistance of the cell turns out to be 0.96 U cm(-2). Up to the knowledge of the authors, this is the first time, that a tubular PEM electrolysis cell is designed, assembled and characterized. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Activity and durability of a polymer electrolyte membrane (PEM) water electrolysis single cell, assembled with porous transport electrodes (PTEs) with a low catalyst loading were investigated for 500 h. A current density of 160 mA/cm2 and a high mass activity of 1368 A/ gIr were achieved while operating at 60 degrees C with 1 mol/L sulfuric acid. The degradation of the cell was characterized using different electrochemical and physicochemical methods before, during and after operation of the electrolysis cell and a mean degradation rate for the cell of 67 mV/h was determined at 15 mA/cm2. To the best of our knowledge this is the first time that long-term performance of a PEM water electrolysis cell assembled with PTEs coated by ALD is investigated.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

This study establishes the applicability of 3D printing (additive manufacturing) towards the generation of titanium alloy scaffolds for water oxidation electrodes. The scaffolds can be subsequently nanostructured by electrochemical anodization to enhance their surface area and coated with iridium as the electrocatalyst. We focus on the characterization of the functional electrodes in process-relevant conditions (1 M H2SO4, 60 circle C, stirring) in terms of their performance and stability in a holistic manner. Various preparative conditions yield various patterns of performance and stability, as quantified by overpotentials eta 10 in steady-state electrolyses, maximum current densities jmax in dynamic voltammetry, surface roughness rf, and by overpotential increase, iridium loss, and jmax decrease after 100 h of operation, on the other hand. In other words, the system is highly flexible and can be adapted to specific constraints depending on the application chosen.

To describe and predict the leaching of Ag, Cu and Sn from waste photovoltaic modules with an electrochemical-assisted process kinetic investigations were performed. In this process, peroxydisulfate is generated from sulfuric acid to oxidize metals. It was found that under the reaction conditions peroxymonosulfate is formed as well and has a major contribution to the leaching process. For Ag, autocatalytic decomposition of the leaching reagents is determined to be a limiting step while for Sn passivation influences the process. The leaching is modeled for three different reaction types, a batch reaction, the reaction in a static H-cell with continuous generation of S₂O₈2− and a fed-batch reaction with an electrochemical flow cell for the production of peroxydisulfate.

A photovoltaic system could supply a single-family house with electrical power, warm water, and room heat if the energy would be distributed over the year to suit the load profile. However, storage systems for this are not state of the art yet. A concrete example is used to estimate which parameters such a power storage system should have. A suitable electrochemical reaction system based on inorganic salt mixtures is proposed. The German Federal Ministry of Education and Research is currently funding the development of a world storage facility based on the same reaction system.

An electrochemical-assisted leaching process using boron-doped diamond (BDD) electrodes was developed to recover valuable metals from photovoltaic modules. With BDD electrodes peroxydisulfate is generated from sulfuric acid to oxidatively dissolve copper, tin and silver from solar cell contacts. Since the oxidant is regenerated in the developed process, no additional hazardous and volatile chemicals are required, and the process can be operated solely by electricity. In addition, the dissolved metals can be electrochemically recovered at the cathode of the same cell.

Membranes play a crucial role in efficiency and longevity of flow batteries. Vanadium flow batteries suffer self-discharge and capacity fading due to crossover of electrolyte components through the membrane from one battery half-cell to the other. We consider the impact of vanadium species crossing ion exchange membranes on state of charge of the battery and we present a simple method to determine crossoverll open circuit potential measurements. State of s. State of charge for the negative and positive half-cell is simulated based on assumptions and simplifications for cation and anion exchange membranes and different crossover parameters. We introduce a crossover index "IndXovr" which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs. Furthermore IndXovr allows statements on crossover amount in dependence on state of operation. Simulated case studies are compared to experimental state of charge values estimated from half-cell potential measurements. Our results reveal that half-cell potential monitoring respectively half-cell SOC estimation, is a simple and suitable tool for the identification of crossover direction and relative amount of crossover in VFB.