Pritam Singh

Vanadium redox flow batteries (VRFBs), a rechargeable type of battery, have gained interest for energy storage due to their energy efficiency, low cross-contamination risk, simple control, high safety, low maintenance costs, flexible design, long cycle life, and environmental friendliness. VRFB studies have focused on addressing the low energy density and thermal instability of traditional electrolytes-hurdles that must be overcome to achieve widespread global adoption. By meeting these technical benchmarks, VRFBs can become a primary, cost-competitive solution for long-duration grid stability. Mixed acid electrolytes have shown promise in enhancing energy efficiency and charge/discharge capacities due to increased solubility for vanadium ions. However, high concentrations of these mixed acids increase the battery system's corrosion risk. Inorganic and organic compounds as additives have been tested as stabilisers for VRFB electrolytes, with some showing limited potential in practical applications. This paper reviews publications on VRFB electrolytes to critically analyse additives' effects on VRFB electrolyte stability, energy density and electrochemical performance. This paper thus summarises the precipitation mechanism associated with the thermal stability of VRFB electrolytes, the effect of various additives in inhibiting this mechanism, and the effect of additives on the electrochemical activities of the electrolyte. Finally, a summary of potential research areas for additives that could be investigated to improve VRFB electrolytes is presented.

The push to integrate more renewable power into the electricity grid has highlighted efficient energy storage systems, such as vanadium redox flow batteries (VRFBs). VRFBs offer an electrochemical energy storage solution with low self-discharge rates, fast response times, prolonged cycle longevity, and eco-friendliness. However, the widespread adoption is hindered by inherent limitations such as lower energy density, more restricted operating temperature range and higher upfront capital costs compared to other energy storage technologies like lithium-ion batteries. These limitations are partly due to costly and stringent purity requirements for vanadium electrolytes and expensive ion exchange membranes. Consequently, in efforts to enhance the properties of the electrolyte and reduce costs by utilising lower-grade vanadium materials, the reduction of impurities is being investigated. These impurities can originate from multiple sources, including the vanadium raw material, the extraction and processing of vanadium from minerals, and failing components within the VRFB. Such impurities affect electrolyte stability, electrochemical reaction kinetics, and cell performance. Furthermore, literature reveals a lack of established industry standards for electrolyte specifications concerning impurity effects in VRFB electrolytes. As a result, the industry relies predominantly on high-purity electrolytes to avoid potential adverse effects from impurities. This approach, however, leads to a significant increase in electrolyte costs. While a few studies have reported that certain impurities present in the VRFB electrolytes can either affect or enhance electrolyte performance, these findings remain preliminary and scarce. This review provides an overview of the effects of impurities in VRFB electrolytes, identifying their sources and impacts on performance.
•Impurities originate from vanadium extraction processing and battery component corrosion.•Impurity effects are linked to electrolyte stability and electrode kinetics.•Cell performance and electrochemical kinetics study define impurity thresholds.•Undefined impurity tolerance levels for vanadium electrolyte increase cost.•An alternative method for producing battery-grade electrolyte is to omit the precipitation stage.

Copper and cobalt are critically important metals for the transition to renewable energy and various aspects of modern life. Their production from primary sources, ores, necessitates metallurgical separation from the unwanted host materials, resulting in the generation of a huge amount of waste. Copper smelting slag is one of these metallurgical wastes, with 39 million tonnes of slag generated and discarded globally each year. These massive amounts of slag occupy a considerable and growing land footprint, often close to residential areas, and present a hazard that potentially releases contaminants into the environment. On the other hand, they also represent a material that often contains a significant residual amount of valuable copper and cobalt. To better understand and address the challenge of reducing the adverse impacts of the waste, as well as the possible commercial opportunity the contained critical metals present, this study reviews global smelting slag production over the last 25 years, its composition, and technical reprocessing options. A summary of the chemical and mineralogical characterization of the copper slag from diverse research is thus provided, as well as a comprehensive overview of the processing strategies for metal recovery from copper slag, such as flotation, pyrometallurgy, and hydrometallurgy. The study demonstrates that a huge amount of smelting slag has been produced, with great variation and complexity, which represents a major potential resource for cobalt and copper metals. The chemical and mineralogical composition of smelting slag varies from location to location, depending on the properties of the feed concentrate, type of fluxes, furnace type, and cooling rates employed during and after the smelting processes. The overview of the production trends and reprocessing techniques shows that while some notable effective options exist or are emerging, further research is needed into the reprocessing of smelting slag waste in order to create economic value, improve energy efficiency in metal production, increase critical metal supply, and reduce negative environmental impacts.

Coal is commonly used as both fuel and reducing agent in producing synthetic rutile from ilmenite (FeTiO3) via the Becher process, which upgrades ilmenite to high-purity TiO2 (>88%). However, coal-based reduction generates significant carbon waste. This study investigated the effect of adding 1–5% w/w potassium hydroxide (KOH), sodium hydroxide (NaOH), and sodium tetraborate (borax) to coal during ilmenite reduction to improve metallisation and reduce carbon burn-off. Results showed that 1% w/w additives significantly increased metallisation to 96% (KOH), 95% (NaOH), and 93% (borax), compared to 80% without additives, while higher concentrations (3–5% w/w) decreased metallisation. Scanning electron microscopy (SEM)analysis showed cleaner particle surfaces and optimal metallisation at 1% w/w, whereas higher additive levels caused agglomeration or sintering due to elevated silica and alumina activity. Additive type also influenced TiO2 quality, with KOH enhancing TiO2 at low concentrations but causing negative effects at higher levels, while NaOH and borax reduced TiO2 quality via sodium-based compound formation. All additives reduced carbon burn-off, with KOH producing the greatest reduction. The iodine number of the carbon residue increased with higher additive concentrations, with KOH achieving 710 mg/g at 1% w/w and 900 mg/g at 5% w/w, making the residue suitable for water treatment. Overall, KOH is the most effective additive for producing high-quality synthetic rutile while minimising carbon waste.

Redox flow batteries (RFBs) are known for their exceptional attributes, including remarkable energy efficiency of up to 80%, an extended lifespan, safe operation, low environmental contamination concerns, sustainable recyclability, and easy scalability. One of their standout characteristics is the separation of electrolytes into two distinct tanks, isolating them from the electrochemical stack. This unique design allows for the separate design of energy capacity and power, offering a significantly higher level of adaptability and modularity compared to traditional technologies like lithium batteries. RFBs are also an improved technology for storing renewable energy in small or remote communities, benefiting from larger storage capacity, lower maintenance requirements, longer life, and more flexibility in scaling the battery system. However, flow batteries also have disadvantages compared to other energy storage technologies, including a lower energy density and the potential use of expensive or scarce materials. Despite these limitations, the potential benefits of flow batteries in terms of scalability, long cycle life, and cost effectiveness make them a key strategic technology for progressing to net zero. Specifically, in Australia, RFBs are good candidates for storing the increasingly large amount of energy generated from green sources such as photovoltaic panels and wind turbines. Additionally, the geographical distribution of the population around Australia makes large central energy storage economically and logistically difficult, but RFBs can offer a more locally tailored approach to overcome this. This review examines the status of RFBs and the viability of this technology for use in Australia.

The growing demand for eco-friendly activated carbon necessitates sustainable production methods. This study investigates the conversion of waste wood into activated carbon using goethite iron ore as an activating agent. A high-temperature rotary furnace was used to activate the carbon at 1373 K. The oxygen released from the iron oxide during the heat treatment reacted with the carbon in the wood, resulting in 49% of activated carbon with BET surface areas between 684 m2/g and 770 m2/g. The activated carbon and char showed type I isotherms with micropore areas between 600 m2/g and 668 m2/g, respectively. Additionally, 92% of the iron in the ore was reduced from ferric to ferrous. The findings demonstrate that goethite iron ore is an effective activating agent for producing wood-based activated carbon while also generating metallic iron as a byproduct. This alternative activation method enhances the sustainability and efficiency of activated carbon production.

The use of coal for ilmenite reduction to produce synthetic rutile is widespread in industry. However, the carbon dioxide emissions associated with coal combustion pose significant environmental concerns. Alternative reductants such as hydrogen have the potential to promote environmentally friendly production of green rutile. This study aimed to assess the technical feasibility of reducing an Australian secondary (weathered) ilmenite using hydrogen, focusing on the effects of reduction temperature and time. The ilmenite was composed of 65 % titanium dioxide, 29 % iron oxide, and 6 % impurities. Samples at each stage of the processing were analysed using X-ray fluorescence spectrometry (XRF) and scanning electron microscopy (SEM). The results revealed that both temperature and time impacted ilmenite reduction, with increasing values of both parameters leading to higher reduction percentages. The maximum reduction percentages were obtained for a reduction time of 240 min at all temperatures, and there was an increase from 62 % at 973 K to 99 % at 1273 K for this reduction time. A reduction percentage of 90 % was obtained at 1273 K with a holding time of 60 min. This study indicates that a minimum temperature of 1073 K is required to achieve a reduction exceeding 90 % for secondary ilmenite. The SEM analysis showed that fine, discrete, metallised iron particles were present on the surface of the reduced secondary ilmenite. The investigation into hydrogen as an alternative reductant demonstrated improved iron–titanium separation in acid leaching compared with the conventional reduction method using coal and resulted in green rutile products with titanium dioxide grades exceeding 96 %, and iron oxide content below 1 %.

Copper smelting slag is a significant potential resource for cobalt and copper. The recovery of copper and cobalt from copper slag could significantly augment the supply of these metals, which are essential to facilitating the transition to green energy while simultaneously addressing environmental concerns regarding slag disposal. However, the complex mineral composition of copper slag poses an enormous challenge. This study investigated the mineralogical and chemical characteristics of copper slag, which are vital for devising the most effective processing techniques. XRD and FESEM-EDS were employed to examine the morphologies of copper slag before and after the reduction process. The effects of borax and charcoal (carbocatalytic) reduction on phase transformation were evaluated. The XRD analysis revealed that the primary phases in the copper slag were Fe2SiO4 and Fe3O4. The FESEM-EDS analysis verified the presence of these phases and yielded supplementary details regarding metal embedment in the Fe2SiO4, Fe3O4, and Cu phases. The carbocatalytic reduction process expedited the transformation of copper slag microstructures from crystalline dendritic to amorphous and metallic phases. Finally, leaching experiments demonstrated the potential benefits of carbocatalytic reduction by yielding high extractions of Cu, Co, and Fe.

The use of coal-derived activated carbon (AC) for water treatment applications demands more sustainable production methods, with chemical activation emerging as a promising alternative to thermal activation due to its higher AC quality, lower carbon burn-off, and higher yield. The study explored the effect of surface area, particle size and acid washing on the quality of AC derived from three seams of lower-rank Collie coal under the same activation conditions with potassium hydroxide (KOH). The quality of AC was determined by surface area and iodine number. The study demonstrates that Collie coal, suitable for AC production via KOH activation, yielded iodine numbers of 640 and 900 mg/g, with yields of 53 and 57 wt.%. Particle size influenced AC yield, with finer particle sizes yielding AC at 57–59 wt.%, whereas coarser ones yielded around 58–65 wt.%. SEM analysis shows the well-developed porous structure in Collie coal-derived activated carbons, with cleaner particles after acid washing. A positive correlation exists between coal surface area and AC iodine numbers, with higher values in coal samples correlating to increased iodine numbers in resulting AC. The regression model’s predicted values yield a coefficient of determination (R²) of 0.99.

The aim of lithium-ion battery (LiB) recyclers is to create a closed-loop process to recover and reuse all the material as secondary sources of material to manufacture new batteries. Global LiB recycling companies apply pyrometallurgy, hydrometallurgy, or direct recycling to meet this goal. Pyrometallurgy is very energy intensive, but hydrometallurgy requires a pretreatment process and a new version of direct recycling that shows more promise for automation would also require pretreatment. Currently, recycling companies appear to favor hydrometallurgy. This review summarizes the current state of development of the pretreatment process involving battery discharging and mechanical treatment series from the literature and its application in the industry, with particular attention on Asia Pacific recyclers. The key pretreatment steps of battery discharging and mechanical treatment are the focus of this review, but pretreatment of the black mass containing cathode material prior to leaching is also included. Discharging is important to reduce the risk of fire during mechanical treatment. An interesting finding is that despite promising laboratory results, there has been no reported commercial application of battery discharging using the submersion method in an electrolyte solution. An efficient mechanical treatment of discharged batteries is essential to remove the impurities which could adversely impact the subsequent LiB processing. Research into mechanical treatment should also include a method to evaluate the liberation of material. This review has highlighted a new potential flowchart for recycling of various cathode types of LiBs.