Dowon Bae

Bromine retention in aqueous redox flow batteries can be improved by complexation, but the resulting gain in stability is often accompanied by losses in electrochemical accessibility and transport. Here, we investigate a ferri‐/ferrocyanide–bromine flow battery based on soluble ferri‐/ferrocyanide and bromine/bromide redox electrolytes. Three bromine/bromide positive‐electrolyte conditions were compared: a pristine bromide electrolyte, an electrolyte containing the bromine‐complexing agent (BCA) N ‐ethyl‐ N ‐methylpyrrolidinium bromide ([EMPyrr]Br), and the same electrolyte after nitrogen presparging. Charge–capacity retention after 70 cycles increased from 23.4% for the pristine electrolyte to 34.9% with [EMPyrr]Br and 66.0% with [EMPyrr]Br and nitrogen presparging. Because neither a nitrogen‐pre‐sparged BCA‐free control nor an alternative inert‐gas control was examined, the higher retention cannot be attributed uniquely to nitrogen; deaeration and sparging‐induced redistribution of bromine‐rich phases may also contribute. Improved retention was accompanied by increased polarization, higher ex situ electrolyte impedance, and an initial voltage rise associated with bromine‐rich phase formation. Postcycling cyclic voltammetry provides qualitative evidence of bromine crossover, while Raman spectroscopy shows condition‐dependent bromine‐related signatures in the sampled liquid phase. These findings show that improved charge–capacity retention does not necessarily translate into better cell performance unless the retained bromine remains electrochemically accessible.

Among various electrolysis technologies, alkaline water electrolysis (AWE) is considered the most mature for industrial scale-up and has robust cost-effectiveness. However, AWE’s cold-start nature requires a ramp-up time (ca. 1 hour). This makes it challenging to integrate with renewable energy sources, which are difficult to predict. Water electrolysis exhibits a pronounced temperature dependence, with overpotential decreasing [1, 2], and some research groups have demonstrated AWEs with external heat sources [1, 3]. Our intended solution is to introduce a chemically resistant solar thermal collector that provides a heated electrolyte to increase reactivity and reduce ramp-up time, and to develop oxygen evolution reaction (OER) catalysts with high-temperature sensitivity. In this work, we discuss key dimensional design aspects of the thermal collector for an alkaline electrolyser and present the performance of transition-metal oxide catalysts (Ni-Co-Fe LDH on Ni plate) for OER under low-grade solar thermal energy (below 80°C). The results show a reduction in overpotential with increasing temperature, with the highest overpotential reduction rate of over 4.6 mV/°C, and our DFT study also supports the observed change in kinetics at elevated temperatures. This study offers an alternative solution to reduce the required electric energy during the ramp-up period by leveraging the benefits of both thermal energy and optimised electrode materials to increase AWE's cost-effectiveness and competitiveness.

Bromine retention in aqueous redox flow batteries can be improved by complexation, but the resulting gain in stability is often accompanied by losses in electrochemical accessibility and transport. Here, we investigate a ferri-/ferrocyanide–bromine flow battery based on soluble ferri-/ferrocyanide and bromine/bromide redox electrolytes. To examine how bromine-management strategies influence retention, accessibility, and transport, three bromine/bromide posolyte conditions were compared. These included a pristine bromide electrolyte, [EMPyrr]Br-containing electrolyte, and [EMPyrr]Br-containing electrolyte with nitrogen sparging. The capacity retention after 70 cycles increased from 24.6% for the pristine electrolyte to 32.5% with [EMPyrr]Br and 67.9% with [EMPyrr]Br and nitrogen sparging. However, the improved retention was accompanied by increased polarisation, resistance growth, and an initial voltage rise associated with bromine-rich phase formation. Post-cycling electrochemical analysis indicates bromine crossover, while Raman spectroscopy reveals changes in bromine speciation during cycling. These findings establish the ferri-/ferrocyanide–bromine system as a model flow-cell platform for distinguishing bromine retention from electrochemical accessibility, while highlighting that improved bromine retention does not necessarily translate into improved cell performance unless the retained bromine remains electrochemically accessible.

Redox flow batteries (RFBs) have attracted considerable attention owing to the development of diverse redox-active species for scalable energy storage. In this study, we present an aqueous ferricyanide/bromine flow battery (FBrFB), employing bromine/bromide (Br2/Br-) and ferri-/ferrocyanide [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ as the anode and cathode redox couples, respectively. The active species are dissolved in a KCl-based aqueous electrolyte, a bromine complexing agent (BCA) is introduced, and the electrolyte conditions are adjusted to enhance the electrochemical stability of Br2. The proposed FBrFB achieved a high coulombic efficiency (~98%) and demonstrated stable operation under standard flow cell conditions with a notable retention of 59.3% after 100 cycles. However, electrochemical impedance spectroscopy revealed increased internal resistance due to the addition of the BCA, indicating a trade-off between enhanced stability and reduced reaction kinetics associated with the formation of a polybromide-rich oily phase. The irreversibility of Br2 was investigated quantitatively under various cycling conditions and states of charge/discharge. Furthermore, bromine crossover was identified as a key contributor to long-term performance degradation and was experimentally verified by detecting Br-related redox peaks in the ferricyanide electrolyte after extended cycling. Future work should focus on suppressing bromine crossover and mitigating the associated kinetic limitations.

A staggering amount of energy is rejected globally from the many diverse heat sources around us, from power plants and industrial processes to smaller-scale electronics and even body heat. An estimated 72% of primary energy is lost once it has been converted, and the majority of this can be categorised as low-grade heat, that is, heat under 100°C. Converting this energy's theoretical maximum, or Carnot limit, would recover 22.3 PWh or 39% of the world’s electricity consumption in 2018. This level of recovery would significantly reduce the carbon footprint in sectors such as electricity generation as more energy could be extracted per unit of emitted greenhouse gas. Given its dominance in the make-up of waste heat, low-grade heat recovery has been a focal point in research in recent years. However, no technology has yet been shown to be suitable for commercialisation for this purpose. Solid state thermoelectric devices have been extensively explored over the years but are still unsuitable for low-grade heat recovery due to their small thermoelectric response (microvolts per Kelvin). Owing to electrode and electrolyte design advancements, electrochemical methods such as the thermally regenerative electrochemical cycle (TREC) have made significant advancements in their thermal coefficients (mV / K) and round-trip efficiencies. Prussian blue analogue (PBA) electrodes, such as CuHCF and NiHCF, have recently garnered attention in this field due to their thermoelectric properties and reversible charge-discharge behaviours.Additives from single ions to larger chaotropic molecules have also been explored in recent literatureand have shown to improve the performance of electrochemical heat recovery systems by maximising the thermoelectric response. Despite these advancements, there has not yet been reports of a system suitable for commercialisation. Currently, different electrolyte chemistries and electrode materials are being explored to determine which combinations have the best performance and are most suitable to be deployed in real world scenarios. The first phase of our research project has been completed by performing a literature-driven screening over 80 redox pair combinations using criteria relevant to TREC systems, such as thermoelectric coefficient, stability and compatibility. The four most promising combinations were analysed further by evaluating their performances in a theoretical TREC scenario. Three of the four pairs have appeared in the literature and have been physically evaluated in varying ways, while one of the identified potentially high-performing combinations has not yet been investigated for TREC purposes in any physical form. Given the interest of PBAs for TREC systems in recent literature, there is a strong case for further exploring the manufacture of these electrodes and how they can be optimally designed for use in flow-cell based heat recovery systems, which previously featured non-active electrodes. In 2025, the focus of our research is to characterise CuHCF and NiHCF electrodes for this purpose as well as explore physical TREC characteristics of the unexplored Fe ^2+/3+ ||Cu ^0/2+ redox pair. For the former topic, the effect of mass loading of CuHCF and NiHCF electrodes are being investigated using electrochemical impedance spectroscopy (EIS) to measure the resistive effects of increased mass loading. Additionally, this will be mapped to the charge density parameters. The outcome will provide critical information on the design of PBA electrodes with optimal mass loading for TREC flow cell systems. Secondly, experimental investigation of Fe ^2+/3+ ||Cu ^0/2+ will shed light on the viability of this previously untested TREC redox pair. Thermoelectric characterisations will be performed to validate the high alpha coefficient identified in the screening stage. A dual loop TREC flow cell system will be deployed to simulate a waste heat recovery scenario and allow meaningful testing of these redox pairs and any future investigations.

Increased attention has been focused on photoelectrochemical redox flow cell systems as a potential integrated technology for simultaneously converting and storing intermittent solar energy. Photoelectrochemical voltammetry and impedance spectroscopy tests were conducted using a single-junction c-Si photoelectrode immersed in Fe(CN)63−/4− under thermal load to evaluate the temperature effect on the thermo-electrochemical performance of silicon-based photoelectrochemical cells. It was observed that the current density significantly increased with temperature as a consequence of enhanced kinetics and electrolyte characteristics, while a detriment to the potential output was identified and predominantly attributed to variations of photovoltaic characteristics. Moreover, it was demonstrated that mass transport enhancement reaches its maximum contribution at 45 °C, followed by a slowdown in the observed trends at higher temperatures, which may lead to improved design development and optimized working conditions.

Correction for ‘Evaluation of redox pairs for low-grade heat energy harvesting with a thermally regenerative cycle’ by José Tomás Bórquez Maldifassi et al., Energy Adv., 2024, 3, 2877–2886, https://doi.org/10.1039/D4YA00368C.

We present a comprehensive survey of the redox couples and end up with 81 different electrolyte combinations to highlight the untapped potential of the thermally regenerative electrochemical cycle.

There is significant recent interest in clean hydrogen production from renewable water electrolysis. In general, alkaline water electrolysis (AWE) operates with a voltage of ca. 2 V due to ohmic and overpotential-derived loss (0.7~0.8 V) [1]. In addition, it requires 0.5~1 hours for the ramping-up, which hinders its wider implementation. Our intended solution is to introduce a heat-tolerant electrolyser integrated with a chemically resistant solar thermal collector, providing a heated electrolyte to increase the reactivity [2] and reduce the ramp-up time. In this work, we discuss key dimensional design aspects of the thermal collector for alkaline electrolyser, considering the control feasibility of operating temperature and fluidic parameters. Furthermore, we also demonstrate our proof-of-concept experimental results using our alkaline electrolyser integrated with the solar-thermal collector at an elevated temperature. The time-dependent current output of the solar-thermal integrated AWE over the first 1 h yielded ca. 70% higher compared to the conventional AWE system. E. Amores, J. Rodríguez, J. Oviedo, A. De Lucas-Consuegra, Open Eng 2017, 7,141–152. B. Zhang, Q. Daniel, M. Cheng, L. Fan, L. Sun, Faraday Discuss 2017, 198, 169–179 Figure 1