The adsorption, electrosorption, and electrodesorption of aqueous, inorganic arsenic on the granular activated carbon (GAC), DARCO® 12×20 GAC was investigated in solutions containing arsenic as the only contaminant, as well as with chromium, nickel and iron. Darco 1220 was selected for these investigations primarily because it is relatively ineffective as a normal (unassisted) arsenic adsorbent in the chosen electrolytes at the low loadings used. It is shown that the application of anodic potentials in the 1.0 – 1.5V range, however, result in enhanced uptake, most probably due to charging of the electrochemical double-layer at the electrode surface. 100% regeneration of electrosorbed arsenic was achieved via electrodesorption at a cathodic potential of 1.50V. The presence of ad–metal ions was observed to have a significant and complex effect on arsenic adsorption, electrosorption, and electrodesorption. In particular, the Cr:As ratio was shown to have complex effects, decreasing adsorption uptake when present as 3:2, but enhancing adsorption when present as 5:1. Nickel was found to have less of an effect than chromium except at the highest anodic potential used of 1.50V, where it exhibited better performance than chromium. The presence of iron significantly enhanced uptake. With a 1.50V anodic potential, the bulk arsenic concentration was reduced to less than detectable limits, well below the USEPA MCL for drinking water. Regeneration efficiency by electrodesorption for the As–Fe system was greater than about 90%.

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Batteries in energy storage systems are exposed to electrical noise, such as alternating current (AC) harmonics. While there have been many studies investigating whether Lithium-ion batteries are affected by AC harmonics, such studies on Nickel Metal Hydride (NiMH) batteries are scarce. In this study a 10 Ah, 12 V NiMH battery was tested with three different harmonic current frequency overlays during a single charge/discharge cycle: 50 Hz, 100 Hz, and 1000 Hz. No effect on battery internal temperature or gas pressure was found, indicating that NiMH battery aging is not affected by the tested harmonic AC frequencies. This can reduce the cost of energy storage systems, as no extra filters are needed to safeguard the batteries. Instead, the capacitive properties of the batteries give the possibility to use the battery bank itself as a high pass filter, further reducing system complexity and cost.

The NiMH battery is resilient against mistreatment and can be charged in a large temperature window. It also has a high power-capability, which has made it popular in for instance power tools. Today, the battery is often found in large scale stationary energy storage, as well as applications which require high power capability. In large scale energy storage, small differences in run cycles can have large total effects on power efficiency and safety. It is therefore desirable to develop better methods of battery management. In recent years, development in system hardware has made it possible to implement on-line models in battery management systems. Such an implementation requires a model capable of working in dynamic conditions simulating voltage, temperature, and pressure. In this study, a model for this purpose is developed and verified. There are two major features that present a challenge when modelling a NiMH-system dynamically: Open Circuit Voltage (OCV) hysteresis and pressure build-up. Handling the hysteresis effect is important to accurately predict voltage and OCV.[1] OCV is especially important, as OCV is used to estimate the battery state of charge (SOC). Pressure build-up can be an important tool for charge termination, as it is the build-up of oxygen pressure and subsequent temperature increase that signal end of charge, Figure 1. In addition, the hydrogen pressure behavior changes with battery age and can be used to estimate end-of-life. OCV hysteresis is when the open circuit voltage at a certain SOC depends on the path taken to reach that charge state. This mean that a battery that has been charged to 50% will have a drastically different OCV than when the same battery has been discharged to 50%. This effect is believed to arise primarily from structural changes in the positive electrode, where the potential of the electrode surface is dependent on the material structure of the surface. An example of open circuit voltage hysteresis in a NiMH cell is found in Figure 1. The gas phase in the battery consists of four gases: nitrogen, hydrogen, oxygen and water. Nitrogen is an inert remnant from the battery manufacture.[2] Water is in equilibrium with the aqueous electrolyte. Hydrogen is in equilibrium with intercalated hydrogen in the negative metal hydride (MH) electrode. Finally, oxygen is the product of a side reaction that occurs at high potentials on the positive electrode. The production of oxygen at the positive electrode and ensuing oxygen recombination on the negative electrode produces a great deal of heat, which heats the battery and produces the temperature response that can be used to set a temperature derivative determined end of charge criteria. However, the presence of oxygen and high temperatures also drives the major aging mechanism: oxidation of the negative electrode. As the negative electrode is consumed, the capacity decreases. This leads to a difference in the hydrogen equilibrium pressure, as reaching a higher intercalation fraction increases the equilibrium pressure. [3] An implementation of a model for system battery management could help improve the following functions for a NiMH system: SOC estimation, SOH estimation, charge termination, fault detection and aging prevention. Therefore, development of a fully dynamic NiMH model has great value in improving over all system function for large scale energy storage applications. This study presents a fully dynamic and experimentally verified model for the NiMH battery that takes both pressure and OCV hysteresis into consideration. References Axén, J. B.; Ekström, H.; Zetterström, E. W.; Lindbergh, G. Evaluation of Hysteresis Expressions in a Lumped Voltage Prediction Model of a NiMH Battery System in Stationary Storage Applications. J. Energy Storage 2022, 48 (January). https://doi.org/10.1016/j.est.2022.103985. Mank, A. J. G.; Belfadhel-Ayeb, A.; Krüsemann, P. V. E.; Notten, P. H. L. In Situ Raman Analysis of Gas Formation in NiMH Batteries. Appl. Spectrosc. 2005, 59 (1), 109–114. https://doi.org/10.1366/0003702052940503. Tserolas, V.; Katagiri, M.; Onodera, H.; Ogawa, H. Thermodynamical Modeling of P-C Isotherms for Metal Hydride Materials. Trans. Mater. Res. Soc. Japan 2010, 35 (2), 221–226. https://doi.org/10.14723/tmrsj.35.221. Figure 1 Left: Open Circuit Voltage of a NiMH cell with exhibited hysteresis behavior. Right: Pressure and Temperature behavior of a NiMH module during a charge discharge cycle. Figure 1

The NiMH battery is an intercalation battery with a positive NiOH electrode and a negative metal hydride electrode. The electrolyte used is a highly alkaline aqueous solution, typically 6:1 KOH:LiOH. As a consequence of this and the reaction potentials of the electrodes, splitting of water into its constituting elements is a part of normal battery operation. When charging at high SOC, as well as when overcharging, oxygen is formed as a side-reaction. It then diffuses through the separator and recombines at the negative electrode. This recombination cycle causes the temperature in the battery to rise, something that is commonly used to terminate battery charging, either through ΔT or -ΔV criteria. In addition to oxygen production at high state of charge, overdischarging of the battery can produce hydrogen at the positive electrode, with a pressure increase at end of discharge indicating a harmful overdischarge of the battery [1]. Hydrogen is also present in the battery gas composition as a consequence of the negative hydrogen intercalation electrode material which is in equilibrium with gaseous hydrogen, an equilibrium which shifts with electrode hydrogen content [2]. Figure Left: A schematic showing the main gas reactions in the metal NiMH battery. Right: An example cycle of a NiMH battery, showing cell voltage (blue), cell gas pressure (green) and cell surface temperature (teal).The internal gas reactions of the NiMH battery have a great impact on battery function, both in regard to energy efficiency and battery aging. While measurements of battery internal total gas pressure occur in some battery systems, measurements of individual constituents would be impractical in the field, as such measurements require laboratory equipment. Therefore, a gas model that can estimate the internal pressure distribution under dynamic conditions would be a valuable tool to study the effects of different application drive cycles, as well as a possibility to further enhance BMS function for battery stability and longevity.There have been many attempts at modeling the gaseous side reactions in the NiMH battery. Some of these models are part of comprehensive battery models, which combines a voltage response model with added side reactions [3,4]. Other models look only at specific gas related processes [5]. However, a limitation with these models is that they are not able to model the battery during dynamic current conditions. The main reason for this is the presence of a strong open circuit voltage hysteresis, something that these models do not capture well.This study presents a model of the gas reactions and heat generation in the NiMH battery. The model is not coupled to a voltage model, instead it simulates internal temperature and pressure when supplied with experimental current, voltage and surface temperature data. This removes the complication of the open circuit voltage hysteresis effect and lets us model dynamic battery behavior. The model is in zero dimensions and based on physical principles. There are several parameter...