High performance solid-state iron-air rechargeable ceramic battery operating at intermediate temperatures (500–650 °C)
Graphical abstract
Introduction
The constant growth of the renewable energy market requires to develop an efficient electrical storage system. In view of a wide-spread utilization of renewable resources, the difference between generation and load will become significant in specific periods while energy cannot be transmitted efficiently through the existing electrical grid [1]. An economic and efficient energy storage system with a long lifetime is strongly necessary [2]. The electrochemical storage appears one of the most appropriate solutions to balance energy demand and offer [3]. Moreover, future implementations will require an enhanced collaboration between households and other stakeholders, such as the distributed system operators and retailers to help with transitioning to a sustainable energy system [4].
Today, the best technology for the electrical energy storage is represented by rechargeable batteries [5]. Other storage systems are based on different approaches, such as mechanical systems (water pumping, flywheels [6]), electrical systems (double sheet capacitors, magnetic superconductors [7]) or utilize other electrochemical devices, e. g. flow batteries [8] or electrolysers for power to gas applications [9], [10], [11], [12], [13]. It is widely accepted that batteries located at the customer site will provide more flexibility versus central battery systems [14], while advanced algorithms may be employed to give a reliable state-of-health estimation for the battery [15].
The most widely used rechargeable batteries are lithium-ion batteries (lithium-cobalt, lithium-phosphorous, lithium-manganese, lithium-nickel, lithium-iron-phosphate, etc.) due to their flexibility, high performance, adaptability to appliance electronics and photovoltaic systems [16]. If for domestic household uses there could be an economic justification for the current costs, between 600 and 1000 €/kWh and 4–8 kWh of energy content, for large systems, to compensate for the variability of production, significantly lower capital costs are required while keeping energy density at suitable levels [17], [18]. The cost of lithium batteries is higher than other types of widely used electrochemical storage systems, such as lead-acid batteries (250–300 €/kWh), but they offer high energy density, much greater discharge depth and much longer lifespan, thus justifying such growing interest [19], [20].
Another well-known class of secondary batteries is the metal-air one. These devices have attracted a lot of attention as possible alternative to lithium-ion batteries to cover the market applications requiring high energy density. On the other hand, they suffer of fast decay in performance upon electrochemical cycling, very poor round-trip efficiency and short cycle-life [21]. The research on metal-air storage is mainly addressed to low temperature zinc-air (Zn-Air), lithium-air (Li-Air), iron-air batteries (Fe-air), according to the different type of anode materials [22], [23], [24].
Zn-Air batteries are promising energy sources because they are cheap and characterized by a low environmental impact [25]. The high specific charge of the zinc electrode affords the possibility to obtain exceptionally high specific capacity. However, both the energetic efficiency and cyclability properties at high power need to be improved. Instead, under low power operation, this technology shows a stable energy efficiency, about 60%, in a hundred cycles [26], [27]. Wang et al. have developed primary Zn–air batteries achieving energy density of 872.3 Wh kg−1 [28]. Chen et al. have shown a performance of 773 Wh kg−1 for Zn-air batteries and charging-discharging cycling stability over 100 cycles [29]. Wang et al. have achieved a specific capacity of 682.6 mA h g−1 for Zn–air batteries [30].
The Li-air, or Li-oxygen, batteries are other promising storage systems due to the very high energy density; the theoretical specific energy density is about 11,000 Wh/kg [31]. One of the most important issue, that presently hinder the diffusion and marketing of such devices, is the negative electrode corrosion due to metallic electrode reaction with water. Humidity is naturally presented in the air and its removal causes technological issues. With the introduction of a protected lithium electrode, the development of this technology could lead to marketing of batteries with energetic density without precedents [16], [32].
Low temperature Fe-air batteries present several advantages such as a low-cost (100 €/kWh), safety characteristics, use of abundant raw materials, environmental friendliness and they are easily scalable. In addition, the risk of short circuiting by dendrite formation in the iron-air battery is much lower than in Zn batteries. The present theoretical energy density is 50–75 Wh/kg. However, the formation of a passive layer on the iron surface increases the cell resistance, and it decreases the utilization efficiency of iron. Recently, composite materials of carbon and iron nanoparticles were reported for the improvement of the utilization efficiency and the discharge capacity [33], [34]. Low temperature Fe-air batteries are a substantially established technology with a reference market.
High temperature Fe-air batteries are new types of ceramic battery systems generally using an internal redox cycle involving hydrogen and water at the anode together with the iron reactions. Their development is still at an infant stage yet, but they promise significant advances [35], [36], [37].
Fe-air batteries operating at 500–800 °C produce high quality heat and if properly designed, can result in high energy densities and low spontaneous discharge. This novel battery system, operating at high temperature, is much less affected by external environmental conditions than in the case of low temperature batteries. If properly designed, these new storage systems can provide high energy density and low spontaneous discharge whilst being characterized by lower cost compared to Li-based batteries [35], [36], [37]. In particular, these systems can eventually utilize waste heat for start-up [38], [39], [40], [41], [42], [43], [44]. In several industrial processes, such as those dealing with steel and cement production, as well as in the ceramic industry, as much as 20–50% of the energy consumed is ultimately lost via waste heat contained in the streams of hot exhaust gases and liquids, and through heat conduction, convection, and radiation from hot equipment surfaces. Waste heat sources cover a wide range from low (<230 °C) to high-temperature (>650 °C). Considering the temperature level and the nature of waste heat, proper recovery methods, which include generating electricity, preheating of combustion air, and space heating, are very useful. Waste heat is essentially free of charge and characterised by zero emissions. Nowadays, most factories, among those producing waste heat, have their industrial buildings equipped with solar panels. Thus, combining the waste heat resource with high temperature batteries to manage the surplus of energy from renewable power sources can play a relevant role in the next generation energy system [45].
Regarding high temperature batteries, Zhao et al. investigated the performance of an iron-air battery that was operated at 550 °C, using cerium oxide nanoparticles incorporated into the Fe-Fe3O4 couple, obtaining a specific discharge energy corresponding to 91% of the theoretical specific energy, with a round-trip efficiency of about 82% [46]. Zhang et al. reported a solid oxide iron-air redox battery (SOMARB) operating at 500 °C suitable for large-scale energy storage; the battery delivered a discharge specific energy of 960 Wh/kg at 80% iron utilization and 600 Wh/kg at 50% utilisation, with an average cycle efficiency of 63% over 25 cycles [47]. Fang et al. showed a concept of Fe-air battery at 800 °C based on a hybrid between the conventional technology of solid oxide fuel and electrolysis cell, with a maximum charge capacity of 30 Ah, round trip efficiency above 95%, a loss of 11% of initial capacity after 130 cycles [48]. Inoishi et al. developed a solid oxide fuel cell concept applied for Fe-air rechargeable battery by using H2/H2O as a mediator for the Fe redox processes. They showed 10 charge-discharge cycles with a capacity above 250 mAh/g [49]. Sakai et al. demonstrated 10 charge-discharge cycles at 600 °C by using Ni-Fe and BaLaCoO electrodes, with capacity of about 700 mAh/g-Fe and energy density of 600 mWh/g-Fe [50].
Most of the concepts developed so far for high temperature Fe-air batteries are based on the use of the H2/H2O couple as redox mediator at the anode where the Fe/FeOX active matter is allocated.
In this case, the electrode is Ni based whereas Fe/FeOX is the reactive anodic matter. The electrochemical charge process includes formation of hydrogen by electrolysis of water vapor in the negative electrode chamber; the following processes occur in the charge mode:O2− migrate to the positive electrode to evolve molecular oxygen whereas the formed hydrogen reduces the Fe-oxide in the anode compartment to metallic Fe.forming water. Water is thus continuously formed until all Fe oxide is reduced.
During charging, metallic Fe reacts with water to form hydrogen. H2 is oxidised to H2O by the O2− ions migrated from the cathode to close the cycle.
As a new concept, Trocino et al. [44] reported a simple configuration for a Fe-air rechargeable battery operating at 800 °C, where both the Ni electrode and the H2/H2O redox processes are avoided and the system is completely operating in the solid state. The main redox processes during the battery charging are:
During discharging, metallic Fe is oxidised to FeOX by consuming O2− coming from the molecular oxygen reduction at the cathode. In this system, the anionic ceramic electrolytes that allow to transport O2− ions are CGO (Gadolinium-doped Cerium Oxide) and LSGM (Lanthanum Gallate doped with Strontium Oxide and Magnesium Oxide). The absence of Ni and H2 in such a system makes reactions kinetics at the anode slower. However, the Fe-air battery based on LSGM showed promising characteristics, such as 1 V of open circuit voltage, electric capacity of 0.3 Ah/g and energy density of 0.22 Wh/g. However, coulombic efficiency was about 42% [44].
In the proposed concept, the presence of a small amount of ceria in the composite anode favoured the ionic percolation of O2− within the Fe electrode and both ceria and gallate electrolytes also acted as charge buffer in the case of overcharging by reversible modification of their redox states [44].
In this work, such solid-state battery concept is further developed for operation at temperatures lower than 800 °C while performance and energy density are significantly increased [44]. The focus of this work is to operate the battery at intermediate temperatures e.g. 650 °C since these operating conditions can reduce degradation issues while allowing to use cheap ferritic steel interconnectors in a battery stack device. Cell conditioning can be eventually carried out at higher temperatures where reactions kinetics are faster just for a few cycles. Thus, the anode matter can be activated more rapidly to form metallic iron similarly to what occurs for other metals involved in solid-oxide electrochemical systems [51].
The operative temperature range explored in this work varies between 500 °C and 800 °C. Excellent performance and cyclability were achieved at 650 °C for the solid-state battery. This concept combines the simplicity of operation of the Fe-air solid-state battery and an intrinsic safety with high energy density and durability. The excellent dynamic behaviour, the absence of any effect from the external environmental conditions, the use of cost-effective ceramic materials, the possibility to produce high quality heat, thus covering the full chain of electrical and thermal energy, make such systems largely appealing for applications related to renewable power sources.
Section snippets
Experimental section
An iron-air battery based on a complete solid-state configuration is shown in Fig. 1.
A co-doped, strontium and magnesium, lanthanum gallate (La0.8Sr0.2Ga0.8Mg0.2O3-LSGM) electrolyte-supported planar button cell was studied. This included an Fe-CGO electrode (Fe2O3-C0.8Gd0.2O2 in the completely discharged state) with a composition made of 70 wt% of Fe and 30 wt% of CGO and strontium-doped lanthanum ferrite-cobaltite (La0.6Sr0.4Fe0.8Co0.2O3-LSFCO) air electrode. The cell had an active area of
Results and discussions
The first test consisted of three charge-discharge cycles from 800 °C to 700 °C to activate the battery by forming metallic iron in a temperature range where reaction kinetics are relatively fast. In this step, the charge current was 1.4 A g−1 and the discharge one was 0.53 A g−1. At 800 °C, a specific capacity of 128 mAh g−1 and a specific energy of 123 mWh g−1, with a voltage efficiency of 75% and a faradaic efficiency of only 9%, were obtained. This was due to the cell conditioning during
Conclusions
In this work a solid-state ceramic iron-air battery for optimal operation at intermediate temperatures (e. g. 600–650 °C) has been reported. The observed electrochemical properties appear appropriate for application in the field of energy storage from renewable power sources. High temperatures 700–800 °C were essentially explored in term of conditioning process with the aim of focusing the attention on the intermediate temperature operation 650–550 °C. The electrochemical properties of this
Acknowledgments
This work was supported by an agreement between the Ministry of Economic Development, Italy (MiSE) and the National Research Council, Italy (CNR) in the framework of a Research Program for the Electric System (AdP-PAR 2015/2017).
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