High-temperature (HT) LiCoO2 recycled from spent lithium ion batteries as catalyst for oxygen evolution reaction
Graphical abstract
HT LiCoO2 synthesized using the cathodes of LIBs as raw materials as catalyst for OER.
Introduction
The oxygen evolution reaction (OER) plays an important role in devices for energy conversion and storage, such as fuel cells, metal-air batteries, and water electrolysers. However, the large- scale use of such devices is limited by the sluggish kinetics of the OER. Research on efficient electrocatalysts is extremely important [[1], [2], [3], [4]]. Pt, IrO2, and RuO2 have conventionally been used as are catalysts for OER. However, owing to the high cost and scarcity of their components, their extensive use is not feasible. Hence, the material cost, electrocatalytic activity, and long-term stability of catalysts for OER must be taken into account [1,3,4]. In alkaline media, the OER process is kinetically favorable and takes place as a multi-step reaction with single-electron transfer at each step. As shown in Eq. (1), O2 detachment requires the transfer of four electrons in a multiple electron transfer mechanism. One electron is involved per step, which causes an accumulation of energy leading to the sluggish kinetics of the OER and a large overpotential that has to be overcome [5]. The main reaction for O2 production requires the transfer of four electrons according to the reaction
An alkaline medium is generally preferred over an acidic one for the electrolysis of water. In an alkaline medium, the materials required for construction are less expensive and less susceptible to corrosion. Relatively inexpensive elements, such as Fe, Mn, and Pb, present lower electrochemical performance for OER compared to more expensive elements, such as Pt, Ir, Ru, and Co. Cobalt is a non-precious metal and forms promising electrocatalysts such as Co3O4 [3,4,[6], [7], [8], [9]]. The redox couple Co3+/Co4+ has been extensively studied for OER application in Co3O4 compounds with different synthesis methods and morphologies [3,[10], [11], [12], [13]]. Cobalt oxide (Co3O4) has Co2+ and Co3+ on the tetrahedral and octahedral sites, respectively. Many studies have shown that the Co2+ would be electro-oxidized into Co3+ during the electrochemical process, and the Co3+ are further oxidized into Co4+ before the onset of the OER. The Co4+ ions thus formed are the actual active OER species [7,14]. From this point of view, many Co compounds, such as sulfides and doped oxides, have been investigated for their potential application in electrocatalysis using the redox couple Co3+/Co4+ [7].
Lithium cobaltate (LiCoO2) is one of the most important Co oxides because it has long been used as cathode material in Li-ion batteries owing to its high specific capacity, long cycle life, and high potential [[15], [16], [17], [18]]. With a view to reducing the cobalt dependence, many other oxides, such as LiNiO2 and LiNi1-xCoxO2, have been studied. Since 2010, LiMn1/3Ni1/3Co1/3O2 has been increasingly used as the cathode in commercial Li-ion batteries [[19], [20], [21]]. Recycling spent Li-ion batteries is an alternative for reducing the environmental impact of metal extraction processes, and it has been proven to be technically feasible [[22], [23], [24], [25], [26], [27], [28]]. LiCoO2 can exist in two forms: low-temperature (LT) LiCoO2 and high-temperature (HT) LiCoO2. LT LiCoO2 has a spinel-type structure with the space group Fd3 m. It is obtained at temperatures below 400 °C. HT LiCoO2 has a hexagonal and lamellar structure with the space group R-3 m. It is obtained at temperatures both below and above 400 °C [15,[29], [30], [31]].
HT LiCoO2 is widely used as cathode material. The HT LiCoO2 used in this study was previously synthesized by recycling a Li-ion battery, and its capacitance was suitable for its applicability as a promising cathode material. However, its application as an electrocatalyst for OER can be studied by considering different synthesis methods and precursor materials. In a previous study by this group, synthesized HT LiCoO2 was characterized using thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and its electrochemical performance as a cathode material was tested [22]. In this work, cobalt is recycled from a spent Li-ion battery as Co(OH)2, which serves as a precursor material for synthesizing HT LiCoO2 using Li2CO3 as a reagent. OER activities of Ni and Pt in alkaline media are widely known, and in this work, they were tested for comparison [5,32]. The HT LiCoO2 was used as a catalyst for OER. Electrochemical tests were accompanied by cyclic voltammetry (CV), chronoamperometry, generation of Tafel slopes for different temperatures, and electrochemical impedance spectroscopy (EIS).
Section snippets
Battery dismantling and synthesis of LiCoO2 using Li2CO3 and Co(OH)2 as precursors
A spent Li-ion battery from an HP notebook was manually dismantled and separated into its basic components. The cathode was separated and dried at 120 °C for 24 h (h), after which it was washed with deionized water at 40 °C and dried at 60 °C for another 24 h. Next, active material was separated from the aluminum substrate. Leaching was performed under stirring for 2 h at 80 °C by dissolving 9.2600 g of the cathode material in 450 mL of H2SO4 solution (0.5 mol L−1) and 50 mL of H2O2 (30% v/v).
Synthesized material characterization
The synthesis of the HT-LiCoO2 used in this paper has already been reported by this group [22]. XRD analysis confirmed the presence of HT LiCoO2 in the final product. Further, Raman spectroscopy confirmed the presence of the phase in HT LiCoO2 without the presence of other phases. XRD patterns of the synthesized powder (HT LiCoO2) was compared with those of LiCoO2 (card no. 16-427) and Co3O4 (card no. 42-1467) from the Joint Committee on Powder Diffraction Standards database files (see
Conclusions
HT LiCoO2 is a very important cathode material for Li-ion batteries, from which Co can be recovered through a recycling process. The recovered Co can be used to synthesize new cathode materials. In this study, the HT LiCoO2 synthesized previously by this group was evaluated for its use as an electrocatalyst for OER.
CV and chronoamperometry tests showed that, in the presence of HT LiCoO2, the OER starts at a smaller potential compared to Ni and Pt owing to the Co3+/Co4+ oxidation. The activation
Acknowledgments
The authors acknowledge NCQP, DQUI-UFES, CAPES, CNPq, FAPES, Laboratório de Ultraestrutura Celular Carlos Alberto Redins (LUCCAR).
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