Renewable energy generated approximately 21% of all the electricity generated in the United States in 2020, the second-largest power generation following natural gas.1 Although this is an astonishing announcement, further energy shifts toward power generation from renewable resources are required to reduce CO2 emissions and mitigate climate change.2,3 One effective strategy to use renewable energy efficiently is to introduce smart grids, which require a large number of stationary energy storage devices.2,4,5 However, the inorganic batteries currently in use are expensive because they use rare metals that are limited resources and are unevenly distributed, sometimes available only in conflict zones.6,7 In addition, inorganic materials are often synthesized under harsh conditions, such as high temperature and pressure, which consumes a large amount of energy.7,8 To make matters worse, the use of toxic metals, such as cobalt, induces severe environmental pollution.7,9,10 Therefore, it would be a better choice to avoid using such inorganic batteries for smart grids.
Organic batteries, the active materials of which are organic compounds, can potentially solve such environmental issues. Firstly, organic energy storage devices consist of ubiquitous light elements such as carbon, oxygen, nitrogen, sulfur, and hydrogen, without restrictions on resource availability.9, 11–13 Secondly, organic compounds are often synthesized under mild conditions from renewable resources, which does not require tons of energy for mass production as inorganic materials do.7,9,10, 13–15 Thirdly, it is easy to tune the theoretical capacity and redox potential by modifying the molecular structure.9,13,16 Carbonyl compounds, especially quinones, have two redox centers in a single molecule, leading to a high capacity of up to 496 mAh g–1. Their redox potentials are adjustable in the range of 1.7–3.2 V vs. Li/Li+ by molecular engineering.17,18 In addition, such small organic molecules have advantages over other polymer-based organic active materials because their production process can be more straightforward and inexpensive. The formation of conjugated polymers fundamentally lowers both redox potentials and theoretical capacities. The general challenges encountered by small organic molecules are their low electric conductivity and intensive dissolution into the electrolyte.19–23 Several approaches have been proposed to overcome these issues.24–29 One effective method is to impregnate quinones in the micropores of porous carbon materials to provide conductive paths and suppress the dissolution of quinones into the electrolyte.20,22,23,25 By Using this strategy, a full-cell redox supercapacitor with a tetrachlorohydroquinone (TCHQ) cathode and a dichloroanthraquinone (DCAQ) anode was proposed, showing an energy density of ~14 Wh kg–1 with excellent rate performance and no capacity loss even after 10000 cycles.22
Although the aqueous quinone supercapacitor has shown satisfactory performance at the fundamental research scale (electrode mass, diameter, and thickness each less than 10 mg, 7 mm, and 100 µm, respectively), some obstacles must be overcome when considering its practical applications. For instance, it should be verified that the practical-size electrodes with sufficient mass loadings still show capacities and voltages comparable to those of the fundamental research-scale electrodes. Another problem is that this type of redox supercapacitor uses acidic aqueous electrolytes (0.5 M H2SO4 aq.), which requires the use of acid-durable materials for cell components. At the fundamental research scale, a beaker and gold mesh are used for the cell container and current collector, respectively, which are strong against acidic solutions. However, beaker cells are made of glass and can be easily broken by an external shock, which is dangerous for practical applications. In addition, gold-mesh current collectors inflate cell costs. Therefore, these materials must be replaced by other inexpensive, acid-durable materials to address safety and cost issues.
This study is the first to develop practical-scale quinone-based aqueous supercapacitors (10 cm × 10 cm × 0.52 cm, 1 g/electrode) that can charge a smartphone. We studied alternative materials for cell containers and current collectors, as well as cell configurations, and have proposed one possible solution. Finally, we fabricated a high-voltage aqueous supercapacitor up to 7.2 V by connecting twelve cells in series. We believe that the knowledge obtained from this study will promote research on a practical application scale, which has been lacking but is essential and must be tackled simultaneously with fundamental research.