Spinel-type solar-thermal conversion coatings on supercapacitors: An effective strategy for capacitance recovery at low temperatures

Solar-thermal conversion is the most direct strategy of solar energy utilization, and spinel-type transition metal oxides as photo-thermal conversion (PTC) materials have attracted great attention because of their attractive optical property and durable stability under strong sunlight or harsh temperature condition. Supercapacitors are important electrochemical energy storage devices, but sluggish ionic mobility and freezing of electrolytes at low temperatures directly lead to low or even no capacitance. Herein, we report an effective strategy to recover the charge storage ability of sandwich-type supercapacitors by employing a series of spinel-type PTC coatings. Under solar irradiation, these PTC coatings can effectively capture sunlight and convert it into thermal energy, thereby increasing the actual operation temperature of supercapacitor devices. At low ambient temperatures (-20 and -30 degrees C), the devices coated with Cu1.5Mn1.5O4 film showed the highest temperature increase (about 60 degrees C) because of the best PTC effect. Consequently, a significant capacitance improvement was achieved (from close to zero up to desirable values), thus enabling the devices to work well at such low ambient temperatures. This works offers a highly effective method to improve low-temperature performance of supercapacitors and, simultaneously, endows a new application of spinel-type PTC coatings for supporting electrochemical energy storage devices.