Abstract
High voltage electric double layer capacitors with reliable cycling performance are in high demand for energy storage applications. In this research, studies of the process-structure-property relations of commercial activated carbon (AC) electrode modified with atomic layer deposition (ALD) have been performed in terms of coating of various oxide compounds. These investigations have yielded similar enhancements of cycling performance at high voltages (3.5V) enabled by the ALD coating of oxides on AC surfaces. The commonalities are the phenomenal increase in capacitance retention of coin cell supercapacitors after 1000 charge/discharge cycles regardless of the electric nature of ALD deposited oxides. Minor differences in coating morphologies and uniformities were observed between the aluminum oxide, titanium oxide, and manganese-cobalt oxide. ALD coating of these oxides of about 20-growth cycle (1-2 nm in thickness) was found to effectively result in reliable cycling performance, lower impedance, and significantly low leakage current. The authors proposed a modified Gouy-Chapman-Stern model to explain the remarkable increase in operating voltage and cycling performance by the increased Stern layer thickness due to the ALD oxide coating. These results demonstrate that the ALD oxide coating has excellent potential for activated carbons for high operating voltage and energy density supercapacitor.
Highlights
An electric double layer capacitor (EDLC) is constructed when both positive and negative electrodes are made of carbon-based materials, such as activated carbon (AC), and electrolytes
To find any reduction in leakage current after atomic layer deposition (ALD) coating on the AC substrate, we have investigated the self-discharge profiles of bare AC and ALD coated ACs after charging them
The AC electrodes coated with 2 nm thick oxides effectively maintain the capacitance retention at >95% after 1,000 cycles (>20% higher than uncoated AC electrodes), demonstrating commonalities in the ALD coating effect
Summary
An electric double layer capacitor (EDLC) is constructed when both positive and negative electrodes are made of carbon-based materials, such as activated carbon (AC), and electrolytes. Atomic layer deposition (ALD) has proven the effectiveness of conformal coating of ultrathin layers at three-dimensional structures of energy materials for the increased cyclic performance at higher voltages owing to the surface passivation of carbons or Li-rich battery electrodes (Chen et al, 2010; Liu et al, 2012; Jung et al, 2013; Gandla and Tan., 2019). Seeing the advantages of ALD, Tan and Zhao (2014) disclosed an innovative approach in their patent application, applying an ultrathin aluminum oxide coating on the commercial AC electrodes using the ALD technique, which leads to a >3.5 V operating window for the EDLCs. The Al2O3-encapsulation added a thin layer of dielectric material on the AC surface, which may increase the thickness of the Stern layer and decreases the electrical leakage. Using a commercial AC electrode as an ALD substrate to deposit these
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