Well-Aligned Hierarchical Graphene-Based Electrodes for Pseudocapacitors with Outstanding Low-Temperature Stability

The relatively poor performance stability of pseudocapacitors over a wide temperature window (i.e. temperature stability), particularly at low temperatures, hinders their practical applications. Here, well-aligned hierarchical pseudocapacitive electrodes are fabricated, featuring run-through channels in a graphene network (GN) as ion-buffering reservoirs, open inter-sheet channels between vertical graphene nanosheets (VGNSs) for fast ion transport and MnO2 nanopetals on VGNSs for efficient interfacial pseudocapacitive reactions. With reduced ion diffusion length and charge-transfer resistance as well as improved ion-transport rate, the capacitance of pseudocapacitive electrodes decreases from 541 to 490Fg(-1) at 1Ag(-1) as the temperature drops from 25 to 0 degrees C, revealing a high capacitance retention of 90.7%. Furthermore, the specific capacitance of a symmetric device based on the hierarchical electrodes at -30 degrees C maintains 80.8% of the room-temperature capacitance. Such outstanding temperature stability is comparable to the state-of-the-art electric double-layer capacitors. Importantly, 86.0% of capacitance is retained after repeated heating and cooling at temperatures ranging from -30 to 60 degrees C for 5000 cycles. Asymmetric supercapacitors with the hierarchical architecture in the positive electrode exhibit stable performance over a wide temperature range. These results demonstrate the rationality of the electrode design for practical energy storage applications in harsh temperature environments.