Date Published: January 15, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Shuhua Yang, Yuanyue Liu, Yufeng Hao, Xiaopeng Yang, William A. Goddard, Xiao Li Zhang, Bingqiang Cao.
The metal oxides/graphene composites are one of the most promising supercapacitors (SCs) electrode materials. However, rational synthesis of such electrode materials with controllable conductivity and electrochemical activity is the topical challenge for high‐performance SCs. Here, the Co3O4/graphene composite is taken as a typical example and develops a novel/universal one‐step laser irradiation method that overcomes all these challenges and obtains the oxygen‐vacancy abundant ultrafine Co3O4 nanoparticles/graphene (UCNG) composites with high SCs performance. First‐principles calculations show that the surface oxygen vacancies can facilitate the electrochemical charge transfer by creating midgap electronic states. The specific capacitance of the UCNG electrode reaches 978.1 F g−1 (135.8 mA h g−1) at the current densities of 1 A g−1 and retains a high capacitance retention of 916.5 F g−1 (127.3 mA h g−1) even at current density up to 10 A g−1, showing remarkable rate capability (more than 93.7% capacitance retention). Additionally, 99.3% of the initial capacitance is maintained after consecutive 20 000 cycles, demonstrating enhanced cycling stability. Moreover, this proposed laser‐assisted growth strategy is demonstrated to be universal for other metal oxide/graphene composites with tuned electrical conductivity and electrochemical activity.
Supercapacitors (SCs) are becoming a critical member of energy storage systems and have been widely used in consumer electronics, hybrid electric vehicles, uninterruptable power supplies, and so on.1, 2, 3 SCs can be classified into two categories: electrical double‐layer capacitors (EDLCs) where charge is stored by charge separation at the electrode–electrolyte interface, and pseudocapacitors that store charge by fast Faradic reactions. EDLCs employing carbon materials have low capacitance, whereas pseudocapacitors utilizing metal oxides (NiO, Co3O4, MnO2, and RuO2) and conducting polymers exhibit low rate capability and short cycle life.4, 5, 6, 7, 8, 9, 10 A strategy to overcome these problems is to grow or anchor metal oxides or conducting polymers onto carbon‐based material to obtain novel composite electrode materials, which derive their benefits from synergistic effects. Despite the promising results and significant progress in this field, rational synthesis of such electrode materials with controllable conductivity and electrochemical activity is still the topical challenge for high‐performance SCs.
In summary, we take the Co3O4/graphene composite as a typical example and have developed a universal in situ laser‐assisted strategy to grow ultrafine metal oxides/graphene composites via simple laser irradiation in solution at ambient conditions. Under laser irradiation, the GO is reduced to LG with a low transfer resistance for efficient charge storage and delivery, while the porous Co3O4 nanorods completely smash into ultrafine Co3O4 nanoparticles with short ionic diffusion distance for fast ion transport. The as‐obtained UCNG composites exhibit high‐specific capacitance and excellent cycling stability. In particular, when the current density is increased from 1 to 10 A g−1, the capacitance of UCNG composites still retains 93.7%, exhibiting outstanding rate capability. ESR measurements and DFT calculations also reveal that new oxygen vacancies defect states are generated after laser irradiation, which serve as electron reservoir during the electrochemical process. We demonstrate that the unique UCNG composite nanostructures with the abundant oxygen vacancies on the ultrafine Co3O4 nanoparticles surfaces and the tightly anchoring of Co3O4 nanoparticles onto the graphene sheets could provide good electrical conductivity, high electrochemical activity, and effective pathways for rapid ionic/electronic transport and fast reversible Faradaic reactions. Combining the remarkable rate capability, outstanding capacitance and excellent cycle stability, we conclude that the UCNG composites are good electrode materials for supercapacitor compared to previous samples in literature. In addition, this novel synthetic method can be extended to other metal oxide/graphene composites, which will have great potentials in not only the energy storage field, but also in numerous other frontiers.
Synthesis of Porous Co3O4 Nanorods (P‐Co3O4): All chemicals were of analytical grade and used without further purification. In a typical synthesis, 0.996 g of cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O) and 1.0 g of polyvinyl‐pyrrolidone (PVP K‐30) were added in 40 mL of polyethylene glycol (PEG‐400). The mixture was kept vigorous stirring at 60 °C until Co(CH3COO)2·4H2O and PVP K‐30 were completely dissolved. Then, 1.0 g of urea (CO(NH2)2) was added to the resultant homogeneous solution. After stirring for 15 min, the reaction mixture from the first step was transferred to a Teflon‐lined stainless autoclave for solvothermal reaction at 160 °C for 20 h. After that, the product was washed with deionized water and ethanol for several times, and then dried in a vacuum oven at 60 °C for 12 h. The as‐prepared powder was annealed at 300 °C for 2 h in air with a heating rate of 1 °C min−1 to acquire porous Co3O4 nanorods (denoted as P‐Co3O4).
The authors declare no conflict of interest.