wileyonlinelibrary.comvolumetric capacitances of carbon-based electrodes are relatively low (e.g., ≈60 F cm −3 for commercial devices [ 7 ] and 250 F cm −3 for low-density carbon [8][9][10][11][12] ). In order to further increase volumetric capacitance, highly pseudocapacitive materials such as low-cost transition metal oxide/hydroxides are commonly introduced. [13][14][15][16][17] However, the charge transfer effi ciency, rate capability, and long-term cycle life of pseudocapacitive layers are typically poor because of their relatively low electrical conductivity. [ 13,[18][19][20] Consequently, many studies have focused on designing 3D templates with high surface area and electrical conductivity for pseudocapacitive metal oxides/hydroxides. [ 2,18,[21][22][23][24][25][26][27] Among these templates, macroporous graphene foam has engendered substantial interest as a promising binder-free and lightweight template. [ 22,[24][25][26][27] For instance, Lou and co-workers fabricated asymmetric supercapacitors on graphene foam/carbon nanotube hybrid fi lms with energy densities reported in the range 2.7-8.7 mWh cm −3 . [ 28 ] He et al. developed graphene foam/MnO 2 composite electrodes with an areal capacitance of 1.42 F cm −2 at low scan rates in a three-electrode confi guration and reported an energy density of 0.046 mWh cm −3 for two-terminal symmetric supercapacitor devices based on such electrodes. [ 29 ] Liu et al. fabricated solid-state graphene foam-based supercapacitor devices with an areal capacitance of the order of 10 mF cm −2 . [ 30 ] However, the surface area, electrical conductivity, and mechanical robustness of the templates must improve further for prospective applications in functional devices with high energy, power densities, and high rate capabilities.Graphitic petals (GPs), [ 31,32 ] containing a few layers of graphene that generally grow vertically to a substrate by catalyst-free microwave plasma chemical vapor deposition (MPCVD), are promising candidates as electrode materials with high surface area and electrical conductivity. For instance, Bo et al. recently summarized the advances of GP-based electrodes for supercapacitors, [ 33 ] and a related atomic mechanism for ion transport within GP channels has also been proposed. [ 34 ] GPs synthesized on various substrates have exhibited outstanding electrochemical performance as binder-free double-layer capacitor electrodes [35][36][37] and as effi cient nanotemplates for pseudocapacitive materials in prior work. [ 2,31,35,38 ] In particular, GPs synthesized on 3D substrates such as Ni foam [38][39][40] and carbon cloth [ 2,17 ] as A hierarchical structure consisting of Ni-Co hydroxide nanopetals (NCHPs) grown on a thin free-standing graphene petal foam (GPF) has been designed and fabricated by a two-step process for pseudocapacitive electrode applications. The mechanical behavior of GPFs has been, for the fi rst time to our knowledge, quantitatively measured from in situ scanning electron microscope characterization of the petal foams during in-pla...
