Ruthenium nanoparticles decorated curl-like porous carbons for high performance supercapacitors

The synthesis of highly dispersed and stable ruthenium nanoparticles (RuNPs; ca. 2–3 nm) on porous activated carbons derived from Moringa Oleifera fruit shells (MOC) is reported and were exploited for supercapacitor applications. The Ru/MOC composites so fabricated using the biowaste carbon source and ruthenium acetylacetonate as the co-feeding metal precursors were activated at elevated temperatures (600–900 oC) in the presence of ZnCl2 as the pore generating and chemical activating agent. The as-prepared MOC carbonized at 900 oC was found to possess a high specific surface area (2522 m2 g−1) and co-existing micro- and mesoporosities. Upon incorporating RuNPs, the Ru/MOC nanocomposites loaded with modest amount of metallic Ru (1.0–1.5 wt%) exhibit remarkable electrochemical and capacitive properties, achiving a maximum capacitance of 291 F g−1 at a current density of 1 A g−1 in 1.0 M H2SO4 electrolyte. These highly stable and durable Ru/MOC electrodes, which can be facily fabricated by the eco-friendly and cost-effective route, should have great potentials for practical applications in energy storage, biosensing, and catalysis.

. The Raman spectra of the MOC and Ru/OMC samples exhibited two main peaks located at 1363 and 1585 cm −1 (Fig. 2b), which may be attributed to vibration bands of carbons in disordered graphite (D band) and the E 2 g mode of the graphite (G band), which is related to vibrations of sp 2 carbon structure in two-dimensional (2D) hexagonal lattice 70 . Moreover, for MOC carbonized at elevated temperatures (T c ≥ 800 o C), an additional vibrational peak corresponding to the overtone of the D band (i.e., the 2D band) at 2712 cm −1 was also observed 71 . Accordingly, the G to D band intensity ratio (I G /I D ) is normally used to assess the crystalline structure of the graphitic carbons, as depicted in Table 1. That an I G /I D ratio of 1.21 was observed for the MOC-900 and Ru/MOC-900, indicating a high degree of graphitization for the PAC supports.
Results obtained from N 2 adsorption/desorption isotherms (77 K) showed that all MOC-based samples exhibit H1-type isotherms (see Fig. 2c; cf. IUPAC classification). The presence of a weak hysteresis loop at P/P 0 of ca. 0.4 together with the notable increase in N 2 uptade at low relative pressures reveal the coexistence of micro-and mesoporosities in these carbon substrates 72 . Accordingly, the specific surface areas and pore volumes responsible for the micro-and mesoporosities may be derived, as depicted in Table 1. Further calculations by density functional theory (DFT) indicate that these MOC materials have an average mesopore size of ca. 4.0 nm (see Supporting Information, Fig. S1). Based on the above results, it is clear that the MOC-900, which possesses the highest BET surface area (S tot = 2522 m 2 g −1 ) and total pore volume (V tot = 1.78 cm 3 g −1 ), exhibits superior textural properties and I G /I D value. Compared to its counterparts carbonized at relatively lower temperautres, it is indicative that the MOC-900 substrate is more suitable for application as electrode material owing to the anticipated higher electrical conductivity and porosity, which are favorable for electron transport and ion diffusion. Upon loading RuNPs onto the MOC-900 support, consistent decreases in both microporous and mesoporous surface area and pore volume with increasing Ru loading were observed (Table 1), indicating the successiful dispersion of metal NPs in both types of pores. This is also supported by the pore size distribution profiles (Supporting Information, Fig. S1), which showed notable decrease in micropores along with narrowing of mesopore distribution.
The role of activating agent, namely ZnCl 2 , during activation of MOC is worthy for further exploration. During the process, the impregnated ZnCl 2 tends to promote dehydration of the carbon substrate, leading to charring and aromatization along with the creation of porosities. It is anticipated that mobile liquid ZnCl 2 (m.p. ∼ 283 o C) should be formed during the initial stage of the activation. Further increasing the activation temperature beyond 750 o C (b.p. of ZnCl 2 ca. 730 o C), strong interactions between carbon atoms and Zn species, which result in considerable collapses between the carbon interlayers to create pores in the matrix, as illustrated in Fig. 3 73 . It has been shown that the generation of micro-and mesoporosities (Table 1) is provoked by the elimination of hydrogen and oxygen atoms from the carbon substrate by ZnCl 2 , leading to the formation of water rather than oxygenated organic species 74 . The amount of activating agent, activation temperature, and subsequent washing by HCl were all found to have profound effect on the evolution of porosity within the MOC. At a fixed activation temperature, a gradual increase in BET surface area (S tot ) of the as-synthesized MOC with increasing dosage of ZnCl 2 was observed (Supporting Information, Fig. S2). For examples, at a low carbonization temperature (600 o C), the MOC prepared in absence of ZnCl 2 exhibited a rather low S tot = 50.6 m 2 g -1 . On the other hand, the surface area of MOC increased from ca. 210 to 718 m 2 g -1 when prepared in the presence of 0.5 and 2.0 g of the activating agent (Supporting Information, Fig. S3). A more pronounced effect was found for activation temperature, for example, upon increasing the temperature from 600 to 900 o C while in presence of a fixed amount of    S tot and S micro denotes total and microporous surface area, respectively; S micro determined by t-plot analysis. (e) Total pore volume in cm 3 g −1 calculated at P/P 0 = 0.99 of the N2 adsorption/desorption isotherm; V tot , V micro , and V meso represents total, microporous, and mesoporous pore volume, respectively, V meso = V tot -V micro . (f) Average pore size determined by non-local DFT calculations. (g) G to D band intensity ratio obtained from Raman data.
The TGA profiles of various MOC and Ru/MOC nanocomposites are displayed in Fig. 2d, their corresponding DTA curves are shown in Fig. S4 of the Supporting Information. Aside from the slight weight-loss below 150 o C due to desorption of physisorbed water, all samples also showed a strong weight-loss at ca. 620 o C, which should be associated with combustion of the MOC material 77 . The nearly complete weight-loss observed for the as-syntheized MOC-T c (T c = 600-900 o C), indicating a complete oxidation of MOC by combustion and that nearly no trace of other ingredients (e.g., Zn species) were present in the PAC materials 78 . By comparison, while the Ru-loaded MOCs also exhibited two distinct weight-loss peaks at 50-150 and 400-620 o C, respectively, a residual weight-loss of ca. 9.8 and 11.6 wt% was observed for the Ru 1.0 /MOC-900 and Ru 1.5 /MOC-900, respectively, indicating the anticipated presence of remanent rutheniumn oxides.
The morphology and structural properties of the as-syntheiszed and Ru-loaded MOCs are examined by using FE-SEM/TEM, as shown in Figs 4 and 5. The as-prepared MOCs clearly possess abundant porosities, which tend to increase with increasing carbonization temperature, as illustrated in the magnified SEM and TEM images (see Supporting Information, Figs. S5 and S6) 79 . This can be seen by the TEM images of MOC-900 sample taken at   (Fig. 5d), a lattice spacing of 0.230 nm was determined, in excellent agreement with the value derived from the XRD data of the (002) lattice plane (Fig. 2a). Moreover, analysis of the energy-dispersive X-ray (EDX) result (Fig. 5e) indicates the existences of various signals corresponding to C Kα (0.2 keV), Cu Lα ,β (0.9 keV), Cu Kα (8.0 keV), Cu Kβ (8.9 keV), Ru Lα ,β (2.6 keV), Ru Lγ (3.2 keV), and Ru Kα (19.2 keV), respectively. The Cu signals arise from diffuse scattering of the Cu grid support 80 . The above results confirm a complete thermal reduction of Ru(acac) 3 to RuNPs, which are uniformly dispersed over the structural framework of the MOC support.
The surface properties of the MOC-900 sample and Ru/MOC composites were further examined by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 6. The XPS survey spectrum of the as-prepared MOC-900 (Fig. 6a) exhibits the anticipated C 1s (282-290 eV) and O1s (530-535 eV) signals. The spectrum near the C 1s and O 1s regions, which are displayed in Fig. 6d,e, respectively, may further be deconvoluted to identify C-C (284.  Fig. 7a, which all exhibited the typical rectangular shape electri double-layer capacitor (EDLC) behavior. Nonetheless, compared to the Ru/MOC electrode, the metal-free MOC-900 electrode showed only weak capacitive behavior. Among the three MOC-based electrodes examined, the Ru 1.0 /MOC-900 electrode was found to exhibit an excellent pseudocapacitive redox property in the potential ranges of 0.4-0.6 V. As shown earlier, the RuNPs embedded in the Ru/OMC-900 are highly dispersed in the carbon support, leading to a notable decrease in total surface area and pore volume with increasing metal loading ( Table 1). As such, the inferior capacitive performance observed for the Ru 1.5 /OMC-900 electrode compared to its counterpart with lower Ru loading (1.0 wt%) may be attributed to hindrance in electron transport and ion diffusion   by the embedded metal. Overall, a maximum capacitance of 291 F g -1 was observed for the Ru 1.0 /MOC-900 electrode comparing to that of its MOC-900 and Ru 1.5 /MOC-900 counterparts, as shown in Fig. 7b.
To further assess the characteristics of the Ru 1.0 /MOC-900 nanocomposite for its practical application as electrode material for supercapacitors, we conducted further electrochemical studies. As can be seen from the CV curves recorded at vaired scan rates from 10 to 500 mV s -1 shown in Fig. 7c, all curves retained the rectangular-shape even at high scan rates, which indicates that the Ru 1.0 /MOC-900 electrode indeed exhibits excellent capacitive property with good electrical conductivity 82 . Moreover, based on the GCD curves measured for the Ru 1.0 /MOC-900 electrode at different current densities from 1 to 20 A g −1 (Fig. 7d), a gradual decrease in the corresponding specific capacitance with current densitiy from 291 to 94 F g −1 was observed (Fig. 7e). The durability of the Ru/MOC electrode was also evaluated. As shown in Fig. 7f, the Ru 1.0 /MOC-900 electrode retained over 90% of its initial capacitance after more than 2000 charge-discharge cycles when recorded at a constant current density of 4 A g −1 , revealing an extraordinary stability and durability. For comparision, the textural and capacitive properties of other activated carbon (AC)-electrodes derived from various biomass feedstock are depicted in Table 2 23,24,28,[45][46][47][48][49][50][51][52] . It is indicative that the Ru/MOC electrode exhibits comparable capacitive performance even with a modest Ru loading of only 1 wt%. The MOC material derived from Moringa Oleifera fruit shells clearly has the advantage of achieving a high surface area (2473 m 2 g −1 ), this together with the use of a activating agent (ZnCl 2 ) and thermal reduction procedures help to facilitate dispersion of the metal (Ru) NPs and formation of micro-and mesoporosities favorable for electron transport and ion diffusion in the MOC matrix, hence, the superior electrochemical performances and excellent stability and durability well-suited for application of high-performance supercapactiors.
To enhance the energy densities, the fabricated symmetric cell supercapacitor was operated at various potential ranges (Fig. 8a) and scan rates (Fig. 8b). Accordingly, the corresponding calculated specific capacitance of 36. 2 to 11. 5 F g -1 was obtained using the known total mass of the electrode. The increase in capacitive current with increasing scan rate may be ascribed due to intercalation and deintercalation of ions 24 . Likewise, the GCD curves recorded at different potential ranges and current densities are displayed in Fig. 8c,d, respectively, which corresponds to a maximum specific capacitance and current density of 31.6 F g -1 and 0.25 A g -1 , respectively. The specific capacitance so obtained is comparable to earlier literature reports 83 .
The long-term stability of the symmetric cell supercapacitor was also tested upto 1000 charge-discharge cycles at a current density of 0.75 A g -1 . As a result, ca. 97% of its initial capacitance was retained after 1000 cycles (Fig. 8e). Finally, the correlation between power and energy densities of such solid-state device was also investigated; a maximum energy density of 6.3 Wh kg -1 at a power density of 250 W kg -1 was achieved (Fig. 8e).
In summary, a series of novel porous activated carbon materials derived from Moringa Oleifera fruit shells via a facile annealing and chemical activation process using ZnCl 2 as activating agent have been developed and exploited as electrode support for electrochemical energy storage. When incorporated with RuNPs by thermal reduction using ruthenium(III) acetylacetonate as the metal precursor, the Ru/MOC materials so fabricated exhibits not only superior textural properties but also excellent capacitive properties with extraordinarly stability and durability. A maximum specific capacitance of 291 F g -1 was achieved for the Ru 1.0 /MOC-900 o C electrode (Ru loading 1.0 wt%; activation temperature 900 o C) at a current density of 1 A g -1 . The results obtained from a cyclic charge-discharge test showed that the same electrode retained more than 90% of it original capacitance after 2000 consecutive test cycles at 4 A g -1 . We believe that these MOC materials, which possess desirable textural properties, co-exsisting micro-and mesoporosities, and good electrical conductivities should render prospective applications in high-performance energy storage devices, biosensors, and catalysis, especially when combining with different metal oxides or conductive polymers as porous composites.

Materials.
Research grade ruthenium(III) acetylacetonate (Ru(acac) 3 , 96%; Acros), zinc chloride (ZnCl 2 ; Sigma-Aldrich), potassium hydroxide (KOH; Sigma-Aldrich)) and poly vinyl alcohol (PVA; Shimakyu) were purchased commercially and used without further purification. Drumstick fruit shells (Moringa Oleifera; Family: Moringaceae) were collected from Theni district, Tamil Nadu, India. All solutions were prepared using doubly distilled water.   All Raman spectra were recorded on a Jobin Yvon T64000 spectrometer equipped with a charge coupled device (CCD) detector cooled with liquid nitrogen. The backscattering signal was collected with a microscope using an Ar + laser centered at 488 nm as the excitation source.

Synthesis of Activated Carbon (MOC).
The Moringa Oleifera fruit shells were first air-dried and cut into pieces, then subjected to microwave irradiation (typically for 1-2 h) to obtain the pyrolytic MO fruit shells (denoted as MOFS). Typically, the activation of the carbon precursor was carried out by impregnating 1.0 g of MOFS in aqueous solution of ZnCl 2 (2.0 g) under sonication, followed by an evaporation treatment at 80 o C. The dried MOFS/ZnCl 2 mixture was then heated at different temperatures (600, 700, 800, and 900 o C) for 5 h under