1 Introduction

With the rapid development of the economy, the burning of coal and fossil resources has brought problems of environmental pollution and resource shortage. Therefore, it is urgent to find low-cost and green alternative energy [1,2,3,4]. Renewable energy sources, composed of wind and solar, tidal, and wave etc. are clean and can be converted into electrochemical energy [5,6,7,8,9]. However, intermittence of these energies endows the development of energy storage devices with significance. As one of them, supercapacitors show the advantages of high powerful density and specific energy, fast charging/discharging, excellent electrochemical stability, and so on [10,11,12,13,14]. Carbon-based materials [15,16,17], metallic oxides [18, 19], and conductive polymer [20, 21] can be ideal electrode materials. Among them, porous carbon materials show excellent supercapacitor performance because of their high specific surface area, outstanding chemical stability, and developed pore structure [18, 22, 23].

Plant biomass is the only sustainable source of organic carbon for the time being. Therefore, the activated porous carbon materials from plant biomass have attracted much attention [24,25,26,27,28,29,30]. In general, specific surface area, aperture distribution, and functional groups on the surface of activated carbon are the main determining factors deciding the performance of their supercapacitors [31,32,33,34]. Activated carbon with high specific surface area and abundant aperture structures facilitate ion diffusion. Abundant carboxyl groups and hydroxyl groups benefit the wetting of electrodes [35,36,37]. The walnut green peel presents a kind of network structure, composed of glucose, naphthoquinones, terpenoids, and polyphenols [38, 39]. And the walnut green peel is rich in carbon elements and oxygen functional groups. This gives a strong hint that walnut green peel is a potential outstanding raw material to prepare activated porous carbon materials as supercapacitor electrodes. In addition, China’s annual output of accessible walnut green peel is more than 350,000 tons [40, 41]. Therefore, making good use of walnut green peel is not only economical but also can effectively reduce environmental pollution.

In this paper, walnut green peel was used as the raw material to prepare activated porous carbon in different activation temperatures by direct pyrolysis and KOH activation [25,26,27,28,29,30]. The intrinsic properties of these activated porous carbons were explored based on their morphology, composition, and pore structure. Under the optimized annealing temperature, the prepared activated porous carbon showed a high specific surface area of 1404.3 m2 g−1 and was demonstrated to show excellent electrochemical performance with a large specific capacitance (236 F g−1 at 0.5 A g−1) in 6.0 M KOH electrolyte in the three-electrode system and had high capacitance retention rate, comparable to the previously reported activated carbons derived from biomass including poultry litter [13], lotus leaf [14], fungal hyphae [16], wheat flour [23], larch [28], tobacco rod [31], lemon peel [34], cotton [42], baobab fruit shell [43], pinecone [44], walnut shell [45], orange peel [46], seaweed [47, 48], onion [48], and rice straw [49]. Table 1 showed their specific capacitance performance.

Table 1 Specific capacitance of reported activated carbon of different biological wastes
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2 Experimental

2.1 Synthesis of activated porous carbon from walnut green peel

Walnut green peel was collected from the China University of Geosciences, Beijing, then washed, dried, and ground into powder. The walnut green peel powder was calcined under N2 at 500 ºC for 2 h to obtain the optimized pre-carbonization products (Fig. S1). The pre-carbonized walnut green peel powder and KOH were thoroughly mixed at a mass ratio of 1:4. After that, the mixture was heated at 600, 700, and 800 ºC for 2 h under N2/Ar to obtain three different carbonization products named WGL-6, WGL-7, and WGL-8 respectively. The products were rinsed to be neutral (pH = 7) and dried to be used. The synthetic process was displayed in Scheme 1.

Scheme 1
scheme 1

The schematic diagram for activated carbon WGLs from walnut green peel

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2.2 Electrochemical measurement

All electrochemical performance tests were performed on a CHI760E electrochemical workstation using a three-electrode system in 6.0 M KOH electrolyte. Platinum and Hg/HgO electrodes were used as the counter electrodes and reference electrodes, respectively. The working electrodes were prepared by coating WGLs, acetylene black, and PVDF on the nickel foam in a certain proportion.

3 Results and discussion

SEM images in Fig. 1a, d, and h Fig. 2 exhibited the morphology of WGL-6, WGL-7, and WGL-8. WGL-6 and WGL-7 showed a sheet-like morphology and the thickness of WGL-7 nanosheets were ~ 70 nm, while WGL-8 exhibited the agglomerated structure. TEM images further revealed the morphology of these carbon materials. TEM images of WGL-6, WGL-7, and WGL-8 were displayed in Fig. 1b, e, and i. The sheet-like morphology with abundant pores can be clearly observed in WGLs. Scattered graphitic structure domains of WGL-7 and WGL-8 can be indicated by obvious graphite layer fringes in high-solution TEM images (Fig. 1f, j), which can facilitate electrical conductivity of carbon materials and charge transfer in supercapacitors. Lattice spacing value of ~ 0.32 nm corresponded to (002) plane of graphite [42]. WGL-6 had no obvious graphite structure domain due to low calcination temperature (Fig. 1c), indicating that its low degree of graphitization element distribution of WGL-7 was investigated by energy dispersive spectrum (EDS) elemental mapping (Fig. 1e–f). Two elements, C (89%) and O (12%), were uniformly distributed in WGL-7.

Fig. 1
figure 1

SEM image (a), TEM image (b), and HR-TEM image (c) of WGL-6; SEM image (d), TEM image (e), HR-TEM image (f) of WGL-7; SEM image (h), TEM image (i), and HR-TEM image (j) of WGL-8

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Fig. 2
figure 2

The elemental mapping images (a–c) of WGL-7

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The crystallite structure of WGL-6, WGL-7, and WGL-8 was demonstrated by X-ray diffraction (XRD) (Fig. 3a). They all showed a broad peak at 20 ~ 30° and a peak at 43°, corresponding to planes (002) and (100) of graphite [50]. The XRD patterns indicated the limited graphitization of WGL-6, WGL-7, and WGL-8. Raman spectra analysis was performed and shown in Fig. 3b. Two significant peaks at ~ 1350 and ~ 1580 cm−1 were assigned to D band and G band of carbon [29] and represented structural defects and graphite carbon structure, respectively. The defects in WGL-6, WGL-7, and WGL-8 were evaluated by the calculated relative strength ratio (ID/IG). The ID/IG values of WGL-6, WGL-7, and WGL-8 were 0.87, 0.91, and 0.93, which demonstrated that all the carbon materials possessed abundant structural defects after activation and calcination. Fourier transform infrared spectroscopy (FT-IR) spectra of WGL-6, WGL-7, and WGL-8 were shown in Fig. 4c . WGLs showed two sharp peaks at ~ 1230 and ~ 1035 cm−1, attributable to the C = O and C–O stretching vibrations [43]. The absorption peaks at ~ 1390 cm−1 can be attributed to the –OH to bend vibrations and peaks from ~ 1550 to ~ 1840 cm−1 were derived from the C = O and C = C in-plane vibration []. FT-IR results indicated that WGLs had abundant hydroxyl groups and carboxyl groups.

Fig. 3
figure 3

XRD patterns (a), Raman spectra (b), Infrared spectra (c), XPS survey spectra (d), and high-resolution C 1 s (e) and O 1 s (f) XPS spectra of WGL-6, WGL-7, and WGL-8

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Fig. 4
figure 4

Contact angle test images of WGL-6 (a), WGL-7 (b), and WGL-8 (c)

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The XPS survey spectra of WGL-6, WGL-7, and WGL-8 were shown in Fig. 3d. Only C 1 s and O 1 s peaks without other obvious impurities were observed. High-resolution C 1 s XPS spectra were displayed in Fig. 3e. Three deconvolved peaks at 284.8, 286.2, and 289.0 eV corresponded to C–C, C–O, and C = O, respectively [43, 51]. The O 1 s spectra of WGL-6, WGL-7, and WGL-8 in Fig. 3f can be curve-fitted as three peaks at 531.4, 532.5, and 533.8 eV, which were assigned to element O in bonds of C = O, C–O, and O = C–OH functional groups [43, 51, 52]. The C element contents of WGL-6, WGL-7, and WGL-8 were 85.7%, 87.0%, and 91.5%. And the O contents were 14.3%, 12.9%, and 8.5%, respectively. The element content based on XPS analysis of WGL-7 agreed with the EDS mapping results. It can be concluded that higher activation temperature contributed to the decrease of O content. It was acknowledged that O-functional content played an important role in the surface characteristics of carbon-based materials. Therefore, contact angle tests (Fig. 4) were carried out to investigate their wettability. The contact angles of WGL-6, WGL-7, and WGL-8 showed in Fig. 4 were 111.7°, 112.8°, and 126.4°, respectively. It was demonstrated that porous carbon WGL-6 and WGL-7 with more oxygen-containing functional groups exhibited higher wettability.

To determine the pore texture properties and specific surfaces area of WGL-6, WGL-7, and WGL-8, BET tests were performed. The nitrogen adsorption–desorption isotherms were shown in Fig. 5a. The isotherms exhibited a feature of type I and type IV isotherms, which demonstrated the presence of both micropores and mesopores in WGL-6, WGL-7, and WGL-8 [44, 52, 53]. The corresponding specific surface areas of WGL-6, WGL-7, and WGL-8 were calculated to be 1159.7, 1404.3, and 1067.9 m2 g−1, respectively. The results verified that activation temperature of 700 ºC showed the highest surface area, which facilitated to enhance the specific capacitance. The pore size distribution (Fig. 5b) calculated by density functional theory (DFT) methods further confirmed the presence of micropores and mesopores in WGL-6, WGL-7, and WGL-8, and the pore size was distributed at mainly between 2 and 10 nm. Based on the BET results, it can be predicted that all WGLs can be potential in a prospective application because of their high specific surface area and abundant apertures, and WGL-7 would exhibit the best super capacitive performance.

Fig. 5
figure 5

N2 adsorption–desorption isotherms (a) and pore size distributions (b) of WGL-6, WGL-7, and WGL-8

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The electrochemical properties of carbon materials were measured by cyclic voltammetry (CV) scans and galvanostatic charge–discharge (GCD) tests. Figure 6a–c showed the CV curves of WGL-6, WGL-7, and WGL-8 at 5–60 mV s−1. All the curves exhibited the approximate rectangle feature, indicating that WGL-6, WGL-7, and WGL-8 showed typical electrochemical double layer capacitors (EDLC) behavior. Figure S2a-S2c gave the charge–discharge curves at 0.5–10 A g−1 of WGL-6, WGL-7, and WGL-8. All curves presented a symmetrical triangle, indicating that WGLs had good capacitance reversibility. Figure 7a compared the CV curves of WGL-6, WGL-7, and WGL-8 at 5 mV s −1. A larger CV integrated area of WGL-7 indicated its better specific capacitance as supercapacitor electrode material. Figure 7b compared the charge–discharge curves of WGL-6, WGL-7, and WGL-8 at 0.5 A g−1. The specific capacitances of WGL-6, WGL-7, and WGL-8 were calculated to 211, 236, and 185 F g−1, respectively. These results demonstrated that WGL-7-modified electrodes displayed the highest capacitance at all current densities. When WGL-7-modified electrodes were applied in a two-electrode symmetric capacitor (Fig. 7c), its specific capacitance reached 57.2 F g−1 at 0.5 A g−1, where the energy density was 31.8 W h kg−1 and the power density was 1003.5 W kg−1. The cycle stability test of WGL-7 at 10 A g−1 was shown in Fig. 7d. After 3000 cycles of charge–discharge, the capacitance retention rate of WGL-7 remained still about 94.4%.

Fig. 6
figure 6

CV curves of WGL-6 (a), WGL-7 (b), and WGL-8 (c) at 5–60 mV s−1

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Fig.7
figure 7

CV curves at 5 mV s−1 (a), GCD curves at 0.5 A g−1 (b) of WGL-6, WGL-7, and WGL-8; c GCD curves of the WGL-7 at 0.5–10A g−1 in a two-electrode system; d the capacitance retention of WGL-7

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To understand their impedance behavior performance, electrochemical impedance spectroscopy (EIS) tests were performed. Nyquist diagrams of WGL-6, WGL-7, and WGL-8 were displayed in Fig. 8a. In the high-frequency range, all WGLs showed small semicircles and their equivalent series resistance (ESR) were all less than 0.1 Ω, illustrating the WGL-modified electrodes exhibited low charge-transfer resistance and negligible series resistance. In the low-frequency region, the straight line of WGL-7 showed the maximum slope compared to WGL-6 and WGL-8, indicating that WGL-7 was more potential in capacitive performance. Figure 8b gave the capacitance-frequency plots of WGL-6, WGL-7, and WGL-8. Specific capacitances of WGL-6, WGL-7, and WGL-8 at the low-frequency of 0.01 Hz were 119, 146, and 114 F g−1, respectively. WGL-7 displayed the highest specific capacitance at the same frequency. The trend was in accordance with the results of CV and GCD tests. Nyquist diagrams can be fitted by the equivalent circuit shown in Fig. 8c.

Fig. 8
figure 8

Nyquist diagrams (a), the relationship of the specific capacitance and the frequency (b), and corresponding equivalent circuit (c) of WGL-6, WGL-7, and WGL-8. Inset in (a) showed the Nyquist plots in high-frequency region

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4 Conclusion

In summary, walnut green peel was successfully converted into activated carbon materials WGLs by the carbonization and KOH activation. WGLs showed high specific surface areas with abundant aperture and structural defects, and were potential in supercapacitors. Among them, WGL-7 activated at 700 ºC had the highest specific surface area (1404.3 m2 g−1) as well as modest oxygen doping and wettability. EIS tests also showed WGL-7 exhibited fast charge-transfer and potential application in capacitive performance. The capacitance of WGL-7-modified electrodes achieved 236 F g−1 at 0.5 A g−1 and showed excellent cycle stabilization in 6 M KOH electrolyte. The study demonstrated that converting walnut green peel into activated porous carbon as electrode materials for supercapacitor provided a potential avenue to reduce environmental pollution caused by walnut green peel and maximize its value.