1 Introduction

Large volumes of production and consumption of coffee, in turn, lead to large volumes of waste coffee generation. It is both environmentally and economically beneficial to utilize the waste coffee in a way that circles back to the user or economy as a higher value product. A recent study indicates that around 50 wt% of the harvested coffee is discarded as spent coffee grounds (waste coffee) [1]. In order to effectively valorize the waste coffee, it is crucial to determine the constituents of these wastes. For this purpose, Mata et al. investigated spent coffee grounds and found out that traditional waste coffee comprises around 50 wt% carbohydrates, 12 wt% lipids, 10 wt% proteins, 2 wt% chlorogenic acids, and 0.5 wt% caffeine [2]. Most of this content turns into carbonaceous materials and hydrocarbon gasses under high temperatures and a protective atmosphere. The pyrolysis of various organic materials (biomass) to obtain high-value carbonaceous materials has been explored in numerous reports [3,4,5,6,7,8,9]. In addition, coffee contains several aromatic groups in its structure and the degree of aromaticity changes with its aroma. This kind of material having aromatic groups can be a good source for fabricating fullerene-like structures and spherical graphene in the presence of metal catalysts by applying heat treatment [10]. In other words, thermal recycling techniques like pyrolysis convert waste sources into value-added carbon-based products with the upcycling approach.

Upcycling is a significant concept to bring an end to the life cycle of materials and open various new application routes. In recent years, there has been a growing tendency to reuse waste coffee directly or carbonize waste coffee by applying the pyrolysis process in energy and composite applications. For instance, Sun et al. reported a two-step process for pyrolysis of coffee shells at 750 °C for 40 min under excess N2 atmosphere to obtain coffee char and a further activation and pyrolysis at 850 °C for 10 min resulting in the formation of activated carbon structure [11]. This obtained material had a well-distributed microporous structure with a surface area of 2817 m2/g that can be used as an adsorbent for water vapor in solar drying systems. In another work, Liu et al. developed a catalytic pyrolysis and alkali activation method by first mixing waste coffee with FeCl3, pyrolyzing at 700 °C under argon atmosphere for 2 h, followed by KOH activation and pyrolysis at 800 °C for 2 h under argon atmosphere [12]. In this study, the synthesized material consisted of 3D porous architecture with 3549 m2/g surface area and showed high specific capacitance and capacitance retention as high-performance electrodes for supercapacitor applications. On the other hand, pretreatment processes with a metal catalyst and KOH treatment are mainly applied on waste coffee or other carbon-based waste resources to activate its surface during thermal treatment, but this leads to other environmental issues such as the accumulation of metal particles and increasing chemical wastes. At this point, environmentally benign and fast approaches can be a solution for recycling waste coffee to minimize the usage of additional chemicals and thus reduce environmental pollution. Another issue is the utilization of these recycled materials in value-added applications such as energy and composite applications and the completion of the value chain. Specifically, the development of electrode materials from biowaste sources has taken great attention to meet the demands in energy storage systems.

There are numerous attempts in the development of bio-based electrodes in energy storage systems. For instance, Purkait et al. demonstrated the conversion of agricultural waste biomass of peanut shells into large area few-layer graphene using KOH activation and pyrolysis [13]. The obtained material was utilized as a supercapacitor electrode providing 58.25 Wh/kg energy density and 37.5 kW/kg power density. In addition, Rufford et al. activated waste coffee grounds with ZnCl2 and pyrolyzed the mixture to fabricate supercapacitor electrodes that exhibit 20 Wh/kg energy density and 368 F/g specific capacitance [14]. Furthermore, Jung et al. produced KOH-activated graphitic porous carbon electrodes by hydrothermal carbonization [15]. The resulting material displayed 97% charge retention and 51 F/g specific capacitance after 1500 cycles at 0.3 A/g. Additionally, the pore size distribution in activated carbon materials is, in most cases, broad and often not optimized owing to incomplete pyrolysis [16]. For instance, Simon et al. reported that longer activation time and higher temperatures led to a larger average pore size that decreased specific surface area and directly affected the electrochemical properties of the electrodes [17]. Therefore, an ideal optimization is required to achieve effective and rapid pyrolysis and get high porous and high carbon content electrode materials from coffee wastes by minimizing CO2 emissions. Moreover, thermal treatment of these kinds of carbon-rich waste sources provides the formation of turbostratic graphene structures, multilayer misaligned, or rotated array of hexagonal carbon atoms. In a recent study, Luong et al. indicated that the formation of turbostratic structure is caused by rapid heating and cooling of the graphitic material such as flash joule heating [18]. This mismatch in the layer conformation results in slight changes in the interlayer distances, crystalline structure, and Raman-active phonon modes [19]. Consequently, there is great potential to use waste coffee as the electrode material to develop sustainable energy storage systems. However, it is essential to attain an ideal structure after the thermal treatment and get a high-power density, reversible discharge/recharge process, and longer lifetime.

Life cycle assessment (LCA) is a technique that comprehensively analyzes and elucidates the effects of the productions, processes, and related technologies on human health and environmental impacts from the cradle-to-grave approach. LCA allows the manufacturers and scientists to identify and find the environmental hotspots [20] for designing their processes to alleviate the hazardous impact on crucial environmental and human health issues [21]. An LCA is to be carried out in compliance with ISO 14040 (2006a) through the stages of goal and scope definition followed by life cycle inventory analysis (LCI), life cycle impact assessment (LCIA), and interpretation of outcomes. The first step of an LCA study is to set the goals and explain scope definition indicating the system boundaries, types of the impacts considered, and functional units practiced [22]. LCI part provides the data of resources, processes, and emissions [23]. After identifying the inputs and processes, LCIA is performed to evaluate the impact by analyzing the data necessary to measure both inputs and outputs found relevant to the processes occurring within the boundaries of the whole commodity system [24]. Characterization factor selection plays a vital role in selecting the impact assessment method while calculating the potential impacts on various subjects such as human health and the environment. Global warming potential (GWP) is one of the most studied methods as an impact indicator [25]. Moreover, climate change, ozone depletion, human toxicity, respiratory inorganics, ionizing radiation, photochemical ozone formation, acidification, eutrophication, ecotoxicity, land use, and resource depletion are some impacts identified by International Reference Life Cycle Data System (ILCD) standard methodology. In the European context, the ILCD handbook on the LCIA methods provides essential sources for the scope of the study [26]. Subsequently, at the last step, the outcomes of studies are interpreted in the manner of data and assumptions that have been made. Eventually, it results in a conclusion that describes the whole system [22]. Despite the advances and growing interest, LCA analysis of graphene and related materials is currently challenging due to the lack of a standardized method for the production process and quality of the products [27, 28].

In the present study, upcycled graphene is produced from the waste coffee by flash pyrolysis in a rotary furnace without activating agents in a simple, fast, and scalable fashion and utilized as an electrode material for the sustainable supercapacitor. This upcycled graphene was assembled with graphene oxide (GO) synthesized by modified Hummers’ method and reduced GO (rGO) [29, 30] by hydrazine treatment utilized as the counter electrode for manufacturing asymmetric supercapacitors, respectively. The electrochemical performance of upcycled graphene was compared with virgin graphenes having different C/O ratios. An ideal supercapacitor configuration was selected based on capacitance values, charge–discharge behaviors, and the results of a life cycle assessment (LCA) study. To the best of our knowledge, there is no comprehensive study for combining virgin and upcycled graphenes in the same supercapacitor system and assessing their environmental impacts by cradle-to-grave technique from raw material to the end-of-life product. A synergistic effect was created by assembling upcycled graphene with virgin graphene materials produced from graphite sources in a hybrid supercapacitor to provide an end to the life cycle of waste coffee. Moreover, consequential process-oriented LCA studies of GO, rGO, and waste coffee-derived graphene were carried out to bring a sustainable approach to graphene manufacturing with the upcycling method and its contribution to the circular economy. Furthermore, a comparative case study brings a new insight on the environmental benefits and impact of producing graphene from flash pyrolysis of waste coffee and fabrication of the supercapacitors, and the assessment of the electrochemical performance of the developed supercapacitors.

2 Materials and methods

2.1 Materials

Waste coffee was collected daily after each consumption and dried in the oven at 80 °C overnight before the pyrolysis process. Graphite flakes (+ 100 mesh, Sigma-Aldrich), hydrazine hydrate (N2H4, 50–60%, Sigma-Aldrich), hydrogen peroxide (H2O2, 30%, Sigma-Aldrich), potassium permanganate (KMnO4, Merck), sulfuric acid (H2SO4, 95–98%, ISOLAB), phosphoric acid (H3PO4, 85%, Sigma-Aldrich), hydrochloric acid (HCl, 37%, ISOLAB), ethanol (C2H6O, 99.9%, ISOLAB), and distilled water were used for the synthesis and reduction of GO. A 6 M aqueous potassium hydroxide (KOH) solution and glass fiber (WHA1825042 GF/F, Whatman) were used to fabricate supercapacitors as electrolyte and separator, respectively.

2.2 Synthesis of GO and rGO

Graphene oxide (GO) was synthesized from graphite by adjusting the process parameters mentioned in the improved Hummers’ method [29, 30]. In this process, graphite flakes (3 g) were mixed with the concentrated H2SO4/H3PO4 (v:v = 9:1) in the presence of KMnO4 (18 g) as an oxidizing agent below 10 °C. Then, the reaction temperature was increased slowly up to 50 °C, and the reaction mixture was kept for 24 h at 50 °C through refluxing. At the end of the reaction, the mixture was poured on the ice bath (800 ml) having 3 ml of H2O2 and washed by centrifugation using distilled water, concentrated HCl (37%), and ethanol to separate GO sheets and remove the unreacted KMnO4. GO was then dried in a vacuum oven at 50 °C for 48 h. In the reduction step, GO was reduced chemically by mixing in an aqueous hydrazine hydrate (the weight ratio of hydrazine hydrate/GO = 1:1) through refluxing for 24 h. Reduced graphene oxide (rGO) powders were obtained by filtration and washed with distilled water several times, then dried at 60 °C for 24 h to remove the residual solvent.

2.3 Upcycled graphene by flash pyrolysis of waste coffee

Upcycled graphene was produced using dried waste coffee and applying direct and flash pyrolysis using a rotating furnace. For this process, a custom-made Protherm rotary furnace was used to perform the pyrolysis process in a short time. Pyrolysis was performed at 1000 °C for 5 s under argon atmosphere with a yield of 15%. The furnace has an angle and rotation control panel, and the speed of material fed into the furnace was adjusted by changing the parameters in a speed and temperature-controlled feed zone. The continuous process was maintained by constantly feeding waste coffee into the rotary furnace and collecting the upcycled graphene at the discharge zone. The temperature was kept constant at 1000 °C at all three furnace zones to provide thermal stability.

2.4 Fabrication of hybrid supercapacitors

Supercapacitor devices were fabricated by using upcycled and virgin graphenes as electrode material binders or other carbonaceous additives. Following previous successful implementations, a custom-made supercapacitor assembly with an asymmetric design comprising two electrodes, separator, and electrolyte was used [31]. Two supercapacitor designs were carried out using upcycled graphene as the first electrode; GO and rGO for the second electrodes, respectively. Highly porous glass microfibers were utilized as the dielectric separator. Additionally, a strong electrolyte comprising a 6 M aqueous KOH solution was used to accommodate the ion transfer due to superior charge transfer properties [32]. Supercapacitor assembly is summarized in Table S1 in the electronic supplementary material (Online Resource 1).

2.5 Material characterization

Various properties of waste coffee and upcycled graphene from flash pyrolysis of coffee were characterized using spectroscopic and microscopic techniques. Raman spectroscopy was carried out using a Renishaw inVia Reflex Raman Microscope with 532-nm edge laser to investigate the molecular structure and vibrational properties of waste coffee and upcycled graphene as well as virgin graphenes. X-ray diffraction (XRD) technique was conducted using a Bruker D2 Phaser diffractometer with a CuKα radiation source. Crystallinity percentages of the samples were calculated using Bruker DIFFRAC.SUITE EVA software. Elemental analysis of each sample was carried out using X-ray photoelectron spectroscopy (XPS) technique. Fourier transform infrared spectroscopy (FT-IR) was used to determine the surface functional groups in the waste coffee, upcycled graphene, GO, and rGO. Electrochemical properties of the supercapacitors were investigated using cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) techniques with BioLogic VMP-300 Multichannel Potentiostat. LCA analysis was performed using SimaPro (release 9.1.1.1) software and ReCiPe 2016 v1.1 midpoint method, Hierarchist version.

3 Results and discussion

3.1 Structural characteristics of virgin and upcycled graphenes

Herein, upcycled graphene with a misaligned turbostratic structure is obtained by flash pyrolysis of waste coffee in a rotary furnace. One of the leading indicators of graphene formation is the characteristic peaks seen in the Raman spectroscopy. Raman spectra of graphene constitute distinct peaks that correspond to specific structural vibrations [33]. The D peak is generally located at around ~ 1350 cm−1 and appears due to the point defects in the planar structure. The G band corresponds to the in-plane stretching vibration of sp2 hybridized C atoms and appears around ~ 1580 cm−1 [34]. The intensity ratio of G and D peaks (IG/ID) gives insight into graphene’s structural order and quality [35]. Figure 1a displays the Raman spectra of waste coffee, upcycled graphene, and their respective IG/ID values. It can be seen that the flash pyrolysis process has led to the formation of graphene-like structures with the IG/ID ratio of 1.08, which is comparable to the chemical reduction [36] or electrochemistry-based [37] graphene synthesis methods, while waste coffee did not show any characteristic peaks. In the literature, especially in the field of electrochemical applications, carbonized biomass-derived materials having significant structural defects are generally named as turbostratic carbon or hard carbon [38]. This hard carbon structure usually contains curved graphene layers in contrast to flat layers in graphite [39]. However, this curved and defective structure causes an increase in D peak indicating an increase in defects [38, 40]. In our work, G peak intensity is higher which confirms a relative order on the hexagonal sp2 bond structure [34]. On the other hand, XRD indicates the formation of turbostratic structures partially with the peaks assigned as seen in Fig. 2. In other words, flash pyrolysis process led to the formation of graphene-like structures due to the occurrence of (002) belonging to main characteristic peak of graphene. Additionally, Raman spectra of graphite precursor, GO, and rGO electrodes were given in Fig. 1b. D peaks for GO and rGO are located at 1349 and 1367 cm−1, respectively, while the G peaks are located around 1590 and 1609 cm−1 with the IG/ID intensity ratios of 1.05 and 0.97 for GO and rGO, respectively. The decrease in the IG/ID ratio results from the decrease in in-plane stretching C sp2 atoms and an increase in the defective vacancy sites due to the chemical reduction [34]. The repeatability of the conversion of waste coffee into upcycled graphene was also validated by checking the quality of the samples from different batches and Raman and XRD characterizations, which are provided in Figures S2 and S3 in the electronic supplementary material, confirmed the reproducibility of the process.

Fig. 1
figure 1

Raman spectra of waste coffee and upcycled graphene from coffee (a) and graphite, GO and rGO (b)

Fig. 2
figure 2

XRD patterns of the waste coffee and upcycled graphene (a) and graphite, GO, and rGO (b)

Changes in the crystalline structure from the waste coffee to upcycled graphene also play a significant role in the electronic properties of the end-product [41]. XRD patterns of the waste coffee and upcycled graphene were investigated to obtain information on the crystalline structures of the samples. Figure 2a shows the XRD patterns of waste coffee and upcycled graphene. Relatively high carbon content in the waste coffee results in the formation of a distinct (002) peak at  = 20° diffraction angle, which is slightly shifted to 2θ = 24° in the upcycled graphene, indicating a wrinkled plane resulting in a turbostratic structure [42]. Moreover, upcycled graphene displays a weak (100) peak at 2θ = 42°, typically observed in turbostratic graphene structures [43]. Furthermore, a less pronounced (002) peak in upcycled graphene suggests a smaller crystalline size in the z-direction [44]. Additionally, XRD patterns of graphite, GO, and rGO are given in Fig. 2b. Also, XRD patterns of virgin graphenes indicate turbostratic graphene peaks around 2θ = 24° and 2θ = 43° regarding (002) and (100) planes, and (001) plane peak in GO. Table 1 summarizes the crystallinities of the waste coffee, upcycled graphene, GO, and rGO. The reproducibility of the conversion from waste coffee to upcycled graphene is confirmed by the multiple XRD patterns obtained from one batch in Figure S2.

Table 1 Crystallinities of waste coffee, upcycled graphene, GO, and rGO

3.2 Surface chemical composition of virgin and upcycled graphenes

X-ray photoelectron spectroscopy (XPS) is a widely used surface chemistry analysis technique to investigate the samples’ elemental composition and molecular structure. As the performance of supercapacitors greatly depends on the surface features of electrode materials and their elemental composition, the determination of these properties using XPS provides beneficial insight into the supercapacitor application. Figure 3 displays the XPS survey spectra of waste coffee and upcycled graphene. It is shown that the C/O ratio of the sample has increased due to the flash pyrolysis process. Figure 4a shows the deconvoluted C1s spectrum of the waste coffee, while Fig. 4b displays the same spectrum for the upcycled graphene. Although most of the C1s peak constitutes of sp2 C–C group at around 284.2 eV, weak C-O peak at around 285.7 eV and C = O peak at around 288.2 eV may suggest an incomplete transformation or slight oxidization by the atmospheric oxygen [45, 46]. Figure 4 also displays the deconvoluted O1s spectrum of the waste coffee (Fig. 4c) and upcycled graphene (Fig. 4d). It can be seen that the majority of the oxygen is present in a C-O formation indicated by the peak around 532 eV. Additionally, C = O groups with the binding energy at around 533.5 eV for both samples are also present in the structure [47,48,49]. Elemental and molecular analyses of waste coffee and upcycled graphene from coffee are summarized in Table 2. XPS analysis results showed that the significant increase in the C/O ratio after the flash pyrolysis confirmed the formation of highly carbonaceous material suitable for supercapacitor electrode application.

Fig. 3
figure 3

XPS survey scans of waste coffee and upcycled graphene

Fig. 4
figure 4

Deconvoluted C1s scans of waste coffee (a) and upcycled graphene (b) and deconvoluted O1s scans of waste coffee (c) and upcycled graphene (d)

Table 2 A summary of the elemental composition of waste coffee and upcycled coffee and their molecular bonds

GO and rGO samples were also investigated using the XPS technique to determine the elemental composition and functional groups on their surfaces. Elemental composition is vital in determining the reduction procedure’s efficiency and restoring the carbon sp2 formation. Figure 5 displays the survey spectra of GO and rGO electrode materials, while Table 3 summarizes the elemental composition and molecular bonds of GO and rGO. According to XPS results, the C/O ratio of GO significantly increased from 1.28 to 5.33 after the reduction process by hydrazine hydrate. In addition, the residual N and S elements were detected due to the treatments by hydrazine hydrate and sulfuric acid. Furthermore, the detailed C1s analysis of GO in Fig. 6a indicates that most of GO constitute of C-O group with the energy around 287.3 eV together with the graphitic backbone of C–C sp2, defect-caused sp3 C–C, and C = O with binding energies at around 284.0 eV, 285.1 eV, and 288.9 eV, respectively [50]. On the other hand, C1s analysis of rGO provided in Fig. 7b indicates significantly restored the formation of C–C sp2 with the binding energy around 284.0 eV. However, there are still several functional groups on the surface, namely C-N, C-O, C = O groups and sp3 C–C caused by the defects at around 285.4, 289.2, 286.6, and 284.6 eV binding energies, respectively [49, 50]. Deconvoluted O1s spectra of GO and rGO, as well as the deconvoluted N1s spectrum of rGO are given in Figure S4 and Figure S5, respectively. Functional groups were also analyzed by Fourier transform infrared (FT-IR) spectroscopy. The conversion from waste coffee to upcycled graphene by flash pyrolysis and GO to rGO and chemical treatment can be clearly detected by changing functional groups in FT-IR spectra provided in Figure S1.

Fig. 5
figure 5

XPS survey scans of GO and rGO

Table 3 A summary of the elemental composition of the GO and rGO materials and molecular bonds in the structure
Fig. 6
figure 6

Deconvoluted C1s spectra of GO (a) and rGO (b)

Fig. 7
figure 7

SEM images of waste coffee before (a, b) and after (c, d) flash pyrolysis at 25,000 × (a, c) and 75,000 × (b, d) magnifications

3.3 Morphological characterization of virgin and waste coffee-derived graphenes

Surface properties of electrode materials play a crucial role in the charge transfer, and it is essential to investigate their surface topography by microscopic techniques. SEM analysis provides to monitor the effect of flash pyrolysis and fast spinning at high temperatures on the waste coffee particles. Figure 7 displays the surface morphology of waste coffee before (Fig. 7a, b) and after the flash pyrolysis in the rotary furnace (Fig. 7c, d), respectively, at different magnifications. As seen in SEM images, flash pyrolysis leads to the sphericalization and size reduction of the particles, and thus the formation of spherical graphene structures with various sizes and conjugation degrees is observed. To date, there are several developed theories on the sphericalization mechanism of carbonaceous materials at high temperatures [51,52,53,54]. The general consensus is that as the temperature increases, aromatic compounds undergo dehydration, polymerization through cross-linking, and carbonization by intermolecular dehydration, respectively, leading to spherical shapes with the lowest surface energy [55]. Moreover, the surface morphology of the GO is also transformed during the reduction process. Figures 8a and b show the surface topography of GO and rGO, respectively. Additional HRSEM images taken at high magnifications provided in Figure S6 display the transformation of the plane and porous structure into spherical geometry after the flash pyrolysis process.

Fig. 8
figure 8

SEM images of GO at 30,000 × (a) and rGO at 15,000 × (b) magnifications

3.4 Electrochemical performance of flash pyrolyzed coffee and graphene-based asymmetric supercapacitors

Charge–discharge (intercalation–deintercalation) behaviors and the Nyquist plots of the upcycled graphene/GO and upcycled graphene/rGO asymmetric supercapacitors were obtained using CV and EIS, respectively. Figure 9a displays the charging behaviors of fabricated supercapacitors for a single cycle, while the inset figure shows the behavior after 50 cycles. CV technique is applied from − 1 V to 1 V to determine the behavior of the supercapacitor, whether if the assembly behaves like an electric double-layer capacitor (EDLC) or displays a faradaic charge mechanism. The formation of humps during the charge–discharge cycle indicated the dominance of faradaic behavior due to a delay while reversing the potential due to the slow kinetic charging process and redox reactions [56]. It is indeed the case for the upcycled graphene/GO device due to the defective surface of GO leading to the delayed discharge, and thus humps in the CV diagram. GO shows typical battery-like redox peaks shown with arrows in the region at 0.6 V and − 0.6 V, whereas rGO sample shows typical pseudocapacitance giving higher performance [31]. As the molecular structure is somewhat reinstituted during the reduction process, charge transfer occurs without delays, as confirmed by the smooth CV curve of the upcycled graphene/rGO device. Figure 9b shows the electron impedance spectroscopy (EIS) measurements obtained from the range of 100 MHz to 10 mHz for the supercapacitor assemblies. The first semicircle in the EIS Nyquist plot indicates the charge transfer resistance (Rct) [31]. Upcycled graphene/GO device displays an Rct around 15 Ω, while the upcycled graphene/rGO device shows a slightly higher resistance around 58 Ω. The main reason EIS data shows higher charge transfer resistance from upcycled graphene/rGO than the GO samples is possibly due to two reasons namely chemical composition and that PEIS technique may only measures EDLC contribution of the supercapacitor.

Fig. 9
figure 9

CV curves (a) and EIS spectra (b) of the fabricated supercapacitors

Additionally, the changes in the specific capacity vs. the potential were investigated for the supercapacitor devices shown in Fig. 10. It is seen that both supercapacitors are capable of effectively charging and discharging while maintaining their specific capacities for the first 50 cycles, giving insight into the stability of the devices. This is an important result while it shows at first that after 50 cycles the degradation is not overwhelming and by increasing the number of the same type of supercapacitor assembly by serial connection it might be possible to have higher specific capacities meaning that the potential increase of energy density, while the capacity is proportional to the energy (E = 1/2CV2).

Fig. 10
figure 10

Cyclic behavior curves of supercapacitor assemblies at the 1st and 50th cycles

In addition, specific capacitance and Coulombic efficiency changes were investigated with the CV technique and shown in Fig. 11, respectively. Upcycled graphene/rGO device showed the specific capacitance values of 0.42 F/g and 0.39 F/g at the 1st and 50th cycles, respectively, displaying 93% of capacitance retention. On the other hand, the upcycled graphene/GO assembly shows 0.25 F/g and 0.16 F/g specific capacitance for the first and 50th cycles, respectively, retaining 64% of the capacitance. Similarly, the supercapacitor efficiency of the Upcycled graphene/rGO device increases as the charging cycles continue, from 80% up to ~ 99.9%. However, the efficiency of the upcycled graphene/GO supercapacitor starts from around 72% and drops to 62%. Even though the electrochemical performance of these supercapacitors seem underwhelming, the obtained spherical nanomaterials can be utilized in other environmental applications, such as waste-water treatment [57, 58] and as a reinforcement material to fabricate green polymer nanocomposites [59] as shown with similar materials.

Fig. 11
figure 11

Specific capacitance (a) and Coulombic efficiency (b) of assembled supercapacitors

3.5 Life cycle assessment analysis of virgin and upcycled graphene-based electrodes

Environmental effects and subsequent sustainability of a particular production process and overall product can be surveyed using life cycle assessment methods. Comparative study on the production routes of virgin graphenes (GO and rGO) and the upcycled graphene from flash pyrolysis of waste coffee is investigated using process-oriented life cycle assessment methods. For this purpose, Ecoinvent 3-allocation cut off by classification and Swiss input and output databases are used. The unit of the analysis covers only one batch of graphene production from waste coffee, GO, and rGO. The end life of the products is selected as municipal solid waste according to the landfill disposal scenario. Volume units are not considered during the waste disposal scenario; therefore, materials that are input as units of masses are evaluated by the system. The LCA study covers the life cycle analysis of the processes, starting from the flash pyrolysis of waste coffee to graphene production used in supercapacitor applications with GO and rGO production processes and their disposal scenarios as waste. Coffee roasting (energy consumption) is not taken into consideration. The input regarding the coffee was the filter coffee that remained on the filter paper. Transportation is also not considered since this study only aimed to elucidate the production technique of graphene. The discussion mainly focuses on the global warming potential and human health, which are the impact factors that are designated as the most crucial factors [25]. Furthermore, a brief discussion on the other factors such as stratospheric ozone depletion, ozone formation, human health, fine particulate matter formation, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity, and human non-carcinogenic toxicity is provided. Ecoinvent database is used for an average evaluation of materials and energy consumption. The contribution of the inputs as an impact is determined by the impact assessment method [60], where various Intergovernmental Panels on Climate Change (IPCC) equivalency factors were considered.

Impact factors resulting from the production of upcycled graphene and virgin graphenes are given in Table 4. It is shown that rGO production has the highest global warming potential of 16 kg CO2 eq which is 5.5 times higher than the impact of upcycled graphene with 2.9 kg CO2 eq. GO has the second-highest global warming potential of 13 kg CO2 eq. Moreover, rGO contributed more to the stratospheric ozone depletion and followed by GO, and graphene obtained from waste coffee, which are 3.68 × 10−5 kg CFC11 (Trichlorofluoromethane) eq, 3.59 × 10−5 kg CFC11 eq, and 2.46 × 10−5 kg CFC11 eq, respectively. rGO production process causes more ozone formation, affecting human health with 0.030 kg NOx eq followed by GO and graphene, which are 0.024 kg NOx eq and 0.007 kg NOx eq, respectively. rGO contributes the most to all 18 characterizations, whereas the graphene from waste coffee has the least. The contribution of rGO to ozone formation is around four times the impact of upcycled graphene. The use of strong acids contributes to the damage inflicted to terrestrial ecotoxicity, freshwater ecotoxicity, and marine ecotoxicity while producing GO and rGO. The dramatic increase in the amount of ecotoxicity impact and human toxicity can result from using hydrazine during the reduction reaction.

Table 4 Environmental impacts of upcycled graphene from waste coffee, GO, and rGO productions

The resulting units of CO2 equivalents are theoretical values relative to the reference value of the material. Eventually, units of the results come out different from each other. Therefore, a normalization ought to be performed to resolve the contradiction of units by dividing the outcomes of the LCIA analysis to the characterization factors that concurrently result in the corresponding impact analysis method. As a result, it becomes easier and more convenient to compare the results. It is not convenient to compare the impact categories without normalization since each ingredient in the analysis has its own multiplication of characterization factors which depend on the impact analysis method [61]. Figure 12 summarizes the normalized characterizations, indicating that marine ecotoxicity, freshwater ecotoxicity, human carcinogenic toxicity, and human non-carcinogenic toxicity are the primary impact categories of the production processes of graphene, GO, and rGO. A more inclusive impact characterization graph is given in Figure S7.

Fig. 12
figure 12

Normalized impact characterization of graphene from waste coffee, GO, and rGO samples

Nowadays, it is vital to consider the impact on global warming potential when planning a production process. Figure 13 displays that the main contributor to global warming potential is the amount of energy (electric) used for the production processes for both raw materials and the final products. The amount of energy used for the rGO and GO is higher due to the complexity and the time consumption of the production routines (centrifuge, heating, drying, and stirring). On the other hand, graphene from waste coffee has a less complex production routine and requires less energy, resulting in less CO2, CH4, and chlorofluorocarbon emission to the environment. Coffee has a higher contribution to GWP than the other substances that are used as raw materials. Moreover, the ethanol used in the GO and rGO contributes significantly to the GWP. The graphene production process results in a less fine particulate matter formation in the environment (0.008 kg PM2.5 eq). Consequently, graphene from waste coffee is less harmful to human health and the environment than GO and rGO.

Fig. 13
figure 13

Impact of production processes and inputs on GWP

4 Conclusion

In the present study, upcycled spherical graphene structures were successfully produced from waste coffee by flash and spinning pyrolysis without using any catalyst or surface treatments. This sustainable and viable approach provides a new source for graphene manufacturing and contributes to the circular economy. SEM confirmed the structural formation of upcycled spherical graphene, and Raman spectroscopy and XPS also monitored the structural transformation from waste to graphene. This upcycled graphene showed a promising electrode performance with functionalized graphene synthesized from graphite in an asymmetric supercapacitor system. Asymmetric supercapacitors with upcycled graphene/GO and upcycled graphene/rGO formations displayed low charge transfer resistances such as 15 Ω and 58 Ω, respectively. In contrast, the latter displayed very high specific capacitance retention of 93% and Coulombic efficiency of ~ 99% over 50 cycles. In other words, the presence of high oxygen content in the counter electrode led to a decrease in the performance of supercapacitors. In addition, a comparative process-oriented life cycle assessment study revealed that the less complex nature of the flash pyrolysis with the absence of hazardous chemicals resulted in a significantly less carbon footprint and global warming potential compared to graphene-related material synthesis, namely GO and rGO. Further optimizations on the processes would lead to even lower CO2 emissions and global warming potential. Moreover, chemical treatments may improve the supercapacitor performance by providing nucleation sites on the waste coffee by tuning the spherical graphene particle size and morphology. Consequently, designing the recycling/upcycling processes by reducing waste like spent coffee and environmental pollution brings new insight into developing supercapacitor electrodes from bio-based sustainable resources and supports the transition to a circular economy. Additionally, upcycled graphene has a potential to replace the similar materials that are being used in environment-related applications such as waste-water treatment and green nanocomposite.