1 Introduction

A continuous industrial growth is associated with rising energy demands and emission of hazardous gases into atmosphere. These negative effects of industrial development can be diminished by reducing emissions of CO2 and other hazardous gases to atmosphere and searching for sustainable energy sources such as H2 or CH4. However, the generation and storage of these fuels are challenging tasks. The U.S. Department of Energy established targets for onboard hydrogen storage systems for light-duty fuel cell vehicles: 0.055 kg H2/kg system for the gravimetric capacity and 0.040 kg H2/L system for the volumetric capacity [1].

Many various materials have been proposed as potential gas adsorbents for commercial applications. Among them activated carbons, metal organic frameworks (MOFs) and graphene materials have attracted a lot of attention; for instance, MOF-210 adsorbed 54.5 mmol CO2/g at 25 °C and 50 bar [2]. In comparison, polyaniline-modified graphene material adsorbed 75 mmol CO2/g at 25 °C and under lower pressure (11 bar) [3]. Activated carbons are among the best CO2 adsorbents at ambient conditions [4,5,6]. For instance, Wickramaratne and Jaroniec [4] reported the CO2 uptake of 8.9 mmol/g at 0 °C and 1 atm for phenolic resin-derived activated carbons. MOFs are considered as potential storage materials for hydrogen, mainly because of their large BET specific surface areas up to ~7000 m2/g; for instance, MOF-174-Mg showed H2 uptake of 4.33 wt% at −196 °C and 20 bar [7].

Activated graphene-based materials also exhibit great gas adsorption properties. A KOH activated graphene oxide (a-rGO) was shown to be very efficient H2 adsorbent [8]. This material adsorbed 7.0 wt% H2 at −196 °C under 40 bar and 4.2 wt% H2 at −80 °C under 120 bar. Nevertheless, a key drawback of the proposed synthesis method is low efficiency—up to 80% loss during activation process. Other activated graphene material had BET specific surface area of 3240 m2/g and showed high gas adsorption for various gases: H2—3.8 wt% (196 °C, 10 bar), CH4—11.3 mmol/g (25 °C, 35 bar), CO2—21 mmol/g (25 °C, 20 bar) [9]. Various activation routes have been explored to develop highly porous graphene materials. Among them, KOH activation was shown to be a very effective route.

It is noteworthy that many effective sorbents were obtained from polymers or waste polymeric materials [10, 11]. Furthermore, composite sorbents obtained by coupling graphene oxide with polymers (e.g., polypyrrole, polyethyleneimine, polyindol) were shown to be effective adsorbents, especially at ambient conditions [12,13,14]. Much less attention has been devoted to the development of sorbents for volatile organic compounds (VOCs) [15,16,17]. Benzene, one of the most common VOCs, is commonly used in the chemical industry as a substrate in organic synthesis and also as a high-energy component of gasoline. Benzene is a subject of air quality assessment because of its well-documented carcinogenicity [18]. Thus, there is an ongoing search for effective adsorbents for benzene vapor and other VOCs. Unfortunately, experimental studies devoted to benzene adsorption are rare, mainly due to the harmfulness of benzene and time-consuming complex measurements. While highly microporous carbons (with pores below 1 nm) are best suited for CO2 adsorption at ambient conditions, high benzene uptake can be achieved for sorbents with large volume of larger micropores and small mesopores [15, 19]. The above mentioned studies indicate that there are still great opportunities in the development of new, highly microporous composites for sorption of various gases such as CO2, H2 and C6H6 by merging polypyrrole with graphene or graphene oxide followed by specially designed activation that generates porosity suitable for the adsorbate used. This topic is discussed in the current work.

2 Experimental

2.1 Chemicals

All chemicals were purchased and used without further purification. Graphite powder (99.99%), potassium permanganate (99.5%), hydrogen peroxide (30%), sulfuric acid (95%), orthophosphoric acid (85%), pyrrole (99.8%), ammonium persulfate (98%), hydrazine monohydrate (80%), potassium hydroxide (85%), potassium citrate monohydrate (99%), hydrochloric acid (35%) and hydrofluoric acid (40%).

2.2 Synthesis of graphite oxide

Graphite oxide was synthesized from graphite powder using a modified Hummer’s method [20, 21]. In brief, 1.5 g of graphite powder was mixed with 9 g of KMnO4 and added slowly to a mixture of concentrated H2SO4 (180 mL) and H3PO4 (20 mL) under stirring. The mixture was than heated to 50 °C and stirred for 12 h. After cooling, it was poured onto ice (~200 mL) with 3 mL of 30% H2O2. Finally, the supernatant was washed copiously with deionized water (DI H2O) by centrifugation and dried under vacuum at 40 °C.

2.3 Synthesis of polypyrrole/reduced graphene oxide

Synthesis of polypyrrole/reduced graphene oxide composite (PPy/rGO) was performed according to the procedure reported by Chandra et al. [12]. In brief, 250 mg of graphite oxide and 1 g of pyrrole were dispersed in 100 mL of DI H2O. Next, 4 g of (NH4)2S2O8 was added to the above mixture and polymerization was conducted for 12 h. The last step in the synthesis of PPy/rGO composite involved reduction of GO with 2.5 mL of N2H4·H2O at 90 °C. Reduced graphene oxide (rGO) without polypyrrole was also prepared.

2.4 Activation of PPy/rGO

Firstly, PPy/rGO was impregnated with 7 M KOH aqueous solution for 24 h. Secondly, after drying it was heated in flowing nitrogen in a tubular furnace at 600–800 °C for 1 h. Thirdly, the resulting material was washed with 8% HCl and next treated with 20% HF in an ultrasonic bath for 5 min. Finally, the product was washed with DI H2O and dried under vacuum at 70 °C to obtain PPy/rGO-KOHx (x denotes the activation temperature in °C). KOH activated rGO was also prepared using the same activation route; the resulting sample was labeled as rGO-KOH700. Additionally, one sample was obtained using potassium citrate as an activation agent. In this case, prior to heating at 700 °C for 1 h in nitrogen flow, PPy/rGO was simply mixed with solid potassium citrate (1:4 PPy/rGO:K weight ratio). Next, the same washing and drying procedures as described above were applied. The product was denoted as PPy/rGO-Cit700.

2.5 Characterization methods

Scanning electron microscopy (SEM), Raman spectroscopy (RS) and X-ray diffraction (XRD) analysis were used to characterize morphology and structure of the rGO, PPy/rGO and PPy/rGO-KOH700 samples. The elemental composition analysis of the samples was performed by energy dispersive spectroscopy method using a FEI Quanta 3D FEG scanning electron microscope equipped with silicon drift detector. RS was conducted on a Renishaw inVia Reflex instrument using 514 nm laser. XRD analysis was carried out on a PANalytical Empyrean diffractometer with Cu Kα X-rays operating at 40 V and 30 mA. Adsorption of N2 and H2 was measured at −196 °C and CO2 at 0 °C using a volumetric adsorption analyzer ASAP 2020 manufactured by Micromeritics Instrument Corp. (Norcross, GA, USA). Benzene adsorption at 20 °C was measured using McBain–Bakr gravimetric method on a home-made apparatus equipped with a quartz-spring balance. Each graphene-based sample was degassed in a vacuum at 100 °C for 24 h prior to adsorption measurements.

3 Results and discussion

Figure 1 shows SEM images of rGO, PPy/rGO and PPy/rGO-KOH700. The images of rGO and PPy/rGO show clearly graphene layers. PPy/rGO-KOH700 shows rather irregular macroporosity (pores above 50 nm) with incorporated highly defected, stacked graphene layers. Table 1 provides the elemental composition (at.%) of the samples. Atomic concentrations in the samples indicate a partial oxidation of graphene surface and trace amounts of impurities. It is noteworthy, that rGO and PPy/rGO show relatively small content of oxygen up to 14.14 at.%. After activation of PPy/rGO the oxygen concentration was further reduced to 10.35 at.%, while nitrogen concentration was reduced from 8.79 to 2.06 at.%.

Fig. 1
figure 1

SEM images of rGO (a), PPy/rGO (b), and PPy/rGO-KOH700 (c, d)

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Table 1 Elemental composition (at.%) of rGO, PPy/rGO and PPy/rGO-KOH700
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The XRD pattern (Fig. 2) shows the (002) diffraction peak at 2θ = 24.59° (rGO), 2θ = 25.87° (PPy/rGO) and 25.52° (PPy/rGO-KOH700) indicating the interlayer distances of 0.36 nm (rGO), 0.34 nm (PPy/rGO) and 0.35 nm (PPy/rGO-KOH700). The (10) diffraction peak at 2θ = 43.16° (rGO), 2θ = 42.95° (PPy/rGO) and 43.52° (PPy/rGO-KOH700) reflects a short range order in stacked graphene layers [22]. This peak is clearly visible for rGO indicating the best ordered structure among the analyzed samples. The Raman spectra of rGO, PPy/rGO and PPy/rGO-KOH700 show intensive D and G bands. The D band is attributed to defects or disorder. High intensity D band (1356 cm−1) in rGO suggests that defects were induced in graphite by oxidation and reduction. The 2D band is the second order of the D band, often referred to as an overtone of the D band. The G band correspond to the sp2 carbon atoms. The G band of rGO is located at 1592 cm−1, while that of PPy/rGO-KOH700 is shifted back to 1586 cm−1 indicating further reduction of rGO during heating up to 700 °C.

Fig. 2
figure 2

XRD pattern (a) and Raman spectra (b) of rGO, PPy/rGO and PPy/rGO-KOH700

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Figure 3 shows the low-temperature nitrogen adsorption–desorption isotherms measured for all samples; additional semi-logarithmic graph of these isotherms is shown in the supporting information as Fig. 1S. Figure 4 displays the corresponding pore size distribution functions (PSDs) for activated composite samples. These PSD functions were calculated based on the low-temperature nitrogen adsorption using the non-local density functional theory method (2D-NLDFT) for carbon slit-shaped pores by taking into account energetic heterogeneity and geometrical corrugation of the surface [23, 24]. The specific surface area (SBET) was calculated using Brunauer–Emmett–Teller method based on the nitrogen isotherms in a relative pressure range of 0.05–0.2 [25]. The total pore volume (Vt) was calculated using volume of nitrogen adsorbed at p/p0 ≈ 0.99. Table 2 shows the structural parameters listed for all samples. The activated composite samples feature high specific surface areas varying from 940 to 2780 m2g. The highest value of SBET (2780 m2/g) and Vt (1.66 cm3/g) was obtained for the composite sample activated with KOH at 700 °C (PPy/rGO-KOH700) (Table 2). This study shows that KOH activation at 700 °C was optimal for achieving high micro-mesoporosity in PPy/rGO. It should be mentioned that the higher activation temperature reduces the efficiency of activation process to 45% yield in the case of PPy/rGO-KOH700. Pore size distribution functions well illustrate structural changes upon activation at different temperatures (Fig. 4). We found that lower temperature in the range of 600–800 °C does not over activate samples, which leads to the creation of large micropores and small mesopores. Maxima on the PSD curves calculated for KOH activated materials appear in the following ranges 0.5–0.8 and 2.0–2.5 nm. In general, all the KOH activated materials possess ultramicropores, supermicropores and small mesopores. Whereas potassium citrate activated sample at 700 °C was highly microporous (92%) and had large volume of ultramicropores (0.31 cm3/g). This shows the importance of activating agent for controlling the range of microporosity in the graphene-based samples.

Fig. 3
figure 3

Low-temperature (−196 °C) N2 adsorption–desorption isotherms measured on all graphene-based materials

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Fig. 4
figure 4

Differential pore size distribution functions calculated for all activated samples

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Table 2 Structural parameters of all samples
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4 Adsorption of CO2, H2 and C6H6

Carbon dioxide adsorption isotherms were measured on the most microporous materials (PPy/rGO-KOH600, PPy/rGO-KOH700 and PPy/rGO-Cit700) at 0 °C up to 850 mmHg (Fig. 5); also semi-logarithmic graph of these isotherms is shown in the supporting information as Fig. 2S. The highest CO2 uptake of 6.8 mmol/g was obtained at 0 °C and 760 mmHg and 4.3 mmol/g at 25 °C and 760 mmHg for PPy/rGO-Cit700 because this material featured the highest fraction of micropores (92%) among all samples studied. At the same conditions PPy/rGO-KOH600 and PPy/rGO-KOH700 adsorbed 5.4 mmol/g and 4.9 mmol/g CO2, respectively. Ultramicropores (<0.7 nm) are mainly responsible for high CO2 uptake at ambient pressure due to the enhanced gas–solid interactions inside fine micropores. Further adsorption studies were conducted for H2 and C6H6 on PPy/rGO-KOH700, which possessed the highest surface area (2780 m2/g). For the purpose of comparison, we also measured C6H6 adsorption isotherm on a commercially available metal organic framework—Basolite Z377 labeled as MOF-Zn, which featured very high surface area (~3800 m2/g). Figure 5 shows H2 adsorption isotherm measured on PPy/rGO-KOH700 at −196 °C up to 850 mmHg and Fig. 6 shows C6H6 adsorption isotherms measured on PPy/rGO-KOH700 and MOF-Zn at 20 °C and relative pressure p/p0 close to unity. The graphene material exhibited great affinity toward H2—11.3 mmol/g (2.3 wt%) at −196 °C under 760 mmHg and C6H6—15.8 mmol/g at 20 °C and p/p0 ~ 1. Note that PPy/rGO-KOH700 shows only a slightly smaller C6H6 uptake (~1 mmol/g) at p/p0 ~ 1 despite its much smaller specific surface area (~1000 m2/g) as compared to that of MOF-Zn. High C6H6 uptake on PPy/rGO-KOH700 was achieved due to its micro-mesoporous structure with small mesopores (2–5 nm). This result agrees well with previous studies of benzene adsorption on activated carbons at ambient conditions [19]; benzene mainly adsorbs in micropores and small mesopores through volume filling mechanism. Table 3 provides CO2, H2, and C6H6 adsorption data measured on PPy/rGO-KOH700 and additionally CO2 adsorption on PPy/rGO-KOH600 and PPy/rGO-Cit700.

Fig. 5
figure 5

CO2 adsorption isotherms measured on the most microporous samples (PPy/rGO-Cit700, PPy/rGO-KOH600, PPy/rGO-KOH700) at 0 °C and H2 adsorption isotherm measured on PPy/rGO-KOH700 at −196 °C

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Fig. 6
figure 6

C6H6 adsorption isotherms measured on PPy/rGO-KOH700 and MOF-Zn at 20 °C

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Table 3 Adsorption uptakes for CO2, H2, at 760 mmHg and C6H6 at a pressure close to the saturation vapor pressure on activated polypyrrole/graphene composites
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To obtain the KOH activated samples we used a slightly modified procedure proposed by Chandra et al. [12], which resulted in composite samples of tailorable structural parameters that are well suited for adsorption of CO2 at 0 °C and 760 mmHg, H2 adsorption at −196 °C, or adsorption of benzene vapor at 20 °C. Treatment of the activated materials with 20% HF (a special caution recommended in experiments involving HF) in an ultrasonic bath was a key to highly develop porosity of the samples. KOH activated polypyrrole/graphene composite reported in ref [12] was microporous (Vt = 0.59 cm3/g, SBET = 1360 m2/g) and able to capture 4.3 mmol/g CO2 at 25 °C and 760 mmHg. No further adsorption measurements (toward H2 or C6H6) were conducted for this adsorbent. In our work, to achieve highly microporous structure we used solid potassium citrate as an activation agent of the polypyrrole/graphene composite, instead of the corrosive, harmful base. The as-prepared sample (PPy/rGO-Cit700) adsorbed the same amount of CO2 (4.3 mmol/g) at ambient conditions (25 °C, 760 mmHg) as above mentioned adsorbent obtained by the another group [12]. This indicates that KOH can be successfully replaced by an environmentally friendly chemical such as potassium citrate. Furthermore, the adsorption data obtained for PPy/rGO-KOH700 are competitive to those reported for other graphene materials. For instance, nanoporous graphene material reported by Chowdhury and Balasubramanian [26] and N-doped graphene aerogel reported by Sui et al. [27] exhibited CO2 uptakes below 3 mmol/g at 0 °C and 1 bar. In the case of hydrogen adsorption, graphene oxide without and with Fe3O4 nanoparticles adsorbed 1.7 and 2.1 wt% of H2, respectively, at −196 °C and 760 mmHg [28]. Only few materials have been tested as potential C6H6 adsorbents; for example, activated saran-derived carbon showed C6H6 uptake up to 11.6 mmol/g at 20 °C and pressure close to the saturation vapor pressure [15].

5 Conclusions

Graphene-based composite materials were synthesized by using polypyrrole and graphene oxide precursors and activated with KOH and potassium citrate. The resulting samples showed high BET surface areas (1650–2800 m2/g), large pore volumes (0.73–1.66 cm3/g) as well as large micropore volumes (0.43–0.78 cm3/g). The KOH activation at 700 °C produced the sample with highest surface area, which showed very high uptakes of hydrogen (11.3 mmol/g at −196 °C and 760 mmHg) and benzene (15.8 mmol/g at 20 °C and relative pressure close to unity). As expected, the best CO2 adsorption (6.8 mmol/g at 0 °C and 760 mmHg) was recorded for the most microporous sorbent (92%), which was obtained by activation with potassium citrate. Our findings confirm that graphene-based composite sorbents exhibit a great potential for gas capture/storage applications.