Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the

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Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the Key Role of Metallic Impurities in Graphene Based Electrocatalysis Vlastimil Mazánek, Jan Luxa, Stanislava Mat#jková, Jan Kucera, David Sedmidubsky, Martin Pumera, and Zden#k Sofer ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07534 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the Key Role of Metallic Impurities in Graphene Based Electrocatalysis Vlastimil Mazáneka, Jan Luxaa, Stanislava Matějkováb, Jan Kučerac, David Sedmidubskýa, Martin Pumeraa, Zdeněk Sofera* a

Department of Inorganic Chemistry, University of Chemistry and Technology Prague,

Technická 5, 166 28 Prague 6, Czech Republic b

Central Analytical Laboratory, Institute of Organic Chemistry and Biochemistry of the Academy

of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic c

Department of Nuclear Spectroscopy, Nuclear Physics Institute of the Academy of Sciences of

the Czech Republic, 250 68 Řež, Czech Republic KEYWORDS: metallic impurities, purification, graphene, electrocatalysis, doping, neutron activation analysis

ABSTRACT: Graphene and its derivatives have been reported in many articles as “metal-free” carbon electrocatalytic materials. Its synthesis procedures are generally based on chemical oxidation of graphite and subsequent thermal or chemical reduction. Since graphene oxide has a large surface area and typically contains variety of oxygen functionalities, metallic ions

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(impurities) from reaction mixtures can be adsorbed on its surface. These impurities can significantly enhance the electrocatalytic activity and thus lead to data misinterpretation provided such impure samples are referred to as “metal free” catalysts. In this paper, we report the synthesis of impurity free graphene which is compared with graphene prepared by standard methods based on thermal and chemical reduction of two graphene oxides. Detailed analysis of graphene prepared by standard methods shows a direct relation between metallic impurities and electrocatalytic activity of graphene. By contrast, impurity-free graphene exhibits a poor electrocatalytic activity.

Over the last decade, graphene and its derivatives have been the most intensively studied metalfree electrocatalytic materials.1 Various derivatives like nitrogen or sulfur doped graphene have been reported as electrocatalytic materials for oxygen reduction reaction,2-5 hydrogen evolution reaction6-9 and many others. In many cases, electrocatalytic activity has been also compared with undoped graphene. A question arises, especially in the case of pure graphene, what is the origin of its electrocatalytic activity. The presence of impurities introduced by synthetic procedures has been recently reported in several papers.10-12 The synthesis of graphene is typically a two-step process based on oxidation of graphite to graphene oxide and its subsequent reduction by various methods. These methods typically include thermal treatment based on high temperature decomposition of oxygen functionalities or chemical methods based on chemical reduction of oxygen functionalities.13 Complex hydrides, hydrazine and many other reagents have been used as reducing agents.14 However, it is usually impossible to completely remove all oxygen functionalities in this process. Thus, the reduced graphene contains various oxygen functionalities which are capable to adsorb a multitude of metallic ions covering the whole periodic table.15-19 Incorporation of manganese into graphene oxide and graphene prepared by permanganate method is a well known example.20,21 However, due to high affinity of metallic ions to graphene oxide and


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its derivatives, all metallic impurities are in general “extracted” from the reagents used in each step of the synthesis. This effect of graphene contamination has been recently reported.10,22,23 Several metallic ions can have a significant effect on the electrochemical activity of such materials, especially for reactions like peroxide reduction or hydrazine oxidation.22 Enhanced electrocatalytic activity towards oxygen reduction reaction has been also often observed for the graphene derived from permanganate methods.10,24-26 Since graphene based materials are in general prepared in twoor more steps, the electrocatalytic activity of “pure” graphene may be influenced by the impurities and unintentional dopants. Standard methods of analysis have many limitations for monitoring of graphene purity. Typical methods like X-ray photoelectron spectroscopy and electron microprobe analysis exhibit a relatively poor detection limit usually not exceeding 0.1 wt.%. Other methods like X-ray fluorescence analysis have a sensitivity below 0.01 wt.%, however, an adequate calibration due to the light matrix of carbon is necessary for exact quantification. Inductively coupled plasma (ICP) method with optical emission (OES) or mass spectroscopy (MS) detection belongs to the most commonly used techniques. Although, these methods are extremely sensitive, the graphene based matrix poses many challenges. For the most accurate quantification down to µg kg-1 levels, the most suitable analytical techniques involve nuclear methods like neutron activation analysis (NAA) or modified inductively coupled plasma methods connected with in-situ high temperature sample decomposition systems (e.g. electrothermal vaporization). A combination of these methods was applied for elemental characterization of impurity-free graphene in this study. More information about detection limits are given in the experimental section. Here, we report the synthesis of ultrapure graphene by the developed purification process based on chemically reduced graphene oxide treatment at temperatures exceeding 2000 °C in a reactive


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halogen atmosphere. Modified electrothermal vaporization (ETV) unit was used for purification of graphene and a combination with an ICP-OES system allowed online monitoring of such process. The purity control was confirmed by a neutron activation analysis. The electrochemical properties of the purified graphene were investigated together with thermally and chemically reduced graphene oxide for comparison and to elucidate the effect of purity and defect density on the electrocatalytic activity. RESULTS AND DISCUSSION The effect of impurities and defects on the graphene properties was investigated in detail using graphene prepared by chemical (N2H4) and thermal (1000 °C in N2) reduction of graphene oxide (GO). Impurity free graphene was prepared from chemically reduced GO by high temperature treatment at 2500 °C in halogenated hydrocarbons atmosphere and subsequent final heating in argon atmosphere. In these experiments, graphenes were prepared from two types of precursors obtained by Hofmann’s synthesis (chlorate based method) and Hummers’ synthesis (permanganate based method). Samples were termed as Ho and Hu indicating the GO precursor and CRG (HoCRG and Hu-CRG) or TRG (Ho-TRG and Hu-TRG) for the chemical and thermal reduction process, respectively. The purified graphenes were termed as Pure (Ho-Pure and Hu-Pure). For the purification, we used a graphite furnace connected to ICP-OES unit for on-line monitoring of the purification process. The heat treatment with duration of about three minutes was repeated three times until no metallic impurities were detected. The intensity signal of the emission lines of manganese, iron and nickel is shown on Figure S1. The temperature dependence indicates a small clean up of iron and manganese even at relatively low temperatures below 500 °C, however, the dominant removal of metals is observed at 800 - 1500 °C, depending on the respective metal and volatility of its halides. For most of the monitored elements, the impurities were removed already


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in the first purification cycle. Only the elements present in larger concentrations like calcium are completely removed after the second thermal treatment. In the third thermal treatment no released impurities were detected. The final treatment was performed in a pure argon atmosphere to minimize any contamination by chlorine or fluorine. The process was based on thermal decomposition of fluorinated and chlorinated hydrocarbons into radicals which reacted with metallic impurities and formed volatile halides. These halides were subsequently transported by argon gas stream to ICP torch in which the process could be monitored. The results from ICP-OES analysis are summarized in Table S1. A significant amount of metallic impurities was detected in both the thermally and chemically reduced GOs. High concentration of manganese (exceeding 1000 mg kg-1) especially in the reduced graphene originating from Hummers method was detected. Other common metals like iron, nickel, cobalt and chromium were detected too and some of them (iron) exceeded 100 mg kg-1. On the other hand, a notably lower concentration of metallic impurities was found in the reduced graphene prepared by Hofmann method, however, these impurities, still exceeding 100 mg kg-1, can dramatically influence electrochemical behaviour of such material. It is not surprising that the reduced GO contained a large amount of alkali metals as well as calcium, magnesium and boron which originated from reagents as well as from glassware used during synthesis. Thermal treatment of the chemically reduced GO led to a suppression of the impurities level below the detection limit of ICP-OES in combination with the ETV unit. As mentioned above, this purity was reached after three-times repeated thermal treatment in ETV unit. Although the graphene was pure (according to the ICP-OES measurement), sub-mg kg-1 traces of impurities can be still enclosed within the graphene structure. Such metallic impurities are not present on the surface, however they can locally influence the electronic structure of graphene by non-covalent interactions.27


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In order to avoid the matrix-enclosing effect, neutron activation analysis (NAA) was employed for assays of all samples (see Table S2). This method is extremely sensitive to most elements and provides detection at mg kg-1 and µg kg-1 levels. The concentrations of metallic element residues were coherent with the results obtained by ICP-OES for on-line monitoring. Interestingly, the trace concentrations of metallic impurities like manganese and iron in sub-mg kg-1 levels were found by NAA showing that the impurities can remain inside graphene sheets, which could not be detected by ICP-OES. Although these traces of impurities remained in graphene, they should not be in contact with the surrounding environment, since otherwise they would be evaporated during the purification procedure. Such a type of impurities should not influence the electrochemical behaviour since these are not in contact with surroundings and are completely covered by graphene. Since NAA is sensitive to a broad range of elements, halogens were also detected in the samples. Namely, in all graphene samples including the purified ones, traces of fluorine and chlorine were detected. Fluorine was detected only in samples Ho-CRG and Ho-Pure, while chlorine was present in all samples in a concentration comparable or higher than in the purified samples. Interestingly, trace levels of alkali metals were also detected in the purified samples which may originate from secondary contamination, e.g. from air or sample storage container. The chemical composition was further studied by XPS showing different degrees of reduction for chemically, thermally reduced samples and purified one. The survey spectra together with C/O ratios are shown in SI (Figure S2). Only carbon and oxygen were detected in all samples. Nitrogen was not detected by XPS in any samples. Nitrogen was detected only by the bulk analysis based on elemental combustion analysis in both hydrazine reduced GOs. The hydrazine reduced graphene contains nitrogen dominantly as pyrazole based heterocycles on the edges and defect sites.28 Such nitrogen based functionalities induce changes in the electronic structure of graphene


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(according to the DFT calculations) but are not supposed to be electrocatalytically active towards ORR. Theoretical calculations show dominant effect of graphitic and pyridinic nitrogen functionalities.29,30 More details on the composition obtained by elemental combustion analysis are given in the SI file (Table S3). The degree of GO reduction was clearly visible from high-resolution XPS spectra of C 1s. The TRG samples contained less oxygen functionalities in comparison with CRG, since the reduction using hydrazine is not capable to remove all oxygen functionalities such as hydroxyl groups (see Figure 1 and Table 1). These spectra also showed a decreased amount of oxygen functionalities in the purified samples, which confirmed reduction/carbonization during the purification process. Moreover, the amount of π-π* interaction in the purified samples was lower than in the chemically reduced precursors suggesting that the recrystallization during the purification procedure should not lead into restacking of graphene sheets. Surprisingly, the amount of single C-C bonds in the purified samples was similar as in the thermally reduced samples. This was in contrast with the results obtained from Raman spectroscopy discussed below.


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Figure 1. High resolution XPS spectra of C 1s were deconvoluted into the same set of carbon bonding states (see the legend). Table 1. Distribution of bonds obtained from deconvolution of C 1s. Sample

C=C

C-C

C-O

C=O

O-C=O

π-π*

Hu-Pure

78.81

13.20

3.03

1.82

1.05

2.09

Ho-Pure

71.18

19.57

4.01

2.19

1.37

1.68

Hu-TRG

69.87

16.25

5.38

3.71

2.73

2.06

Ho-TRG

69.98

16.78

4.51

4.17

1.58

2.98

Hu-CRG

64.83

17.23

7.30

5.53

2.93

2.18

Ho-CRG

57.54

22.02

8.79

7.12

2.36

2.17


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Besides the composition, the purification process had also a significant impact on the structure of the reduced graphene. Compared to thermally and especially chemically reduced graphene, a substantial improvement of lattice structure was observed. The TEM images showed individual single and few layered graphene sheets; however, these sheets were strongly wrinkled due to the presence of defects in the case of chemically and thermally reduced graphene samples (Figure 2). This wrinkled structure was considerably eliminated in the case of purified samples, moreover, lattice fringes can be observed in their high resolution TEM images. The structure improvement was also confirmed by selective area electron diffraction (SAED) where the sharp diffraction patterns were observed for the purified samples (Figure 3). These observations indicated a suppression of defect density and complex recovery of graphene structure. This is an interesting observation since the total time the sample was exposed to temperatures over 1000 °C was about 6 minutes only. In general, diffraction patterns observed on samples originating from Hummers GOs were more diffuse than those from Hoffman GO. These diffusive diffraction patterns are indicative of structural defects induced by harsher conditions of Hummers graphite oxidation compared to Hofmann method. The significant recovery of the graphene structure was further confirmed by Raman spectroscopy.


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Figure 2. TEM images obtained with different magnification.


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Figure 3. SAED obtained with the same magnification, scale bars are 200 µm. The Raman spectra of graphene are dominated by D-band and G-band located around 1270 cm-1 and 1580 cm-1, respectively. The D band intensity can be correlated with the structural defects associated with sp3 hybridized carbon atoms, while the G-band originates from in-plane vibration of sp2 hybridized carbon atoms from graphene skeleton. In addition, other vibration bands like G*, G’, D+D’ and 2G can be observed at 2460, 2700, 2940 and 3250 cm-1, respectively.31 The ratio of D- and G-band intensity is usually applied for the comparison of defect density within the graphene


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materials.13 The Raman spectra are shown in Figure S3 and the ID/IG rations are summarized in Table S4. The highest ID/IG ratio was observed for the chemically reduced graphene samples and only in this case the intensity of D-band exceeded the G-band. The highest D-band intensity originated from defects as well as the high amount of remaining oxygen functionalities since the chemical reduction is less effective than the thermal one. For the thermally reduced graphene, the observed ID/IG ratio was around 1 showing an improved structure which was mainly based on low amount of oxygen functionalities. However, these materials still contained a significant number of defects. The purification procedure at high temperatures led to almost complete disappearance of the D-band which confirmed a recovery of graphene structure indicated by a decrease of ID/IG ratio below 0.05. Moreover, with such a low density of defects being comparable with CVD graphene,31 one would expect a dramatically reduced heterogeneous electron transfer (HET) rate in the case of outer-sphere probe. However, we have found comparable HET rates for both reduced and purified samples which are discussed in the electrochemical section. This effect is apparently caused by relatively high concentration of edges within the purified graphene sheets that dominate and drive the heterogeneous electron transfer. The electrochemistry of the reduced and purified graphene was thoroughly investigated using various redox probes especially those highly sensitive to metallic ions. First, we have studied the inherent electrochemistry behaviour to avoid any interference with a reduction or oxidation of any functionality present on the graphene surface. The inherent electrochemical activity consisting of electrochemical reduction and oxidation of graphene surface, was negligible due to a successful removal of most electrochemically active oxygen functionalities (see Figure S4). Even Hu- and Ho-CRG exhibiting high amount of oxygen functionalities, did not show any inherent redox


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activity. This indicates that all functional groups at the surface could not be electrochemically reduced in the used potential range. Heterogeneous electron transfer (HET) rate was measured using both inner [Fe(CN)6]3-/4- and outer-sphere [Ru(NH3)6]2+/3+ redox probe. Figure S5 shows the resulting voltammograms, whereas the calculated peak-to-peak separation values (ΔE) are summarized in Figure 4. It should be noted that the outer-sphere redox probes are influenced only by the amount of electrochemically active sites, i.e. the amount of edges and defects in the graphene skeleton. Let us note that the edges and defects dominate the electron transfer process, while the basal plane activity is negligible.32 Therefore, they are insensitive to the amount of oxygen functionalities. All obtained ΔE values were closely aligned. The inner-sphere redox probe is also influenced by the amount of electrochemically active sites, but on the other hand, it is also very sensitive to the presence of oxygen functionalities. Both purified samples had the largest value of ΔE (over 300 mV). The TRG samples exhibited less than half of the ΔE values of purified samples. In conclusion, the purified graphene possesses almost the same amount of electrochemically active sites as the chemically reduced precursors as well as the thermally reduced graphene. The increased ΔE values in the case of inner sphere probe suggested a loss of oxygen functionalities, which was in good agreement with the XPS measurement.


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Figure 4. Heterogeneous electron transfer represented by peak-to-peak separation values (ΔE). Two redox probes were used A) [Ru(NH3)6]2+/3+ and B) [Fe(CN)6]3-/4-. Finally, the electrocatalytical activity was tested towards several reactions from which the oxygen reduction reaction (ORR) is probably the most reported electrocatalytical application of doped graphene. In the case of ORR, plenty of articles dealing with doping of graphene have claimed the synthesis of “metal-free” catalyst. However, several recent works have revealed that improvement in ORR electrocatalysis is caused by a synergistic effect of impurities and the nonmetal doping rather than by the doping itself.10 As expected, the best performance was demonstrated for samples prepared from Hummers GO which contained manganese impurities


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(Figure 5). Meanwhile, Hu-TRG reached the highest current which was caused by the higher degree of exfoliation (larger surface area) compared to Hu-CRG. By contrast, Hu-CRG exhibited the lowest observed overpotential. This observed electrocatalytic activity disappeared after the purification process since the LSV curve of Hu-Pure laid below the GC one. A similar behaviour was observed in the case of samples prepared from Hofmann GO. The big difference in activity of HO and HU samples can be only explained in terms of different amount of manganese impurities. Therefore, the manganese impurities content is probably the crucial element for the ORR electrocatalysis on graphene based materials.

Figure 5. Oxygen reduction reaction was tested in air saturated 0.5 M KOH solution. The second industrially important reaction is the hydrogen evolution reaction (HER). LSV polarization curves are displayed in Figure S6. Differences in HER activity were less pronounced in comparison with ORR. Both purified graphenes exhibited a higher overpotential in comparison with the graphene reduced by chemical and thermal method.


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Hydrazine, cumene hydroperoxide (CHP), and hydrogen sulfide were used as metallic impurity sensitive redox probes (see Figure 6).11,22 Interestingly, hydrazine was the only substance which showed quite great electrocatalytical activity even on the purified samples. However, some of the previous works have reported that the oxidation of hydrazine can take place on the bare carbon electrodes.33,34 The first oxidation peak maximum ranging from 0.7 to 0.9 V was followed by the second one between 1.4 - 1.8 V. A similar behavior has been previously reported and the less positive peak was associated with the direct oxidation of N2H4, while the second peak was interpreted in terms of a more complex reaction mechanism involving radical intermediate formation.35 However, the changes of electrochemical activity were clearly visible on the oxidation of hydrazine where the lowering of oxidation overpotential was clearly indicated for several unpurified graphene samples. Interestingly, the highest activity including peak current as well as overpotential reduction was observed for the Ho-TRG sample followed by Ho-CRG and Hu-CRG. These results reveal that not only manganese impurities present in Hu-TRG and Hu-CRG can dominate the electrocatalytic activity, but also other impurities like Fe present in Ho-CRG and HoTRG can drive the electrocatalytic activity.11 NaHS had the peak maximum around 0.6 V on the bare GC electrode.22 In the case of graphene samples, the peak maximum was shifted to more anodic values (around 0.7 V for Ho samples and around 1.2 V for Hu samples). Although the Hu samples exhibited a higher overpotential, they overperformed the Ho samples in the current peak maximum which was most evident in comparison of CRG samples. In the case of purified samples, only a negligible activity was observed similarly to the Ho-CRG sample exhibiting a very low amount of impurities even before the treatment.


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The small reduction peak of cumene hydroperoxide was clearly observed on both CRG and TRG. On the other hand, not even a trace of any peak could be found on CV curves of Pure samples, although the reduction of cumene hydroperoxide is catalyzed in the presence of metallic impurities. The catalytic oxidation of cumene hydroperoxide was not observed on the purified graphene without any metallic impurities. This clearly manifests the key role of metallic impurities on the electrocatalytic activity of graphene.

Figure 6. Cyclic voltammograms were recorded in the presence of a) 5 mM hydrazine, b) 5 mM NaHS and c) 5mM cumene hydroperoxide in 50 mM PBS (pH=7.2). CONCLUSION We have demonstrated that the reactive thermal treatment of graphene led to a suppression of metallic impurities and oxygen functionalities concentration as well as to a recovery of graphene skeleton. Together with these morphological and chemical changes, heterogeneous electron transfer rate was only slightly slowed down, but on the other hand, the electrocatalytical activity almost disappeared after this treatment. Our findings confirmed the key role of metallic impurities in graphene based electrocatalysis with the exception of hydrazine oxidation. Moreover, the biggest influence was observed for ORR reaction. Despite previous claims of many authors, these findings clearly demonstrated that the metal-free graphene is not catalytic active towards the ORR.


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EXPERIMENTAL SECTION Materials Graphite oxides was prepared from graphite microparticles (2-15 m, 99.9995%), cumene hydroperoxide and sodium hydrogen sulfide from Alfa Aesar, Germany. Sulfuric acid (98%), nitric acid (68%), potassium chlorate (99%), potassium permanganate (99.5%), sodium nitrate (99.5%), hydrogen peroxide (30%), hydrochloric acid (37%), silver nitrate (99.5%), barium nitrate (99.5%), potassium hydroxide (>99.9%), hydrazine hydrate (99%), potassium ferrocyanide, hexaammineruthenium chloride, methanol (>99.9%) and N,N-dimethylformamide (DMF) were obtained from Penta, Czech Republic. Nitrogen (99.9999%) was obtained from SIAD, Czech Republic. Synthesis procedure Graphite oxide prepared according to the Hofmann method was termed HoGO.12 Sulfuric acid (98 %, 87.5 mL) and nitric acid (68 %, 27 mL) were added to a reaction flask (Pyrex beaker with thermometer) containing a magnetic stir bar. The mixture was then cooled by immersion in an ice bath for 30 min. Graphite (5 g) was then added to the mixture with vigorous stirring motion. While keeping the reaction flask in the ice bath, potassium chlorate (55 g) was slowly added to the mixture. Upon the complete dissolution of potassium chlorate, the reaction flask was then loosely capped to allow the escape of the gas evolved and the mixture was continuously stirred for 96 hours at room temperature. The mixture was poured into 3 L of deionized water and decanted. Graphite oxide was then redispersed in HCl solution (5 %, 3 L) to remove sulfate ions and repeatedly centrifuged and redispersed in deionized water until a negative reaction on chloride and sulfate ions was achieved. Graphite oxide slurry was then dried in a vacuum oven.


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The second graphite oxide was synthesized similarly to the Hummers method and was termed as Hu-GO.12 Graphite (5 g) and sodium nitrate (2.5 g) were stirred with sulfuric acid (98 %, 115 mL). The mixture was then cooled to 0 °C. Potassium permanganate (15 g) was then added over a period of two hours. During the next four hours, the reaction mixture was allowed to reach room temperature before being heated to 35 °C for 30 min. The reaction mixture was then poured into a flask containing deionized water (250 mL) and heated to 70 °C for 15 minutes. The mixture was then poured into deionized water (1 L). The unreacted potassium permanganate and manganese dioxide were removed by the addition of 3% hydrogen peroxide. The reaction mixture was then allowed to settle and decant. The obtained graphite oxide was then purified by repeated centrifugation and redispersing in deionized water until a negative reaction on sulphate ions was achieved. Graphite oxide slurry was then dried in a vacuum oven. Chemically reduced graphene denoted “CRG” was prepared by reduction with hydrazine hydrate. 1 g of graphite oxide was dispersed in 1 L of deionized water by ultrasonication (150 W, 60 minutes). The suspension was alkalized to pH~10 by 1 M KOH. 10 mL of hydrazine hydrate was added to the reaction mixture and the solution was kept under reflux for 24 hours. The obtained CRG was separated from the reaction mixture by suction filtration using a nylon membrane with 0.45 μm porosity and repeatedly washed with deionized water and methanol. Prior to further use, CRG was dried in a vacuum oven at 60 °C for 48 hours. The thermal reduction-exfoliation of graphite oxide was performed at 1000 °C in quartz glass reactor in nitrogen atmosphere. Accordingly, samples were termed “TRG”. Graphite oxide (0.1 g) was placed in a porous quartz glass capsule connected to magnetic


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manipulator inside vacuum tight tube furnace with controlled atmosphere. The sample was flushed with nitrogen by repeated evacuation of tube furnace to remove any traces of oxygen. Subsequently the reactor was filled with nitrogen or hydrogen, respectively and the sample was quickly inserted by magnetic manipulator in to the preheated furnace and held in the furnace for 12 minutes. The flow of nitrogen during the exfoliation procedure was 1000 mL.min-1 to remove the byproducts of exfoliation procedure. Purification of graphene was performed using graphite high temperature furnace of electrothermal vaporization unit coupled with ICP-OES. For the annealing, a 250 L high purity graphite boat was used. The heating to temperature 2500 °C was performed in 95 s temperature program and cooling down to room temperature within 3 minutes. The heating was performed in argon with the addition of gaseous dichlor-difluormethane acting as the halogen source and forming volatile halides when combined with metallic impurities. The presence of metallic impurities was monitored by connected ICP-OES. The annealing was performed three times where no metallic impurities were detected upon the third cycle. The last annealing step was performed in pure argon atmosphere to remove both chlorine and fluorine from preceding purification procedure. Characterization The elemental composition was measured by inductively coupled plasma optical emission spectroscopy (ETV ICP-OES) by using Spectro ARCOS spectrometer (SPECTRO Analytical Instruments, Germany). The spectrometer used the Paschen-Runge configuration with an optimized Rowland circle polychromator, measuring simultaneously in the broad spectral range of 130-770 nm using 32 linear CCD detectors. The detection limits reach the µg kg-1 levels for the elements determined. For the trace concentrations of selected elements (Ag, Al, As, Ba, Ca, Cd, Ce, Co, Cr, Cu, Fe, K, La, Mg, Mn, Mo, Na, Ni, Sc, Se, Sr, Ti, V and Zn), an


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electrothermal vaporization unit RTV 4000c (Spectral Systems Peter Perzl, Germany) was used. Solid and aqueous calibration standards were obtained from Analytica (Czech Republic). More details about the method and its capabilities can be found in literature.36 Samples of HU-Pure, HO-Pure, HU-TRG, HO-TRG, HU-CRG, and HO-CRG with masses of 5.975 mg, 29.26 mg, 9.080 mg, 3.360 mg, 60.36 mg, and 59.60 mg, respectively, for neutron activation analysis (NAA) were packed for irradiation into disk-shaped polyethylene (PE) capsules with a 20-mm diameter made by sealing of acid-cleaned polyethylene foils with 0.2 mm thickness. Irradiations were carried out in the LVR-15 experimental reactor of the Research Centre Řež, Ltd. Both short-time (1 min.) and long-time (3 h) irradiations were carried out at thermal neutron fluence rates 3−4 × 1013 cm−2 s−1 as given in our previous publication, where the relative calibration employed using synthetic multielement standards (MES), and choice of decay and counting times are also described.26 Gamma-ray spectra of samples, standards and monitors were measured with coaxial high-purity germanium (HPGe) detectors. For counting of short-lived radionuclides, a coaxial HPGe detector with relative efficiency of 21 % and full width at half maximum (FWHM) resolution 1.80 keV, both for the 1332.5 keV photons of 60Co was employed, whereas for counting of medium- and long-lived radionuclides another coaxial HPGE detector with relative efficiency of 21 % and FWHM resolution 1.85 keV was used. Both HPGe detectors were interfaced to a Canberra Genie 2000 computer controlled gamma-spectrometry analyzer through a chain of associated linear electronics, which included a Canberra 599 Loss Free Counting module to correct the variable count-rate and dead time. Canberra Genie 2000 software was used for evaluation of gamma-ray spectra. Nuclear parameters of radionuclides measured have already been given earlier.37


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For quality control purposes, a standard reference material of the US National Institute of Standards and Technology NIST SRM 1515 Apple Leaves was co-analyzed. Agreement of our values with NIST certified values within uncertainty margins proved the accuracy of our results. The only exception was a significantly higher value for Na compared with the NIST certified value, which, however, has previously been found in our laboratory and other laboratories, as well.38 The difference between matrix composition of the materials studied in this work and that of NIST SRM 1515 does not invalidate the proof of accuracy because of matrix independence of NAA. The concentration obtained on NIST SRM 1515 standard and certified concentration are shown in Table S5. Detection limits are dependent on individual element cross-section towards neutron and sample mass used for analysis without any effect of matrix. More details can be found in literature.39,40 High resolution transmission electron microscopy (HR-TEM) was performed using an EFTEM Jeol 2200 FS microscope (Jeol, Japan). A 200 keV acceleration voltage was used for measurement. Elemental maps and EDS spectra were acquired with SDD detector XMaxN 80 TS from Oxford Instruments (England). Sample preparation was attained by drop casting the suspension (1 mg mL-1 in water) on a TEM grid (Cu; 200 mesh; Formvar/carbon) and dried at 60 °C for 12 h. High resolution X-ray photoelectron spectroscopy (XPS) was performed using an ESCAProbeP

spectrometer

(Omicron

Nanotechnology

Ltd,

Germany)

with

a

monochromatic aluminium X-ray radiation source (1486.7 eV). Wide-scan surveys of all elements were performed, with subsequent high-resolution scans of the C 1s and O 1s. Relative sensitivity factors were used to evaluate the carbon-to-oxygen (C/O) ratios from the survey spectra. The samples were placed on a conductive carrier made from a high


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purity silver bar. An electron gun was used to eliminate sample charging during measurement (1-5 V). inVia Raman microscope (Renishaw, England) in backscattering geometry with CCD detector was used for Raman spectroscopy. DPSS laser (532 nm, 50 mW) with applied power of 5 mW and 50x magnification objective was used for the measurement. Instrument calibration was achieved with a silicon reference which gives a peak position at 520 cm−1 and a resolution of less than 1 cm-1. The samples were suspended in deionized water (1 mg/ml) and ultrasonicated for 10 min. The suspension was deposited on a small piece of silicon wafer and dried. The electrochemical characterization by means of cyclic voltammetry was performed using an Autolab PGSTAT 204 (Metroohm, Switzerland). All glassy carbon electrodes were cleaned by polishing with an alumina suspension to renew the electrode surface then washed and wiped dry prior to any use. Modified glass carbon electrode as a working electrode, KCl saturated Ag/AgCl reference electrode and platinum counter electrode were used for the measurement. The samples were dispersed in methanol to obtain a 1.5 mg mL-1 suspension. The suspension was then sonicated for 5 min at room temperature before every use. A cleaned GC electrode was then modified by coating with a 1 μL aliquot of the suspension and left to dry at ambient temperature to give a layer of randomly dispersed material on the GC surface. The modified GC electrodes, saturated Ag/AgCl reference electrode, and platinum counter electrode were then placed into an electrochemical cell containing the electrolyte solution, and the measurements were subsequently taken. The electrolytes used were 100 mM, pH 7.2 phosphate buffer solution (PBS) as the blank buffer electrolyte as well as the


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supporting solution for the redox probes. The following probes were used: potassium hexacyanidoferrate, hexaammineruthenium chloride, hydrazine monohydrate, sodium hydrogensulfide and cumene hydroperoxide, all of them in final concentration of 5 mmol/L. Moreover, 0.5 M KOH and H2SO4 were used in the case of ORR and HER, respectively. All measurements were performed for two consecutive scans at a scan rate of 100 mV s-1.

ASSOCIATED CONTENT Supporting Information. ICP-OES record of the annealing process, elemental composition obtained by ICP-OES and NAA, Raman spectra with ID/IG ratios, inherent electrochemistry and CVs of HET probes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Zdeněk Sofer, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) ACKNOWLEDGMENT This work was supported by Czech Science Foundation (GACR No. 16-05167S) and by specific university research (MSMT No. 20-SVV/2019). This work was created with the financial support of the Neuron Foundation for science support. This work was supported by the project Advanced


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Functional Nanorobots (Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). NAA analysis was supported by Center of Accelerators and Nuclear Analytical Methods infrastructure (Ministry of Education, Youth and Sports of the Czech Republic Project LM2015056). REFERENCES 1. Li, J. C.; Hou, P. X.; Liu, C., Heteroatom‐Doped Carbon Nanotube and Graphene‐Based Electrocatalysts for Oxygen Reduction Reaction. Small 2017, 13, 1702002. 2. Jeon, I. Y.; Zhang, S.; Zhang, L.; Choi, H. J.; Seo, J. M.; Xia, Z.; Dai, L.; Baek, J. B., Edge‐Selectively Sulfurized Graphene Nanoplatelets as Efficient Metal‐Free Electrocatalysts for Oxygen Reduction Reaction: the Electron Spin Effect. Adv. Mater. 2013, 25, 6138-6145. 3. Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L., Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321-1326. 4. Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H., Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and its Excellent Electrocatalysis. ACS Nano 2011, 5, 4350-4358. 5. Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X. A.; Huang, S., Sulfur-Doped Graphene as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction. ACS Nano 2011, 6, 205-211. 6. Tian, Y.; Ye, Y.; Wang, X.; Peng, S.; Wei, Z.; Zhang, X.; Liu, W., Three-Dimensional NDoped, Plasma-Etched Graphene: Highly Active Metal-Free Catalyst for Hydrogen Evolution Reaction. Appl. Catal., A 2017, 529, 127-133. 7. Tian, Y.; Wei, Z.; Wang, X.; Peng, S.; Zhang, X.; Liu, W.-m., Plasma-Etched, S-Doped Graphene for Effective Hydrogen Evolution Reaction. Int. J. Hydrogen Energy 2017, 42, 41844192. 8. Tabassum, H.; Zou, R.; Mahmood, A.; Liang, Z.; Guo, S., A Catalyst-Free Synthesis of B, N Co-Doped Graphene Nanostructures with Tunable Dimensions as Highly Efficient Metal Free Dual Electrocatalysts. J. Mater. Chem. A 2016, 4, 16469-16475. 9. Ito, Y.; Cong, W.; Fujita, T.; Tang, Z.; Chen, M., High Catalytic Activity of Nitrogen and Sulfur Co‐Doped Nanoporous Graphene in the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2015, 54, 2131-2136. 10. Mazánek, V.; Matějková, S.; Sedmidubský, D.; Pumera, M.; Sofer, Z., One‐Step Synthesis of B/N Co‐doped Graphene as Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: Synergistic Effect of Impurities. Chem. Eur. J. 2018, 24, 928-936. 11. Ambrosi, A.; Chee, S. Y.; Khezri, B.; Webster, R. D.; Sofer, Z.; Pumera, M., Metallic Impurities in Graphenes Prepared from Graphite can Dramatically Influence their Properties. Angew. Chem. Int. Ed. 2012, 51, 500-503. 12. Lum, Y.; Kwon, Y.; Lobaccaro, P.; Chen, L.; Clark, E. L.; Bell, A. T.; Ager, J. W., Trace Levels of Copper in Carbon Materials Show Significant Electrochemical CO2 Reduction Activity. ACS Catal. 2015, 6, 202-209.


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13. Jankovský, O.; Marvan, P.; Nováček, M.; Luxa, J.; Mazánek, V.; Klímová, K.; Sedmidubský, D.; Sofer, Z., Synthesis Procedure and Type of Graphite Oxide Strongly Influence Resulting Graphene Properties. Appl. Mater. Today 2016, 4, 45-53. 14. Kuila, T.; Mishra, A. K.; Khanra, P.; Kim, N. H.; Lee, J. H., Recent Advances in the Efficient Reduction of Graphene Oxide and its Application as Energy Storage Electrode Materials. Nanoscale 2013, 5, 52-71. 15. Li, Y.; Du, Q.; Liu, T.; Peng, X.; Wang, J.; Sun, J.; Wang, Y.; Wu, S.; Wang, Z.; Xia, Y., Comparative Study of Methylene Blue Dye Adsorption Onto Activated Carbon, Graphene Oxide, and Carbon Nanotubes. Chem. Eng. Res. Des. 2013, 91, 361-368. 16. Sitko, R.; Turek, E.; Zawisza, B.; Malicka, E.; Talik, E.; Heimann, J.; Gagor, A.; Feist, B.; Wrzalik, R., Adsorption of Divalent Metal Ions from Aqueous Solutions Using Graphene Oxide. Dalton Trans. 2013, 42, 5682-5689. 17. Mi, X.; Huang, G.; Xie, W.; Wang, W.; Liu, Y.; Gao, J., Preparation of Graphene Oxide Aerogel and its Adsorption for Cu2+ Ions. Carbon 2012, 50, 4856-4864. 18. Li, Z.; Chen, F.; Yuan, L.; Liu, Y.; Zhao, Y.; Chai, Z.; Shi, W., Uranium (VI) Adsorption on Graphene Oxide Nanosheets from Aqueous Solutions. Chem. Eng. J. 2012, 210, 539-546. 19. Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I.-C.; Kim, K. S., Water-Dispersible Magnetite-Reduced Graphene Oxide Composites for Arsenic Removal. ACS Nano 2010, 4, 3979-3986. 20. Ye, R.; Dong, J.; Wang, L.; Mendoza-Cruz, R.; Li, Y.; An, P.-F.; Yacamán, M. J.; Yakobson, B. I.; Chen, D.; Tour, J. M., Manganese Deception on Graphene and Implications in Catalysis. Carbon 2018, 132, 623-631. 21. Klímová, K. i.; Pumera, M.; Luxa, J.; Jankovský, O. e.; Sedmidubský, D.; Matějková, S.; Sofer, Z. k., Graphene Oxide Sorption Capacity Toward Elements Over the Whole Periodic Table: a Comparative Study. J. Phys. Chem. C 2016, 120, 24203-24212. 22. Ambrosi, A.; Chua, C. K.; Khezri, B.; Sofer, Z.; Webster, R. D.; Pumera, M., Chemically Reduced Graphene Contains Inherent Metallic Impurities Present in Parent Natural and Synthetic Graphite. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 12899-12904. 23. Jankovský, O.; Libánská, A.; Bouša, D.; Sedmidubský, D.; Matějková, S.; Sofer, Z., Partially Hydrogenated Graphene Materials Exhibit High Electrocatalytic Activities Related to Unintentional Doping with Metallic Impurities. Chem. Eur. J. 2016, 22, 8627-8634. 24. Wang, L.; Ambrosi, A.; Pumera, M., “Metal‐free” Catalytic Oxygen Reduction Reaction on Heteroatom‐Doped Graphene is Caused by Trace Metal Impurities. Angew. Chem. Int. Ed. 2013, 125, 14063-14066. 25. Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S., Exploration of the Active Center Structure of Nitrogen-Doped Graphene-Based Catalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 7936-7942. 26. Sheng, Z.-H.; Gao, H.-L.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H., Synthesis of Boron Doped Graphene for Oxygen Reduction Reaction in Fuel Cells. J. Mater. Chem. 2012, 22, 390395. 27. Khomyakov, P. A.; Giovannetti, G.; Rusu, P. C.; Brocks, G.; J. van den Brink, J.; Kelly, P.J., First-Principles Study of the Interaction and Charge Transfer between Graphene and Metals. Phys. Rev. B 2009, 79, 195425-12. 28. Park, S.; Hu, Y.; Hwang, J. O.; Lee, E.-S.; Casabianca, L. B.; Cai, W.; Potts, J. R.; Ha, H.-W.; Chen, S.; Oh, J., Chemical Structures of Hydrazine-Treated Graphene Oxide and Generation of Aromatic Nitrogen Doping. Nat. Commun. 2012, 3, 638.


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29. She, Y.; Chen, J.; Zhang, C.; Lu, Z.; Ni, M.; Sit, P. H.-L.; Leung, M. K., Nitrogen-Doped Graphene Derived from Ionic Liquid as Metal-Free Catalyst for Oxygen Reduction Reaction and its Mechanisms. Appl. Energy 2018, 225, 513-521. 30. Reda, M.; Hansen, H. A.; Vegge, T., DFT Study of Stabilization Effects on N-Doped Graphene for ORR Catalysis. Catal. Today 2018, 312, 118-125. 31. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R., Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Letters 2010, 10, 751-758. 32. Yuan, W.; Zhou Y.; Li Y.; Peng, H.; Zhang, J.; Liu, Z.; Dai, L.; Shi, G., The Edge- and Basal-Plane-Specific Electrochemistry of a Single-Layer Graphene Sheet. Sci. Rep. 2013, 3, 2248-7. 33. Wiesener, W., Untersuchung zur Anodischen Hydrazin-Oxidation an Porösen Kohleelektroden unter Verwendung von Nickel-und Nickelboridkatalysatoren. Electrochim. Acta 1970, 15, 1065-1077. 34. Zhukova, O.; Lazareva, L.; Artem'yanov, A., Electrooxidation of Phenylhydrazine on Carbon Electrodes. Russ. J. Electrochem. 2001, 37, 629-631. 35. Aleksandrova, T.; Skvortsova, L.; Kiryushov, V., Electrochemical Behavior of Hydrazine at Mechanically Renewed Solid Electrodes. J. Anal. Chem. 2008, 63, 994-998. 36. Resano, M.; Vanhaecke, F.; de Loos-Vollebregt, M., Electrothermal Vaporization for Sample Introduction in Atomic Absorption, Atomic Emission and Plasma Mass Spectrometry—a Critical Review with Focus on Solid Sampling and Slurry Analysis. J. Anal. Atom. Spectrom. 2008, 23, 1450-1475. 37. Kučera, J.; Soukal, L., Homogeneity Tests and Certification Analyses of Coal Fly Ash Reference Materials by Instrumental Neutron Activation Analysis. J. Radioanal. Nucl. Chem. 1988, 121, 245-259. 38. Bitewlign, T. A.; Chaubey, A. K.; Beyene, G. A.; Melikegnaw, T. H.; Mizera, J.; Kameník, J.; Krausová, I.; Kučera, J., Instrumental Neutron Activation Analysis of Environmental Samples from a Region with Prevalence of Population Disabilities in the North Gondar, Ethiopia. J. Radioanal. Nucl. Chem. 2017, 311, 2047-2059. 39. Greenberg, R., Pushing the Limits of NAA: Accuracy, Uncertainty and Detection Limits. J. Radioanal. Nucl. Chem. 2008, 278, 231-240. 40. Alfassi, Z.B., Instrumental Multi-Element Chemical Analysis, Springer, Dordrecht, 1998.


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ToC Figure


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Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the

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