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. 2020 Sep 15;5(38):24367–24378. doi: 10.1021/acsomega.0c02673

Spinach-Derived Porous Carbon Nanosheets as High-Performance Catalysts for Oxygen Reduction Reaction

Xiaojun Liu 1, Casey Culhane 1, Wenyue Li 1, Shouzhong Zou 1,*
PMCID: PMC7528166  PMID: 33015453

Abstract

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Biomass-derived porous carbon materials are effective electrocatalysts for oxygen reduction reaction (ORR), with promising applications in low-temperature fuel cells and metal–air batteries. Herein, we developed a synthesis procedure that used spinach as a source of carbon, iron, and nitrogen for preparing porous carbon nanosheets and studied their ORR catalytic performance. These carbon sheets showed a very high ORR activity with a half-wave potential of +0.88 V in 0.1 M KOH, which is 20 mV more positive than that of commercial Pt/C catalysts. In addition, they showed a much better long-term stability than Pt/C and were insensitive to methanol. The remarkable ORR performance was attributed to the accessible high-density active sites that are primarily from Fe–Nx moieties. This work paves the way toward the use of metal-enriching plants as a source for preparing porous carbon materials for electrochemical energy conversion and storage applications.

1. Introduction

Electrochemical energy conversion and storage play a key role in sustainable and clean energy production. Among different electrochemical devices, fuel cells and metal–air batteries show great promises. Oxygen reduction reaction (ORR) is the cathode reaction of these devices and is one of the bottlenecks of their practical applications because of its sluggish kinetics that limits their power output. Platinum-based carbon-supported catalysts have long been recognized as the most effective electrocatalysts for ORR. However, the scarcity and high cost of Pt, together with the lack of long-term stability and the vulnerability to surface poisoning by various chemicals such as methanol and carbon monoxide, call for the development of non-Pt group metal (NPGM) catalysts. To address these issues, carbon-based catalysts have attracted increasing attention because of their excellent stability, resistance to surface poisoning, and use of low-cost and earth-abundant elements. A variety of NPGM catalysts have been developed by doping carbon with various heteroatoms such as N, S, P, B, or co-doping with N and Fe, or N and Co.1−4 Among them, iron- and nitrogen-co-doped carbon materials exhibited the best performance for ORR, and some of these catalysts showed key performance descriptors that are superior to Pt catalysts.1,2

Given the great promise these catalysts hold, it is of importance to explore approaches for obtaining them in a sustainable fashion. A reasonable starting point would be to use inexpensive and readily available materials as sources for carbon, nitrogen, and iron, as opposed to sometimes expensive and/or toxic chemicals. In this regard, biomass is an attractive renewable natural resource because it is typically rich in carbon and nitrogen, and some forms contain metal elements such as iron or cobalt. These metal elements are key to the formation of highly active catalytic centers for ORR.5−7 Progress has been made in this direction. Doped carbon ORR catalysts have been made from plants,8−14 plant products,15−18 biomass wastes of animal or human origin,19,20 fungi,21,22 and microorganisms.23,24 Of particular interest among these biomass resources are plants because some plants are able to selectively store metals.25 Therefore, they can be used to remove toxic metals from contaminated soil or water (phytoremediation)26,27 or enrich valuable metals from mineral wastes (phytomining).28 It is envisioned that doped carbons with high catalytic activity can be obtained from these metal-rich plants.

As a first step toward this direction, we chose spinach to conduct proof-of-principle studies because spinach contains appreciable amounts of N and Fe, among other elements such as S and P,29,30 which are important elements for carbon-based ORR catalysts. We found that by using spinach as the precursor, melamine as a nitrogen promoter, and NaCl/KCl as the pore producer, multielement-doped carbon with high ORR activity can be obtained through pyrolysis at 900 °C under Ar. The resultant carbon materials have a sheet-like structure with a specific area of 289.6 m2 g–1 and a high density of active sites for ORR, as demonstrated by the high oxygen reduction current peak in the cyclic voltammograms. The polarization measurements further show that the obtained carbon sheets have ORR performance superior to Pt/C in alkaline solutions in terms of half-wave potential, methanol tolerance, and long-term stability. They are also active ORR catalysts in acidic media. The Fe–Nx moiety is the main active center, as supported by the results from the X-ray photoelectron spectroscopy (XPS) measurements and the acid etching studies, as well as from the observations of poisoning effects of cyanide and fluoride. Control experiments revealed that the presence of micro- and mesopores and the sufficiently high density of Fe–Nx active sites are the main contributors to the remarkable ORR performance. This study paves an avenue to preparing high-performing NPGM ORR catalysts by using metal-enriching plants as the precursor.

2. Results and Discussion

2.1. Synthesis and Characterization of Porous Carbon Nanosheets

A schematic illustration of the preparation procedure of spinach-derived carbon nanosheets is shown in Scheme 1. Fresh spinach leaves were first cleaned and blended into juice, which was then freeze-dried and ground into a fine powder. Afterward, the spinach powder, melamine, NaCl, and KCl were dissolved and mixed in water and heated to 120 °C to form a uniform mixture. An elevated temperature is needed to dissolve melamine because of its relatively low solubility at room temperature.31 The mixture was then cooled down quickly in liquid nitrogen and freeze-dried to obtain spinach–salt–melamine composites. During this process, the uniformity of the mixture was preserved. The composites were then pyrolyzed at high temperatures, and the metal particles were leached out by soaking in a heated sulfuric acid solution. Finally, the obtained carbon powder was pyrolyzed again at 900 °C for 1 h and denoted as M+S+C900-900.

Scheme 1. Schematic of the Procedure for Preparing Spinach-Derived Carbon Nanosheets.

Scheme 1

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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs of the spinach-derived carbon prepared by pyrolyzing at 900 °C (M+S+C900-900) are shown in Figure 1. M+S+C900-900 consists of agglomerates of graphene-like carbon sheets with wrinkles and defects. There are many voids created by the dissolution of NaCl/KCl salt crystals. These macropores provide passages for transporting the reactants to active sites, which are beneficial for improving the catalytic activity (vide infra). The high-magnification TEM image in Figure 1e demonstrates that M+S+C900-900 has crystalline domains with a lattice distance of about 0.36 nm, corresponding to the (002) direction of the graphitic carbon lattice. The corresponding selected-area electron diffraction pattern (the inset in Figure 1e) exhibits a sixfold symmetry, confirming that M+S+C900-900 has a graphite-like structure.

Figure 1.

Figure 1

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Representative SEM (a–c) and TEM (d,e) images of M+S+C900-900. The inset in (e) is an electron diffraction pattern of M+S+C900-900.

Figure 2a shows the X-ray diffraction (XRD) patterns of M+S+C900-900. The two main peaks at around 25.0 and 44.1° can be assigned to the (002) and (101) planes of graphite (JCPDS card no. 41-1487), respectively,15,22 which indicate that graphite-like structures were formed after the high-temperature pyrolysis. This result is in good agreement with the TEM observations. To further characterize the structure of M+S+C900-900, a Raman spectrum was recorded. As shown in Figure 2b, two relatively strong peaks were observed at 1332 and 1590 cm–1, which can be assigned to the D and G bands, respectively.32 The G band arises from the Raman-active in-plane bond-stretching of the sp2 carbon atoms and has an E2g symmetry. The D band is attributed to the ring breathing mode of A1g symmetry. This mode is Raman-inactive in perfect graphite and graphene but becomes active in defective graphite and graphene.32−34 The observation of these Raman peaks suggests the formation of defective graphite structure after pyrolysis, which is further confirmed by the appearance of a weak broad band centered at 2725 cm–1, assignable to the overtone of the D band.32 The position of the G and D bands and the lower ID/IG ratio (0.92) suggest the carbon nanosheets are more crystalline than the Vulcan XC (ID/IG = 2.1) used in commercial Pt/C.34,35 Surface area and pore distribution are the critical parameters for carbon-based catalysts. Figure 2c displays the nitrogen adsorption–desorption isotherm of M+S+C900-900, which exhibits a type IV sorption isotherm with a type H4 hysteresis loop. The corresponding Brunauer–Emmett–Teller (BET) surface area is 289.6 m2 g–1. At low relative pressures (<0.1 P/P0), a sharp increase indicates the presence of micropores. The observation of the type H4 hysteresis loop extending from P/P0 = 0.45 to 1.0 signals the existence of narrow slit-like pores, large mesopores in a matrix with much smaller pores, and pores of irregular shape and broad size distribution.36 In addition, an obvious tail appeared at P/P0 = ∼1.0, implying the formation of macropores as well.37−39 This conjecture is corroborated with the pore size distribution (PSD) calculated using the density functional theory (DFT) method shown in Figure 2d, which shows the presence of micropores, mesopores, and macropores. The slit-like pores may arise from the sheet-like structure of M+S+C900-900, as shown in the TEM images in Figure 1. The presence of mesoporous and macroporous structures has been shown to facilitate the diffusion and transport of reactants to the active sites for ORR.40−42

Figure 2.

Figure 2

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Structural characterizations of M+S+C900-900. (a) XRD. (b) Raman spectrum. (c) N2 adsorption–desorption isotherm. (d) PSD. Inset: zoom-in of the PSD. (e) XPS survey spectrum. (f–i) High-resolution XPS spectrum of: (f) C 1s, (g) N 1s, (h) S 2p, and (i) Fe 2p.

XPS measurements were employed to characterize the chemical states of different elements in M+S+C900-900. As shown in Figure 2e, the full survey XPS spectrum reveals that M+S+C900-900 is mainly composed of carbon, oxygen, and nitrogen and trace amounts of iron, sulfur, and phosphorus. The atomic percentage of each element is summarized in Table S1. The high-resolution C 1s, N 1s, S 2p, and Fe 2p XPS spectra are shown in Figure 2f–i. The C 1s spectrum can be resolved into four peaks at the binding energies of 284.5, 285.6, 286.5, and 289.0 eV (Figure 2f), representing graphitic carbon, C–N, C–O, and O–C=O species, respectively.43 The N 1s spectrum can be fitted by multicomponent peaks. The peaks at 397.6 and 398.2 eV (Figure 2g) can be assigned to imine and pyridinic N.44,45 The peak at 399.3 is attributed to Fe-coordinated N (Fe–Nx),46,47 which is highly ORR-active.45,48,49 The other peaks at 399.9, 400.9, and 402.6 eV arise from pyrrolic N, graphitic N, and oxidized N, respectively.10,47,50 As shown in Figure 2h, the high-resolution S 2p XPS spectrum can be deconvoluted into several peaks. The two peaks at binding energies of around 162.0 and 163.0 eV can be attributed to the spin–orbit coupling of S 2p3/2 and S 2p1/2 for thiol sulfurs.51,52 The peaks at binding energies of 163.9 and 165.1 eV can be assigned to the spin–orbit coupling positions of 2p3/2 and 2p1/2 for thiophene S that has been reported active for ORR.53,54 A strong peak at the binding energy of 164.7 eV is from sulfur in conjugated C=S bonds.55 The peaks at 165.8, 166.7, 167.5, and 168.0 eV are assigned to the oxidized sulfur groups (−C–SOx–C–, x = 2–4, 165.0–171.5 eV).56,57 The Fe 2p spectrum (Figure 2i) displays two broad features centered around 711 and 725 eV, and each can be deconvoluted into two peaks at 710.6, 713.6 and at 723.0, 725.3 eV, respectively. These binding energies are very close to those observed on various iron oxides58 and an iron-coordinated nitrogen-doped carbon catalyst.45 Following these previous reports, we assign the 710.6 and 723.0 eV peaks to Fe 2p3/2 and Fe 2p1/2 from an Fe(II) species and the 713.6 and 725.3 eV peaks to Fe 2p3/2 and Fe 2p1/2 of an Fe(III) species. The peak at 716.8 eV is identified as a satellite peak corresponding to Fe 2p3/2.53 The P2p signal is too weak for a meaningful peak fitting.

2.2. ORR Activity and Stability

To test the ORR performance of M+S+C900-900, cyclic voltammograms were first recorded in 0.1 M KOH saturated with N2 and O2, respectively (Figure 3a). The current was normalized to the geometric area of the glassy carbon supporting electrode. A quasi-rectangular voltammogram was observed in N2-saturated 0.1 M KOH, which suggests that only capacitive charging current was present. In contrast, when the solution was saturated with O2, a very strong reduction peak appeared at +0.88 V, indicating the effective electrochemical reduction of oxygen. The high magnitude of the O2 reduction peak current density at a relatively low scan rate (10 mV s–1) suggests the existence of abundant accessible active sites on the catalyst. Rotating ring-disk electrode (RRDE) measurements were carried out to gain further insights into the ORR performances of M+S+C900-900. A representative result is shown in Figure 3b along with that from a commercial Pt/C (20 wt % Pt) of the same mass loading as a benchmark. For clarity, only the disk current density is shown. Clearly, M+S+C900-900 exhibited a much higher ORR activity than Pt/C. Thus, M+S+C900-900 has a half-wave potential (E1/2) of +0.88 V, which is 20 mV more positive than that of Pt/C, whereas the two catalysts have a similar ORR onset potential (Eo). Similar ORR activities were also obtained on carbon materials derived from spinach purchased from different supermarkets (Figure S1), suggesting the high ORR performance is insensitive to the source of spinach. Compared with other NPGM ORR catalysts, including those derived from biomass, M+S+C900-900 is the most active, especially after the differences in surface area and catalyst loading are considered (Table S2). For example, mesoporous carbon derived from okara showed an ORR E1/2 of +0.86 V, and the same was observed on lysine-derived nitrogen-doped carbon hollow cubes.17,59 These values are 20 mV more negative than that of M+S+C900-900. As mentioned in the outset, a variety of biomass have been used as sources for preparing carbon-based ORR catalysts.60 One advantage of spinach is that it naturally contains a high amount of Fe, which has been extensively demonstrated to be an important component of ORR active sites.1,2 In the present case, no additional Fe source was used in preparing the carbon nanosheets, unlike okara- and catkin-derived carbons where FeCl3 was used to increase the Fe content.11,17 This notion suggests that high ORR-performing plant-derived carbon catalysts containing active metal centers could be obtained from selected metal-enriching plants, such as those proposed for phytoremediations26,27 or phytomining.28

Figure 3.

Figure 3

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Oxygen reduction studies of M+S+C900-900 in 0.1 M KOH. (a) Cyclic voltammograms recorded in N2- and O2-saturated solutions; scan rate: 10 mV s–1. (b) Linear sweep voltammetry (LSV) curves obtained in O2-saturated solutions; electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1. (c) Plot of the number of electron transfer (n) vs the electrode potential. (d) LSV curves recorded in an O2-saturated solution at various rotation rates. (e) Koutecky–Levich (K–L) plots. (f) Tafel plots. Results from Pt/C are included for comparison.

The high ORR activity of M+S+C900-900 was further demonstrated by the number of electron transfer (n) and the HO2– percent yield involved in ORR. The n value and HO2– percent yield at different potentials were calculated by using eqs 3 and 4, respectively. As shown in Figure 3c, for M+S+C900-900, the average n was ca. 3.91, comparable to that of Pt/C (n = 3.97), and the HO2– yield was <5% in the potential range from +0.20 to +0.80 V (see Figure S2). To further examine the ORR activity, we also performed RRDE measurements at various rotation speeds from 400 to 2500 rpm. As expected from the K–L equation (eq 1), the limiting current density increased with the increasing rotation speed (Figure 3d). The corresponding K–L plots (j–1 vs ω–1/2) derived from the RRDE voltammograms are presented in Figure 3e. The K–L plots from +0.61 to +0.81 V display good linearity with a similar slope, indicating the first-order reaction kinetics for ORR with respect to the dissolved oxygen concentration. To shed light on the mechanism of the ORR on M+S+C900-900, Tafel analysis was performed. As shown in Figure 3f, the Tafel slope for M+S+C900-900 is 61.5 mV dec–1, which is lower than that for Pt/C of 63.1 mV dec–1. These values are close to the theoretical value of 60 mV dec–1 when the transfer of the first electron is the rate-determining step in the ORR process.47 M+S+C900-900 also showed a strong ORR activity in acidic media. As shown in Figure S3, the LSV curve of M+S+C900-900 has an E1/2 of +0.75 V, which is only 98 mV more negative than that from Pt/C in O2-saturated 0.1 M HClO4.

Selectivity and durability of the catalysts are important performance metrics for their practical applications. To investigate the methanol crossover effects, chronoamperometric responses (i–t curves) were recorded in O2-saturated 0.1 M KOH at +0.88 V with the addition of methanol into the electrolyte solution to make up a 1 M methanol solution. As shown in Figure 4a, M+S+C900-900 is insensitive to methanol, indicating the excellent selectivity to ORR over methanol oxidation, whereas Pt/C shows a high methanol oxidation current, as indicated by the appearance of a large oxidation current (opposite to the ORR current) after the addition of methanol. The durability of M+S+C900-900 and Pt/C was also evaluated and compared by chronoamperometric measurements, with an electrode rotation speed of 900 rpm in O2-saturated 0.1 M KOH. As depicted in Figure 4b, the resultant chronoamperometric profile for M+S+C900-900 exhibited a nearly invariant current response after the initial 15% drop within the first hour. The current remained at 83% of the initial value even after 10 h of continuous operation. In stark contrast, the activity of Pt/C kept decreasing, and only 57% of the initial current remained after 10 h under the same experimental conditions. The result indicated a markedly higher stability of M+S+C900-900 than the commercial Pt/C catalyst in the alkaline solution. M+S+C900-900 also exhibited a remarkable long-term stability. The RDE measurements showed almost no change of limiting current and half-wave potential after the M+S+C900-900 modified electrode was immersed in the electrolyte for 2 months (Figure 4c). In contrast, the commercial Pt/C displayed a 44 mV negative shift of the half-wave potential in the same period (Figure 4d).

Figure 4.

Figure 4

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Methanol tolerance and stability comparison between M+S+C900-900 and Pt/C in O2-saturated 0.1 M KOH. (a) Current–time (i–t) responses at +0.88 V, with the addition of 1 M methanol at 1200 s. (b) Normalized current–time profiles at +0.88 V, at a rotation rate of 900 rpm. (c,d) LSV curves obtained with freshly prepared catalysts (solid traces) and after storage in the electrolyte solution for 60 days (dashed traces): (c) M+S+C900-900 and (d) Pt/C. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

2.3. ORR Active Sites

To probe the nature of the ORR catalytic active sites, we measured the ORR activity of M+S+C900-900 in 0.1 M KOH containing 10 mM KCN. As can be seen in Figure 5a, the addition of KCN negatively shifts the onset and half-wave potentials by ∼73 and ∼79 mV, respectively. In addition, the diffusion-limited current decreased 6% at +0.4 V. These observations indicate that the iron centers are the active sites, and the decreased activity is attributed to the strong binding of CN– ions to the iron centers.53,61 This assertion is further confirmed by the 41 mV negative shift of E1/2 after the addition of 5 mM of NaF to O2-saturated 0.1 M KOH (Figure 5b). The decrease of ORR activity by these anions has been attributed to their high affinity to the iron centers that compete with O2 binding to these sites.62−64 This hypothesis cannot explain the observation of the minute decrease of diffusion-limited current accompanied with the tens of millivolts negative shift of E1/2 in the present and previous studies by others.62,63 Recent Mössbauer spectroscopic and in situ X-ray absorption spectroscopic studies by Mukerjee and others revealed that the active site for Fe–N–C catalysts is an Fe2+–N4 moiety that is formed by the reduction of Fe3+–N4.65,66 Therefore, the ORR activity in terms of E1/2 is dictated by the Fe2+/3+ redox potential. This active site model is consistent with the earlier studies of Fe and Co porphyrins by Anson and co-workers67,68 and can reconcile the large negative shift of E1/2 and the small decrease of the limiting current. Thus, the negative E1/2 arises from the much larger binding constant of these anions to Fe3+ than to Fe2+, as has been shown extensively in iron porphyrins and hemes,69,70 and thereby the formation of the ORR active center Fe2+–N4 is shifted to more negative potentials. The small decrease of the limiting current can be accounted for by the competitive binding of these anions and O2 to the Fe2+–N4 center. The critical role of Fe species in the high ORR activity was further demonstrated by the chemical etching of M+S+C900-900 with 12 M HCl. The ORR activity of M+S+C900-900 diminished markedly after the etching, with an 81 mV negative shift of the onset potential and a 17% decrease of the limiting current (Figure S4). These results clearly indicate that the Fe species play an indispensable role in the high ORR activity of M+S+C900-900 and provide additional support for the hypothesis of the Fe2+–N4 active center discussed above.

Figure 5.

Figure 5

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(a) LSV curves of M+S+C900-900 recorded before (red) and after (black) the addition of (a) 10 mM KCN and (b) 5 mM NaF in O2-saturated 0.1 M KOH. Electrode rotation rate: 1600 rpm and potential scan rate: 10 mV s–1.

2.4. Factors Affecting ORR Activity

To aid in dissecting how different factors in the synthesis procedure contribute to the high performance of the obtained carbon-based catalysts, various controlled experiments were performed. The influence of the pyrolysis temperature on the electrocatalytic properties was first explored. As shown in Figure 6a, the ORR activity for the catalysts prepared under the same protocol (first heat treatment followed by an acid wash and second heat treatment) but with different pyrolysis temperatures (800 °C for M+S+C800-800 and 1000 °C for M+S+C1000-1000) is significantly inferior to that of M+S+C900-900, which displays more negative half-wave potentials and lower limiting current densities. The higher activity of M+S+C900-900 can at least partly be attributed to its higher surface area and pore volume (see Figure S5 and Table S3), which could expose more active sites for ORR, and facilitate mass transport through the porous structures. In addition, the variation of chemical speciation at different temperatures can strongly influence the catalyst activity. As disclosed by the XPS measurements, the percentage of the overall N and O contents decreased with the increasing pyrolysis temperature, whereas the Fe and C contents increased (Table S1). These observations agree well with that reported on catalysts prepared by pyrolyzing polyaniline.71 Further inspection of the high-resolution N 1s XPS spectrum revealed that catalysts prepared at 900 °C have the highest percentage of graphitic and Fe–N species (Table S4). As discussed above, the Fe–N species is the most active site in NPGM catalysts. This result adds an explanation for the optimal pyrolysis temperature at 900 °C.71,72

Figure 6.

Figure 6

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Results from controlled experiments. (a) Effects of pyrolysis temperature. (b,c) Effects of second heat treatment at 900 °C: (b) LSV and (c) EIS. (d) Effects of different reagents on the starting materials. The LSV curves were recorded in O2-saturated 0.1 M KOH with a rotation speed of 1600 rpm and a potential scan rate of 10 mV s–1.

We also attempted to explore the effects of the second heat treatment in the catalyst preparation on the catalytic activity by comparing the ORR activities of the samples before and after the second heat treatment. As shown in Figure 6b, the sample subjected to the second heat treatment (M+S+C900-900) exhibited dramatically improved activity in both kinetic-controlled and mass transport-controlled potential regions. The onset potential shifted from +0.91 V for M+S+C900 to +0.98 V for M+S+C900-900, and the limiting current density also increased significantly. We would like to point out that the sample without the second heat treatment was acid-washed; therefore, the observed difference was from the second heat treatment alone. It has been shown by others that a second heat treatment could remove oxygen-containing functional groups and corrosive carbon species from the material surface and improve the degree of graphitization, thereby further enhancing the ORR activity.72,73 The increased degree of graphitization can increase the electric conductivity of the catalyst and therefore improve the catalytic activity.10,72,74 The electrochemical impedance spectroscopy (EIS) results in Figure 6c show that the serial resistance (Rs) of M+S+C900-900 (47.6 Ω) is lower than that of M+S+C900 (62.9 Ω), indicating the conductivity of the catalysts improved after the second heat treatment. We also varied the temperature of the first heat treatment from 800 to 1000 °C but kept the second heat treatment at 900 °C (Figure S6). Interestingly, the ORR onset potentials for these samples are largely the same, unlike those in Figure 6a where the onset potentials for the samples prepared at 800 and 1000 °C are significantly more negative than the one prepared at 900 °C, suggesting that the second heat treatment at 900 °C is critical for obtaining the high ORR activity. It has been postulated that the second heat treatment may repair the disrupted active sites.71,73 We speculate that the lower temperature is unable to repair the disrupted active sites, and the higher temperature may destroy the Fe–N active sites. Detailed surface characterizations are needed to further digest these results.

Control experiments were also performed to reveal the roles of NaCl/KCl and melamine in forming the high-performance spinach-derived carbon materials. From the ORR polarization curves shown in Figure 6d, on the sample prepared under identical conditions as that for M+S+C900-900, but in the absence of NaCl/KCl and melamine (M–S–C900-900), the onset potential was significantly shifted to a negative potential at +0.84 V, and the limiting current was reduced to half of that observed on M+S+C900-900. The sample prepared with the addition of melamine, but no NaCl/KCl, to spinach showed slightly improved ORR onset potential and limiting current (M+S–C900-900). Adding NaCl/KCl mixture to spinach in the absence of melamine (M–S+C900-900) further increased the ORR limiting current. These performance differences can be understood in terms of surface area, pore formation, and nitrogen content in the samples. Nitrogen adsorption–desorption isotherms clearly reveal that NaCl/KCl crystals play a major role in forming mesopores and increasing the surface area (Table S3). The BET surface area for M–S–C900-900 is only 40.3 m2 g–1, with a pore volume of 0.33 cm3 g–1. Most pores are micropores. With the addition of the NaCl and KCl salts, the surface area increased to 175.5 m2 g–1 with a pore volume of 0.70 cm3 g–1 (M–S+C900-900). Most pores are mesopores. Chen and co-workers have shown that NaCl can facilitate the formation of mesopores and act as a template to form macropores.75,76 Our results agree well with these previous studies. A close inspection of Table S3 further reveals that melamine also plays a role in increasing the surface area and pore volume. The surface area of M+S–C900-900 is more than 60% higher than that of M–S–C900-900. Similarly, the surface area of M+S+C900-900 is about 65% higher than that of M–S+C900-900. The only difference in the two pairs is the absence of melamine in M–S–C900-900 and M–S+C900-900. Accompanied with the increasing surface area is the pore volume. The micropore and mesopore areas increased significantly after the addition of melamine. These structural changes most likely arise from the formation of volatile compounds in melamine decomposition.77,78 It has been shown that a combination of micropores and mesopores in Fe, N-doped carbon material can boost the ORR activity.79 The micropores are where the active sites reside,79−81 and the mesopores facilitate the mass transport of the reactants. Melamine has also been extensively used as a source of nitrogen in the formation of high-performance Fe, N-doped ORR catalysts82−85 because its decomposition at elevated temperatures releases NH3 gas, which is commonly used for doping nitrogen to carbon materials.1 As revealed by the XPS measurements (Figure S7 and Tables S1 and S4), samples prepared with the addition of melamine (M+S–C900-900 and M+S+C900-900) have significantly higher nitrogen contents than the one without melamine (M–S–C900-900 and M–S+C900-900), which increase the density of the active sites.

3. Conclusions

We have developed an effective synthetic strategy to fabricate spinach-derived heteroatom-doped porous carbon sheets. These materials exhibit ORR performance superior to Pt/C in alkaline media in terms of half-wave and onset potentials, longtime, stability and resistance to methanol poisoning. They also show ORR activity close to that of Pt/C in acidic media. The control experiments reveal the active centers as Fe–Nx moieties and the important roles of micro- and mesopores. Given that many plants have the ability to accumulate metals, we envision that they can be used not only for metal remediation from contaminated environments but also as a natural precursor for the preparation of heteroatom-doped carbon materials as catalysts for various reactions and electrode materials for electrochemical energy storage.86

4. Experimental Section

4.1. Chemicals

Spinach was purchased from various local supermarkets. Sodium chloride (NaCl, ≥99.5%), potassium chloride (KCl, ≥99%), potassium cyanide (KCN, ≥98.0%), sodium fluoride (NaF, ≥99%), Nafion solution (5%), and potassium hydroxide (KOH, 85%), were purchased from Sigma-Aldrich. Double-distilled perchloric acid (HClO4, 70%) and sulfuric acid (H2SO4, 95–98%) were procured from GFS Chemicals. Melamine (99%) was purchased from Alfa Aesar. Commercial 20 wt % Pt/C was obtained from Fuel Cell Store. Water was supplied from a Milli-Q water purification system (18.2 MΩ cm).

4.2. Catalyst Preparation

The spinach leaves were washed with water and ground into juice using a kitchen blender. The juice was freeze-dried for 48 h with a Labconco FreeZone freeze-dryer and ground into powder with a mortar and pestle. A 0.50 g of the obtained spinach powder, 0.50 g melamine, 3.0 g KCl, 3.0 g NaCl, and 20.0 mL deionized water were added into a beaker and mixed with magnetic stirring. The beaker was covered with a parafilm, and the mixture was stirred and heated to 120 °C until a uniform mixture was formed. The resulting mixture was quickly transferred to centrifuge tubes and frozen with liquid nitrogen and then freeze-dried for 48 h. The samples were then collected and ground into fine powders. Subsequently, the composite powders were pyrolyzed in an Ar atmosphere according to the following heating program: (1) at 500 °C for 2 h at a ramp rate of 5 °C min–1 to dehydrate and carbonize the composite powders; (2) at 900 °C for 2 h to further carbonize and graphitize the material. Finally, the obtained sample was soaked in 0.5 M H2SO4 at 85 °C for 24 h to remove metallic particles, followed by filtration with water and freeze-drying for 24 h. The freeze-dried sample was calcined at 900 °C for 1 h in Ar flow. The obtained sample is denoted as M+S+C900-900, where M and S represent melamine and salt, respectively, and the presence or absence of melamine and salt is presented with “+” and “–”. The first “900” indicates the first pyrolysis temperature and the second “900” denotes the second pyrolysis temperature. Similar notation method is used for control samples, as noted in the text.

4.3. Characterization

SEM observation was performed on a field emission scanning electron microscope (Hitachi S-4800). High-resolution TEM images were obtained using a JEOL-2100F microscope operated under an accelerating voltage of 200 kV. XRD data were collected at room temperature using a Bruker D8 diffractometer with Cu Kα radiation (λ = 0.1541 nm). Raman spectra were recorded on a confocal LabRAM HR800 spectrometer (Horiba) with an excitation wavelength of 514.5 nm from an argon ion laser. N2 adsorption–desorption isotherms and PSD were obtained at 77 K using a Micromeritics ASAP 2020 Plus instrument. The specific surface areas of the samples were calculated using the BET method. The PSD was calculated via the DFT model. XPS measurements were conducted on a PHI X-tool instrument with Al Kα radiation.

4.4. Electrochemical Measurements

The electrochemical measurements were carried out on a CHI700C electrochemical workstation (CH instruments Inc.) with a three-electrode two-compartment glass cell at room temperature. A platinum wire and an Ag/AgCl (1 M KCl) electrode were used as the counter and reference electrodes, respectively. A RRDE with a glassy carbon disk (5.0 mm in diameter) surrounded by a platinum ring (with a collection efficiency of 20%) served as the working electrode. The working electrode was placed in the main compartment of the electrochemical cell that is separated from the compartment housing the counter and reference electrodes by a fine glass frit to minimize contaminations. To prepare the catalyst ink, 2 mg of the carbon sample was ultrasonically dispersed in 1.0 mL of a 7:3 (v/v) ethanol/water mixture solvent along with 20 μL of Nafion solution (5%) for 30 min. Then, 15 μL of the catalyst ink was dropped onto the glassy carbon disk and dried at room temperature for 30 min at a catalyst loading of 0.15 mg cm–2. For comparison, commercial Pt/C was loaded onto the glassy carbon electrode surface using the same procedure and catalyst loading. The RRDE tests were carried out in N2- or O2-saturated 0.1 M KOH or 0.1 M HClO4, and the scan rate was 10 mV s–1. In all of the measurements, the Ag/AgCl reference electrode was calibrated with respect to a homemade reversible hydrogen electrode (RHE), using E(RHE) = E(Ag/AgCl) + 0.982 V in 0.1 M KOH and E(RHE) = E(Ag/AgCl) + 0.288 V in 0.1 M HClO4. The disk current density was normalized to the geometric surface area of the glassy carbon electrode.

The LSV curves were recorded in O2-saturated 0.1 M KOH, with varying electrode rotation rates from 400 to 2500 rpm. The K–L plots (j–1 vs ω–1/2) were analyzed at various potentials according to eqs 1 and 2

4.4. 1
4.4. 2

where j, jK, and jD are the measured, kinetic, and diffusion-limited current densities, respectively. ω is the angular velocity of the rotating electrode, n is the number of electron transfer involved in the ORR, F is the Faraday constant (96485 C mol–1), D is the oxygen diffusion coefficient (1.9 × 10–5 cm2 s–1), ν is the kinematic viscosity of the electrolyte solution (0.01 cm2 s–1), and CO2 is the bulk concentration of oxygen (1.2 × 10–3 mol L–1).

The number of electron transfer (n) and HO2– percent yield at different potentials were calculated by eqs 3 and 4

4.4. 3
4.4. 4

where ID represents the disk current, IR is the ring current, and N is the RRDE collection efficiency (20%).

Acknowledgments

This work was partially supported by the American University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02673.

  • Elemental contents in different carbon catalysts determined by XPS measurements; comparison of the catalytic performances of M+S+C900-900 and noble metal-free carbon materials in the literature; results from nitrogen adsorption−desorption analysis; comparison of nitrogen species in different carbon catalysts; LSV curves of M+S+C900-900 from different spinach; hydrogen peroxide yield of M+S+C900-900 and Pt/C during the ORR process; RDE voltammograms for ORR of M+S+C900-900 and 20 wt % Pt/C in 0.1 M HClO4; LSV curves of M+S+C900-900 before and after acid leaching in O2-saturated 0.1 M KOH; N2 adsorption–desorption isotherms at 77 K for different samples; LSV curves of different catalysts prepared under different first heat treatments and the same second heat treatment at 900 °C; and XPS survey spectra and deconvolutions of N1s spectra of M+S+C800-800, M-S+C900-900, M-S-C900-900, M+S-C900-900, and M+S+C1000-1000 (PDF)

Author Present Address

† Department of Chemistry, Oakland University, Rochester, MI, 48309, USA.

Author Present Address

‡ Department of Electrical & Computer Engineering, Texas Tech University, Lubbock, TX 79409, USA.

Author Contributions

X.L.: investigation, data curation, formal analysis, writing—original draft. C.C.: data curation, validation, review and editing. W.L.: data curation, validation, review and editing. S.Z.: conceptualization, resources, supervision, funding acquisition, writing—review and editing.

The authors declare no competing financial interest.

Supplementary Material

ao0c02673_si_001.pdf (1.6MB, pdf)

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