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Selective H2O2 production on surface-oxidized metal-nitrogen-carbon electrocatalysts
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Selective H2O2 production on surface-oxidized metal-nitrogen-carbon electrocatalysts ⁎
Minhee Suka,1, Min Wook Chunga,1, Man Ho Hanb, Hyung-Suk Ohb, , Chang Hyuck Choia, a b
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School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxygen reduction reaction Hydrogen peroxide Selectivity Me-N-C catalysts Fenton reaction Oxygen functional groups
The electrochemical synthesis of hydrogen peroxide (H2O2) can provide an attractive alternative to the current anthraquinone redox process, as it combines on-site chemical and electricity productions. A major challenge in the electrochemical H2O2 synthesis is the catalyst design which leads to a selective two-electron pathway in the oxygen reduction reaction (ORR) without dissociation of the OeO bond. In the present work, we report that the partial oxidation of metal-nitrogen-carbon catalysts (Me-N-C, Me = iron, cobalt and manganese) can modify their ORR mechanisms from a four- to a two-electron pathway. Spectroscopic measurements reveal that ex situ H2O2 treatment introduces abundant oxygen functionalities on the Me-N-C surface without considerable changes to their bulk properties, such as crystallinity, degree of defects, surface area, and metal contents. Decreased H2O2 reduction kinetics on the oxidized catalysts confirm that the dissociation of the OeO bond is strongly suppressed by the newly introduced oxygen functionalities. Among the three central metal candidates, the cobalt-nitrogencarbon catalyst shows the highest H2O2 selectivity of > 85%. This work provides a new simple guideline for designing Me-N-C catalysts for the efficient electrochemical synthesis of H2O2.
1. Introduction Hydrogen peroxide is a significant commodity chemical that is widely used in textile and paper manufacturing, water-cleaning processes, chemical synthesis, and hydrogen storage [1,2]. Currently, H2O2 is almost exclusively manufactured by the anthraquinone redox process, i.e., hydrogenation of anthraquinone with H2 and subsequent oxidation by O2 in an organic solvent, and this process is optimized for the large-scale production of highly concentrated H2O2 (ca. 70 wt%) [3]. Despite its high efficiency, the anthraquinone process involves a multistep sequence requiring significant energy input and generates substantial waste. In addition, the instability of the concentrated H2O2 poses safety problems for transport, and diluted H2O2 solution is typically utilized for most applications. Hence, there is an increasing interest in the development safe and decentralized H2O2 production methods [4–14]. One promising alternative route is the electrochemical two-electron reduction of oxygen (i.e., O2 + 2H+ + 2e− → H2O2) [7–14], which is safer and proceeds under mild conditions (ambient temperature and pressure) [15]. Moreover, the electrochemical process can be coupled with renewable energy sources as well as practical fuel cell devices for simultaneous electricity generation.
The prevention of a competitive four-electron pathway (i.e., O2 + 4H+ + 4e− → 2H2O) of ORR, which is typically catalyzed by noble metal catalysts in purpose of efficient electricity generation, is a critical key to achieve the successful production of H2O2 [10–14]. On the noble metal surface (e.g., Pt and Pd), oxygen molecules are believed to be adsorbed as a ‘side-on’ configuration rather than an ‘end-on’ adsorption configuration, leading to weakened OeO bond and consequently facile water production [16,17]. In principle, the dissociative adsorption of O2 requires adjacent atomic metal sites (i.e., ensemble) which provide separate binding sites for each oxygen atom in oxygen molecule. Hence, single atomic noble metal catalysts (e.g., Pt1-S4 [10] and Pt1/TiN [11]) and noble metal alloys with inert elements (e.g., Pt-Hg [12], Pd-Au [13] and Pd-Hg [14]) have been extensively suggested to eliminate the accessible active metal ensembles. On the isolated noble metal atom, it has been predicted that the selective chemisorption of oxygen molecule as the end-on configuration can prohibit the undesirable dissociation of OeO bond and increase selectivity towards the two-electron pathway of ORR [10–14]. In this sense, atomically dispersed transition metals ligated with nitrogen-doped carbon, i.e., Me-N-C catalysts where Me = Fe, Co, Mn, etc., can be reasonably presumed to electrochemically catalyze oxygen
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Corresponding authors. E-mail addresses: [email protected] (H.-S. Oh), [email protected] (C.H. Choi). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cattod.2019.05.034 Received 20 January 2019; Received in revised form 16 April 2019; Accepted 14 May 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Minhee Suk, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.034
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a Talos F200X (FEI) at an operating voltage of 200 kV. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) measurements were carried out using a Regulus 8230 (Hitachi) at an operating voltage of 5 kV. The Raman spectra were collected by Alpha 300S (WlTec) with 532 nm laser excitation. The X-ray photoelectron spectroscopy (XPS) measurements were carried out using a K-Alpha (Thermo Scientific) equipped with monochromatic X-ray generated by an Al K-alpha source. The XPS data were analyzed using XPSPEAK41 software with a ± 0.1 eV deviation in binding energy. The XPS-N1s spectra of Me-N-C were fitted into four N species: pyridinic-N (398.5 eV), pyrrolic-N (400.1 eV), graphitic-N (401.1 eV), and pyridinic-oxide (403.7 eV) [43]. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were carried out using an ICAP 7000 series (Thermo Scientific). The surface area was determined by nitrogen physisorption measurements (Quantachrome Autosorb-1Q) and the Brunauer-Emmett-Teller model (BET) formulations were used.
molecules to H2O2. Contrary to this preliminary prediction, however, the Me-N-C catalysts (particularly Fe-N-C) generally follow the fourelectron pathway on active MeNxCy moieties [18–30]. With heterogeneity of the Me-N-C catalysts including bulk Me and doped N species as well as active MeNxCy moieties, this discordance was explained by tandem catalysis on multitudinous catalytic sites, i.e., H2O2 production on one site and consecutive H2O2 reduction on the other (or identical) site known as the ‘2e− + 2e−’ or ‘2e− × 2e−’ mechanism [31–36]. After synthetic achievements of the Fe-N-C catalysts solely comprised of the atomic FeNxCy moieties [37], however, the dominant four-electron ORR pathway on the FeNxCy site was verified and a minor production of H2O2 on the atomic Fe site could not be explained by the simple ensemble effects [38]. Otherwise, Wang group predicted in their computational study that cleavage of the OeO bond on the atomic Fe site becomes kinetically possible via cooperative dissociation pathway, during which one end of the oxygen molecule (not bound to the atomic Fe) is attracted to the nearest carbon atom [39,40]. Considering the cooperative dissociation mechanism on the MeNxCy moiety, pre-occupation of the nearest carbon site, to prevent communication with one end of the adsorbed oxygen molecule, can be a promising strategy to magnify H2O2 production on the Me-N-C catalysts. Here, we have thus tried to confirm the above presumption and concurrently develop cost-effective Me-N-C catalysts (Me = Fe, Co and Mn) for electrochemical H2O2 production. Surface carbon atoms on the MeN-C catalysts were modified by Fenton(-like) reactions with H2O2. This treatment introduced oxygen-functional groups on the carbon surface without significant changes in the crystallinity, degree of defects, surface area and metal contents of the Me-N-C catalysts. With the increased surface oxygen content, the two-electron ORR pathway on the Me-N-C catalysts also increased. Among the metal candidates, Co-N-C showed the highest selectivity of > 85% toward H2O2 production. These insights thus provide that the electrochemical production of H2O2 on transition metal atom catalysts can be achieved if rational strategies optimizing pre-occupation of the carbon sites and surface modification are developed.
2.3. Electrochemical characterizations The electrochemical properties were investigated using a VMP-300 potentiostat (Bio-Logic) in a three-electrode cell equipped with a saturated Ag/AgCl as a reference electrode (RE-1A, EC-Frontier) and a graphite rod as a counter electrode. The electrolyte was 0.1 M HClO4, which was prepared from concentrated HClO4 (70%, Sigma-Aldrich). Prior to any electrochemical measurements, the Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) scale by calibration in a H2-saturated electrolyte against a Pt electrode. The catalyst inks were prepared by dispersing the 10 mg of catalysts in 3,979 μL of the solvent (3,617 μL of DI water, 282 μL of isopropanol and 80 μL of Nafion solution (5 wt%)). After the sonication of the suspension for 1 h, working electrodes were prepared by dropping 10 μL of the Me-N-C ink onto a glassy carbon disk (0.13 cm2) of a rotating Pt-ring disk electrode (RRDE, 012613, ALS Co.). The catalyst loading was set to 200 μg cm−2. The ORR polarization curve was measured at a 10 mV s−1 scan rate and a 900 rpm rotation speed in an O2-saturated electrolyte. To remove the non-Faradaic capacitance responses, polarization curves measured using the same procedure but in an Ar-saturated electrolyte were subtracted from the results. H2O2 formation during ORR was studied by polarizing the Pt-ring electrode at 1.2 VRHE. H2O2 selectivity was calculated by the following equation: H2O2 (%) = 200 × (IR/N)/ (ID + (IR/N)), where IR is the ring current, ID is the disk current and N is the collection efficiency (0.378, as determined by Fe2+/3+ redox calibration). The hydrogen peroxide reduction reaction (PRR) polarization curves were measured at a 10 mV s−1 scan rate and a 900 rpm rotation speed in an Ar-saturated electrolyte containing 10 mM H2O2.
2. Experimental 2.1. Catalyst synthesis The Me-N-C catalysts were prepared with Me-acetate salts, 1,10phenanthroline (phen) and ZnII zeolitic imidazolate framework (ZIF-8, Basolite Z1200 from Sigma-Aldrich). The precursor powders (1 g), containing Me/phen/ZIF-8 with a mass ratio of 0.5/20/80, were homogenized in a ZrO2 crucible with 100 ZrO2 balls (5 mm diameter) in a ball-miller for four cycles of 30 min each at 400 rpm. The mixture of the catalyst precursors was then pyrolyzed at 1050 ℃ in N2 for 1 h, resulting in pristine Me-N-C catalysts. Note that previous works with identically or similarly synthesized Me-N-C catalysts revealed the predominance of atomically dispersed MeNxCy moieties (e.g., porphyrinlike MeN4C12) on carbonaceous supports [37,41,42], which are known to be the main active sites for the ORR. The H2O2 treatment of the catalysts (0.1 g) was carried out in a 0.01 M HClO4 solution (300 mL) at 100 ℃ with a reflux condenser for 8 h. The 5 mL of H2O2 solution (30 wt %, Sigma-Aldrich) was injected into the solution and replenished every hour. After the oxidation reaction, the catalyst powder was collected by filtration, washed with 1 L of deionized (DI) water (> 18 MΩ, Satorious) and dried at 70 ℃ in an oven.
3. Results and discussion 3.1. Bulk characteristics of Me-N-C catalysts before and after H2O2 treatment We first investigated the bulk properties of the Me-N-C catalysts before and after H2O2 treatment. The XRD patterns of the pristine MeN-C catalysts represented two broad peaks at ca. 25° and 44°, which correspond to the (002) and (101) planes of graphite, respectively (Fig. 1a). Other discernable peaks were not shown in the XRD patterns, inferring a negligible amount of bulk Me phases such as metallic Me, oxides, and carbides. Note that in literature (and our previous works) Mössbauer and X-ray absorption spectroscopy studies of Fe-N-C and CoN-C catalysts [37,41], which were synthesized in the same manner as in the present study, showed a predominant presence of atomic Fe and Co moieties ligated with nitrogen functional groups on the carbonaceous support without a discernable bulk Me species. In the Raman spectra, all the pristine Me-N-C catalysts showed two intense D- and G-band peaks at ca. 1350 and 1590 cm−1 (Fig. 1b), respectively. Because the G-
2.2. Physical characterizations The X-ray diffraction (XRD) patterns were obtained using an Empyrean (Malvern Panalytical) equipped with Cu K-alpha radiation. The XRD was measured at an accelerating voltage of 45 kV and a current of 40 mA with a scan rate of 2° min−1 and a step size of 0.013°. Transmission electron microscopy (TEM) analysis was carried out using 2
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Fig. 1. (a) XRD patterns, (b) Raman spectra of pristine and oxidized Me-N-C catalysts. (c–e) TEM and (f–h) SEM images of (c, f) Co-N-C, (d, g) Mn-N-C, and (e, h) FeN-C catalysts (left) before and (right) after H2O2 treatment. Composition data measured by EDX was indicated in each SEM image. 4 band arises from the E2g vibrational mode in the D6h symmetry group of the graphite crystal planes, and the D-band originates from the lattice distortion in sp2-hybridized carbon [44], their intensity ratios (ID/IG) are typically used as a parameter indicating the extent of carbon disorder. Intense D-band signals with ID/IG values of ca. 0.95 ( ± 0.01) for the pristine Me-N-C catalysts showed a highly disordered graphitic structure, as also confirmed by the broad graphite peaks in their XRD patterns. After the H2O2 treatment under harsh oxidizing conditions (i.e., 3.5 wt% H2O2, 100 ℃ and 8 h), however, the H2O2-treated Me-N-C catalysts showed almost identical bulk properties with those of the pristine catalysts. Modifications of the XRD and Raman patterns (including ID/IG values) after the treatment were marginal (Fig. 1a and b). The TEM and SEM images (Fig. 1c–h) show a fully carbonaceous structure without appreciable metal clustering, and an imperceptible change in morphology before and after H2O2 treatment. In addition, N2adsorption isotherms revealed that the H2O2 treatment did not lead to considerable modifications of the surface area of the Me-N-C catalysts either (Table 1, deviation ≤3%). The metal content, measured by ICPAES, were ca. 1.4, 1.3, and 1.5 wt% for pristine Co-N-C, Mn-N-C, and Fe-N-C, respectively. The metal contents slightly decreased to 1.1, 1.2, and 1.3 wt% after the treatment, while the changes were also moderate.
were found before and after the H2O2 treatment except for the Fe-N-C catalyst, which showed additional small contributions of the C]O and/ or OeC]O functionalities after the H2O2 treatment [45,46]. Modifications in the nitrogen functionalities of the Me-N-C catalysts after the treatments were studied using the XPS-N1s analysis (Fig. 2b). The nitrogen content of the catalysts, calculated from the XPS spectra, were ca. 4.5 wt% regardless of the Me precursors and H2O2 treatments (Fig. 3a). The XPS-N1s fitting results confirmed a predominant presence of pyridinic- and graphitic-N moieties (Fig. 3b). The XPS-N1s signals and fitting results were modified insignificantly after the H2O2 treatment, indicating no oxidation of nitrogen functionalities including those ligating the active Me moieties. Overall, the XRD, Raman, TEM, SEM, and XPS-C1s/N1s measurements of the samples showed almost identical spectra/diffractograms before and after the H2O2 treatment, indicating no considerable modifications in defect sites and other carbon/nitrogen moieties, which could possibly effect on the ORR pathway of the Me-NC catalysts [43,47,48]. Exceptionally, one remarkable change after the treatments was the oxygen content (Fig. 2c), increasing from ca. 4–6 wt% for pristine MeN-C catalysts to ca. 9–17 wt% for H2O2-treated catalysts measured by XPS-O1s (Fig. 3a). This appreciable increase in oxygen content was also observed in EDX measurements (Fig. 1f–h), ca. 4–6 wt% oxygen content in the pristine Me-N-C catalysts but ca. 7–13 wt% in the treated samples. The XPS-O1s signals were fitted with two peaks at 531.6 and 533.0 eV, which correspond to the C]O and CeO components, respectively [49]. The H2O2 treatment increased the content of the former component more prevailingly than that of the latter one (Fig. 3c). Compared with our previous work performed with relatively mild H2O2 treatment conditions (i.e., 2 h and 20–70 ℃) [50], which also showed increment in both the C]O and CeO content, the regnant C]O formation in the present work implies a more oxidized surface of carbon, resulting from the harsher oxidizing conditions (i.e., 8 h and 100 ℃). Substantial surface oxidation occurred on the Fe-N-C catalysts compared with that on the Mn-N-C and Co-N-C catalysts. Considering the fact that the H2O2-treatment conditions were identical except for the type of central metal ion (i.e., Fe, Co and Mn) in the Me-N-C catalysts, the differences in the extent of surface oxidation may be due to the different levels of reactive oxygen species (ROS) generated from Fenton or Fenton-like reactions between the H2O2 and Me ions [51]. In the classical Fenton reaction producing hydroxyl radicals from H2O2 and ferrous ions, the reaction kinetics is very fast, having a reaction
3.2. XPS measurements of Me-N-C catalysts before and after H2O2 treatment Other possible changes were further investigated using XPS (Fig. 2), a more surface-sensitive technique than XRD and Raman spectroscopy. The XPS-C1s of the pristine Me-N-C catalysts showed a clear peak at 284.5 eV (Fig. 2a), which emerges from the CeC bond of the sp2-hybridized carbon [45,46]. No significant changes in the XPS-C1s spectra Table 1 BET surface area and metal content of Me-N-C catalysts before and after oxidation process. Catalyst
Co-N-C
Mn-N-C
Fe-N-C
BET surface area (m2 g−1)
Pristine H2O2-treated
763 729
664 682
680 716
Metal content (wt%)
Pristine H2O2-treated
1.4 1.1
1.3 1.2
1.5 1.3
3
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Fig. 2. XPS spectra of pristine and H2O2-treated Me-N-C catalysts: (a) C1s, (b) N1s, and (c) O1s.
constant of k = 63–76 M−1s−1. In contrast, the kinetics of Fenton-like reaction on non-ferrous ions (e.g., Co and Mn) is relatively slow, and the reaction can often only occur under specialized reaction conditions. Hence, the ROS level generated by non-ferrous ions is expected to be lower than that by ferrous ions, resulting in more significant changes in the XPS-O1s spectrum of the Fe-N-C catalyst after H2O2 treatment than those of the Co-N-C and Mn-N-C catalysts. This result is in line with the XPS-C1s spectrum of the H2O2-treated Fe-N-C catalyst showing C]O
and/or OeC]O functionalities while their contributions were not clearly discernable in the XPS-C1s spectra of other Me-N-C catalysts (Fig. 2a).
3.3. Electrochemical characteristics of Me-N-C catalysts before and after H2O2 treatment The electrocatalytic activity and selectivity toward ORR were then
Fig. 3. (a) Compositions of oxygen, nitrogen and metal in the catalysts. The former two elements were measured by XPS, while the metal composition was obtained from ICP-AES due to the poor signal-to-noise ratio of the XPS-Me2P spectra. Contents of (b) nitrogen and (c) oxygen functional groups calculated from the XPS fitting results. In all the figures, the values from pristine (left bar) and H2O2-treated Me-N-C catalysts (right bar) were indicated concurrently. 4
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Fig. 4. (a–c) ORR polarization curve and (d–f) selectivity toward H2O2 formation of the Me-N-C catalysts before and after H2O2 treatment: (a, d) Co-N-C, (b, e) Mn-NC, and (c, f) Fe-N-C.
It has been theoretically expected that the reduction of *OOH intermediate to either *HOOH or *O is a key step to determine ORR selectivity and that the OeO bond cleavage at a single atomic Me moiety requires a considerable reorganization energy [52]. Hence, as shortly noted in the Introduction section, the cooperative dissociation pathway, i.e., an OeO bond cleavage assisted by the nearest carbon atom, has been recently considered by the Wang group [39,40] in order to understand a predominant H2O production on atomic FeNxCy moieties of Fe-N-C catalysts [38]. When the surface of the Fe-N-C catalysts is oxidized, however, this pathway could become less favorable due to a weakened iron oxophilicity (i.e., electronic effect) and/or a pre-occupation of the nearest carbon site by oxygen functionalities (i.e., steric effect) [50], leading to an enhanced H2O2 production on the Fe-N-C catalysts. Beside these electronic and steric effects on the surface-oxidized Me-N-C catalysts, in the present work, the highest H2O2 selectivity on the surface-oxidized Co-N-C catalysts could be explained by a relatively lower oxophilicity of the Co-N4 complexes (i.e., the weak binding energy of oxygen and ORR intermediates) than that of the FeN4 and Mn-N4 complexes. Considering previous case studies of molecular MeN4 catalysts (e.g., porphyrin and phthalocyanine) [53], the molecular catalysts were divided into two main categories according to their ORR selectivity: i) Fe and Mn, which have a strong binding affinity for O2 that facilitates the dissociation of the OeO bond and ii) Co (or Cr and Ni), which bind with O2 weakly and do not break the OeO bond efficiently. Simiarly, in the pyrolized systems (i.e., Me-N-C catalysts), computational calculations also predicted much weaker O2-adsorption energy (−0.80 to −1.26 eV) on CoNxCy sites than that on FeNxCy sites (ca. −1.84 eV) [41]. Beside the high selectivity towards the two-electron ORR pathway, the slow kinetics of the consecutive H2O2 reduction reaction (PRR, H2O2 + 2H+ + 2e− → 2H2O) is also of importance for the production of concentrated H2O2 solution in electrochemical systems. Furthermore, in the presence of some site-blocking spectator species and the low content of active metal sites, H2O2 diffusion into the bulk electrolyte could be artificially favored with its subsequent decomposition in the RRDE experiments [54]. Therefore, effects of the chemical oxidations of the Me-N-C catalysts on their PRR activity were additionally
evaluated in a 0.1 M HClO4 electrolyte at an electrode rotation of 900 rpm (Fig. 4). The ORR polarization curves of the pristine Me-N-C catalysts show an onset potential (Eonset, defined here as the potential at a current density j of 0.1 mA cm−2) of 0.81, 0.76, and 0.85 VRHE for CoN-C, Mn-N-C, and Fe-N-C, respectively (Fig. 4a–c). The j values measured at 0.8 VRHE, at which the ORR polarization curves were mainly controlled by their kinetics rather than mass transports, were −0.17, −0.02, and −0.53 mA cm−2 for Co-N-C, Mn-N-C, and Fe-N-C, respectively. In the RRDE measurements, the pristine Fe-N-C showed a predominant H2O production during the ORR with a relatively minimal formation of H2O2 less than 10% (Fig. 4f). Otherwise, the pristine Co-NC and Mn-N-C released considerable amounts of H2O2 in the ORR, i.e., < 60 and < 35% H2O2 in an overall potential range, respectively (Fig. 4d and e). The varied H2O2 selectivity was also supported by the different diffusion current density (jd) (Fig. 4a–c), which is a function of the number of electrons transferred during ORR (at this condition jd is ca. −4.5 mA cm−2 when four electrons are transferred). The measured values of jd were ca. −3, −3.5 and −4 mA cm−2 when the electrode changed in order of Co-N-C, Mn-N-C, and Fe-N-C, respectively. With the highest activity among the candidates with a dominant four-electron ORR pathway, the Fe-N-C catalysts verified their potential applications in PEMFCs as reported so far [20–30]. However, the H2O2-treated Me-N-C catalysts showed completely different electrocatalytic properties. After the chemical oxidations, their Eonset was negatively shifted to 0.74, 0.63, and 0.81 VRHE for Co-N-C, Mn-N-C, and Fe-N-C, respectively (Fig. 4a–c). Considering the fact that the thermodynamic equilibrium potential between oxygen and water (EO02/H2O ) is ca. 1.23 VRHE (i.e., O2 + 4H+ + 4e− ↔ 2H2O), and that between oxygen and H2O2 is much lower, as EO02/H2O2 = ca. 0.69 VRHE, the negative shifts of the Eonset as well as the decreased jd values inferred modifications of their ORR catalysis toward H2O2 formation rather than water production. More clearly, the RRDE measurements of the H2O2-treated Me-N-C catalysts showed enhanced H2O2 productions of ca. 50 and 30% for H2O2-treated Mn-N-C and Fe-N-C catalysts during the ORR, respectively. In particular, the H2O2-treated Co-N-C catalyst predominantly followed two-electron ORR pathway with a maximum selectivity of ca. 87%. 5
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Fig. 5. PRR activity of the Me-N-C catalysts before and after the H2O2 treatment: (a) Co-N-C, (b) Mn-N-C and (c) Fe-N-C.
investigated with a 10 mM H2O2 electrolyte. As can be seen in Fig. 5, electrochemical reduction current was observed for all the catalysts electrochemically reduced H2O2 at a potential below ca. 0.8 VRHE regardless of the H2O2 treatment. However, the H2O2-treated Me-N-C catalysts showed significantly decreased PRR kinetics. Mn-N-C and CoN-C did not reach the PRR current density of −1 mA cm−2, even at a very low potential of 0.1 VRHE. With high H2O2 selectivity in the ORR and low PRR activity, the surface-oxidized Co-N-C catalyst is likely to be the most suitable catalyst for electrochemical H2O2 productions.
[7] [8]
[9]
[10]
4. Conclusions [11]
We synthesized three Me-N-C catalysts (Me = Fe, Mn and Co) and introduced abundant oxygen functionalities on their surface through H2O2 treatment without significant modifications to their bulk properties. The formation of the C]O and CeO moieties on the Me-N-C catalysts led to an enhanced two-electron ORR pathway producing H2O2, rather than the four-electron pathway producing H2O. In particular, the oxidized Co-N-C catalysts revealed the highest H2O2 selectivity of > 85% among the three candidates. On the other hand, the kinetics of the electrochemical H2O2 reduction to H2O significantly declined on the oxidized catalysts. Considering the cooperative dissociation ORR mechanism [39,40,50], we presumed that the promoted two-electron ORR pathway was due to weakened oxophilicity at active MeNxCy moieties (i.e., electronic effect) and/or a pre-occupation of the nearest carbon site by oxygen functionalities (i.e., steric effect) after the carbon oxidations. Consequently, the surface oxidation of the Me-N-C catalysts can be considered a promising approach for the electrochemical synthesis of H2O2, while simultaneously generating electricity by combining with practical fuel cell devices.
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Acknowledgements
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This work was supported by GIST Research Institute(GRI) grant funded by the GIST in 2019. [21]
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Selective H2O2 production on surface-oxidized metal-nitrogen-carbon electrocatalysts ⁎
Minhee Suka,1, Min Wook Chunga,1, Man Ho Hanb, Hyung-Suk Ohb, , Chang Hyuck Choia, a b
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School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxygen reduction reaction Hydrogen peroxide Selectivity Me-N-C catalysts Fenton reaction Oxygen functional groups
The electrochemical synthesis of hydrogen peroxide (H2O2) can provide an attractive alternative to the current anthraquinone redox process, as it combines on-site chemical and electricity productions. A major challenge in the electrochemical H2O2 synthesis is the catalyst design which leads to a selective two-electron pathway in the oxygen reduction reaction (ORR) without dissociation of the OeO bond. In the present work, we report that the partial oxidation of metal-nitrogen-carbon catalysts (Me-N-C, Me = iron, cobalt and manganese) can modify their ORR mechanisms from a four- to a two-electron pathway. Spectroscopic measurements reveal that ex situ H2O2 treatment introduces abundant oxygen functionalities on the Me-N-C surface without considerable changes to their bulk properties, such as crystallinity, degree of defects, surface area, and metal contents. Decreased H2O2 reduction kinetics on the oxidized catalysts confirm that the dissociation of the OeO bond is strongly suppressed by the newly introduced oxygen functionalities. Among the three central metal candidates, the cobalt-nitrogencarbon catalyst shows the highest H2O2 selectivity of > 85%. This work provides a new simple guideline for designing Me-N-C catalysts for the efficient electrochemical synthesis of H2O2.
1. Introduction Hydrogen peroxide is a significant commodity chemical that is widely used in textile and paper manufacturing, water-cleaning processes, chemical synthesis, and hydrogen storage [1,2]. Currently, H2O2 is almost exclusively manufactured by the anthraquinone redox process, i.e., hydrogenation of anthraquinone with H2 and subsequent oxidation by O2 in an organic solvent, and this process is optimized for the large-scale production of highly concentrated H2O2 (ca. 70 wt%) [3]. Despite its high efficiency, the anthraquinone process involves a multistep sequence requiring significant energy input and generates substantial waste. In addition, the instability of the concentrated H2O2 poses safety problems for transport, and diluted H2O2 solution is typically utilized for most applications. Hence, there is an increasing interest in the development safe and decentralized H2O2 production methods [4–14]. One promising alternative route is the electrochemical two-electron reduction of oxygen (i.e., O2 + 2H+ + 2e− → H2O2) [7–14], which is safer and proceeds under mild conditions (ambient temperature and pressure) [15]. Moreover, the electrochemical process can be coupled with renewable energy sources as well as practical fuel cell devices for simultaneous electricity generation.
The prevention of a competitive four-electron pathway (i.e., O2 + 4H+ + 4e− → 2H2O) of ORR, which is typically catalyzed by noble metal catalysts in purpose of efficient electricity generation, is a critical key to achieve the successful production of H2O2 [10–14]. On the noble metal surface (e.g., Pt and Pd), oxygen molecules are believed to be adsorbed as a ‘side-on’ configuration rather than an ‘end-on’ adsorption configuration, leading to weakened OeO bond and consequently facile water production [16,17]. In principle, the dissociative adsorption of O2 requires adjacent atomic metal sites (i.e., ensemble) which provide separate binding sites for each oxygen atom in oxygen molecule. Hence, single atomic noble metal catalysts (e.g., Pt1-S4 [10] and Pt1/TiN [11]) and noble metal alloys with inert elements (e.g., Pt-Hg [12], Pd-Au [13] and Pd-Hg [14]) have been extensively suggested to eliminate the accessible active metal ensembles. On the isolated noble metal atom, it has been predicted that the selective chemisorption of oxygen molecule as the end-on configuration can prohibit the undesirable dissociation of OeO bond and increase selectivity towards the two-electron pathway of ORR [10–14]. In this sense, atomically dispersed transition metals ligated with nitrogen-doped carbon, i.e., Me-N-C catalysts where Me = Fe, Co, Mn, etc., can be reasonably presumed to electrochemically catalyze oxygen
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Corresponding authors. E-mail addresses: [email protected] (H.-S. Oh), [email protected] (C.H. Choi). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cattod.2019.05.034 Received 20 January 2019; Received in revised form 16 April 2019; Accepted 14 May 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Minhee Suk, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.05.034
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a Talos F200X (FEI) at an operating voltage of 200 kV. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) measurements were carried out using a Regulus 8230 (Hitachi) at an operating voltage of 5 kV. The Raman spectra were collected by Alpha 300S (WlTec) with 532 nm laser excitation. The X-ray photoelectron spectroscopy (XPS) measurements were carried out using a K-Alpha (Thermo Scientific) equipped with monochromatic X-ray generated by an Al K-alpha source. The XPS data were analyzed using XPSPEAK41 software with a ± 0.1 eV deviation in binding energy. The XPS-N1s spectra of Me-N-C were fitted into four N species: pyridinic-N (398.5 eV), pyrrolic-N (400.1 eV), graphitic-N (401.1 eV), and pyridinic-oxide (403.7 eV) [43]. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were carried out using an ICAP 7000 series (Thermo Scientific). The surface area was determined by nitrogen physisorption measurements (Quantachrome Autosorb-1Q) and the Brunauer-Emmett-Teller model (BET) formulations were used.
molecules to H2O2. Contrary to this preliminary prediction, however, the Me-N-C catalysts (particularly Fe-N-C) generally follow the fourelectron pathway on active MeNxCy moieties [18–30]. With heterogeneity of the Me-N-C catalysts including bulk Me and doped N species as well as active MeNxCy moieties, this discordance was explained by tandem catalysis on multitudinous catalytic sites, i.e., H2O2 production on one site and consecutive H2O2 reduction on the other (or identical) site known as the ‘2e− + 2e−’ or ‘2e− × 2e−’ mechanism [31–36]. After synthetic achievements of the Fe-N-C catalysts solely comprised of the atomic FeNxCy moieties [37], however, the dominant four-electron ORR pathway on the FeNxCy site was verified and a minor production of H2O2 on the atomic Fe site could not be explained by the simple ensemble effects [38]. Otherwise, Wang group predicted in their computational study that cleavage of the OeO bond on the atomic Fe site becomes kinetically possible via cooperative dissociation pathway, during which one end of the oxygen molecule (not bound to the atomic Fe) is attracted to the nearest carbon atom [39,40]. Considering the cooperative dissociation mechanism on the MeNxCy moiety, pre-occupation of the nearest carbon site, to prevent communication with one end of the adsorbed oxygen molecule, can be a promising strategy to magnify H2O2 production on the Me-N-C catalysts. Here, we have thus tried to confirm the above presumption and concurrently develop cost-effective Me-N-C catalysts (Me = Fe, Co and Mn) for electrochemical H2O2 production. Surface carbon atoms on the MeN-C catalysts were modified by Fenton(-like) reactions with H2O2. This treatment introduced oxygen-functional groups on the carbon surface without significant changes in the crystallinity, degree of defects, surface area and metal contents of the Me-N-C catalysts. With the increased surface oxygen content, the two-electron ORR pathway on the Me-N-C catalysts also increased. Among the metal candidates, Co-N-C showed the highest selectivity of > 85% toward H2O2 production. These insights thus provide that the electrochemical production of H2O2 on transition metal atom catalysts can be achieved if rational strategies optimizing pre-occupation of the carbon sites and surface modification are developed.
2.3. Electrochemical characterizations The electrochemical properties were investigated using a VMP-300 potentiostat (Bio-Logic) in a three-electrode cell equipped with a saturated Ag/AgCl as a reference electrode (RE-1A, EC-Frontier) and a graphite rod as a counter electrode. The electrolyte was 0.1 M HClO4, which was prepared from concentrated HClO4 (70%, Sigma-Aldrich). Prior to any electrochemical measurements, the Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) scale by calibration in a H2-saturated electrolyte against a Pt electrode. The catalyst inks were prepared by dispersing the 10 mg of catalysts in 3,979 μL of the solvent (3,617 μL of DI water, 282 μL of isopropanol and 80 μL of Nafion solution (5 wt%)). After the sonication of the suspension for 1 h, working electrodes were prepared by dropping 10 μL of the Me-N-C ink onto a glassy carbon disk (0.13 cm2) of a rotating Pt-ring disk electrode (RRDE, 012613, ALS Co.). The catalyst loading was set to 200 μg cm−2. The ORR polarization curve was measured at a 10 mV s−1 scan rate and a 900 rpm rotation speed in an O2-saturated electrolyte. To remove the non-Faradaic capacitance responses, polarization curves measured using the same procedure but in an Ar-saturated electrolyte were subtracted from the results. H2O2 formation during ORR was studied by polarizing the Pt-ring electrode at 1.2 VRHE. H2O2 selectivity was calculated by the following equation: H2O2 (%) = 200 × (IR/N)/ (ID + (IR/N)), where IR is the ring current, ID is the disk current and N is the collection efficiency (0.378, as determined by Fe2+/3+ redox calibration). The hydrogen peroxide reduction reaction (PRR) polarization curves were measured at a 10 mV s−1 scan rate and a 900 rpm rotation speed in an Ar-saturated electrolyte containing 10 mM H2O2.
2. Experimental 2.1. Catalyst synthesis The Me-N-C catalysts were prepared with Me-acetate salts, 1,10phenanthroline (phen) and ZnII zeolitic imidazolate framework (ZIF-8, Basolite Z1200 from Sigma-Aldrich). The precursor powders (1 g), containing Me/phen/ZIF-8 with a mass ratio of 0.5/20/80, were homogenized in a ZrO2 crucible with 100 ZrO2 balls (5 mm diameter) in a ball-miller for four cycles of 30 min each at 400 rpm. The mixture of the catalyst precursors was then pyrolyzed at 1050 ℃ in N2 for 1 h, resulting in pristine Me-N-C catalysts. Note that previous works with identically or similarly synthesized Me-N-C catalysts revealed the predominance of atomically dispersed MeNxCy moieties (e.g., porphyrinlike MeN4C12) on carbonaceous supports [37,41,42], which are known to be the main active sites for the ORR. The H2O2 treatment of the catalysts (0.1 g) was carried out in a 0.01 M HClO4 solution (300 mL) at 100 ℃ with a reflux condenser for 8 h. The 5 mL of H2O2 solution (30 wt %, Sigma-Aldrich) was injected into the solution and replenished every hour. After the oxidation reaction, the catalyst powder was collected by filtration, washed with 1 L of deionized (DI) water (> 18 MΩ, Satorious) and dried at 70 ℃ in an oven.
3. Results and discussion 3.1. Bulk characteristics of Me-N-C catalysts before and after H2O2 treatment We first investigated the bulk properties of the Me-N-C catalysts before and after H2O2 treatment. The XRD patterns of the pristine MeN-C catalysts represented two broad peaks at ca. 25° and 44°, which correspond to the (002) and (101) planes of graphite, respectively (Fig. 1a). Other discernable peaks were not shown in the XRD patterns, inferring a negligible amount of bulk Me phases such as metallic Me, oxides, and carbides. Note that in literature (and our previous works) Mössbauer and X-ray absorption spectroscopy studies of Fe-N-C and CoN-C catalysts [37,41], which were synthesized in the same manner as in the present study, showed a predominant presence of atomic Fe and Co moieties ligated with nitrogen functional groups on the carbonaceous support without a discernable bulk Me species. In the Raman spectra, all the pristine Me-N-C catalysts showed two intense D- and G-band peaks at ca. 1350 and 1590 cm−1 (Fig. 1b), respectively. Because the G-
2.2. Physical characterizations The X-ray diffraction (XRD) patterns were obtained using an Empyrean (Malvern Panalytical) equipped with Cu K-alpha radiation. The XRD was measured at an accelerating voltage of 45 kV and a current of 40 mA with a scan rate of 2° min−1 and a step size of 0.013°. Transmission electron microscopy (TEM) analysis was carried out using 2
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Fig. 1. (a) XRD patterns, (b) Raman spectra of pristine and oxidized Me-N-C catalysts. (c–e) TEM and (f–h) SEM images of (c, f) Co-N-C, (d, g) Mn-N-C, and (e, h) FeN-C catalysts (left) before and (right) after H2O2 treatment. Composition data measured by EDX was indicated in each SEM image. 4 band arises from the E2g vibrational mode in the D6h symmetry group of the graphite crystal planes, and the D-band originates from the lattice distortion in sp2-hybridized carbon [44], their intensity ratios (ID/IG) are typically used as a parameter indicating the extent of carbon disorder. Intense D-band signals with ID/IG values of ca. 0.95 ( ± 0.01) for the pristine Me-N-C catalysts showed a highly disordered graphitic structure, as also confirmed by the broad graphite peaks in their XRD patterns. After the H2O2 treatment under harsh oxidizing conditions (i.e., 3.5 wt% H2O2, 100 ℃ and 8 h), however, the H2O2-treated Me-N-C catalysts showed almost identical bulk properties with those of the pristine catalysts. Modifications of the XRD and Raman patterns (including ID/IG values) after the treatment were marginal (Fig. 1a and b). The TEM and SEM images (Fig. 1c–h) show a fully carbonaceous structure without appreciable metal clustering, and an imperceptible change in morphology before and after H2O2 treatment. In addition, N2adsorption isotherms revealed that the H2O2 treatment did not lead to considerable modifications of the surface area of the Me-N-C catalysts either (Table 1, deviation ≤3%). The metal content, measured by ICPAES, were ca. 1.4, 1.3, and 1.5 wt% for pristine Co-N-C, Mn-N-C, and Fe-N-C, respectively. The metal contents slightly decreased to 1.1, 1.2, and 1.3 wt% after the treatment, while the changes were also moderate.
were found before and after the H2O2 treatment except for the Fe-N-C catalyst, which showed additional small contributions of the C]O and/ or OeC]O functionalities after the H2O2 treatment [45,46]. Modifications in the nitrogen functionalities of the Me-N-C catalysts after the treatments were studied using the XPS-N1s analysis (Fig. 2b). The nitrogen content of the catalysts, calculated from the XPS spectra, were ca. 4.5 wt% regardless of the Me precursors and H2O2 treatments (Fig. 3a). The XPS-N1s fitting results confirmed a predominant presence of pyridinic- and graphitic-N moieties (Fig. 3b). The XPS-N1s signals and fitting results were modified insignificantly after the H2O2 treatment, indicating no oxidation of nitrogen functionalities including those ligating the active Me moieties. Overall, the XRD, Raman, TEM, SEM, and XPS-C1s/N1s measurements of the samples showed almost identical spectra/diffractograms before and after the H2O2 treatment, indicating no considerable modifications in defect sites and other carbon/nitrogen moieties, which could possibly effect on the ORR pathway of the Me-NC catalysts [43,47,48]. Exceptionally, one remarkable change after the treatments was the oxygen content (Fig. 2c), increasing from ca. 4–6 wt% for pristine MeN-C catalysts to ca. 9–17 wt% for H2O2-treated catalysts measured by XPS-O1s (Fig. 3a). This appreciable increase in oxygen content was also observed in EDX measurements (Fig. 1f–h), ca. 4–6 wt% oxygen content in the pristine Me-N-C catalysts but ca. 7–13 wt% in the treated samples. The XPS-O1s signals were fitted with two peaks at 531.6 and 533.0 eV, which correspond to the C]O and CeO components, respectively [49]. The H2O2 treatment increased the content of the former component more prevailingly than that of the latter one (Fig. 3c). Compared with our previous work performed with relatively mild H2O2 treatment conditions (i.e., 2 h and 20–70 ℃) [50], which also showed increment in both the C]O and CeO content, the regnant C]O formation in the present work implies a more oxidized surface of carbon, resulting from the harsher oxidizing conditions (i.e., 8 h and 100 ℃). Substantial surface oxidation occurred on the Fe-N-C catalysts compared with that on the Mn-N-C and Co-N-C catalysts. Considering the fact that the H2O2-treatment conditions were identical except for the type of central metal ion (i.e., Fe, Co and Mn) in the Me-N-C catalysts, the differences in the extent of surface oxidation may be due to the different levels of reactive oxygen species (ROS) generated from Fenton or Fenton-like reactions between the H2O2 and Me ions [51]. In the classical Fenton reaction producing hydroxyl radicals from H2O2 and ferrous ions, the reaction kinetics is very fast, having a reaction
3.2. XPS measurements of Me-N-C catalysts before and after H2O2 treatment Other possible changes were further investigated using XPS (Fig. 2), a more surface-sensitive technique than XRD and Raman spectroscopy. The XPS-C1s of the pristine Me-N-C catalysts showed a clear peak at 284.5 eV (Fig. 2a), which emerges from the CeC bond of the sp2-hybridized carbon [45,46]. No significant changes in the XPS-C1s spectra Table 1 BET surface area and metal content of Me-N-C catalysts before and after oxidation process. Catalyst
Co-N-C
Mn-N-C
Fe-N-C
BET surface area (m2 g−1)
Pristine H2O2-treated
763 729
664 682
680 716
Metal content (wt%)
Pristine H2O2-treated
1.4 1.1
1.3 1.2
1.5 1.3
3
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Fig. 2. XPS spectra of pristine and H2O2-treated Me-N-C catalysts: (a) C1s, (b) N1s, and (c) O1s.
constant of k = 63–76 M−1s−1. In contrast, the kinetics of Fenton-like reaction on non-ferrous ions (e.g., Co and Mn) is relatively slow, and the reaction can often only occur under specialized reaction conditions. Hence, the ROS level generated by non-ferrous ions is expected to be lower than that by ferrous ions, resulting in more significant changes in the XPS-O1s spectrum of the Fe-N-C catalyst after H2O2 treatment than those of the Co-N-C and Mn-N-C catalysts. This result is in line with the XPS-C1s spectrum of the H2O2-treated Fe-N-C catalyst showing C]O
and/or OeC]O functionalities while their contributions were not clearly discernable in the XPS-C1s spectra of other Me-N-C catalysts (Fig. 2a).
3.3. Electrochemical characteristics of Me-N-C catalysts before and after H2O2 treatment The electrocatalytic activity and selectivity toward ORR were then
Fig. 3. (a) Compositions of oxygen, nitrogen and metal in the catalysts. The former two elements were measured by XPS, while the metal composition was obtained from ICP-AES due to the poor signal-to-noise ratio of the XPS-Me2P spectra. Contents of (b) nitrogen and (c) oxygen functional groups calculated from the XPS fitting results. In all the figures, the values from pristine (left bar) and H2O2-treated Me-N-C catalysts (right bar) were indicated concurrently. 4
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Fig. 4. (a–c) ORR polarization curve and (d–f) selectivity toward H2O2 formation of the Me-N-C catalysts before and after H2O2 treatment: (a, d) Co-N-C, (b, e) Mn-NC, and (c, f) Fe-N-C.
It has been theoretically expected that the reduction of *OOH intermediate to either *HOOH or *O is a key step to determine ORR selectivity and that the OeO bond cleavage at a single atomic Me moiety requires a considerable reorganization energy [52]. Hence, as shortly noted in the Introduction section, the cooperative dissociation pathway, i.e., an OeO bond cleavage assisted by the nearest carbon atom, has been recently considered by the Wang group [39,40] in order to understand a predominant H2O production on atomic FeNxCy moieties of Fe-N-C catalysts [38]. When the surface of the Fe-N-C catalysts is oxidized, however, this pathway could become less favorable due to a weakened iron oxophilicity (i.e., electronic effect) and/or a pre-occupation of the nearest carbon site by oxygen functionalities (i.e., steric effect) [50], leading to an enhanced H2O2 production on the Fe-N-C catalysts. Beside these electronic and steric effects on the surface-oxidized Me-N-C catalysts, in the present work, the highest H2O2 selectivity on the surface-oxidized Co-N-C catalysts could be explained by a relatively lower oxophilicity of the Co-N4 complexes (i.e., the weak binding energy of oxygen and ORR intermediates) than that of the FeN4 and Mn-N4 complexes. Considering previous case studies of molecular MeN4 catalysts (e.g., porphyrin and phthalocyanine) [53], the molecular catalysts were divided into two main categories according to their ORR selectivity: i) Fe and Mn, which have a strong binding affinity for O2 that facilitates the dissociation of the OeO bond and ii) Co (or Cr and Ni), which bind with O2 weakly and do not break the OeO bond efficiently. Simiarly, in the pyrolized systems (i.e., Me-N-C catalysts), computational calculations also predicted much weaker O2-adsorption energy (−0.80 to −1.26 eV) on CoNxCy sites than that on FeNxCy sites (ca. −1.84 eV) [41]. Beside the high selectivity towards the two-electron ORR pathway, the slow kinetics of the consecutive H2O2 reduction reaction (PRR, H2O2 + 2H+ + 2e− → 2H2O) is also of importance for the production of concentrated H2O2 solution in electrochemical systems. Furthermore, in the presence of some site-blocking spectator species and the low content of active metal sites, H2O2 diffusion into the bulk electrolyte could be artificially favored with its subsequent decomposition in the RRDE experiments [54]. Therefore, effects of the chemical oxidations of the Me-N-C catalysts on their PRR activity were additionally
evaluated in a 0.1 M HClO4 electrolyte at an electrode rotation of 900 rpm (Fig. 4). The ORR polarization curves of the pristine Me-N-C catalysts show an onset potential (Eonset, defined here as the potential at a current density j of 0.1 mA cm−2) of 0.81, 0.76, and 0.85 VRHE for CoN-C, Mn-N-C, and Fe-N-C, respectively (Fig. 4a–c). The j values measured at 0.8 VRHE, at which the ORR polarization curves were mainly controlled by their kinetics rather than mass transports, were −0.17, −0.02, and −0.53 mA cm−2 for Co-N-C, Mn-N-C, and Fe-N-C, respectively. In the RRDE measurements, the pristine Fe-N-C showed a predominant H2O production during the ORR with a relatively minimal formation of H2O2 less than 10% (Fig. 4f). Otherwise, the pristine Co-NC and Mn-N-C released considerable amounts of H2O2 in the ORR, i.e., < 60 and < 35% H2O2 in an overall potential range, respectively (Fig. 4d and e). The varied H2O2 selectivity was also supported by the different diffusion current density (jd) (Fig. 4a–c), which is a function of the number of electrons transferred during ORR (at this condition jd is ca. −4.5 mA cm−2 when four electrons are transferred). The measured values of jd were ca. −3, −3.5 and −4 mA cm−2 when the electrode changed in order of Co-N-C, Mn-N-C, and Fe-N-C, respectively. With the highest activity among the candidates with a dominant four-electron ORR pathway, the Fe-N-C catalysts verified their potential applications in PEMFCs as reported so far [20–30]. However, the H2O2-treated Me-N-C catalysts showed completely different electrocatalytic properties. After the chemical oxidations, their Eonset was negatively shifted to 0.74, 0.63, and 0.81 VRHE for Co-N-C, Mn-N-C, and Fe-N-C, respectively (Fig. 4a–c). Considering the fact that the thermodynamic equilibrium potential between oxygen and water (EO02/H2O ) is ca. 1.23 VRHE (i.e., O2 + 4H+ + 4e− ↔ 2H2O), and that between oxygen and H2O2 is much lower, as EO02/H2O2 = ca. 0.69 VRHE, the negative shifts of the Eonset as well as the decreased jd values inferred modifications of their ORR catalysis toward H2O2 formation rather than water production. More clearly, the RRDE measurements of the H2O2-treated Me-N-C catalysts showed enhanced H2O2 productions of ca. 50 and 30% for H2O2-treated Mn-N-C and Fe-N-C catalysts during the ORR, respectively. In particular, the H2O2-treated Co-N-C catalyst predominantly followed two-electron ORR pathway with a maximum selectivity of ca. 87%. 5
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Fig. 5. PRR activity of the Me-N-C catalysts before and after the H2O2 treatment: (a) Co-N-C, (b) Mn-N-C and (c) Fe-N-C.
investigated with a 10 mM H2O2 electrolyte. As can be seen in Fig. 5, electrochemical reduction current was observed for all the catalysts electrochemically reduced H2O2 at a potential below ca. 0.8 VRHE regardless of the H2O2 treatment. However, the H2O2-treated Me-N-C catalysts showed significantly decreased PRR kinetics. Mn-N-C and CoN-C did not reach the PRR current density of −1 mA cm−2, even at a very low potential of 0.1 VRHE. With high H2O2 selectivity in the ORR and low PRR activity, the surface-oxidized Co-N-C catalyst is likely to be the most suitable catalyst for electrochemical H2O2 productions.
[7] [8]
[9]
[10]
4. Conclusions [11]
We synthesized three Me-N-C catalysts (Me = Fe, Mn and Co) and introduced abundant oxygen functionalities on their surface through H2O2 treatment without significant modifications to their bulk properties. The formation of the C]O and CeO moieties on the Me-N-C catalysts led to an enhanced two-electron ORR pathway producing H2O2, rather than the four-electron pathway producing H2O. In particular, the oxidized Co-N-C catalysts revealed the highest H2O2 selectivity of > 85% among the three candidates. On the other hand, the kinetics of the electrochemical H2O2 reduction to H2O significantly declined on the oxidized catalysts. Considering the cooperative dissociation ORR mechanism [39,40,50], we presumed that the promoted two-electron ORR pathway was due to weakened oxophilicity at active MeNxCy moieties (i.e., electronic effect) and/or a pre-occupation of the nearest carbon site by oxygen functionalities (i.e., steric effect) after the carbon oxidations. Consequently, the surface oxidation of the Me-N-C catalysts can be considered a promising approach for the electrochemical synthesis of H2O2, while simultaneously generating electricity by combining with practical fuel cell devices.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgements
[20]
This work was supported by GIST Research Institute(GRI) grant funded by the GIST in 2019. [21]
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