Influence of Precursor Functional Groups on the Formation and

Article pubs.acs.org/JPCC

Influence of Precursor Functional Groups on the Formation and Performance of Iron-Coordinating Ordered Mesoporous Carbons as Fuel Cell Catalysts C. Janson† and A. E. C. Palmqvist*,† †

Chalmers University of Technology, Department of Chemistry and Chemical Engineering, SE-412 96 Göteborg, Sweden S Supporting Information *

ABSTRACT: We examine the influence of precursor functional groups on the formation and electrocatalytic performance of iron ion-chelating ordered mesoporous carbon (Fe-OMC) fuel cell catalysts. First, we study whether the active sites in these catalysts consist of Fe−Nx or Fe−Ox chelates. To verify this, catalysts were prepared from two different molecular precursors (furfurylamine and furfuryl alcohol) and the functional groups’ (−NH2 vs −OH) role in the formation of iron ionchelating active sites in the catalysts was established. From electrochemical tests and EPR spectroscopy, conspicuously different behaviors were obtained for the catalyst prepared from furfurylamine compared to that prepared from furfuryl alcohol. It was unambiguously established that the amine group is central to the formation of electrocatalytically active sites in Fe-OMC catalysts and that these are of the Fe−Nx-OMC type. Additional Fe-OMC catalysts were prepared with the purpose to determine the influence of the two precursors on the formation of the carbon matrix. By complementing the furfurylamine with the more readily polymerizing furfuryl alcohol and using a mixture of the two as precursor solution in the synthesis, an overall improvement over the pure furfurylamine was achieved. The mixture gave a catalyst with a larger pore volume and surface area, a higher conductivity, and a higher oxygen conversion rate.

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INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are of great interest as energy conversion devices, thanks to their high potential to efficiently convert chemical energy to electricity. However, PEMFCs have seen a slow commercial development and limited market introduction due to high cost, mainly a result of the current use of expensive platinum electrocatalysts. In order to make fuel cells economically competitive in comparison to other energy conversion devices, the cost of the catalyst has to be reduced. Therefore, it is of interest to develop new, inexpensive noble metal-free catalysts with similar properties as the platinum-based catalysts, in terms of catalyst conversion efficiency and operation stability. Several promising catalyst materials have been introduced containing inexpensive, abundant elements, such as transition metals, nitrogen and carbon, which have shown considerable activity for the oxygen reduction reaction (ORR) in PEMFCs the last decades.1−14 One such catalyst is the transition metal-chelating ordered mesoporous carbons (TM-OMCs) presented by Dombrovskis et al. in 2013.11 The EXAFS analysis of the atomic local structure of the catalytically active site for the oxygen reduction reaction in the TM-OMC catalyst, suggests that it consists of iron−nitrogen (Fe−Nx) chelates bound to the carbon matrix.12 However, it is difficult to unambiguously distinguish between Fe−Nx bonds and Fe−Ox bonds with EXAFS due to their similar bond lengths, and in principle, both could constitute catalytically active sites. To identify the nature of the bond type, © XXXX American Chemical Society

it would be interesting to produce catalysts in which formation of each of these chelates (Fe−Nx or Fe−Ox) is enabled by use of two different precursors and then compare their activity. Each precursor would require the ability to form the desired chelate and the OMC structure. In the published synthesis of the TM-OMCs, the carbon precursor furfurylamine (see Figure 1a) was found suitable for making carbon materials function-

Figure 1. Molecular structures of furfurylamine (a) and furfuryl alcohol (b).

alized with nitrogen heteroatoms. Besides that, the molecular structure of furfurylamine allows, in principle, for the formation of both Fe−Nx and Fe−Ox chelates when mixing it with an iron salt. Furthermore, Joo et al. presented in an earlier study an efficient method for making highly ordered mesoporous carbons for electrocatalytic purposes using the structurally related furfuryl alcohol (see Figure 1b).15 The molecular structure of furfuryl alcohol allows, in principle, for the formation of Fe−Ox chelates. Thus, we found these two Received: May 12, 2017 Revised: September 15, 2017

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DOI: 10.1021/acs.jpcc.7b04588 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

950 °C for 2 h. Electrochemical experiments were made in a single cell PEM fuel cell and by rotating disk electrode experiments (RDE). Physical characterization was made by nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance spectroscopy (EPR), Raman spectroscopy, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and nitrogen physisorption. NMR measurements were done on furfurylamine/iron chloride and furfuryl alcohol/iron chloride precursor solutions with a furfurylamine/furfuryl alcohol to iron molar ratio of about 1000:1. Detailed information about equipment and experimental parameters is presented in the Supporting Information.

carbon precursors (furfurylamine and furfuryl alcohol) very well suited to assess the potential roles of nitrogen and oxygen in the formation of iron ion-chelating active sites in Fe-OMC catalysts. In addition, it is likely that the two precursors are differently suitable to develop an optimal carbon matrix structure. Thus, in principle, one may be better suited to form the active site and the other better suited to form a carbon matrix with superior conductivity and mass transport properties. Therefore, catalysts were also synthesized from mixtures of the two precursors to evaluate if such mixtures would produce catalysts with an improved combination of active sites and porous carbon matrix properties, thereby rendering enhanced catalytic performance over a wider power range of the fuel cell’s operating conditions.

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RESULTS AND DISCUSSION Role of the Precursor Functional Groups on the Formation of Iron Chelating Active Sites. To identify whether the catalytically active sites of the Fe-OMC catalysts consist of Fe−Nx or Fe−Ox chelates, one strategy is to investigate when, during the synthesis, the possible iron− nitrogen or iron−oxygen coordination occurs and how it is developed throughout the synthesis. By solution state nuclear magnetic resonance (NMR) spectroscopy we could study the molecular interactions in the precursor mixing step (the first stage of the synthesis), in a similar way as previously described.16 A number of possible coordination modes between an iron ion and furfurylamine or furfuryl alcohol exists. Both furfurylamine and furfuryl alcohol have two donor sites, one at the nitrogen versus the oxygen on the functional groups (amine and alcohol) and one at the ether oxygen in the furan ring. Each ligand (furfurylamine or furfuryl alcohol) can either coordinate to only one of the iron sites (monodentate), as schematically shown in Figure 2a,b,d,e, or to two of the iron sites simultaneously (bidentate), schematically shown in Figure 2c,f. Here, NMR spectroscopy was used to identify which of the interactions that actually occur in the two different precursor alternatives. Figure 3 shows NMR data obtained immediately after the precursor solutions were prepared. It should be noted that the NMR experiments were measured on solutions with low iron concentration to allow for analysis of the molecular interactions in the precursor mixtures. High concentrations of paramagnetic materials (such as iron(III) and some iron(II) complexes), decrease the T2 relaxation, which broadens the NMR signal. Besides that, too high iron concentrations in the furfuryl alcohol precursor solution makes the NMR signal uninterpret-

EXPERIMENTAL SECTION Mesoporous silica (KIT-6) was prepared and used as a template for the formation of mesoporous carbon catalysts. Four catalysts were prepared combining furfurylamine (≥99% Sigma-Aldrich), furfuryl alcohol (98% Sigma-Aldrich), and anhydrous iron chloride (98% Sigma-Aldrich) in different ways. The precursors were mixed together in the order shown in Table 1. Typically, a mixture of reactants 1 and 2 were well Table 1. Details of the Catalysts Prepared and Mixing Order of the Reactants catalyst name

reactant 1

reactant 2

reactant 3

FuNH2-Fe FuNH2-Fe-FuOH FuOH-Fe-FuNH2 FuOH-Fe

furfurylamine furfurylamine furfuryl alcohol furfuryl alcohol

FeCl3 FeCl3 FeCl3 FeCl3

furfuryl alcohol furfurylamine

mixed for 15 min followed by the addition and mixing of reactant 3. The iron to furfurylamine/furfuryl alcohol molar ratio was about 1 to 180. After mixing, most of the iron chloride was dissolved but some small pieces of undissolved salt remained. The liquid part of the mixture, was subsequently impregnated in the silica template and soaked overnight, followed by a heat treatment in air at 100 °C for 2 h. A second impregnation was made followed by a heat treatment in air at 160 °C for 2 h. The sample was then pyrolized in an inert atmosphere at 950 °C for 2 h. Thereafter, the silica template was removed by acid washing with 40% HF for 20 h at room temperature. Finally, an additional acidic treatment was made with 0.5 M H2SO4 (95−98% Sigma-Aldrich), for 6 h at 80 °C followed by another pyrolysis treatment in inert atmosphere at

Figure 2. Schematic representations of possible coordination configurations of an iron ion and furfurylamine (a−c) and an iron ion and furfuryl alcohol (d−f). L represents either a ligand coordinated to the iron or a vacant coordination site of the iron. B


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Figure 3. 1H NMR of furfurylamine (a) and furfuryl alcohol (b), each mixed with iron chloride at a molar ratio of 1000/1. Pure furfurylamine (red) and furfuryl alcohol (blue), respectively at the bottom and the mixtures directly after mixing with iron chloride (light red and light blue) at the top. The numbers 1−5 indicate which proton in the molecule that corresponds to which peak.

Figure 4. 1H NMR of furfurylamine (a) and furfuryl alcohol (b), each mixed with iron chloride with a molar ratio of 1000/1. Furfurylamine directly after mixing with iron chloride (light red), after 3 days (red), and after 44 days (dark red), and furfuryl alcohol directly after mixing (light blue), after 4 days (purple), and after 7 days (dark purple).

able since iron chloride catalyzes the polymerization reaction of furfuryl alcohol at room temperature as will be discussed below. A peak shift analysis was performed by comparing the 1H NMR peak positions in the two precursor solutions containing iron chloride to the pure solutions of furfurylamine and furfuryl alcohol, respectively (see Figure 3). The largest shifts were observed for peak 5 in both precursor solutions. This establishes that in the furfurylamine precursor solution, the complex formation primarily involves the amine group in coordination with the iron ion as illustrated in Figure 2a. Correspondingly, in the furfuryl alcohol precursor solution, the complex formation involves the alcohol group suggesting an iron−oxygen interaction as in Figure 2d. Noticeable is the relatively large shift also shown for peak 4 in the furfuryl alcohol precursor solution (see Figure 3b), indicating a relatively larger influence of the iron on these protons than observed in the furfurylamine solution. The reason for this could be that a

polycondensation process starts immediately upon mixing furfuryl alcohol and iron chloride (discussed below), in which the iron chloride acts as a catalyst. In the polycondensation reaction, water and formaldehyde are released.17,18 During the cleavage of water, the iron will act in close proximity to both protons 4 and 5 and, therefore, cause the larger shift for peak 4 in the furfuryl alcohol case. In addition, NMR experiments measured over several days, confirmed that a polymerization (polycondensation) reaction starts when furfuryl alcohol is mixed with iron chloride at room temperature (as shown in Figure 4b). Proton peaks from oligomers/polymers appeared after 4 days, in agreement with polymerization. After 7 days, the polymerization had proceeded far enough that the signal broadened beyond interpretation because of the high viscosity of the mixture, causing a decrease in the isotropic molecular tumbling. Notable is that a rather low amount of iron chloride was needed to catalyze the reaction C


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Figure 5. EPR spectra of FuNH2-Fe (red) and FuOH-Fe (blue) at 120 K (a) and 5K (b).

planned. For the purpose of this communication, the following analysis is sufficient. The EPR spectra shown in Figure 5 are measurements of Fe-OMCs prepared from solutions containing higher concentrations of iron compared to the solutions used in the NMR experiments (see Experimental Section). Several EPR features appeared in the spectra of the catalyst prepared from furfurylamine (FuNH2-Fe), but only a small feature appeared in the spectra of the catalyst prepared from furfuryl alcohol (FuOH-Fe). The features in the FuNH2-Fe catalyst are consistent with presence of paramagnetic iron species and confirm coordination of iron in the structure. The EPR signal for the FuOH-Fe catalyst is visible only in the measurements at 5 K and could be associated with iron coordination although of a significantly lower concentration than that in FuNH2-Fe. The EPR spectra for FuOH-Fe thus suggest that this material lacks significant paramagnetic iron coordination. The possible presence of EPR-silent iron(II)-species cannot be completely ruled out, but we hypothesize that the concentration is very low, since otherwise we would expect features from coexisting EPR-visible iron(III) species to appear as chelated iron relatively easy changes between the two oxidation states in air. From the EPR spectra, it is, however, abundantly clear that the two different catalysts have different ability to coordinate iron ions and consequently feature different iron local structures. We showed above that in the initial stage of the formation of the FuOH-Fe catalysts, the iron ions interact with the alcohol group of the furfuryl alcohol in solution, whereby a polycondensation reaction is initiated. From the EPR results it appears that at some point during the formation of the material, the iron coordination is lost and that the iron is likely washed out during the acid treatments possibly leaving the material essentially devoid of Fe−Ox chelates. For the FuNH2Fe catalyst, we showed above that the iron ions interact strongly with the amine groups in the synthesis solution, and the EPR results show that the iron remains coordinated in the formed material even after the elevated temperature treatment and the acid washing.16 The iron−nitrogen interaction clearly is strong enough to keep the iron coordinated in the formed structure throughout the entire synthesis procedure. We thus conclude that nitrogen is important in providing ironcoordinating function of the Fe-OMC and that the catalyst contains Fe−Nx chelates as was suggested in the previous EXAFS study on similar TM-OMC catalysts.12 If it is the Fe−Nx chelates that are responsible for the electrocatalytic oxygen reduction reaction activity of the FeOMC, a significant difference in fuel cell performance between the two prepared catalysts should be observed. Indeed, we find

and eventually solidify the sample. It remains unclear if the iron only catalyzes the reaction or stays coordinated to the alcohol also after the polymerization, but considering the large difference in concentrations it is most likely the former. In the furfurylamine precursor solution, no signs of polymerization can be observed at room temperature upon introduction of the iron salt (see Figure 4a). Instead the iron−nitrogen molecular interaction appears to increase somewhat with time, possibly related to changes of the iron speciation in the solution affecting the paramagnetic influence of the iron on the peak shifts.16 To summarize the NMR results, it is clear that iron ions interact with the donor sites of the functional groups of the precursors and that there is no indication of a specific interaction between the iron and the oxygen of the furan ring in either of the two precursors. The iron−nitrogen interaction in the amine appears stable over time in the solution mixture, whereas in contrast, the iron−oxygen interaction in the alcohol is associated with a polycondensation reaction that contributes to the development of the molecular speciation with time in the mixture. Subsequent steps in the synthesis of Fe-OMC catalysts include impregnating the porous silica template with the precursor mixture followed by carbonization of the carbon precursor and removal of the silica template. The carbonization step involves some degree of polycondensation/polymerization of the precursor at an elevated temperature and a subsequent pyrolysis/carbonization treatment of the polycondensate at high temperature. Removal of the silica template is typically done by HF acid treatment and after that a sulfuric acid washing step is applied to remove any undesired and soluble iron species that are not bound or coordinated to the carbonbased matrix. The development of the iron coordination during these process steps cannot be followed by regular solution NMR. Instead, we employed other methods to evaluate the structure and properties of the resulting catalyst materials to establish if there are catalytically active (or inactive) Fe−Nx or Fe−Ox chelates incorporated in the carbon matrix. To study coordination of iron in the catalyst materials, electron paramagnetic resonance (EPR) spectroscopy was used. EPR allows for identification of species with unpaired electrons, such as iron(III) and some iron(II) complexes and was here mainly employed to determine the presence or absence of such iron-coordination in the materials. In principle, the method allows for information to be extracted about structural variations of the iron- chelating sites, the oxidation- and spin state of the catalytically active sites, and the electronic structure of the carbon matrix and a deeper study of these aspects is D


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Figure 6. Polarization plots in a single cell PEM fuel cell (a), RDE experiments (b), Koutecky−Levich plots at 0.4 V (c), and number of electrons transferred at different voltage (d) of FuNH2-Fe (red) and FuOH-Fe (blue).

and that the bond shown in previous EXAFS studies corresponds to Fe−Nx chelates and not to Fe−Ox species. Based on the obtained results in this study and previous EXAFS studies,11,12 we conclude that the responsible ORR active site in Fe-OMCs are Fe−Nx chelates. Influence of Carbon Precursor on the Formation of the Carbon Matrix Structure. Despite the fact that furfuryl alcohol showed poor ability to form ORR active sites in the previous section it may be better suited for formation of the carbon matrix. The choice of carbon precursor influences the carbon matrix’s structure and properties. In Fe-OMCs, carbon matrix properties such as size, volume, and structure of the pores, surface composition, and type of carbon hybridization (ratio between sp2 and sp3) will influence mass transport and resistivity of the catalysts and, thereby, their performance in fuel cells, especially at medium and high current densities. On the other hand, it is unclear whether or not the presence of nitrogen heteroatom dopants in the carbon matrix has a positive or negative impact on its properties and since the furfurylamine has been found above to incorporate nitrogen chelating groups in the carbon matrix it may also influence the matrix’s other properties. By mixing furfuryl alcohol into the synthesis of Fe-OMCs prepared from furfurylamine, we hoped to enable formation of an active catalyst with improved carbon properties compared to the FuNH2-Fe catalyst. With such a mixture the hypothesis is that furfurylamine would form active chelates with the iron chloride and the furfuryl alcohol would render an improved carbon matrix. To evaluate this, Fe-OMC catalysts were synthesized with mixtures of furfurylamine, furfuryl alcohol and iron chloride employing two different reagent mixing orders. In FuOH-Fe-FuNH2, the iron salt was first dissolved in the furfuryl alcohol before the furfurylamine

such a difference in Figure 6a, which compares polarization plots from single cell fuel cell measurements performed with catalysts FuNH2-Fe (red) and FuOH-Fe (blue), respectively. As shown, there is a huge performance difference over the entire power range and also in the kinetically controlled region at low currents confirming large differences between the activities of the two catalysts’ active sites. The extended higher potential at higher currents of FuNH2-Fe further concurs a higher concentration of active sites, whereas the low potential at higher currents of FuOH-Fe is indicative of a low number of active sites with limited activity. Additionally, rotating disk electrode (RDE) measurements at four different rotation speeds were performed to study the catalysts’ ORR activity. The RDE polarization plot in Figure 6b is in agreement with the fuel cell results and the half potentials for the catalysts were calculated to 0.54 V (FuNH2-Fe) and 0.25 V (FuOH-Fe), respectively. To further study the catalysts, their Koutecky− Levich plots were obtained from the current densities at different rotation speeds (see Figure 6c). From the Koutecky− Levich plots the theoretical number of electrons transferred during the ORR for each catalyst could be calculated. In Figure 6d the number of electrons transferred is plotted against the voltage. As shown in Figure 6c and 6d the catalyst FuNH2-Fe follows a direct four-electron transfer mechanism indicating an ORR active catalyst, whereas the catalyst FuOH-Fe ranges from zero to three electrons transferred with decreasing potential confirming the poor ORR performance of that catalyst. From the fuel cell and RDE results, we can conclude that the catalyst containing nitrogen exhibits much higher catalytic activity and selectivity compared to the nitrogen-free catalyst. This confirms previous hypothesis11,12 that the nitrogen is involved in the formation of active sites in Fe-OMC catalysts E


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The Journal of Physical Chemistry C was added, whereas in FuNH2-Fe-FuOH, the salt was first dissolved in the furfurylamine before the furfuryl alcohol was added. As such, the mixed precursor catalysts may influence the conversion rate of the catalyst under fuel cell operating conditions outside the activation loss region of the polarization plot and we look for enhanced catalytic performance also under medium and high power output conditions of the fuel cell. Under those conditions, limited by either Ohmic or mass transport losses of the catalyst, the properties of the carbon matrix become increasingly important in addition to the active site properties. It is important to tune pore and surface properties of the catalysts to obtain an efficient access to active sites and high mass transport rates. The pore sizes, pore volumes, and surface areas of the catalysts were determined by nitrogen physisorption, and the results are presented in Table 2. The

solution can hinder the impregnation of the silica template, and consequently the specific surface and micropore areas of the OMC become lower. In the FuNH2-Fe catalyst, the polymerization process was slower and did not start at room temperature. At the relatively low iron to furfurylamine ratio used, the polymerization may not have finished completely during the heating step at 160 °C, because the concentration of iron was not sufficiently high. Instead the polymerization proceeded further during the pyrolysis step which has been shown to increase the pore size and decrease the specific surface area.11 Accordingly, this was found for catalyst FuNH2Fe shown in Table 2. When mixing the two precursors, as in catalyst FuNH2-Fe-FuOH and FuOH-Fe-FuNH2, both the impregnation and the polymerization are favored. The polymerization is slow enough to keep the viscosity low, to not limit the impregnation, but fast enough to complete the polymerization and fix the meso-structure during the 160 °C heating step. Therefore, an improved meso-order (see Supporting Information, Figure S1), and larger surface and micropore areas were obtained in these catalysts. To enable transport of electrons to the active site through the carbon matrix the carbon’s conductive properties are of high importance. Graphitic carbons are conductive because of their conjugated system in the molecular structure. The conjugated system can be disturbed by defects in the structure involving grain boundaries, in-plane substituted heteroatoms, vacancies, and/or sp3-hybridization. It should be noted that in a three-dimensional meso-structure, certain sp3-hybridization is necessary to attain the structure. Raman spectroscopy is a suitable tool for characterizing the nature of defects in low dimensional carbons and here it was employed together with Xray diffraction to investigate how the carbon local structure varies with type of carbon precursor used in the synthesis. The most significant peaks showed in the Raman spectra in Figure 7a,b, are the two Raman-allowed peaks (G and 2D), and the two Raman-forbidden peaks (D and D′), which are activated in graphitic materials. The D and D′ peaks are activated by single-phonon intervalley and intravalley scattering

Table 2. Specific Surface Area (SBET), T-Plot Micropore Area, Average Pore Diameter (⌀ads), and Pore Volume (Vp) of the Prepared Catalysts Calculated from the Nitrogen Physisorption Curves catalyst

SBET (m2 g−1)

T-plotmicropore (m2 g−1)

Vp (cm3 g−1)

⌀ads (nm)

FuNH2-Fe FuNH2-Fe-FuOH FuOH-Fe-FuNH2 FuOH-Fe

733 1045 1291 865

85.7 190 278 34.2

1.01 1.19 1.43 0.88

4.77 3.98 3.94 3.32

highest specific surface and micropore areas were achieved by the mixed samples FuOH-Fe-FuNH2 and FuNH2-Fe-FuOH, followed by FuOH-Fe and then FuNH2-Fe. The differences can be explained by different polymerization reaction mechanisms of the two carbon precursors. It was shown above (see Figure 4), that furfuryl alcohol with the addition of iron chloride, starts to polymerize already at room temperature and that the polymerization thereby increases the viscosity of the precursor solution. In the synthesis of FuOH-Fe, the polymerization was visible immediately after the reactants were mixed. The viscous

Figure 7. Raman spectra (a), Raman spectra close up (b) of FuNH2-Fe (red), FuNH2-Fe-FuOH (yellow), FuOH-Fe-FuNH2 (green), and FuOH-Fe (blue), and schematic representation of nitrogen-dopant forming vacancy defects (c). F


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Figure 8. Polarization curves (a), zoom in of low current density region (b), power vs potential (c), and Nyquist plot at 0.7 V (d) measured in a single cell PEM fuel cell, and polarization curves (e) and number of electrons transferred at different voltage (f) measured with RDE of FuNH2-Fe (red), FuNH2-Fe-FuOH (yellow), and FuOH-Fe-FuNH2 (green).

of sp3-hybridization. In Figure 7b, it is shown that the G peak shifts to higher frequencies in the following order FuNH2-Fe < FuOH-Fe < FuNH2-Fe-FuOH = FuOH-Fe-FuNH2. This agrees with previous results, where the G peak blue shifts when a graphitic structure goes from a planar to a more strained structure.21 This result is also corroborated by the XRD data (see Supporting Information, Figure S2), where the intensity of the graphene stacking order peak (002) follows the same trend with the FuNH2-Fe being the most graphitic. The reduction of the 2D peak intensity with increasing amount of defects/dopants22 also indicates higher density of defects in the two mixed samples (FuNH2-Fe-FuOH and FuOH-Fe-FuNH2), which is not unexpected since they probably consist of mixtures of the defects in FuNH2-Fe and FuOH-Fe. In the Fe-OMCs made with only or partly furfuryl alcohol, it may be that some peaks (x and y around 1150 and 1450 cm−1, respectively) underlie the G and the D peak, which can be attributed to coexistence of sp2 and sp3 phases as in conjugated nonaromatic chains23,24 and to amorphous carbons, connecting graphitic parts.25 To evaluate the effect of the use of mixed precursors on the electrochemical properties of the catalysts prepared their performance was studied in a single cell fuel cell and with RDE experiments. The performance of the prepared catalysts FuNH2-Fe, FuNH2-Fe-FuOH, and FuOH-Fe-FuNH2 are compared in Figure 8. The mixed catalyst FuNH2-Fe-FuOH (yellow), showed higher fuel cell performance compared to FuNH2-Fe in all regions except at very low currents in the polarization curve, see Figure 8a,b. The observation that

process, where the defects contribute in the missing momentum to satisfy the Raman resonant process. Each of these four peaks, are more or less influenced by defects. The carbon structures of the two Fe-OMCs synthesized with only furfurylamine or furfuryl alcohol and iron chloride (red and blue, respectively) are expected to be the most different and consist of at least two different types of defects. The D′ peak is much more distinct in FuNH2-Fe than in FuOH-Fe. By comparing the intensity ratio of the D and the D′ peaks the nature of defects in graphene can be investigated, where I(D)/ I(D′) values ∼3.5 are associated with boundary defects, ∼6 to vacancy defects, and ∼13 to sp3-hybridization defects.19 We find the I(D)/I(D′) to be around 3 for FuNH2-Fe and clearly higher for FuOH-Fe, suggesting different compositions of defects in the two. In the mixed samples, the G and the D′ peaks merged together causing difficulties to distinguish between the two. Nitrogen is expected to become incorporated in the carbon structure as a dopant forming vacancy-type of defects and in addition to be part of the iron chelating active sites as schematically shown in Figure 7c. Another indication for vacancy-type (p-type) of doping is that the G peak sharpens and that the 2D peak blue-shifts for FuNH2-Fe compared to FuOH-Fe, suggesting that less electrons are involved.20 Furthermore, presence of boundary defects in the FuNH2-Fe is also a possibility and could be explained by the less reactive furfurylamine giving less cross-linking throughout the carbon matrix. For the catalysts including furfuryl alcohol in the precursor mixture, the defects possibly can be explained by higher degree G


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Furthermore, when introducing amine in the precursor, nitrogen becomes incorporated in the carbon matrix and may influence its conductivity. A nitrogen-doped carbon is also more surface active and therefore more hydrophilic. This may assist proton conductivity to the active sites, but also prevent mass transport of water out from the catalyst. In contrast, by adding furfuryl alcohol to the synthesis the carbon matrix likely becomes more hydrophobic (since the −OH groups are released during the polymerization), favoring the mass transport of water. It is not fully understood how the sp3hybridization defects seen in the catalysts (with the addition of furfuryl alcohol in the precursor) would affect the conjugation and consequently the conductivity. However, since the furfuryl alcohol seems to connect the carbon matrix more efficiently compared to the furfurylamine the conjugated system appears to be more complete, which could explain the lower resistivity in these samples and the more facile electron transfer in the material. This would result in an increased access to the active sites for the electrons, that is, an increased ORR activity and a more efficient catalyst.

FuNH2-Fe showed higher performance at currents below 1 mA/cm2 indicates that it has a slightly higher average activity per active site. Consequently, the improved performance of FuNH2-Fe-FuOH in the other regions is thus explained by improvements in the carbon support. The other mixed catalyst, FuOH-Fe-FuNH2 (green), did not reach as high performance and this could be explained by the mixing order of the precursors. In this sample, the furfuryl alcohol and iron chloride were first mixed together and since the polymerization starts immediately some of the iron ions might be inhibited from the furfurylamine to form active sites in the next mixing step. Therefore, it is likely that the number of active sites in the catalyst FuOH-Fe-FuNH2 is lower compared to the other two catalysts. Differences between the catalysts can also be seen in the impedance results, see Figure 8d. The semicircle in the middle/ low frequency region is attributed to the electrode processes at the cathode, which are dominant due to slower reaction kinetics of the ORR. The magnitude of the circle depends on the charge transfer resistance of the catalyst and its double layer capacitance.26 Besides this, the number of active sites and the activity of each site will also influence the impedance spectra. Assuming the number of active sites and their efficiency are close to equal in all catalysts (or a bit higher in the case of FuNH2-Fe), then the differences in catalytic performance between the catalysts (at 0.7 V) are caused by different resistance in the carbon structure and not by differences related to the active site. The smallest semicircles were obtained for FuNH2-Fe-FuOH followed by FuOH-Fe-FuNH2, indicating lower charge transfer resistance and double layer capacitance in these catalysts. The polarizations curves obtained from the RDE experiments shown in Figure 8e provide trends that are in agreement with the fuel cell results. The half potentials for the catalysts were calculated to 0.54 V (FuNH2-Fe), 0.64 V (FuNH2-Fe-FuOH), and 0.61 V (FuOH-Fe-FuNH2), respectively. The slightly lower activity for the FuNH2-Fe in the kinetic region may be caused by a more problematic electrode preparation of this catalyst compared to the other catalysts. In Figure 8f, the number of electrons transferred is shown for the three catalysts and it is clear that all three follow a direct four-electron transfer mechanism with high selectivity. Both the polarization curves and the impedance tests, thus confirm that the addition of furfuryl alcohol in the precursor mixture decreases the resistivity of the formed catalysts. To summarize, by judicious choice of carbon precursors we can produce Fe-OMC catalysts with different carbon matrix properties. By mixing furfuryl alcohol into the synthesis of FeOMCs prepared from furfurylamine we were able to enhance the catalyst’s carbon properties and therefore also its catalytic performance. The highly reactive nature of furfuryl alcohol enables strong bond formation in three dimensions during the polymerization step which seems to be important in order to obtain the cubic meso-ordered structure and to achieve a high surface area. To obtain a cubic mesostructured carbon the coexistence of sp2- and sp3-hybridization is necessary. The larger surface area in FuNH2-Fe-FuOH allows for increased active site density, resulting in an increased ORR rate. When only using furfurylamine as carbon precursor, as in FuNH2-Fe, a poorer meso-order and lower surface area were achieved due to a different polymerization reaction mechanism and a less reactive precursor. A less reactive precursor could also result in poorly interconnected grains of the OMC structure.

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CONCLUSIONS In this study, we identified whether the active sites of Fe-OMC catalysts consist of Fe−Nx or Fe−Ox chelates. Two carbon precursors (furfurylamine and furfuryl alcohol) were used to verify the effects of the amine and alcohol functional groups of the precursors on the formation of iron chelating active sites in Fe-OMC catalysts. The catalyst prepared from furfurylamine was shown through a combination of EPR and EXAFS to exhibit Fe−Nx coordination. All catalysts formed using the amine showed catalytic activity with high selectivity for the direct four-electron transfer mechanism of the oxygen reduction reaction, whereas the catalyst prepared from furfuryl alcohol showed absence of iron coordination and no activity. It is thus clear that the nitrogen in the precursor is crucial in order to form Fe−Nx chelates and that these constitute the desired active sites. The choice of carbon precursor was found to also influence the formation of the carbon matrix structure and its properties. By preparing catalysts from mixtures of furfurylamine and furfuryl alcohol improved oxygen reduction rates were achieved compared to the catalyst prepared from only furfurylamine. These improvements were correlated to the polymerization behavior of the precursors and their effects on properties of the carbon structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04588.

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Experimental parameters and SAXS and XRD plots (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. Janson: 0000-0002-2428-869X Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS The Swedish Energy Agency, Energimyndigheten, is acknowledged for partly funding this research through project 38340-1. Assoc. Prof. Lars Nordstierna (Chalmers University of Technology), Assoc. Prof. Anna Martinelli (Chalmers University of Technology), Ph.D. Ilia Kaminker (University of California, Santa Barbara), and Ph.D. Tim Fellinger (Technical University of Münich) are gratefully acknowledged for assistance with NMR, Raman, and EPR spectroscopy and RDE measurements, respectively. A portion of this research was carried out at beamline I711, MAX-lab synchrotron radiation source, Lund University, Sweden. Funding for the beamline I711 project was kindly provided by VR and The Knut och Alice Wallenbergs Stiftelse.

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