Probing Enhanced Site Activity of CoâFe Bimetallic Subnanoclusters

http://pubs.acs.org/journal/aelccp

Probing Enhanced Site Activity of Co−Fe Bimetallic Subnanoclusters Derived from Dual Cross-Linked Hydrogels for Oxygen Electrocatalysis Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 00:36:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Panpan Li,†,§,‡ Zhaoyu Jin,#,‡ Yumin Qian,† Zhiwei Fang,† Dan Xiao,*,§ and Guihua Yu*,† †

Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States § Department of Architecture and Environment, Sichuan University, Chengdu 610065, People’s Republic of China # Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Great efforts have been devoted to unveiling structural contributions of transition heterometallic materials to enhanced catalysis, while effective in situ approaches are still urgently demanded to probe the intrinsic activity on individual electrolyteaccessible sites. Here, we report cobalt−iron subnanoclusters (CoFePPy) incorporated on nitrogen-rich nanotubular carbon from dual cross-linked polypyrrole hydrogels. The bimetallic catalyst exhibits obviously improved oxygen electrocatalysis largely attributed to the electronic interaction between cobalt and iron at the atomic scale. Furthermore, the site information and catalytic kinetics of CoFe-PPy were in situ quantified by a surface-interrogation scanning electrochemical microscopy (SI-SECM) technique. Consequently, CoFePPy not only provides higher active site density but also achieves rapid kinetics to activate O2, greatly promoting the oxygen electroreduction turnover frequency compared with monometallic catalysts. Our work offers an opportunity to investigate important catalytic parameters of bimetallic electrocatalysis and highlights the heterometallic interaction at the subnanometer scale to promote enhanced electrocatalytic reactivity.

O

metal-based catalysts for both ORR and OER. Among them, iron (Fe), cobalt (Co), nickel (Ni), and manganese (Mn) were reported to show bifunctional performance in an active sequence of Fe > Co > Mn > Ni for ORR7,8 and Ni(Fe)OxHy > Co(Fe)OxHy > FeOxHy−AuOx > FeOxHy > CoOxHy > NiOxHy > MnOxHy for OER.9 In comparison to monometallic catalysts, bimetallic (or multimetallic) catalysts are reported to show enhanced catalytic performance in both ORR and OER via tuning the redox properties of metal sites.10 On the basis of previous literature, heterometallic catalysts reveal distinctive catalytic properties ascribed to the special phase11 and the electronic structure variation,12,13 which were further confirmed by structural analysis including in situ Raman spectra, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) as well as first-principles density

xygen-involved electrochemical catalysis, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is considered a cornerstone of sustainable energy conversion and storage devices, including artificial photosynthesis, water electrolysis, fuel cells, and rechargeable metal−air batteries.1−4 However, the relatively high energy barriers of oxygen electrocatalysis due to sluggish kinetics and high overpotentials limit practical applications. Noble metal-based materials such as platinum (Pt), ruthenium (Ru), or iridium (Ir) are widely known as the best catalysts for ORR or OER, while main drawbacks are the high cost from their scarcity and poor stability upon long-term working.5 Thus, designing cost-effective and bifunctional catalysts for both ORR and OER with superior activity and sufficient stability is of great importance to achieve highly efficient energy conversion/storage devices, especially for metal−air batteries.2,6 As an alternative to expensive precious metals, considerable efforts have been devoted to exploring non-noble transition © XXXX American Chemical Society

Received: April 25, 2019 Accepted: July 1, 2019 Published: July 1, 2019 1793

DOI: 10.1021/acsenergylett.9b00893 ACS Energy Lett. 2019, 4, 1793−1802

Letter

Cite This: ACS Energy Lett. 2019, 4, 1793−1802

Letter

ACS Energy Letters Scheme 1. Schematic Illustration of the Synthesis for CoFe-PPy Hydrogels

Figure 1. Morphology and elemental distribution. SEM images of CoFe-PPy before (a; the inset shows the CoFe-PPy hydrogels) and after calcination (b; the inset shows the TEM image of CoFe-PPy); TEM image obtained at the edge of the nanotubes (c); EDS mapping (d); HAADF-STEM image showing subnanoclusters (marked with red circles) homogeneously dispersed on the carbon matrix and their size (diameter) distribution in the inset (e) and the corresponding EELS spectra (f).

functional theory (DFT) calculations.14,15 These advanced characterizations may help optimize the binding ability of reaction intermediates at the active sites.10,16,17 Nevertheless, there are still two major issues that remain to be addressed for further understanding and improving bimetallic oxygen electrodes. First, in situ investigation of electrolyte-accessible sites that really take part in the electrocatalysis, as well as their kinetics, is rarely reported. Second, there is still a great challenge to control the size of heterometallic clusters with highly homogeneous dispersion because multiple metal

precursors are prone to form bulk aggregation or even cause phase segregation after post-treatment. Up to now, forming metal−organic framework (MOF)based precursors18−21 with different types of metalloligands has been considered an essential approach to uniformly incorporate dual or multiple desirable metal species into a nitrogen (N)-doped carbon matrix. However, serious aggregation of precursors is inevitable in MOFs due to the dense metal sites and complicated preparation of the metal complexes with special molecular structures. Alternatively, physically or chemically cross-linked three-dimensional (3D) polymer 1794


Letter

ACS Energy Letters

Figure 2. Structural analysis. XPS N 1s (a), Co 2p (b), and Fe 2p (c).

and CoII(bpdc)3 to form polypyrrole (PPy) hydrogels. These cross-linkers can be attracted by the positively charged N on PPy chains through an electrostatic interaction. Therefore, the metal centers are efficiently constrained and separated by PPy backbones, which ensures homogeneous distribution of the Co−Fe active sites in the subsequent calcination. After carbonization at 800 °C, metal complexes were pyrolyzed and transformed into M−N−C (M refers to metal) according to previous works.31−34 Therefore, adjacent Fe and Co atoms are likely to form Co−Fe subnanoclusters that coordinate with nitrogen and are uniformly dispersed on the carbon matrix (CoFe-PPy). Figure 1a shows the scanning electron microscopy (SEM) image of the dual cross-linked PPy hydrogel (the inset of Figure 1a) that exhibits nanotubular structure. After calcination, the as-prepared heterometallic catalyst (CoFe-PPy) also maintains a similar morphology with dual cross-linked PPy hydrogels (Figure 1b), revealing a nanotubular structure with the diameter of ∼200 nm (the inset of Figure 1b). From the high-resolution transmission electron microscopy (HR-TEM) image in Figure 1c, CoFe-PPy possesses a highly disordered structure with randomly oriented lattice fringes, and no obvious aggregated particles are observed, suggesting that Co and Fe species are more likely in the amorphous state. The morphologies of monometallic and bimetallic PPy samples are compared in Figure S2. To confirm the existence of Co and Fe, energy-dispersive X-ray spectroscopy (EDS) was conducted to illustrate the element distribution. Figure 1d shows that C, N, Co, and Fe are homogeneously dispersed on the carbon matrix. An aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image coupled with electron energy loss spectroscopy (EELS) was

hydrogels were reported to be an outstanding catalyst support to load heterometallic active sites22,23 owing to the excellent charge/mass/ion transport, large surface areas, and satisfied conductivity.24−30 Thus, in our work, Co−Fe subnanoclusters were homogeneously decorated on nanotubular carbon supports via constructing dual cross-linked hydrogels, which reveals enhanced bifunctional activities for oxygen electrocatalysis compared with the monometallic materials. Specifically, carbon nanotubular structures incorporating Co and Fe metal centers were generated by theelectrostatic interaction between the pyrrole monomers (or oligomers) and carboxyl group-modified metal complexes, which subsequently calcinated at a high-temperature to in situ transform Co−Fe subnanoclusters. To further understand the enhanced electrocatalytic mechanism, the surface-interrogation scanning electrochemical microscopy (SI-SECM) technique was adopted to in situ quantify surface-active species and kinetics on individual sites. By these means, active intermediates on Co−Fe catalysts can be detected at a relatively lower overpotential. In addition, the much higher turnover frequency (TOF) value for ORR was obtained on the bimetallic Co−Fe clusters compared to the monometallic catalysts. Moreover, we also determined the reaction rate constant of oxygen binding ability by SI-SECM to evidence the superiority of bimetallic catalysts. The dual cross-linked hydrogels were prepared as indicated in Scheme 1, where two different metal-centered molecules containing six carboxyl groups (FeII(bpdc)3 and CoII(bpdc)3; the detailed synthesis was described in Figure S1 and the Experimental Section) were simultaneously mixed with pyrrole monomers in isopropyl alcohol (IPA) solutions. By adding initiators (ammonium persulfate (APS)), pyrrole monomers were immediately polymerized and cross-linked by FeII(bpdc)3 1795


Letter

ACS Energy Letters

Figure 3. Bifunctional activities for oxygen electrocatalysis. LSV curves collected with a RDE (1600 rpm) at 10 mV s−1 (a); the bar diagram of the potential difference (ΔE) between EJ10 for OER and E1/2 for ORR (b).

originates from the nitrogen bonding to the metal center that is absent in pure PPy. Additionally, Co-PPy exhibits a relatively low metal peak (N2) compared to CoFe-PPy and Fe-PPy due to the lower metal contents (recorded in Tables S1 and S2). In addition, Co 2p and Fe 2p spectra of CoFe-PPy are also compared with monometallic catalysts. From Co 2p spectra in Figure 2b, CoFe-PPy mainly presents three peaks at around 780.11, 782.66, and 787.64 eV ascribed to Co2+, CoNx species, and the satellite peak of Co2+,39,40 implying the existence of a metal−nitrogen bond (Co−N) that is a favorable active site for ORR. In contrast, Co-PPy exhibits an additional binding state at 778.80 eV belonging to Co0, which is because the abundant Co content may result in the aggregation of adjacent Co atoms and thus generates metallic Co species. As for Fe 2p spectra, CoFe-PPy and Fe-PPy demonstrate two main peaks (710.66, 723.69 eV) and a small satellite peak (718.20 eV), suggesting that the oxidation state is Fe2+.41 There are still two relatively weak peaks (714.31, 726.08 eV) assigned to Fe3+, presumably due to the oxidation of the sample in the ambient environment.42 Overall, CoFe-PPy reveals similar features to Fe-PPy, but a slight peak shift is observed due to the electronic interaction between Co and Fe atoms, which aggrees with the result from a previous report.43 Specifically, the electronic structure of parent transition metal compounds can be greatly affected by the electronic affinity of metal substituents, further tuning the energy of antibonding states, the redox potential, and the oxygen electrocatalysis.10 The oxygen electrocatalytic property was evaluated in oxygen-saturated 0.1 M KOH with a three-electrode configuration using a rotating disk electrode coated with 338 μg cm−2 samples, a Ag/AgCl (KCl-saturated solution) electrode, and a graphite rod as working, reference, and counter electrodes, respectively. Polarization curves were collected by linear sweep voltammetry (LSV) at a scan rate of 10 mV s−1 with a rotating speed of 1600 rpm in Figure 3a. Here, the bimetallic sample (CoFe-PPy) shows improved activities in both OER and ORR, revealing a much higher current density and a relatively low onset potential. As one of the most important parameters, the potential difference (ΔE = EJ10 − E1/2) between the OER potential at 10 mA cm−2 (EJ10) and the half-wave potential in ORR (E1/2) was compared to investigate the enhanced bifunctional activity of the hetero-

further applied to investigate the atomic dispersion and the local environment of Co and Fe at an atomic scale. As shown in Figure 1e, bright spots corresponding to clusters with heavier metal assembled by several atoms (emphasized by red circles) are scattered randomly, which represents an average diameter of 0.4−0.6 nm from the histogram of the size distribution in the inset. To study the elemental composition of subnanoclusters, local EELS was conducted in a small region marked with red circles in Figure 1e. Due to the atom resolution of the electron probe, the signal in Figure 1f mainly originated from metal atoms and its adjacent atoms, where C, N, Co, and Fe were contained in a subnanocluster. The coexistence of N, C, Co, and Fe at the Angström scale as well as the statistic average diameter of the clusters suggests that several metal atoms probably bound with N form the M−N−C (M referred to Co and Fe) coordinating structure35,36 and act as the active site for reducing oxygen. X-ray diffraction (XRD) was conducted to study the structure of as-prepared samples in Figure S4a, where two broad peaks at around 25.41 and 43.45° can be attributed to the carbon matrix, demonstrating the poor crystallinity and the amorphous nature.37 To further confirm the elemental composition and the electronic interaction between dissimilar elements, X-ray photoelectron spectra (XPS) were obtained, as shown in Figure 2, which includes N 1s, Co 2p, and Fe 2p spectra. From the survey scan XPS in Figure S4b, CoFe-PPy is mainly composed of O, N, C, Co, and Fe, among which metal contents are much lower than the others. The metal contents of selected samples were also investigated by EDS analysis (Table S1) and inductively coupled plasma-optical emission spectrometry (ICP-OES, Table S2), revealing the total metal content of around 1 wt % and closed Co:Fe weight ratios. The amount of C and N reaches approximately 80 and 15 wt %, respectively, which is consistent with the XPS result. Furthermore, as shown in Figure 2a, N 1s spectra of CoFePPy, Co-PPy, and Fe-PPy were extracted and deconvoluted into five speices, including pyridinic nitrogen (N1, 398.3 ± 0.2 eV), Co(or Fe)-N (N2, 398.9 ± 0.2 eV), pyrrolic nitrogen (N3, 399.8 ± 0.2 eV), graphitic nitrogen (N4, 400.9 ± 0.2 eV), and pyridinic nitrogen oxide (N5, 402−406 eV).8,38 The control sample, pure PPy (after the pyrolysis), showed only four components (N1, N3, N4, and N5) because N2 mainly 1796


Letter

ACS Energy Letters

Figure 4. Oxygen reduction reaction. CV curves obtained in N2- and O2-saturated 0.1 M KOH at 10 mV s−1 (a); polarization curves collected at 10 mV s−1 with 1600 rpm (b); corresponding Tafel plots (c); calculated kinetic current density and charge transfer number based on Koutecky−Levich plots (d); and long-term electrochemical durability (e) and methanol resistance test (f) at 0.85 V vs RHE.

metallic catalyst. From Figure 3b, a much lower ΔE value (0.85 V) was achieved by CoFe-PPy, while those of Pt/C, Co-PPy, and Fe-PPy were around 1.10, 1.12, and 1.13 V, respectively, indicating that CoFe-PPy outperforms most of the state-of-theart bifunctional catalysts listed in Table S3. Furthermore, LSV curves of samples prepared with different metal molar ratios were also evaluated and are compared in Figure S5a. It clearly displays that introducing a dissimilar metal can lead to a significant improvement in the current density and onset potential for water oxidation. Here, the best result was recorded by CoFe-PPy with the same Co to Fe molar ratio. Moreover, CoFe-PPy-2 was selected for the longterm electrochemical stability test by collecting the current at 1.7 V vs RHE (Figure S5b), exhibiting favorable retention of about 87% after a continuous measurement up to 40 h. Even though the OER performance was optimized by incorporating two dissimilar metals into one carbon matrix, it was still less impressive when compared with other transition metal oxidebased materials. That is possibly due to a very low metal content of 1−2% as the active sites for OER. For another, the enhanced OER mechanism of heterometallic catalysts has been well discussed through in situ structural analysis11,13 and theoretical calculation.15 In light of the above reasons, we mainly focused on detailed investigation of ORR in this work. As for the oxygen reduction process, cyclic voltammetry (CV) curves were measured in N2- and O2-saturated 0.1 M KOH aqueous solution, as shown in Figure 4a, where apparent oxygen reduction peaks were recorded with added O2, indicating that as-prepared samples were all active for the ORR. Among these catalysts, CoFe-PPy-2 is the most

outstanding one in terms of the relatively high onset potential (0.96 V vs RHE), suggesting that the active intermediates can be generated with a low energy comsuption. That is also confirmed from LSV curves in Figure 4b, in which the bimetallic catalyst demonstrates better ORR activity than monometallic catalysts or pure PPy (without any metals), even outperforming commercial Pt/C. Considering our prepared CoFe-PPy with a total metal content of less than 1 wt % (Table S2), it is able to be comparable to commercial Pt/C (20 wt %). According to the Butler−Volmer equation, its kinetics should be roughly several times higher than that of Pt/ C, while they show a similar apparent current. To systematically compare activities with various metal molar ratios in the preparation process, Tafel plots (in Figure 4c) were extracted from LSV curves in Figure S6, and corresponding Tafel slopes are summarized in Table S4. Two commonly identified Tafel slopes include a smaller slope located at a low overpotential (η) and a larger one above 100 mV dec−1 at a relatively high η.44 Notably, CoFe-PPy-2 reveals a Nernstian Tafel slope close to 60 mV dec−1, which is linked to a typical redox ORR mechanism of transition metals, suggesting O−O bond splitting as the rate-determining step.45 As mentioned in previous work,45,46 the high Tafel slopes originate from insufficient catalyst utilization because of mass transport losses at high current densities. In comparison, much different Tafel slopes from 60 mV dec−1 were measured for Co-PPy (49.60 mV dec−1) and Fe-PPy (85.70 mV dec−1), implying that the reaction rate was determined in dissimilar steps. More interestingly, similar Tafel slopes were observed in commercial Pt/C and CoFe-PPy-2, suggesting that these two catalysts 1797


Letter

ACS Energy Letters

Figure 5. SI-SECM analysis. CVs (at 0.2 mV s−1 in N2-saturated KOH) indicating the redox ORR mechanism (a); titrated gravimetric site density (b) and the schematic illustration of the SI-SECM setup (the inset); TOF values (the number of O2 per site per second) derived from the gravimetric site density (c); Pseudo-first-order reaction rate constant determined by the slope from the linear relationship between the logarithmic active site concentration and delay time (d).

for 20 min at 0.85 V (vs RHE) in O2-saturated 0.1 M KOH. It indicates that bimetallic subnanoclusters would be stable for the ORR. With respect to the contribution of the carbon support, although it is less likely to directly take part in the catalysis, the location of nitrogen dopants is believed to be the favorable site to bind and stabilize the metal, avoiding the possible Ostwald ripening of atoms or clusters into particles with post-treatment, as well as forming the M−N−C structure. It is known as one of the most active species for the ORR that undergoes the four-electron transfer pathway and even can compete with Pt.49 Additionally, carbon backbones provide a much larger surface area for loading highly dispersed catalysts, thus greatly decreasing the use of metal. As we mentioned above, bimetallic catalysts have been widely investigated due to the enhanced activity for oxygen electrocatalysts. Nevertheless, there are relatively few studies devoted to in situ exploration of kinetics and intrinsic activity of heterometallic catalysts. For another, the SI-SECM technique provides us a straightforward approach to quantify and qualify surface-active sites and intermediates in real time, which have been well established to study the electrocatalytic mechanism.50−52 As indicated in Figure 5a, samples exhibit an apparent redox peak in deaerated 0.1 M KOH at a very slow scan rate, confirming the redox-active species coupled ORR mechanism,49 which offers us an opportunity to detect active intermediates using SI-SECM. In addition, CoFe-PPy demonstrates a more positive reduction peak, which suggests that active intermediates can be generated with a relatively low energy supply, thus achieving a better overpotential. This point was further proved by the SI-SECM titration.

exhibit a comparable electrocatalytic mechanism for the oxygen reduction. Koutecky−Levich plots in Figure S7h were obtained by collecting current densities (at 0.6 V vs RHE) with different rotating speeds from 400 to 2500 rpm. Then, the electron transfer number and kinetic current densities (Jk) were calculated according to these plots and are summarized in Figure 4d for comparison. Pt/C- and Fe-dominated samples exhibit a relatively high Jk and a near-four-electron ORR pathway. On the basis of previous reports,17,47 Co-based catalysts were more likely to go through a lower-efficiency ORR process (mainly two-electron transfer) than Fe- and Mnbased materials because of the weak oxygen adsorption ability, leading to the inferior ORR activity. As one of the most essential parameters, long-term electrochemical stability for ORR was also evaluated in air- or O2-saturated 0.1 M KOH to indicate the potential practical application of as-prepared samples. From Figures 4e and S8, CoFe-PPy reveals no obvious current decay after continuous electrocatalysis for 20 h at 0.85 V vs RHE and 5000 cycles of CV in O2-saturated electrolyte. Methanol crossover and poison effects are critical issues for ORR catalysts when applied in methanol fuel cells.41,48 From Figure 4f, CoFe-PPy still maintains a stable reduction current density when adding 0.5 M methanol, while Pt/C is immediately occupied for catalyzing methanol oxidation and losses the activity for oxygen reduction. The result implies that CoFe-PPy owns a superior resistance toward methanol poisoning compared to Pt/C. CoFe-PPy after ORR was characterized by XPS, shown in Figure S9. There are subtle changes observed in Co 2p3/2 and Fe 2p spectra before and after the ORR test by bulk electrolysis 1798


Letter

ACS Energy Letters

(the titrated concentration of active sites after tdelay) and tdelay, which means that the oxygen chemisorption on catalysts follows a pseudo-first-order reaction, expressed as below

The SI-SECM setup is indicated in Figure 5b, and the interelectrode distance (∼5 μm) can be controlled by the positive feedback approaching curve associated with the simulated one (Figure S11), which is clearly described in the Experimental Section. From titration curves in Figure S12a−c, the feedback current reveals considerable growth upon a more negative substrate potential applied in all samples, demonstrating that increasing active intermediates were generated. Obtained CA curves were integrated to yield the site density, representing the real active sites per gram evolved in the catalytic reaction, and are plotted against the applied substrate potential, as shown in Figure 5b. Here, the mass loading of catalysts deposited on Pt UME (the substrate electrode) was obtained by comparing the electric double-layer capacitance (CEDLC) of samples on Pt UME and that on glass carbon, as shown in Figure S10. Notably, CoFe-PPy is able to produce active intermediates at the lowest overpotential (less than 200 mV) and achieves an improved site density compared to monometallic catalysts, which is consistent with CVs from Figure 5a. The plateau below 0.8 V indicates the oxidation of total Fe(II) or Co(II) species generated on the substrate, showing maximum site densities of 21.57, 40.68, and 28.20 μmol g−1 for Co-PPy, CoFe-PPy, and Fe-PPy, respectively. Besides, TOF values are of great importance for evaluating catalysts because the intrinsic activity on each site can be quantified. However, most papers described TOF values of electrocatalysts using total metal contents of bulk materials instead of the electrolyte-accessible active sites, leading to the underestimate of the intrinsic catalytic property.53 Thus, in our case, the more reliable TOF of samples was evaluated based on the titrated Fe(II) or Co(II) active sites that were actually involved in the catalysis. For the ORR process, the bimetallic catalyst delivers an improved TOF (0.79 site−1 s−1 at 0.8 V vs RHE) at the kinetically controlled region compared to monometallic catalysts (Figure 5c). It suggests that more dioxygen molecules can be reduced on the individual active site in a unit time, which is comparable to the value (∼1.0 site −1 s −1 at 0.8 V) determined by the CO pulse chemisorption/desorption.53 For the OER process, TOF analysis based on the all-metal content measured by SISECM is provided in Figure S13 and Table S5, where bimetallic catalysts (CoFe-PPy) show considerable enhancement of the intrinsic OER activity compared with Co-PPy and Fe-PPy. Specifically, the TOF value (0.45 s−1) at 1.58 V vs RHE (η = 0.35 V) for CoFe-PPy is in good agreement with the previous results (0.48−0.61 s−1).54 Apart from the site information investigation, catalytic kinetics is also especially concerned. SI-SECM is a powerful technique to in situ and time-resolved quantify the intermediate species generated on the surface of catalysts. By delaying the time of redox titrant generation (Figure S12d−f), we determined the binding rate between Fe(II)/Co(II) and molecular dioxygen via plotting the logarithmic active site number against the delay time. Specifically, the titrant (FcMeOH+) was generated at the tip only after the delay time (tdelay), during which the only substance reacting with Fe(II) or Co(II) species was oxygen, as illustrated in the inset of Figure 5b. Once the tip produced FcMeOH+ at an applied oxidation potential, the remaining active intermediates (Fe(II) or Co(II) species) could be immediately oxidized by the titrant. At the same time, the tip collected a positive feedback response, yielding the residual active species. The result in Figure 5d indicates a linear relationship between ln[Cactive sites]

k′

M(II) + O2 V M(III)−O*2 k″

−

d(M(II)) = k′[M(II)] dtdelay

ln[M(II)]tdelay = −k′ × tdelay + ln[M(II)]0

where k′ is the pseudo-first-order reaction rate constant reflecting the oxygen binding ability on active sites. From Figure 5d, an impressively high rate constant of 0.22 s−1 was found on CoFe-PPy compared to 0.14 s−1 from Fe-PPy, while Co-PPy shows the lowest value of about 0.10 s−1. The different values of k′ imply that the bimetallic catalyst exhibits a much faster binding rate to oxygen in contrast to monometallic catalysts, thus facilitating the oxygen reduction process. Aiming to support some evidence on the scientific insight of the topic, we carried out some discussion based on the electrochemical study and the SI-SECM result. First of all, the catalyst with only Fe (Fe-PPy) shows much better performance than that with Co (Co-PPy) in terms of the onset potential, half-wave potential, electron transfer number, and oxygen-binding kinetics for the ORR due to the favorable activity on Fe(II) instead of Co(II). Regarding the CoFe-PPy, the onset potential in Figure 4b is close to that of Fe-PPy, implying probably the same active species of Fe(II) that is responsible for the ORR. In addition, Table S2 shows the total content and ratio of Co and Fe in samples, where CoFe-PPy-1 and CoFe-PPy-2 contain much less Co but deliver better ORR activity (Figure S6) than samples with a higher content of cobalt. Therefore, it is more likely that the ORR would be driven by Fe(II) in CoFe-PPy catalysts especially in the low overpotential region. In light of the function of Co, we deduce that it could serve as an inducer to modulate the electronic structure of iron.10 As we observed in XPS spectra of Fe 2p, there is a slightly positive shift of the binding energy after the introduction of Co into Fe-PPy, suggesting the electronic interaction. This interaction between two metals in such a small cluster would result in a remarkable difference in the redox feature of the active species during the ORR process. Besides, the presence of Co also plays an essential role in the formation of a nanotubular carbon support. Figure S3 demonstrates SEM images of samples containing various metal contents. We see only aggregated nanoparticles for FePPy but uniform nanotubes once Co is introduced. It could be attributed to the special cross-linking between Co(bpdc)3 and PPy in the precursor. The nanotubular substrate is very beneficial to expose more active sites in the electrolyte because of the high specific area compared to that of the particle shape. Combining the result of the active site from SI-SECM and total site from ICP-OES, the availability ratio of metal can be calculated by dividing them, which is clearly improved for samples with Co. On the other hand, the OER activity promoted by the bimetallic structure could be considered as the formation of CoFe oxyhydroxides under OER conditions. In alkaline conditions, the low-valent metal clusters coordinated to nitrogen would transform into high-spin metal ions, such as Co(III/IV) or Fe(III/IV), which are likely to bond OH− from the bulk solution and deprotonate (oxyhydroxide) 1799


Letter

ACS Energy Letters

21777108). P.L. acknowledges financial support from the China Scholarship Council (No. 201706240160) and thanks Jiwoong Bae for helpful discussion.

for the stabilization at the resting state. The superior oxygenevolving performance for bimetallic oxyhydroxides has been studied in previous reports, where Fe would be the OER active site and cobalt species basically offer a conductive, high-surface area, chemically stabilizing host.9 In summary, a CoFe bimetallic subnanocluster catalyst was homogeneously incorporated on carbon nanotubes derived from dual cross-linked PPy hydrogels. The electrostatic interaction between positively charged N on pyrrole monomers and carboxyl groups on cross-linkers enables a high distribution and supramolecular confinement of Co and Fe species on PPy backbones. Thus, uniform Co−Fe subnanoclusters can be achieved by taking advantage of this intermolecular interaction via efficiently suppressing the serious aggregation in the following pyrolysis. The as-prepared CoFe-PPy presents improved oxygen electrocatalysis for both OER and ORR, even outperforming commercial Pt/C (20%). More interestingly, the underlying mechanism of the enhanced ORR performance was investigated by in situ and timedependent titration of the active intermediates using the SISECM technique in terms of thermodynamics and kinetics. As a result, the bimetallic catalyst was proved to generate active intermediates at a relatively lower overpotential than that of monometallic catalysts. Besides, more active sites were formed on CoFe-PPy, while its intrinsic activity on individual sites was also higher than that of Co-PPy and Fe-PPy. Ultimately, this work offers opportunities on the design of heterometallic catalysts based on the strong interaction between two species at the subnanometer scale.

■

■

(1) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design principles for oxygenreduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries. Nat. Chem. 2011, 3, 546−550. (2) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the rational design of nonprecious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 2015, 8, 1404−1427. (3) Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J. J. Oxygen electrochemistry as a cornerstone for sustainable energy conversion. Angew. Chem., Int. Ed. 2014, 53, 102−121. (4) Lai, J.; Huang, B.; Tang, Y.; Lin, F.; Zhou, P.; Chen, X.; Sun, Y.; Lv, F.; Guo, S. Barrier-free interface electron transfer on PtFe-Fe2C Janus-like nanoparticles boosts oxygen catalysis. Chem. 2018, 4, 1153−1166. (5) Bu, L.; Shao, Q.; Pi, Y.; Yao, J.; Luo, M.; Lang, J.; Hwang, S.; Xin, H.; Huang, B.; Guo, J.; Su, D.; Guo, S.; Huang, X. Coupled s-p-d exchange in facet-controlled Pd3Pb tripods enhances oxygen reduction catalysis. Chem. 2018, 4, 359−371. (6) Gasteiger, H. A.; Marković, N. M. Just a dreamor future reality? Science 2009, 324, 48−49. (7) Cheng, F.; Chen, J. Metal-air batteries: from oxygen reduction electrochemistry to cathode catalysts. Chem. Soc. Rev. 2012, 41, 2172−2192. (8) Wang, X.; Cullen, D. A.; Pan, Y.; Hwang, S.; Wang, M.; Feng, Z.; Wang, J.; Engelhard, M. H.; Zhang, H.; He, Y.; Shao, Y.; Su, D.; More, K. L.; Spendelow, J. S.; Wu, G. Nitrogen-coordinated single cobalt atom catalysts for oxygen reduction in proton exchange membrane fuel cells. Adv. Mater. 2018, 30, 1706758. (9) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. Revised oxygen evolution reaction activity trends for first-row transition-metal (oxy)hydroxides in alkaline media. J. Phys. Chem. Lett. 2015, 6, 3737−3742. (10) Kuznetsov, D. A.; Han, B.; Yu, Y.; Rao, R. R.; Hwang, J.; Román-Leshkov, Y.; Shao-Horn, Y. Tuning redox transitions via inductive effect in metal oxides and complexes, and implications in oxygen electrocatalysis. Joule 2018, 2, 225−244. (11) Louie, M. W.; Bell, A. T. An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2013, 135, 12329−12337. (12) Liang, Y.; Wang, H.; Zhou, J.; Li, Y.; Wang, J.; Regier, T.; Dai, H. Covalent hybrid of spinel manganese-cobalt oxide and graphene as advanced oxygen reduction electrocatalysts. J. Am. Chem. Soc. 2012, 134, 3517−3523. (13) Li, N.; Bediako, D. K.; Hadt, R. G.; Hayes, D.; Kempa, T. J.; von Cube, F.; Bell, D. C.; Chen, L. X.; Nocera, D. G. Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 1486−1491. (14) Song, J.; Zhu, C.; Xu, B. Z.; Fu, S.; Engelhard, M. H.; Ye, R.; Du, D.; Beckman, S. P.; Lin, Y. Bimetallic cobalt-based phosphide zeolitic imidazolate framework: CoPx phase-dependent electrical conductivity and hydrogen atom adsorption energy for efficient overall water splitting. Adv. Energy Mater. 2017, 7, 1601555. (15) Liu, K.; Zhang, C.; Sun, Y.; Zhang, G.; Shen, X.; Zou, F.; Zhang, H.; Wu, Z.; Wegener, E. C.; Taubert, C. J.; Miller, J. T.; Peng, Z.; Zhu, Y. High-performance transition metal phosphide alloy catalyst for oxygen evolution reaction. ACS Nano 2018, 12, 158−167. (16) Chen, X.; Sun, S.; Wang, X.; Li, F.; Xia, D. DFT study of polyaniline and metal composites as nonprecious metal catalysts for oxygen reduction in fuel cells. J. Phys. Chem. C 2012, 116, 22737− 22742.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00893. Experimental section, calculation details, morphology (SEM and TEM images) and structure (XRD and XPS analysis) characterizations, elementary composition, electrochemical measurements, and SI-SECM titration tests (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Y.). *E-mail: [email protected] (D.X.). ORCID

Zhaoyu Jin: 0000-0003-0840-3931 Zhiwei Fang: 0000-0001-8826-8834 Dan Xiao: 0000-0001-5295-0540 Guihua Yu: 0000-0002-3253-0749 Author Contributions ‡

P.L. and Z.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS G.Y. acknowledges funding support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0019019 and a Camille Dreyfus Teacher− Scholar Award. D.X. acknowledges financial support from the National Natural Science Foundation of China (No. 1800


Letter

ACS Energy Letters (17) Zagal, J. H.; Koper, M. T. Reactivity descriptors for the activity of molecular MN4 catalysts for the oxygen reduction reaction. Angew. Chem., Int. Ed. 2016, 55, 14510−14521. (18) Guan, B. Y.; Lu, Y.; Wang, Y.; Wu, M.; Lou, X. W. Porous ironcobalt alloy/nitrogen-doped carbon cages synthesized via pyrolysis of complex metal-organic framework hybrids for oxygen reduction. Adv. Funct. Mater. 2018, 28, 1706738. (19) Wurster, B.; Grumelli, D.; Hötger, D.; Gutzler, R.; Kern, K. Driving the oxygen evolution reaction by nonlinear cooperativity in bimetallic coordination catalysts. J. Am. Chem. Soc. 2016, 138, 3623− 3626. (20) Lin, Q.; Bu, X.; Kong, A.; Mao, C.; Bu, F.; Feng, P. Heterometal-embedded organic conjugate frameworks from alternating monomeric iron and cobalt metalloporphyrins and their application in design of porous carbon catalysts. Adv. Mater. 2015, 27, 3431−3436. (21) Wang, J.; Liu, W.; Luo, G.; Li, Z.; Zhao, C.; Zhang, H.; Zhu, M.; Xu, Q.; Wang, X.; Zhao, C.; Qu, Y.; Yang, Z.; Yao, T.; Li, Y.; Lin, Y.; Wu, Y.; Li, Y. Synergistic effect of well-defined dual sites boosting the oxygen reduction reaction. Energy Environ. Sci. 2018, 11, 3375− 3379. (22) Fu, G.; Chen, Y.; Cui, Z.; Li, Y.; Zhou, W.; Xin, S.; Tang, Y.; Goodenough, J. B. Novel hydrogel-derived bifunctional oxygen electrocatalyst for rechargeable air cathodes. Nano Lett. 2016, 16, 6516−6522. (23) Yang, J.; Wang, X.; Li, B.; Ma, L.; Shi, L.; Xiong, Y.; Xu, H. Novel iron/cobalt-containing polypyrrole hydrogel-derived trifunctional electrocatalyst for self-powered overall water splitting. Adv. Funct. Mater. 2017, 27, 1606497. (24) Shi, Y.; Peng, L.; Ding, Y.; Zhao, Y.; Yu, G. Nanostructured conductive polymers for advanced energy storage. Chem. Soc. Rev. 2015, 44, 6684−6696. (25) Shi, Y.; Yu, G. Designing hierarchically nanostructured conductive polymer gels for electrochemical energy storage and conversion. Chem. Mater. 2016, 28, 2466−2477. (26) Zhao, F.; Shi, Y.; Pan, L.; Yu, G. Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications. Acc. Chem. Res. 2017, 50, 1734−1743. (27) Zhao, Y.; Liu, B.; Pan, L.; Yu, G. 3D nanostructured conductive polymer hydrogels for high-performance electrochemical devices. Energy Environ. Sci. 2013, 6, 2856−2870. (28) Zhao, F.; Bae, J.; Zhou, X.; Guo, Y.; Yu, G. Nanostructured Functional Hydrogels as an Emerging Platform for Advanced Energy Technologies. Adv. Mater. 2018, 30, 1801796. (29) Bae, J.; Li, Y.; Zhang, J.; Zhou, X.; Zhao, F.; Shi, Y.; Goodenough, J. B.; Yu, G. A 3D nanostructured hydrogel-frameworkderived high-performance composite polymer lithium-ion electrolyte. Angew. Chem., Int. Ed. 2018, 57, 2096−2100. (30) Li, P.; Jin, Z.; Peng, L.; Zhao, F.; Xiao, D.; Jin, Y.; Yu, G. Stretchable all-gel-state fiber-shaped supercapacitors enabled by macromolecularly interconnected 3D graphene/nanostructured conductive polymer hydrogels. Adv. Mater. 2018, 30, 1800124. (31) Cheng, Q.; Han, S.; Mao, K.; Chen, C.; Yang, L.; Zou, Z.; Gu, M.; Hu, Z.; Yang, H. Co nanoparticle embedded in atomicallydispersed Co-N-C nanofibers for oxygen reduction with high activity and remarkable durability. Nano Energy 2018, 52, 485−493. (32) Leonard, N. D.; Wagner, S.; Luo, F.; Steinberg, J.; Ju, W.; Weidler, N.; Wang, H.; Kramm, U. I.; Strasser, P. Deconvolution of Utilization, Site Density, and Turnover Frequency of Fe−Nitrogen− Carbon Oxygen Reduction Reaction Catalysts Prepared with Secondary N-Precursors. ACS Catal. 2018, 8, 1640−1647. (33) Zhang, M.; Wang, Y.-G.; Chen, W.; Dong, J.; Zheng, L.; Luo, J.; Wan, J.; Tian, S.; Cheong, W.-C.; Wang, D.; Li, Y. Metal (Hydr)oxides@Polymer Core−Shell Strategy to Metal Single-Atom Materials. J. Am. Chem. Soc. 2017, 139, 10976−10979. (34) Li, Q.; Chen, W.; Xiao, H.; Gong, Y.; Li, Z.; Zheng, L.; Zheng, X.; Yan, W.; Cheong, W.-C.; Shen, R.; Fu, N.; Gu, L.; Zhuang, Z.; Chen, C.; Wang, D.; Peng, Q.; Li, J.; Li, Y. Fe Isolated Single Atoms

on S, N Codoped Carbon by Copolymer Pyrolysis Strategy for Highly Efficient Oxygen Reduction Reaction. Adv. Mater. 2018, 30, 1800588. (35) Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G. Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nat. Catal. 2018, 1, 935−945. (36) Deng, D.; Chen, X.; Yu, L.; Wu, X.; Liu, Q.; Liu, Y.; Yang, H.; Tian, H.; Hu, Y.; Du, P.; Si, R.; Wang, J.; Cui, X.; Li, H.; Xiao, J.; Xu, T.; Deng, J.; Yang, F.; Duchesne, P. N.; Zhang, P.; Zhou, J.; Sun, L.; Li, J.; Pan, X.; Bao, X. A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature. Sci. Adv. 2015, 1, No. e1500462. (37) Panomsuwan, G.; Saito, N.; Ishizaki, T. Nitrogen-doped carbon nanoparticle-carbon nanofiber composite as an efficient metal-free cathode catalyst for oxygen reduction reaction. ACS Appl. Mater. Interfaces 2016, 8, 6962−6971. (38) Lai, Q.; Zhu, J.; Zhao, Y.; Liang, Y.; He, J.; Chen, J. MOF-Based Metal-Doping-Induced Synthesis of Hierarchical Porous Cu−N/C Oxygen Reduction Electrocatalysts for Zn−Air Batteries. Small 2017, 13, 1700740. (39) Chen, Y.-Z.; Wang, C.; Wu, Z.-Y.; Xiong, Y.; Xu, Q.; Yu, S.-H.; Jiang, H.-L. From bimetallic metal-organic framework to porous carbon: High surface area and multicomponent active dopants for excellent electrocatalysis. Adv. Mater. 2015, 27, 5010−5016. (40) Chao, S.; Bai, Z.; Cui, Q.; Yan, H.; Wang, K.; Yang, L. Hollowed-out octahedral Co/N-codoped carbon as a highly efficient non-precious metal catalyst for oxygen reduction reaction. Carbon 2015, 82, 77−86. (41) Li, J.; Chen, J.; Wan, H.; Xiao, J.; Tang, Y.; Liu, M.; Wang, H. Boosting oxygen reduction activity of Fe-N-C by partial copper substitution to iron in Al-air batteries. Appl. Catal., B 2019, 242, 209− 217. (42) Lin, L.; Zhu, Q.; Xu, A.-W. Noble-metal-free Fe-N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions. J. Am. Chem. Soc. 2014, 136, 11027−11033. (43) Zhao, X.; Pattengale, B.; Fan, D.; Zou, Z.; Zhao, Y.; Du, J.; Huang, J.; Xu, C. Mixed-node metal-organic frameworks as efficient electrocatalysts for oxygen evolution reaction. ACS Energy Letters 2018, 3, 2520−2526. (44) Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K. Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion. Sci. Rep. 2015, 5, 13801. (45) Chlistunoff, J. RRDE and voltammetric study of ORR on pyrolyzed Fe/polyaniline catalyst. On the origins of variable Tafel slopes. J. Phys. Chem. C 2011, 115, 6496−6507. (46) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science 2009, 324, 71−74. (47) Chen, R.; Li, H.; Chu, D.; Wang, G. Unraveling oxygen reduction reaction mechanisms on carbon-supported Fe-phthalocyanine and Co-phthalocyanine catalysts in alkaline solutions. J. Phys. Chem. C 2009, 113, 20689−20697. (48) Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-Doped Graphene Quantum Dots with Oxygen-Rich Functional Groups. J. Am. Chem. Soc. 2012, 134, 15−18. (49) Masa, J.; Xia, W.; Muhler, M.; Schuhmann, W. On the role of metals in nitrogen-doped carbon electrocatalysts for oxygen reduction. Angew. Chem., Int. Ed. 2015, 54, 10102−10120. (50) Ahn, H. S.; Bard, A. J. Surface interrogation of CoPi water oxidation catalyst by scanning electrochemical microscopy. J. Am. Chem. Soc. 2015, 137, 612−615. (51) Ahn, H. S.; Bard, A. J. Surface interrogation scanning electrochemical microscopy of Ni1−xFexOOH (0 < x < 0.27) oxygen evolving catalyst: kinetics of the “fast” iron sites. J. Am. Chem. Soc. 2016, 138, 313−318. (52) Liang, Z.; Ahn, H. S.; Bard, A. J. A study of the mechanism of the hydrogen evolution reaction on nickel by surface interrogation 1801


Letter

ACS Energy Letters scanning electrochemical microscopy. J. Am. Chem. Soc. 2017, 139, 4854−4858. (53) Sahraie, N. R.; Kramm, U. I.; Steinberg, J.; Zhang, Y.; Thomas, A.; Reier, T.; Paraknowitsch, J.-P.; Strasser, P. Quantifying the density and utilization of active sites in non-precious metal oxygen electroreduction catalysts. Nat. Commun. 2015, 6, 8618. (54) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt−iron (oxy)hydroxide oxygen evolution electrocatalysts: the role of structure and composition on activity, stability, and mechanism. J. Am. Chem. Soc. 2015, 137, 3638−3648.

1802


Probing Enhanced Site Activity of CoâFe Bimetallic Subnanoclusters

Recommend Documents

Probing Enhanced Site Activity of CoâFe Bimetallic Subnanoclusters