Iron(Oxides) Heterostructures for Efficient Oxygen Evolution

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Letter

Cobalt/Iron (Oxides) Heterostructures for Efficient Oxygen Evolution and Benzyl Alcohol Oxidation Reactions Yiyin Huang, Rui Yang, Ganesan Anandhababu, Jiafang Xie, Jiangquan Lv, Xiaotao Zhao, Xueyuan Wang, Maoxiang Wu, Qiaohong Li, and Yaobing Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01071 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Energy Letters

Cobalt/Iron

(Oxides)

Heterostructures

for

Efficient

Oxygen

Evolution and Benzyl Alcohol Oxidation Reactions Yiyin Huang, Rui Yang, Ganesan Anandhababu, Jiafang Xie, Jiangquan Lv, Xiaotao Zhao, Xueyuan Wang, Maoxiang Wu, Qiaohong Li,* Yaobing Wang* CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials, State Key Laboratory of Structural Chemistry, Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ABSTRACT Design of advanced electrocatalysts for oxygen evolution reaction (OER) and the alternative reaction is of prime importance to splitting water for hydrogen generation. Herein, cobalt/iron (oxides) heterostructures with interface engineering for regulating surface structure properties towards enhanced OER and benzyl alcohol oxidation (BAO) are demonstrated. Interface engineering triggers generation of local crystallinity and defective oxygen, enabling the material to export 50 mA cm−2 for OER at the overpotential of 329 mV, and continuous 20 h operation without apparent decay. Further, BAO is also boosted on the heterostructures, further propelling water splitting to export 10 mA cm−2 at a voltage of only 1.42 V. Theoretical calculation reveals that the defective sites dominated by interfaces facilitate adsorption/dissociation of intermediates during electrocatalysis. The findings in this work, place Fe/Co (oxides) heterostructures an excellent

bifunctional

OER/BAO

catalyst,

and

also

provide

a

promising

interface-regulated-electrocatalysis strategy for development of other advanced heterostructures towards various applications.

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Hydrogen is an ideal alternative fuel with high gravimetric energy density, and splitting water to producing hydrogen shows great significance due to the environment-friendly nature, low energy consumption and high-purity product.1 The overall water splitting is usually impeded by the sluggish kinetics of anode reaction, namely oxygen evolution reaction (OER), which hence becomes a flourishing research field for the past few years. OER involves four-step processes, in which adsorption and dissociation of OH* and OOH* are usually the rate-determining steps.2 An ideal catalyst should govern the reaction of these intermediates with low activation energies. To this end, various OER materials such as metal chalcogenides, nitrides, phosphide, boride, as well as metal-organic frameworks (MOF) materials have been designed for anodes.2 However, the issues arisen from poor conductivity of most MOF materials still remains unsolved. It compels researchers to focus on the design of ultrathin MOF materials rather than bulks for better contact with conductive substrates, while this kind of synthesis usually involves complex processes and is not applicable for every target MOF. On the other hand, the widely-used metal compounds were revealed to be unstable during OER and the irreversible transformation into oxides/hydroxides occur at high potentials, which are ultimately deemed as the real active sites;3 In this regard, exploitation on advanced metal oxides or hydroxides would be more forthright and effective for splitting water. In addition to this strategy, substitution of OER with a thermodynamically more favorable reaction at the same potential, for example, benzyl alcohol (BA) electro-oxidation (BAO) reaction, is also a feasible route to hydrogen production.4,5 Proceeding of this paired reaction requires an efficient catalyst too. 2 ACS Paragon Plus Environment

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Recently, many different Fe-/Co-based materials with unique structure and morphology were developed.6-9 For design of efficient catalysts, one should pay attention to the surface structure characteristic because of its close correlation with electro-catalytic performance. One of characteristic categories is defective surface structures which could tailor the surface electronic properties and gap states,10,11 thus assuring a fast charge transfer and optimal adsorption and activation energy for intermediates during electrocatalysis. The defective surface structures include atom defects, such as heteroatom doping and anion/cation vacancies, and structure variation involving dislocations, stepped surfaces, boundaries, etc.12 Modulation over structure variation as well as atom defects remains a huge challenge by now. Thus, exploitation of new approach to preparation of advanced catalysts with structure variation and coupling atom defects is highly desirable. In this pursuit, emergent properties usually occurred in the interfaces of heterostructures, such as surface compression/stretch strain,13 charge transfer,14 and generation of new interfacial moities,15 have enlighten us. We assume that the strong atom interplay in the interfaces, especially for heat-coupled heterostructures, provides a new possibility for creation of atom rearrangements, and thus probably resulting in the desired surface defective structure characteristic for efficient electrocatalysis. Herein, the interface-modulated-electrocatalysis route was first demonstrated, by assembly of ultrathin Co (oxides) nanosheets and iron (oxides) nanochains for fabrication of Fe/Co (oxides) heterostructures. The well-defined interfaces were revealed to trigger creation of both surface structure variation and atom defects under heat treatment, namely local crystallinity and defective oxygen species, respectively. It was proven that the emerging surface structure characteristic is capable of dominating the overall OER activity of the heterostructures, yielding a current density of 50 mA cm−2 at overpotential of 329 mV, with continuous 20 h operation at 10 mA cm−2 without apparent decay. Moreover, such unique structure characteristic is demonstrated to boost BAO reaction. With this merit, the voltage of water splitting at 10 mA cm−2 could be further reduced to 1.42 V in the presence of benzyl alcohol. 3 ACS Paragon Plus Environment


Density functional theory (DFT) calculation revealed that the oxygen-defective metal sites adjacent to the interfaces are the highly active sites. Interface engineering is thus demonstrated to be a feasible approach to optimizing the surface structure characteristic of heterogeneous materials towards enhanced electrocatalytic reactions.

Fig. 1. (a) High-angle annular dark-field (HAADF) spherical aberration corrected Scanning Transmission Electron Microscope (Cs-corrected STEM) image of Co/Fe200; (b) the corresponding energy dispersive spectrometer (EDS) elemental mapping images of Co/Fe200;

(c) High-resolution XPS spectra of O 1s for the

synthetic materials;(d) X-ray powder diffraction (XRD) patterns of Fe/Co200, Fe200 and Co200; (e) High-angle annular dark field (HAADF)-STEM image of the Fe/Co200 showing the boundary between Fe (oxides)and Co(oxides), as well as the 4 ACS Paragon Plus Environment

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crystallization of Co around the boundary; (f) atomic-scale EDS mapping of Fe, Co and O in the interface region. Scale bar: 5 nm. Synthesis of the target two-dimensional (2D) oxide heterostructures, consisting of two componential forms of Fe and Co, first underwent magnetism-assisted assembly (see Fig. S1, Supplementary Information) and then heat-induced interface coupling processes. Since the resultant metallic Fe and Co by sodium borohydride reduction would suffer partial surface oxidation in air, thus the two materials are assigned to Fe (oxides) and Co (oxides). Magnetism-assisted assembly of these two materials was visualized by the magnetic attraction phenomenon in Fig. S2 (Supplementary Information), and demonstrated by the ferromagnetic behavior of iron/cobalt (oxides) precursors in vibrating sample magnetometer (VSM) measurements in Fig. S3. Furthermore, since FeO and Fe2O3 are non-magnetic phase,16 it can be rationally concluded that there was α-Fe in the Fe25 sample, with the content of around 20 wt.% determined.17 The atomic ratio of Fe/Co/O in the assembled complex for annealing was determined to be 1:0.76:2.32 (Table S1) by Energy dispersive spectrum (EDS) analysis. Thus, the complex can be named after FeCo0.76O2.32. Heat-induced interface coupling of the as-prepared Fe25 with Co25 to form Fe/Co200 was witnessed by Cs-STEM and TEM imaging in Fig. 1a and Fig. S4 in Supplementary Information, with further evidence by the elemental mapping images in Fig. 1b. A large amount of nanochain-like morphology and 2D nanosheet structures, probably attributed to Fe (oxides) and Co (oxides), respectively, could be clearly observed in Fig. S5 and S6 (Supplementary Information). The pure nanosheets of Co (oxides) with a thickness of approximately 2.0 nm were confirmed by atomic force microscopy (AFM) in Fig. S7. Uniformly distributed oxygen along with Co and Fe elements is shown in Figs. 1b and S4 in Supplementary Information. Particularly, a heterogeneous outer layer on Fe (oxides) nanochain was easily to found in Fig. S6 (Supplementary Information). The coupled Fe/Co200 with pore size ranging from 2 to 40 nm gives a Brunauer–Emmett–Teller (BET) specific surface area of 37 m2 g−1 as shown in Fig. S8, comparable to many of recently reported transition metal materials, such as Ni2Fe1 5 ACS Paragon Plus Environment


nanofoams (40.8 m2 g−1),18 Co2B (11.9 m2 g−1)19 and CoO nanoplates (51.5 m2 g−1).20 All in all, such desired 2D nanosheet/nanochain architecture with local combination in interface is successfully obtained by simple assembly and annealing. The interface combination in Fe/Co200 would induce some interfacial interaction. Thus, surface-sensitive X-ray photoelectron spectra (XPS) were first applied to investigate the surface electron transfer and defect formation. Co, Fe and O species can be found in the elemental makeup of Fe/Co200 in Fig. S9 (Supplementary Information). After deconvolution as shown in Fig. S10, the components can be determined. For Fe constitution in Fe/Co200, there are mainly metallic Fe, Fe2O3 and Fe(OH)3, while metallic Co and CoO consist of Co forms. These oxides may exist outside because of surface oxidation of the materials in air. The case for Fe system was confirmed by TEM images as aforementioned. As shown in Fig. S10, negative shift of binding energy (BE) by about 0.7~1.1 eV in Co region for Fe/Co200 was observed relative to Co200. Meanwhile, the BE of Fe in Fe/Co200 also shift negatively. This phenomenon is mainly attributed to the decreased amount of oxygen in the materials, for example, from 47.8 wt.% on Fe200 and 46.8 wt.% on Co200, to 34.7 wt.% on Fe/Co200 as determined by XPS analysis. Besides, the generation of more zero-valent α-Fe, similar to the case of Fe/Co300 as revealed by Mössbauer spectroscopy in Fig. S11 and Table S2, may also cause the negative shift in BE. The negative shifts of BE in both Fe and Co suggested that high-energy theory for interpretation of enhanced OER by accelerating adsorbed –OOH species, was inapplicable in this Fe/Co200 system.15,21 Modification of electronic structure has also been studied in other systems with controlled composition, such as in Co1−xFex(OOH) and Ni–Fe composites.22,23 Their mode for interface regulation was expressed via bridging O2– via e–-e– repulsion/π-donation.24 The decreased oxygen content implies the formation of more defective oxygen, which probably modulates spin states of d-orbital in both Co and Fe towards favorable adsorption/desorption for OER intermediates, in a sense reminiscent of regulation over OER by NiCo-UMOFNs.24 In addition to oxygen content change, the interfacial strong interaction was further 6 ACS Paragon Plus Environment

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supported by the shift of O 1s peak (531.2 eV) in Fe/Co200 as seen in Fig.1c, to the position in-between Fe200 (530.7 eV) and Co200 (532.1 eV). This phenomenon is similar to the CoO/Mn3O4 system,15 evidencing the generation of possible interfacial species such as Fe-O-Co moities. Besides, it also indicated the formation of abundant defective oxygen under interface coupling based on the defective oxygen position located at 531.1 eV. Such defective oxygen may induce electron delocalization nearby, which not only enables the low-coordinated metal center more active toward the adsorption of H2O for OER, but also improves the conductivity of the catalyst because the electrons are easily excited into the conduction band, also making for OER.25 Another derived interfacial interaction is structure variation, which was examined by XRD analysis shown in Fig.1d. It was found that annealing at 200 °C still cannot render the crystallization of Co, while it enhanced crystallization of Fe in Fe200 as indicated by the Fe (110) (JCPDS 65-4899) peak at 44.7°. The negative shift of the peak towards Co (111) (JCPDS 15-0806) was observed in Fe/Co200, suggesting crystallization of Co in the material. This Co (111) crystal also matches with the selected area electron diffraction in Fig. S4. In order to further verify this, aberration-corrected STEM images of Co (oxides) moieties in the Fe/Co200 were analyzed compared to Co200, as presented in typical Figs.1e and S12 (Supplementary Information). Lots of crystalline regions in Co (oxides) sides adjacent to amorphous Fe2O3 can be observed in Fe/Co200, as marked by white dotted lines in the figures. Meanwhile, some amorphous regions were still observed. In contrast, 2D Co200 nanosheets only displayed very few crystalline regions in Fig. S13 (Supplementary Information). Moderate crystallinity, not only gives rise to higher electron conductivity, but also generally signifies more grain boundaries, edges at steps or kinks, under-coordinated sites and point/line defects. Such structure variation are usually highly reactive towards enhanced catalytic activities.26-29 Such a crystallinity has been excluded to be originated from magnetism induced by Fe (oxides) moieties, as demonstrated in Fig. S14 (Supplementary Information). In consequence, it can be reasonably concluded that Co crystallization was caused by the compact atomic-scale 7 ACS Paragon Plus Environment


contact of Fe/Co (oxides) interface under annealing (see Fig. 1f, atomic-scale EDS mapping), which also modulated interface electron transfer and defective oxygen content as aforementioned. Overall, we can get a clear picture on the structure and interface regulation of the heterostructures: Co nanosheets and Fe nanochains were prepared with dominated crystal planes of Co (111) and Fe (110), respectively. Partial oxidation in air occurred on the two materials, to form crystalline/amorphous Co and crystalline/amorphous Fe covered by their amorphous oxides, respectively. The heat-coupled Fe/Co200 heterostructures via combination of these two induces generation of abundant defective oxygen and a crystallization effect of Co in the heterostructures.

Fig. 2. (a) Linear sweep voltammetry(LSV) of Fe/Co200, Fe200 and Co200; (b) 20 h chronopotentiometry test of Fe/Co200 sample at 10 mA cm-2; (c) Tafel plot of different materials; (d) OER Faradaic efficiency of the Fe/Co200 catalyst in 1.0 M KOH at 20 mA cm-2. Interface-regulated performance was first witnessed by linear sweep voltammetry (LSV) curves in Fig. 2a, showing superior OER activity of Fe/Co200 compared to 8 ACS Paragon Plus Environment

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Fe200 and Co200. The overpotential is only 302 mV at 10 mA cm-2, and 329 mV at 50 mA cm-2, respectively, which surpasses that of Ir/C (430 mV at 50 mA cm-2), as shown in Fig. S15 (Supplementary Information). The excellent performance of Fe/Co200 is comparable to some advanced OER materials such as Li1.2Ni0.8Fe0.2O2 (302 mV at 10 mA cm−2),30 Co/CoOx/perovskite nanofibres (410 mV at 10 mA cm−2),31 and Co2B-500 (380 mV at 10 mA cm−2),19 pinpointing the great effect by interface interaction. Furthermore, the TOF of Fe/Co200 was calculated to be 0.037 s−1 (see Supplementary Information) at an overpotential of 0.35 V, also comparable to many state-of-the-art Co- or Fe-based materials, such as CoNi nanowires (0.0086 s−1, 0.3 V of overpotential, 1 M KOH) and AuFe alloys (0.09 s−1, 0.59 V of overpotential, 1 M KOH),32,33 again highlighting the strong coupling of the individual metal oxides. The 20 h chronopotentiometry test at 10 mA cm−2in Fig. 2b, with an only 25 mV decline observed for Fe/Co200, suggests such interfacial effect is also durable. To check structural and compositional stability of Fe/Co200, XRD, XPS and TEM analyses were performed after stability test. It is found that the metallic peak in the XRD pattern becomes smaller after stability test, and the pattern becomes roughening, both suggesting formation of more amorphous phase probably due to the transformation of metallic Fe/Co to oxides (see Fig. S16, Supplementary Information). XPS spectra on the Fe and Co peaks shift positively after OER stability test, also indicating the generation of high-valence metal oxides. Similar transformation was found in metal sulfide and boride.3 Besides, TEM images (see Fig. S17, Supplementary Information) revealed the slight change in nanosheet morphology, suggesting the structure stability of Fe/Co200 during stability test. Tafel plot in Fig. 2c indicates the lowest Tafel slope of 45 mV dec−1 for Fe/Co200, superior to many other OER catalysts (Table S3), not only manifesting its fast kinetic processes for OER, but also revealing that the rate-determining step for OER was switched to the electron-proton reaction of Fe/Co200, from MOH ad + OH − → MO − + H 2O

MOH ad + OH − → MO + H 2O + e

on

on Fe200 and Co200.2 Finally,

Fe/Co200 generates nearly 100% faradic efficiency for O2 production during OER, as 9 ACS Paragon Plus Environment

ACS Energy Letters

shown in Fig. 2d.

450

400

350

90 45 60 30 30 15

0

Percentage of O2 / %

Fe/Co (oxides) Fe (oxides) Co (oxides)

500

Crystallinity of Co / %

b

Overpotential / mV

a

300

300

400

600

200

Temperature / oC

300

100

50

25

1.7 1.6 1.5

Co200

75

with BA without BA

Fe200

Fe/Co200 Co200 Fe200

200

d

Current density / mA cm-2

c

25

Fe/Co200

200

Temperature / oC

Potential / mV vs. RHE

25

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 mA cm-2 1.42 V

1.48 V

0

800

1.0

Temperature / oC

1.2

1.4

1.6

Voltage / V

Fig. 3. (a) Overpotential comparison of Fe/Co (oxides), Fe (oxides) and Co (oxides) without and with annealing at current density of 50 mA cm−2; (b) the relationship between crystallinity level of Co, percentage of defective oxygen (O2) species and annealing temperature in Fe/Co (oxides). For determination on crystallinity of Co, the maximum height of peak at 44.2~44.7° was defined as 100% and the relative height of Co peak at 44.2° was calculated as the percentage. For determination on percentage of O2, deconvolution of O 1s spectrum was done as shown in Fig. S29 (Supplementary Information) and the data were summarized in Table S5. (c) O2-TPD profiles of Fe/Co200, Co200 and Fe200 samples from 25 to 900 °C at 10 °C min−1. (d) Water splitting of Fe/Co200 with and without 15 mM BA in 1 M KOH, the inset is oxidation potential comparison at 10 mA cm−2. In order to optimize the interface effects, Fe/Co25 and Fe/Co300 samples, as well as their single metal (oxides) counterparts were also prepared for comparison. As shown in Figs. 3a and S18-S21 (Supplementary Information), the Fe/Co200 catalyst still yields the best performance relative to other materials, as judged from LSV 10 ACS Paragon Plus Environment

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ACS Energy Letters

curves, Tafel plots and potential-time curves. The influences on performance in the aspects of ultrasound treatment, electrochemically active surface area (ECSA) from evaluation in 0.15 M TBA-PF6/CH3CN solution34 and resistance (Table S4) were all excluded (see Figs. S22-S24, Supplementary Information), which enable us to correct interface effects to the activity enhancement. Interface effects, arisen from annealing at different temperature, supported by unconsolidated Fe/Co25 and close-knit Fe/Co300 in Figs. S25-S27 (Supplementary Information), induce change in binding energy (Fig. S28, Supplementary Information), defective oxygen and structure variation with different levels. For determination of oxygen defect contents, deconvolution of O 1s spectra was done as shown in Fig. S29 (Supplementary Information). Four O species, namely lattice oxygen (O1), defect sites with a low oxygen coordination (O2), hydroxyl groups or surface-adsorbed oxygen (O3) and adsorbed oxygen in molecular water (O4), were found and quantized in Table S5 (Supplementary Information).25 Meanwhile, the crystallinity of Co in Fig. S30 (Supplementary Information) was also fitted.

Fig. 4. (a). The OER process on Fe/Co (oxides)model; (b) The Gibbs free energy change (∆Gi/eV) of each reaction step involved in the OER processes and the overpotential for Co (oxides), Fe (oxides), Fe/Co (oxides) and defective Fe/Co (oxides) models. Based on the aforesaid discussion, the two important interface-modulated bi-defect parameters, namely level of local crystallinity of Co and defective O species, were corrected with the activity change so as to investigate the activity-determining 11 ACS Paragon Plus Environment


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mechanism. As shown in Fig. 3b, crystallinity and O2 level increase with the elevated annealing temperature, while activity of Fe/Co (oxides) already achieved maximum at 200 °C. This phenomenon suggests both surface structure variation and defective oxygen regulated the overall OER activity of the complex. However, effects of local crystallinity (structure variation) on activity cannot be simulated, mainly due to the difficulty

in

construction

of

the

representative

amorphous

model.

Thus

interface/oxygen defect-derived variation of electronic structure, adsorption energies and Gibbs free energy in those elementary steps (equation (1)~(4)) of water oxidation reaction on Co (oxides), Fe (oxides) and Fe/Co (oxides) models were investigated by DFT calculations. It was found that the first step of water adsorption to generate adsorbed OH* tended to occur on oxygen-defective metal sites. For Fe/Co (oxides) model, the more expedient unsaturated Fe sites, stabilized by CoO substrate, were studied compared to unsaturated Co sites in Co (oxides), and Fe sites in Fe (oxides). The four-step transient state configuration was shown in Figs. 4a and S31 (Supplementary Information). The Gibbs free energy change and overpotential in the OER process were listed in Fig. 4b. For the models, the formation of O* requires the largest energy and becomes the rate-limiting step for water oxidation reaction, which agrees with the Tafel analysis. According to overpotential values, obviously, the OER is easier to occur on Fe/Co (oxides) than on Co (oxides) and Fe (oxides), due to the suitable interactions with OH*, O* and OOH*. In addition, the defective Fe/Co (oxides) with oxygen vacancy could further reduce the energy barrier of forming O* as shown in the Fig. 4b. The last step of OER is O2 desorption, which was evaluated by O2 temperature-programmed desorption (O2-TPD) test. Apparently, the oxygen desorption behavior of Fe/Co200 was integrated into the low-temperature region relative to Fe200 and Co200 as shown in Fig.3c. This interesting phenomenon also highlights enhancement in O2 desorption capability of Fe/Co200, which renders high OER activity as well as stability by expediting release of evolved O2 from surface active sites.35-37 Such interface effects can be extended to other catalytic and metal systems, which 12 ACS Paragon Plus Environment

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ACS Energy Letters

was first demonstrated by BAO which can also act as an efficient paired reaction for producing hydrogen from H2O.4 As can be seen from Fig. 3d and Fig. S32 (Supplementary Information), LSV curves of Fe200 and Co200 only show slight difference upon the addition of 15 mM BA, suggesting their limited electro-catalytic oxidation ability towards BA. However, Fe/Co200 gives apparently enhanced BAO with a peak in the CV test (Fig. S32c), which enables it to export an evidently negative shift of potential to only 1.438 V vs. RHE so as to afford 10 mA cm−2, approximately 100 mV smaller than that of OER. Fe/Co200 also provides a useable stability for BAO (Fig. S32d). Based on the negligible influence of BA and its oxidized products on the counter electrode Pt (see Fig. 33, Supplementary Information), and this outstanding BAO activity on Fe/Co200 electrode, the voltage required for water splitting at 10 mA cm−2 can be reduced from 1.48 V (overall water splitting) to 1.42 V, as demonstrated in Fig. 3d. Further, benzoic acid and benzaldehyde were detected, and the production rates for benzoic acid and benzaldehyde were calculated to be 475 µmol h-1, and 175 µmol h-1, with the Faradic efficiencies of 25.5% and 4.7%, respectively (see Fig. 33, Supplementary Information). Meanwhile, the Faradic efficiency of H2 production maintains at 99.4%. These results reveal that interface effect can not only enhance OER for overall water splitting, but also promote BAO for more advantageous water splitting for hydrogen production. On the other hand, interface effects also occur on Ni/Co metal (oxides) system. As depicted in Fig. S34 (Supplementary Information), the interfacial coupling of Ni (oxides) and Co (oxides) at 200 °C yielded a negative shift of ca. 20 mV in potential at the current densities of 10 and 50 mA cm−2 for OER, relative to individual Co (oxides). This potential gap even got bigger at 50 mA cm−2 compared to Ni200. Given the general applicability of this interface-regulated-activity route, it is therefore attractive to develop other interface coupling systems for various kinds of electrochemical and catalytic applications.

In summary, engineering the interface of 2D cobalt/iron (oxides) by annealing has been demonstrated to be an efficient route for modulation over 13 ACS Paragon Plus Environment


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defective surface structures. The advanced 2D Fe/Co200 heterostructures were shown to be an excellent electrocatalyst, with exporting 50 mA cm−2 at an overpotential of only 329 mV for OER, and decreasing oxidation potential to 1.438 Vvs. RHE at 10 mA cm−2 for BAO. The interface-modulated structure and defective oxygen in the heterostructures boost the reaction of intermediates, especially for OH* dissociation, in a low energy barrier. It is thus anticipated that

such effective

interface-regulated defective

surface

structures in

heterostructures could be further expanded to various electrochemical and catalytic applications. Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Schematic synthesis, Magnetism test, AFM, TEM, SEM, BET, XPS, Mössbauer absorption spectra and XRD analysis; Other electrochemical data and DFT models. Author information Corresponding Author *E-mail: [email protected]; [email protected] ORCID Yaobing Wang: 0000-0001-6354-058X Notes The authors declare no competing financial interest. Acknowledgements

This work was supported by the One Hundred Talents Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Nos. 21601190 and 21501173), the Natural Science Foundation of Fujian Province 14 ACS Paragon Plus Environment

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(No. 2018J05030), the Science and Technology Planning Project of Fujian Province (No. 2014H2008), the Strategic Priority Research Program, CAS (No.XDB20000000) and National Key R&D Program of China (No. 2016YFB0100100). Author contributions The contribution of authors to this work: Yiyin Huang is in charge of the whole route design, experiments and preparation of this manuscript; Qiaohong Li is in charge of DFT calculation; Yaobing Wang revises the paper; Others help with the physical characterization.

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Iron(Oxides) Heterostructures for Efficient Oxygen Evolution

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