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Operando Insight into Oxygen Evolution Kinetics on MetalFree Carbon-Based Electrocatalyst in an Acidic Solution Xu Zhao, Hui Su, Weiren Cheng, Hui Zhang, Wei Che, Fumin Tang, and Qinghua Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09315 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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ACS Applied Materials & Interfaces

Operando Insight into Oxygen Evolution Kinetics on Metal-Free Carbon-Based Electrocatalyst in an Acidic Solution Xu Zhao, Hui Su, Weiren Cheng*, Hui Zhang, Wei Che, Fumin Tang, and Qinghua Liu* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China

*E-mail: [email protected]; [email protected]

Abstract Operando insight into the catalytic kinetics under working conditions is important for further rationalizing the design of advanced catalysts towards efficient renewable energy applications. Here, we enable the ubiquitous carbon material as an efficient acidic oxygen evolution reaction (OER) electrocatalyst, synthesized via a facile and controllable “amino-assisted polymerization and carbonization” strategy. This as-developed

metal-free

amino-rich

hierarchical-network

carbon

framework

(amino-HNC) directly supported on carbon paper can catalyze OER at a quite low overpotential of 281 mV and a small Tafel slope of 96 mV dec-1 in an acid solution, and maintain ~98% of its initial catalytic activity after 100 h oxygen evolution operation. By using the operando synchrotron infrared spectroscopy, a crucial structurally-evolved H2N-(*O-C)-C, formed by adsorbing the *O intermediate on the active H2N-C=C moiety, is observed on amino-HNC electrocatalysts during OER process in the acid medium. Furthermore, theoretical calculations reveal that the optimization of the sp2 electronic structure of C=C by amino radicals could effectively lower the kinetic formation barrier of *O intermediate on H2N-C=C moiety, contributing to a prominent acidic oxygen-involved catalysis.

Keywords: Metal-free electrocatalyst; Amino-rich carbon framework; Operando technique; Fourier transform infrared spectroscopy; Oxygen evolution reaction

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Introduction Oxidation of water into molecular oxygen driven by electric power is critically central at the heart of renewable and sustainable energy carrier systems, such as water electrolyzers and carbon-dioxide reduction cells, in modern society.1-4However, the sluggish multi-electron kinetics of the electrochemical oxygen evolution reaction (OER) in aqueous solution urgently requires a cost-effective and highly-active electrocatalyst to substantially promote the limited energy conversion efficiency of these advanced energy carrier systems, especially under an acidic condition in which relatively more efficient devices of water electrolyzers and carbon-dioxide reduction cells have been theoretically predicted.3,5,10-13 In addition, proton exchange membrane (PEM), as a kind of material that can work under high current density, 6,7 has a small size and high efficiency, and generates hydrogen with a purity of up to 99.999% and a sustainable hydrogen production line, is considered to be the most promising hydro-electrolysis technology.8,9 However, the working environment of PEM needs to be better reflected in strong acid medium. At present, the available electrocatalysts in an acidic medium are mainly the high-cost noble-metal oxides, such as RuOx and IrOx, all of which yet show large overpotential (~500 mV at 10 mA cm-2) and poor long-term durability.14,15 Fortunately, the metal-free carbon-based functional materials with strong earth-abundance and acid-resistance capability have been regarded as potential alternatives of OER electrocatalysts relative to the expensive noble-metal based catalysts under acidic conditions.16,17 Nonetheless, further promotion of the activity and durability of the active sites for the metal-free carbon-based catalysts is still highly desired to boost the energy conversion efficiency towards scalable application of the current sustainable energy devices.18,19 In general, the development of efficient metal-free catalysts heavily relies on the comprehensive understanding and precise control of the real active sites at atomic and molecular levels.20,21 Up to date, via atomic-structure engineering and surface functionalization, several innovative carbon-based nanomaterials have been made as prominent acidic OER electrocatalysts. By atomic tailoring of sp2 coupling, some non-metal, such as N, B, P etc., doped porous carbon frameworks have been revealed to show significantly improved OER activity in alkaline solution, comparable to the benchmarking noble oxides of RuOx and IrOx catalysts.22-24 Moreover, through 2


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molecular-level surface decoration, oxo-group modified carbon frameworks have been demonstrated to effectively accelerate the oxygen reduction kinetics and prolong the catalytic durability in alkaline medium.25,26 Despite these advances, it will lead to the reconstruction of the initial active sites under practical working conditions due to the electrochemical sensitivity of the active sites, and it is still facing great challenges to further improve the catalytic activity and operation durability of the metal-free carbon-based electrocatalysts.27 Furthermore, owing to the structural complexity and probing difficulty for light elements, the evolution of active centers and crucial catalytic intermediates during OER process are very hard to detect, significantly hindering the atomic-level understanding and design of active metal-free carbon-based catalysts.28,29 Fortunately, with the fingerprint identification of polarized surface radical-group and high sensitivity of molecular conformational changes, the Fourier-transform infrared spectroscopy (FTIR) technique, especially the operando synchrotron radiation FTIR (SR-FTIR), could offer a suitable means to capture the surface dynamic structure evolution of light-element active sites under realistic reaction conditions. Hence, it is highly anticipated to reveal the nature of the active sites and the intermediate evolution kinetics on carbon electrocatalysts under working conditions by using this advanced technique.30-32 Here, we have turned the ubiquitous carbon material into an abundant and low-cost amino-rich hierarchical-network carbon framework (amino-HNC) directly supported on the carbon paper (CP) to making an active acidic OER electrocatalyst, synthesized via a facile and controllable “amino-assisted polymerization and carbonization” strategy. When employed in water oxidation in acid medium, this amino-rich carbocatalyst shows prominent OER performance and strong operation stability, the best metal-free electrocatalysts reported so far and much superior to commercial RuO2 catalysts.19,33. Using our developed operando synchrotron infrared spectroscopy in conjunction with theoretical simulations, a practical active structure of H2N-C=C moiety by binding the in-plane six-member carbon-ring and out-plane amino radical was identified, and a crucial *O intermediate linking to the N-activated carbon to form the H2N-(*O-C)-C active structure was observed during the OER process in acid solution, manifesting the realization of a fast and efficient evolution kinetics of 4eoxygen evolution process. The analyses of electronic structure indicate that the 3



electronic affinity competition within the N-C bonds could optimize the sp2 electronic structure of C=C in the H2N-C=C group and then enable this N-activated C=C in the amino-HNC catalyst with high oxygen-involved electrocatalytic activity and strong long-term durability in the acid medium. Experimental Section Synthesis of amino-HNC catalyst. The metal-free amino-rich hierarchical-network carbon (amino-HNC) electrocatalyst was synthesized directly on the carbon paper (CP), undergoing two crucial steps of underpotential electrodeposition and high-temperature carbonization. Firstly, the Polyaniline (PAni) nanofibers were prepared by electrodeposition method using a three-electrode system in the aniline electrolyte. Typically, the 2×4 cm2 CP was heated to 500 °C for 2 h in air using the muffle furnace and then the concentrated nitric acid was used to wash the CP under ultrasound for 30 minutes at room temperature. After that, the acid-treated CP was used as the working electrode. The saturated calomel electrode (SCE) was used as the reference electrode in a three-electrode electrochemical system with a mixture of 7 ml concentrated nitric acid, 88 ml deionized water and 5 ml aniline as the electrolyte. The electrochemical deposition of polyaniline (PAni) on the CP was conducted at a constant voltage of 0.7 V (SCE) for 600 s. Then, this deposited CP was washed with DI water three times and then dried at 80 °C for 2 h to obtained PAni deposited CP sample (PAni-CP). Finally, this PAni-CP sample was heating at 900 °C under inert N2 atmosphere for 3 h to gain the amino-modified hierarchical-network carbon framework (amino-HNC). Morphology and Structure Characterizations. A Gemini scanning electron microscopy (SEM) 500 microscope at a voltage of 5 kV was used to obtain the SEM images of the samples and the SEM mapping was performed at a voltage of 8 kV. A Philips X’Pert Pro Super X-ray diffractometer with Cu Kα radiation was used to measure the X-ray diffraction (XRD) patterns of the samples. An ESCALAB 250 photoelectron spectrometer with Al K Alpha radiation was used to acquire the X-ray photoelectron spectra (XPS) of the samples. The BL12B-a beamline of the National Synchrotron Radiation Laboratory (NSRL, China) was used to measure the C and N K-edge X-ray absorption near-edge spectra (XANES).

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Operando SR-FTIR characterizations. Operando synchrotron FTIR measurements were operated at the BL01B beamline of NSRL, China through a homemade reaction IR setup. An FTIR spectrometer (Bruker 66v/s) and a liquid nitrogen cooled MCT detector were used to collect the FTIR spectra. The resolution of the FTIR spectra is 0.25 cm-1 and the IR measurement range is in 15 to 4000 cm-1. All the FTIR spectra were collected in the potential range of 1.25–1.50 V with an interval of 0.05 V by averaging 514 scans at a resolution of 2 cm−1. Results and Discussion The successful synthesis of metal-free amino-rich hierarchical-network carbon framework (amino-HNC) catalysts was demonstrated by the scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) measurements.34,35 Figure 1a shows that hierarchical carbon frameworks with interlaced branch structure are well distributed over the carbon fiber of carbon paper support. The X-ray diffractions (XRD) pattern of amino-HNC in Figure S1 displays two main peaks located at 26.5° and 54.6°, which can be well indexed to the (002) and (004) plane of graphite, respectively.36 The elemental distribution was analyzed by scanning electron microscopy-coupled quantified energy dispersive X-ray spectroscopy (SEM-EDS) mappings. As the SEM-EDS mapping images of a single carbon fiber shown in Figure 1b, the N element was

homogeneously

distributed

thorough

the

amino-HNC

catalysts

after

high-temperature carbonization of polyaniline deposited carbon paper (PAni-CP). Moreover, the N 1s spectrum of amino-HNC displays three fitted peaks located at 400.9, 399.6 and 398.4 eV, which can be assigned to the graphitic N, H2N-C and pyridinic N, respectively.37,38 This result evidently indicates the linkage of rich NH2 functional group to the edged carbon in the amino-HNC, consistent well with the C 1s spectra in Figure S2a. Furthermore, the three main peaks of 285.5, 288.7 and 292.8 eV in the C K-edge XANES spectra of amino-HNC presented in Figure 1d could be indexed to the 1s→hybrid 2p excitation of *, H2N-C and *, further confirmed the presence of H2N-C=C moieties within the amino-HNC catalyst.39,40 Moreover, the excitation energy of C=C bonds on the amino-HNC is clearly moved to high energy side by 0.2 eV compared with pure CP, suggesting the activated H2N-C=C moiety as 5



the possible active sites. And this picture is further proven by the result of N K-edge XANES spectra of the amino-HNC (Figure S3).41 These above analyses undoubtedly demonstrate that hierarchical carbon framework with rich surface amino-group is successfully deposited on the carbon paper for amino-HNC catalysts. In comparison, the SEM images of pure CP and the full XPS spectrum of pure CP and amino-HNC before OER have been supplemented (Figure S18 and Figure S19). And the ratio of both the amino-HNC before OER and the pure CP is shown in Table S2.

(b)

(a)

10 μm

200 nm

CN C

N

2 μm

(d)

(c) Graphite N 400.9 eV

N 1s

*

Pyridine N 398.4 eV

402

400

398

396





amino-HNC

Intensity (a.u.)

399.6 eV

404

C-NH2

0.2 eV

H2N-C

406

C K-edge 

amino-HNC

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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394

CP

285

Binding energy (eV)

290

295

300

Energy (eV)

Figure 1. (a) SEM image of amino-HNC. The inset shows the enlarged SEM image. (b) SEM-EDS mapping images for amino-HNC. (c) N 1s XPS spectra for amino-HNC. (d) C K-edge XANES spectra of amino-HNC and CP.

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(b) 1.8

75

CP amino-HNC IrO2

Potential (V vs. RHE)

-2

Current density (mA cm )

(a) 100

50

25 H2O/O2 0 1.0

1.2

=281 mV

1.4

=527 mV

1.6

1.8

2.0

1.7

amino-HNC CP IrO2

(c)

-1

79 mV dec

1.5 1.4

-1

96 mV dec

1.3

-0.4 -0.2

2.2

(d) Faradaic efficiency (%)

G=28.25 kJ/mol -2

-0.8 G=57.48 kJ/mol

-1.2 -1.6 3.2

3.3

3.4

-1

0.2

0.4

0.6

0.8

1.0

-2

-0.4

3.1

0.0

Log [JK(mA cm )] amino-HNC CP

0.0

-1

201 mV dec

1.6


Ln [J0 (mA cm )]

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


3.5

O2 100 75 50 25 0

3.6

1.50

1000/T (K )

1.65

1.80


Figure 2. (a) OER polarization curves and (b) Tafel slopes of CP, and amino-HNC electrodes. (c) The Arrhenius plots of amino-HNC and CP, and (d) Faradaic efficiencies of amino-HNC for O2 production in different potential.

To access the intrinsic catalytic activity, the oxygen-involved catalytic performance of amino-HNC was tested in 0.5M H2SO4 solution and compared to pure CP counterpart. As the OER linear sweep voltammogram (LSV) curves shown in Figure 2a, the amino-HNC catalysts could deliver an unexpectedly high acidic OER activity with an outstanding overpotential of 281 mV at a current density of 10 mA cm-2, and is ~240 mV and ~220 mV lower than that of pure CP (527 mV) and IrO2 (502 mV). In comparison, the current of the LSV curves in Figure 2a were normalized by the BET surface area (Figure S20a) to exclude the contribution of the increased surface area to the significantly enhanced electrocatalytic activity of amino HNC for OER. Figure S20(b) shows the BET normalizing LSV curves. After the BET normalization, the OER activity of amino-HNC is also superior to IrO2. By comparing the SEM-EDS mapping images for amino-HNC before and after reaction (Figure S11), it can be concluded that the efficient acid OER performance originates from the intrinsic activity of amino-HNC rather than any impurities introduced during operation. 7



According to literature report, this is the most efficient metal-free electrocatalysts for acidic OER performance so far, which is also much superior to the state-of-the-art noble-metal electrocatalysts working in the acidic medium, and even the metal-free carbon-based catalysts in alkaline solution (Table S1). Furthermore, the high catalytic activity of amino-HNC can be reflected by its Tafel slopes. The amino-HNC shows a smaller Tafel slope of 96 mV dec-1 relative to CP (201 mV dec-1) and close to IrO2 (79 mV dec-1) (see Figure 2b), demonstrating much more favorable OER reaction kinetics for amino-HNC. To further explore the thermodynamic OER activation energy, the temperature-dependent LSV measurements were performed for amino-HNC and pure CP (Figure S4a, b), and the Tafel curves (Figure S5a, b) are derived from LSV curves to calculate the exchange current density j0 at different temperatures. Based on the Arrhenius plots shown in Figure 2c, the activation energy for amino-HNC is down to 28.25 KJ mol-1, which is only half of that for CP (~57.48 KJ mol-1) suggesting a significant promotion in OER kinetics after amino-group integration.42,43 Accordingly, the amino-HNC offers an excellent four-electron (4e-) OER with Faradaic efficiencies of 96~98% under various potentials in acidic solution (Figure 2d, S8), indicating an efficient OER activity and high stability for amino-HNC (Figure S9). Bedsides, the electrochemical double-layer capacitance (Cdl) measured from the scan-rate dependent CVs for the amino-HNC catalyst is 0.063 mF (Figure S6) and the ECSA is calculated to 1.8 cm-2, and the normalized current density (Figure S7) confirms an outstanding intrinsic activity for amino-HNC catalyst. Moreover, the almost unchanged morphology, atomic and electronic structure after long-term OER operation further confirm a robust OER durability in acidic medium for amino-HNC electrocatalysts (Figure S10-13). The above results clearly confirm that an efficient 4e- OER performance as well as outstanding long-term durability under acidic environment is realized in the amino-HNC catalyst after amino-group modification. In order to further understand the efficient catalytic activity, the operando synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy with a homemade top-plate cell was performed on the amino-HNC electrocatalyst under realistic acidic OER working conditions.44 For the amino-HNC catalyst under open-circuit (O.C.) potential as shown in Figure 3a, there are several obvious absorption bands located at ~1267 cm-1, ~1130 cm-1 and over the range of 850–1100 cm-1, which can be attributed to the stretching mode of C-N, C-C and bending 8


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vibration mode of =C-H on amino-decorated in-plane sp2 carbon-ring structure of amino-HNC, respectively.45-47 Notably, when applied with a high potential of 1.35 V that is slightly larger than the OER onset potential, a new vibrational absorption band of 1546 cm-1 was evidently obtained on the amino-HNC, which can be well assigned to the vibration stretching modes of *O-C species. In addition, after the application of -1.35 V voltage for a long period of 3000 s, the absorption peaks at 1546 cm-1 and 1255 cm-1 remained unchanged. This demonstrates the stability of the C-O* and C-N bonds in acid OER tests (Figure S15). The before-and-after comparison of sample morphology and performance test was conducted about 7.5 h in 0.5M H2SO4. And the XPS full spectrum and the SEM-EDS mapping image for the amino-HNC and pure carbon after OER (Figure S21, Figure S22 and Figure S23) indicate that both the C in CP and the C, N in amino-HNC are stable in morphological and distribution. Meanwhile, there is no obvious new absorption band in the range of 1500-1600 cm-1 for pure CP under potential even larger than 1.35 V (Figure S14). Interestingly, this new vibrational band of 1546 cm-1 completely disappears when the applied bias on the electrode returns to open-circuit potential, evidently inferring the emergence of the crucial H2N-(*O-C)-C evolution structure by adsorbed *O intermediates onto the H2N-C=C active sites during the acidic OER catalytic process. (a)

(b)

0.5 M H2SO4-O.C.

High *

0.5 M H2SO4-O.C. (A.R.)

C-O (a. u.) Absorption

Absorption (a. u.)

0.5 M H2SO4-1.35 V vs. RHE *

C-O

C-N

=C-H

1.5 0 1.4 0 1.3 5

1.3 0 1.2 V 5V O. C.

1546

Low V

V

V

1000

Wave numb er (cm -1 )

800

Wavenumber (cm )

(c) -1

20

Wavenum ber (cm -1 )

9


1350

1.5

1300

1.4


0V 1.5 V 0 4 1. 5V 1.3 0V 1.3 5V 1.2 C. O.

1267

1250

0

Low

1200

10

High

1248

1150

-1

1546 cm -1 1267 cm

1.3

Wavenumber excursion (cm )

(d)

-0.2 0.0

1400

-1

1450

1200

150 0

1400

155 0

1600

) ion (a. u. Absorpt

1800

160 0

16 50

C-C

Absorption difference (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



Figure 3. (a) Operando IR measurement results, (b) IR signal in 1400–1650 cm-1 and (c) IR signal in 1175-1350 cm-1 for amino-HNC during the OER processes. (d) The evolution of 1546 cm-1 absorption band and the red shift of 1267 cm-1 absorption band for amino-HNC.

More importantly, the vibrational band intensity at 1546 cm-1 for amino-HNC shows potential-dependent relationship, and would increase with the increasing potentials of > 1.30 V as shown in Figure 3b, corresponding to the cumulative *O species under higher potentials. It is noted that, for the amino-HNC during OER process, a potential-dependent red-shift of vibrational band for H2N-C strength modes from ~1267 to ~1248 cm-1 is simultaneously found (Figure 3c), inferring a charge-migration from C to H2N driven conformational variation of H2N-C modes when the applied bias increasing from 1.25 to 1.50 V.47 Further, Figure S16 shows both the C-O* and C-N bonds changed and moved towards the direction of low wave number. Meanwhile, there are also some shifts in other peaks, such as 1100 cm-1, which is caused by the redistribution of voltage driving charge after the application of voltage but had no evident impact on the reaction. Thus, these results indicate that it is a four-electron transfer process caused by H2N-(*O-C)- C and H2N-C=C is the active site. To clearly clarify the relation between the active structure evolution and the appearance of *O intermediates during OER catalytic process, the potential-dependent variation of vibrational bands at ~1546 and 1267 cm-1 is plotted in Figure 3d. It can be observed that the evident electron transfer effect over H2N-C=C units is obviously observed before the appearance of adsorbed *O on active sites at low potential of 1.25 V. When the applied bias becomes larger (>1.30 V), the electron transfer effect and *O accumulation on amino-HNC both rapidly enhanced in proportion to the increased applied potentials, suggesting a close relationship between the electron transfer and *O intermediates formation on the H2N-(C=C) active site. These results essentially reveal that the electron extraction from C to NH2 group promotes the generation of the key *O intermediates on H2N-(C=C) active sites to form H2N-(*O-C)-C, contributing to a highly-efficient 4e- oxygen-involved electrocatalysis.

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(a)

CP

(b)

Total

*

20

O

1.2

10 0 30

1.6 H2O

Total N 2p

Ef

amino-HNC

Energy (eV)

DOS (a.u.)

30

DOS (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


20

0.8

0.4

10 0 -4

-3

-2

-1

0

1

Energy (eV)

2

3

0.0

4

CP

(c) I

II H+,

H+, e-

amino

ΔG1=1.47 eV

H

N

C

O

e-

ΔG2=1.28 eV

OER O2 H2O

amino-HNC

ΔG3=1.11 eV H+, e-

H2O III

ΔG4=1.06 eV H+, eIV

Figure 4. (a) Calculated band structure plots of amino-HNC and CP, (b) Calculated adsorption energy of H2O and formation energy of *O intermediates for amino-HNC and CP, and (c) the 4e- OER mechanism for amino-HNC in acidic medium.

To get a further understanding of the mechanism of OER catalytic for amino HNC, the first-principles calculations were performed. As shown in Figure 4a, the electronic structure calculations show that the electron density of amino-HNC around the Fermi level is significantly increased relative to that of pure CP, which is essentially attributed to the sp2 electronic structure optimization of H2N-C=C moiety by amino-group, and would facilitate the adsorption of surface oxo-species as well as intermediate formation during the oxygen-involved catalytic process. Indeed, the energy calculation results in Figure 4b reveal that the adsorption energy of water molecule on amino-HNC has been enhanced by 0.55 eV relative to pure CP. Moreover, the formation energy of key intermediate *O on amino-HNC, corresponding to the OER rate-determining step (RDS) in acidic medium, is significantly reduced to ~0.37 eV, much lower than that of CP (~1.41 eV), indicating the thermodynamic formation of the key *O intermediate on active H2N-C=C moiety to facilitate the H2N-(*O-C)-C 11



structure evolution. Moreover, the kinetics of O-O coupling for amino-HNC will be greatly promoted to form *OOH intermediates on active sites, and then oxygen molecule is smoothly released companied by a proton-coupled electron transfer process in acidic medium. Accordingly, as indicated in Figure 4c, the free energy change of H2O deprotonation on the amino-activated C sites of H2N-(H2O-C)-C has been evidently reduced with a moderate ΔG1 of 1.47 eV (Step 1). Subsequently, the formation of *O intermediates on the active sites, forming H2N-(*O-C)-C structure, undergoes a lowered ΔG2 of 1.28 eV (Step 2) relative to pure CP (1.48 eV), showing that the overcoming of OER RDS for amino-HNC. And then, the molecular oxygen is easily produced following by the thermodynamically favorable generation of *OOH (Step 3; ΔG3 of 1.11 eV) and *OO (Step 4; ΔG4 of 1.06 eV) intermediates on the active H2N-(C=C) sites. It is of note that the potential barrier of the RDS for the H2N-(C=C) is only 0.29 V, significantly smaller than that of pyridine-N (0.41 V) and graphite-C (0.51 V) (Figure S17). This result theoretically strengthens that the H2N-(C=C) serves as most of the active sites in amino-HNC. These proposed OER catalytic mechanism of amino-HNC seems quite similar to homogeneous catalysts reported in previous studies and consistent well with the operando SR-FTIR results, corresponding to an efficient 4e- OER process in acidic medium.13,44,48 Conclusions In summary, we developed a metal-free amino-rich hierarchical-network based on the ubiquitous carbon (amino-HNC) as an efficient acidic OER electrocatalysts. This amino-HNC electrocatalyst shows prominent OER performance in acid solution with a small OER overpotential and strong operation stability, the best metal-free electrocatalysts reported so far and much superior to commercial RuO2 catalysts. By using the operando SR-FTIR technique, an evolved structure of H2N-(*O-C)-C, formed by adsorbed *O intermediate on the active H2N-C=C moiety, is observed for the first time on the metal-free amino-HNC electrocatalyst during the realistic OER working condition in acid solution. Moreover, the theoretical calculation results unravel that the surface amino-group could effectively activate sp2 electronic structure of C=C in H2N-C=C moiety, promoting the adsorption of surface oxo-species and formation of key *O intermediates towards a fast and efficient 4e- OER kinetics in acidic medium. These results not only promise the stable and economic metal-free 12


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amino-rich carbon-based catalyst as acidic-available OER electrocatalyst, but also provide meaningful insight into the active sites evolution and oxygen-involved catalytic mechanism to rationalize the design of advanced electrocatalyst towards globally sustainable energy storage and conversion.

Acknowledgements This work was supported by the National Key Research and Development Programme of China (2017YFA0402800), the National Natural Science Foundation of China (11875257, 21603207, U1532265, 11621063, and 21533007), and the Fundamental Research Funds for the Central Universities (WK2310000070). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXXXX. Additional data and methods; Figures S1–S23, Table S1-S2; supporting references (PDF)

References (1) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. (2) Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward Sustainable Fuel Cells. Science 2016, 354, 1378-1379. (3) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L. Homogeneously Dispersed Multimetal Oxygen-Evolving Catalysts. Science 2016, 352, 333-337. (4) Liu, J. K.; Cheng, W. R.; Tang, F. M.; Su, H.; Huang, Y. Y.; Hu, F. C.; Zhao, X.; Jiang, Y.; Liu, Q. H.; Wei, S. Q. Electron Delocalization Boosting Highly Efficient Electrocatalytic Water Oxidation in Layered Hydrotalcites. J. Phys. Chem. C 2017, 121, 21962-21968. (5) Cheng, W. R.; Zhang, H.; Zhao, X.; Su, H.; Tang, F. M.; Tian, J.; Liu, Q. H. A Metal-vacancy-solid-Solution NiAlP Nanowall Array Bifunctional Electrocatalyst

13



Page 14 of 19

for Exceptionalall-pH Overall Water Splitting. J. Mater. Chem. A 2018, 6, 9420-9427. (6) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrog. Energy 2013, 38, 4901-4934. (7) Thosar, A. U.; Agarwal, H.; Govarthan, S.; Lele, A. K. Comprehensive Analytical Model for Polarization Curve of A PEM Fuel Cell and Experimental Validation. Chem. Eng. Sci. 2019, 206, 96-117. (8) Szendrei, A.; Sparks, T. D.; Virkar, A. Three and Four-Electrode Electrochemical Impedance Spectroscopy Studies Using Embedded Composite Thin Film Pseudo-Reference Electrodes in Proton Exchange Membrane Fuel Cells. J. Electrochem. Soc. 2019, 166(12), 784-795. (9) Vivona, D.; Casalegno, A.; Baricci, A. Validation of A Pseudo 2D Analytical Model for High Temperature PEM Fuel Cell Impedance Valid at Typical Operative Conditions. Electrochim. Acta 2019, 310, 122-135. (10)Liang, Y.; Wang, H.; Diao, P.; Chang, W.; Hong, G.; Li, Y.; Gong, M.; Xie, L.; Zhou, J.; Wang, J. Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 15849-15857. (11)Lin, Y. X.; Feng, W. J.; Zhang, J. J.; Xue, Z. H.; Zhao, T. J.; Su, H.; Hirano, S. I.; Li, X. H.; Chen, J. S. A Polyimide Nanolayer as a Metal‐Free and Durable Organic Electrode toward Highly Efficient Oxygen Evolution. Angew. Chem. 2018, 130, 12743-12746. (12)Xue, Z.; Li, Y.; Zhang, Y.; Geng, W.; Jia, B.; Tang, J.; Bao, S.; Wang, H. P.; Fan, Y.; Wei, Z. W. Modulating Electronic Structure of Metal‐Organic Framework for Efficient Electrocatalytic Oxygen Evolution. Adv. Energy Mater. 2018, 8, 1801564. (13)Xiao, H.; Shin, H.; Goddard, W. A. Synergy between Fe and Ni in the optimal performance of (Ni, Fe) OOH catalysts for the oxygen evolution reaction. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 5872-5877. (14)Frydendal, R.; Paoli, E. A.; Knudsen, B. P.; Wickman, B.; Malacrida, P.; Stephens, I. E.; Chorkendorff, I. Benchmarking the Stability of Oxygen Evolution Reaction

Catalysts:

The

Importance

of

Monitoring

ChemElectroChem 2014, 1, 2075-2081. 14


Mass

Losses.

Page 15 of 19 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


(15)Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. (16)Mirzakulova, E.; Khatmullin, R.; Walpita, J.; Corrigan, T.; Vargas-Barbosa, N. M.; Vyas, S.; Oottikkal, S.; Manzer, S. F.; Hadad, C. M.; Glusac, K. D. Electrode-Assisted Catalytic Water Oxidation by a Flavin Derivative. Nat. Chem. 2012, 4, 794-801. (17)Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J.-P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A. Oxygen and Hydrogen Evolution Reactions on Ru, RuO2, Ir, and IrO2 Thin Film Electrodes in Acidic and Alkaline Electrolytes: A Comparative Study on Activity and Stability. Catal. Today 2016, 262, 170-180. (18)Lai, J.; Li, S.; Wu, F.; Saqib, M.; Luque, R.; Xu, G. Unprecedented Metal-Free 3D Porous Carbonaceous Electrodes for Full Water Splitting. Energy Environ. Sci. 2016, 9, 1210-1214. (19)Zheng, Y.; Jiao, Y.; Qiao, S. Z. Engineering of Carbon‐Based Electrocatalysts for Emerging Energy Conversion: From Fundamentality to Functionality. Adv. Mater. 2015, 27, 5372-5378. (20)Li, J.; Wang, Y.; Zhou, T.; Zhang, H.; Sun, X.; Tang, J.; Zhang, L.; Al-Enizi, A. M.; Yang, Z.; Zheng, G. Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14305-14312. (21)Liu, J.; Wang, H.; Antonietti, M. Graphitic Carbon Nitride “Reloaded”: Emerging Applications beyond (Photo) Catalysis. Chem. Soc. Rev. 2016, 45, 2308-2326. (22)Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation. Nat. Commun. 2013, 4, 2390. (23)Cheng, Y.; Tian, Y.; Fan, X.; Liu, J.; Yan, C. Boron Doped Multi-Walled Carbon Nanotubes as Catalysts for Oxygen Reduction Reaction and Oxygen Evolution Reaction in Alkaline Media. Electrochim. Acta 2014, 143, 291-296. (24)Liu, Z. W.; Peng, F.; Wang, H. J.; Yu, H.; Zheng, W. X.; Yang, J. Phosphorus‐Doped Graphite Layers with High Electrocatalytic Activity for the O2 Reduction in an Alkaline Medium. Angew. Chem. Int. Ed. 2011, 50, 3257-3261. 15



(25)Lu, X.; Yim, W.-L.; Suryanto, B. H.; Zhao, C. Electrocatalytic Oxygen Evolution at Surface-Oxidized Multiwall Carbon Nanotubes. J. Am. Chem. Soc. 2015, 137, 2901-2907. (26)Cheng, N.; Liu, Q.; Tian, J.; Xue, Y.; Asiri, A. M.; Jiang, H.; He, Y.; Sun, X. Acidically Oxidized Carbon Cloth: A Novel Metal-Free Oxygen Evolution Electrode with High Catalytic Activity. Chem. Commun. 2015, 51, 1616-1619. (27)Hu, C.; Dai, L. Carbon‐Based Metal‐Free Catalysts for Electrocatalysis beyond the ORR. Angew. Chem. Int. Ed. 2016, 55, 11736-11758. (28)Cheng, Y.; Xu, C.; Jia, L.; Gale, J. D.; Zhang, L.; Liu, C.; Shen, P. K.; Jiang, S. P. Pristine Carbon Nanotubes as Non-Metal Electrocatalysts for Oxygen Evolution Reaction of Water Splitting. Appl. Catal. B-Environ. 2015, 163, 96-104. (29)Lu, X.-F.; Liao, P.-Q.; Wang, J.-W.; Wu, J.-X.; Chen, X.-W.; He, C.-T.; Zhang, J.-P.; Li, G.-R.; Chen, X.-M. An Alkaline-Stable, Metal Hydroxide Mimicking Metal–Organic Framework for Efficient Electrocatalytic Oxygen Evolution. J. Am. Chem. Soc. 2016, 138, 8336-8339. (30)Hung, S.-F.; Chan, Y.-T.; Chang, C.-C.; Tsai, M.-K.; Liao, Y.-F.; Hiraoka, N.; Hsu, C.-S.; Chen, H. M. Identification of Stabilizing High-Valent Active Sites by Operando High-Energy Resolution Fluorescence-Detected X-ray Absorption Spectroscopy for High Efficient Water Oxidation. J. Am. Chem. Soc. 2018, 140, 17263-17270 (31)Su, X.; Wang, Y.; Zhou, J.; Gu, S.; Li, J.; Zhang, S. Operando Spectroscopic Identification of Active Sites in NiFe Prussian Blue Analogues as Electrocatalysts: Activation of Oxygen Atoms for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2018, 140, 11286-11292. (32)Zheng, X.; Zhang, B.; De Luna, P.; Liang, Y.; Comin, R.; Voznyy, O.; Han, L.; de Arquer, F. P. G.; Liu, M.; Dinh, C. T. Theory-Driven Design of High-Valence Metal Sites for Water Oxidation Confirmed Using In Situ Soft X-ray Absorption. Nat. Chem. 2018, 10, 149-154. (33)Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444. (34)Sun, Z. H.; Yang, X. Y.; Wang, C.; Yao, T.; Cai, L.; Yan, W. S.; Jiang, Y.; Hu, F. 16


Page 16 of 19

Page 17 of 19 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


C.; He, J. F.; Pan, Z. Y.; Liu, Q. H.; Wei, S. Q. Graphene Activating Room-Temperature Ferromagnetic Exchange in Cobalt-Doped ZnO Dilute Magnetic Semiconductor Quantum Dots. ACS Nano 2014, 8, 10589-10596. (35)Peng, Y. H.; He, J. F.; Liu, Q. H.; Sun, Z. H.; Yan, W. S.; Pan, Z. Y.; Wu, Y. F.; Liang, S. Z.; Cheng, W. R.; Wei, S. Q. Impurity Concentration Dependence of Optical Absorption for Phosphorus-Doped Anatase TiO2. J. Phys. Chem. C 2011, 115, 8184-8188. (36)Yanyan, N.; Xiaogang, S.; Manyuan, C.; Xiaoyong, W.; Zhenhong, L.; Lifu, Y. Graphitized Whisker-Like Carbon Nanotubes as Electrodes for Supercapacitors. Chin. J. Mate. Res. 2016, 30, 538-544. (37)Wang, Z. L.; Sun, K.; Henzie, J.; Hao, X.; Li, C.; Takei, T.; Kang, Y. M.; Yamauchi, Y. Spatially Confined Assembly of Monodisperse Ruthenium Nanoclusters in a Hierarchically Ordered Carbon Electrode for Efficient Hydrogen Evolution. Angew. Chem. Int. Ed. 2018, 57, 5848-5852. (38)Niedziałkowski, P.; Ossowski, T.; Zięba, P.; Cirocka, A.; Rochowski, P.; Pogorzelski, S.; Ryl, J.; Sobaszek, M.; Bogdanowicz, R. Poly-L-Lysine-Modified Boron-Doped Diamond Electrodes for the Amperometric Detection of Nucleic Acid Bases. J. Electroanal. Chem. 2015, 756, 84-93. (39)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. (40)Usachov, D.; Vilkov, O.; Gruneis, A.; Haberer, D.; Fedorov, A.; Adamchuk, V.; Preobrajenski, A.; Dudin, P.; Barinov, A.; Oehzelt, M. Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties. Nano Lett. 2011, 11, 5401-5407. (41)Chuang, C.-H.; Ray, S. C.; Mazumder, D.; Sharma, S.; Ganguly, A.; Papakonstantinou, P.; Chiou, J.-W.; Tsai, H.-M.; Shiu, H.-W.; Chen, C.-H. Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study. Sci. Rep-UK 2017, 7, 42235. (42)Zhu, L.; Cai, Q.; Liao, F.; Sheng, M.; Wu, B.; Shao, M. Ru-Modified Silicon Nanowires as Electrocatalysts for Hydrogen Evolution Reaction. Electrochem. Commun. 2015, 52, 29-33. 17



Page 18 of 19

(43)Shao, X. L.; Zhao, J. S.; Zhang, K. L.; Chen, R.; Sun, K.; Chen, C. J.; Liu, K.; Zhou, L. W.; Wang, J. Y.; Ma, C. M. Two-Step Reset in the Resistance Switching of the Al/TiOx/Cu Structure. ACS Appl. Mater. Interfaces 2013, 5, 11265-11270. (44)Cheng, W. R.; Zhao, X.; Su, H.; Tang, F. M.; Che, W.; Zhang, H.; Liu, Q. H. Lattic-Strained Metal-Organic-Framework Arrays for Bifunctional Oxygen Electrocatalysis. Nat. Energy 2019, 4, 115-122. (45)Guoxing, L.; Jingshan, Z.; Ke, S.; Qiang, W.; Ming, W. Study on Mechanical Property

and

Thermal

Stability

of

In-Situ

Nanocomposites

of

Polyurethane/Oxidized Graphene. Chin. J. Mater. Res. 2015, 28, 901-908. (46)Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. The Role of Oxygen During Thermal Reduction of Graphene Oxide Studied by Infrared Absorption Spectroscopy. J. Phys. Chem. C 2011, 115, 19761-19781. (47)Hu, X.; Long, Y.; Fan, M.; Yuan, M.; Zhao, H.; Ma, J.; Dong, Z. Two-Dimensional Covalent Organic Frameworks as Self-Template Derived Nitrogen-Doped Carbon Nanosheets for Eco-Friendly Metal-Free Catalysis. Appl. Catal. B-Environ. 2018, 244, 25-35. (48)Brodsky, C. N.; Hadt, R. G.; Hayes, D.; Reinhart, B. J.; Li, N.; Chen, L. X.; Nocera, D. G. In Situ Characterization of Cofacial Co(IV) Centers in Co4O4 Cubane: Modeling the High-Valent Active Site in Oxygen-Evolving Catalysts. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 3855-3860.

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Table of Contents Graphic

Operando SR-FTIR:

Absorption (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


*

C-O

1800

1600

1400

Wavenumber (cm-1)

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1200

Operando Insight into Oxygen Evolution Kinetics ... - ACS Publications

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