Supplementary Material

In-situ Boron and Nitrogen Doping in Flue Gas Derived Carbon Materials for Enhanced Oxygen Reduction Reaction

Seoyeon Baik,a Bong Lim Suh,a Ayeong Byeon,a Jihan Kima*, and Jae W. Leea*

a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro (373-1 Guseong-dong), Yuseong-gu, Daejeon, Republic of Korea

* Corresponding Authors E-mail: jaewlee@kaist.ac.kr, jihankim@kaist.ac.kr

S1. N-doping confirmation

Table S1. Atomic % of FlueBNPC85 measured by EA (C, N, H, S).

C N H S

80.30% 2.49% 0.91% 0.12%

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Figure S1. XPS survey graph for BPC, FlueBNPC50, and their heat-treated samples (T-BPC and T-FlueBNPC50).

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Figure S2. B1s spectra measured by XPS for (a) non heat-treated samples and (b) heat-treated samples.

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Figure S3. Deconvoluted B1s spectra for (a) non heat-treated samples and (b) heat-treated samples.

Figure S4. N1s spectra measured by XPS for N-detectable samples.

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S2. Mechanism study for nitrogen-doping from N2 gas

The final product synthesized by the procedure described in section 2.4 was analysed by XPS to confirm additional N-doping. Doping of nitrogen and the formation of BN bonds were confirmed by the XPS N1s spectra (Figure S5) and B1s spectra (Figure 4). In the B1s spectra, post-treated sample shows the same peak position as FlueBNPC, which indicates that nitrogen is doped after post treatment.

Figure S5. N1s spectra materials synthesized from BPC, B2O3, N2 gas and H2 gas.

S3. Electrochemical analysis results

The synthesized materials were electrochemically tested by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. The results are given in Figure S6.

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Figure S6. (a) CV measurements of T- FlueBNPC samples, (b) RDE linear sweep voltammographs for the ORR in 1.0 M of O2-saturated NaOH solution with a rotation rate of 2500 rpm, (c) electron transfer number measured by RRDE method with a rotation rate of 2500 rpm at a scan rate of 10 mV s-1 in 1 M NaOH.

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S3.1 CO2 conversion under CO2/N2/O2 mixture, CO2/N2/O2/H2O mixture and its electrochemical performance

Generally, the volumetric composition in actual coal-fired flue gas contains 76-77 % N2, 12-13 % CO2, 6-7 % H2O, and 4-5 % O2. Therefore, to reproduce coal-fired flue gas, a ternary mixture consisting of 15 % CO 2, 80 % N2

and 5 % O2 was also used for CO2 conversion and its evaluation as an ORR catalyst. Furthermore, CO 2 conversion was also carried out using the ternary gas mixture with H2O. The amount of H2O involved in the CO2 conversion experiment was about 10 % of the mixture gas of CO2/N2/O2. Although the reported H2O/(CO2+N2+O2) ratio is 6.6 %, a somewhat greater amount of water was added because it was added when starting the pyrolysis. Therefore, it showed ORR activity comparable to those derived from CO2/N2 mixture (Figure S7). Although the latter current density somewhat different from those of the other two samples, the ORR activity was quite comparable for the three resulting Flue 1050 samples because of the similar ORR peak positions.

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Figure S7. (a) CV and (b) RDE curve of T-FlueBNPCs produced from gas mixture of CO2 /N2 (15%/85%), CO2 /N2/O2 (15%/80%/5%), and CO2 /N2/O2/H2O (13.5%/72%/4.5%/10%).

S4. Further characteristics

Figure S8. XRD curve for BPC and FlueBNPCs .

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Figure S9. XRD peaks of FlueBNPC50 which were processed by the program ‘X-Ray Diffractometer Laboratory’ and their allocation results.

S5. Computational Methods

S5.1 XPS spectra simulations

A comparative study between PC-OBC and PC-ONBC was conducted using natural bond orbital (NBO) 1,2

analysis carried out using the Hartree-Fock method at the level of 3-21G, implemented in the NBO 3.1 program within the GAUSSIAN 09 package3. The s character analysis from the NBO was used to examine the re-hybridization effect from the strain.

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Figure S10. (a) C1s spectra of T-BPC and T-FlueBNPC50 measured by XPS and (b) simulated C1s XPS spectra corresponding to PC-OBC(red) and PC-ONBC (black).

Figure S10 (b) illustrates the XPS spectra of the PC-ONBC (black curve), and PC-OBC (red curve) structures, which were obtained using the binding energy (where the core orbital energies were taken from the Natural Bond Analysis) profiles in the C1s region. As seen in Figure S10 (b), the XPS spectrum of PC-ONBC contains more components with lower binding energy than the spectrum of PC-OBC, showing good agreement with the experimental data from Figure S10 (a). Accordingly, the simulated XPS spectrum data provides reasonable justifications for using these model structures to compute the Gibbs energy to assess ORR activity.

S5.2 Gibbs energy estimation

Using the same PC-OBC and PC-ONBC structures from the previous section, we used density functional theory (DFT) at the level of B3LYP4,5/6-31G(d,p) to compute the Gibbs energy (ΔG=ΔH-TΔS) for both the 2-electron and the 4-electron reaction paths (Figure S11). As with the XPS calculations, all of the simulations were carried out using the GAUSSIAN 09 package3 considering solution effect.

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Figure S11. (a) Reaction pathways of the PC-OBC depicted for 2-electron pathway and (b) reaction pathways of the PC-ONBC depicted for 4-electron pathway. Here, “R” denotes the rate determining step; adsorption step; and tagging of 1, 2, 3, 4, reduction steps in the 4-electron pathway, while 2’ denotes the 2nd reduction step in the 2-electron pathway (C: grey, O: red, N: blue, B: pink, H: white).

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