Metal-Free Oxygen Evolution and Oxygen Reduction Reaction

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A Metal-free OER/ORR Bifunctional Electrocatalyst in Alkaline Media: From Mechanisms to Structure-Catalytic Activity Relationship Chi Ho Lee, Byeongsun Jun, and Sang Uck Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04608 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Sustainable Chemistry & Engineering

A Metal-free OER/ORR Bifunctional Electrocatalyst in Alkaline Media: From Mechanisms to StructureCatalytic Activity Relationship Chi Ho Lee†, Byeongsun Jun †, and Sang Uck Lee†,‡,*

†

‡

Department of Bionano Technology, Hanyang University, Ansan, 15588, Korea

Department of Chemical and Molecular Engineering, Hanyang University, Ansan, 15588, Korea

*Corresponding author. Email: [email protected]

1 ACS Paragon Plus Environment

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Abstract We have systematically investigated a metal-free bifunctional electrocatalyst of heteroatom-doped carbon nitride (XY-C3N4, where X and Y indicate the dopant and doping site on C3N4, respectively) for oxygen evolution and oxygen reduction reactions (OER and ORR), considering the possible reaction pathways based on the Eley-Rideal (ER) mechanism as well as the doping effects on electrocatalytic activity. In this work, the relative stability of 𝑂∗ and 𝑂𝑂𝐻∗ intermediates was a key factor determining the ORR pathway; accordingly, ORR follows a two-step reaction pathway governed by 𝑂∗ rather than a four-step reaction pathway governed by 𝑂𝑂𝐻∗ . In addition, we found that P and S co-doped C3N4 shows superior OER/ORR activity with synergistic geometric and electronic effects, which coordinatively increase unsaturated sp3C via structural deformation and improve electrical conductance by modulating the electronic structure with extra electrons from dopants. In particular, PCSC-C3N4 (C3N4 with P and S codoped at the carbon site) shows better bifunctional performance of OER/ORR with competitive overpotentials at 0.42 V and 0.27 V, respectively, compared to conventional Pt and RuO2 catalysts. Therefore, our theoretical investigations suggest that PCSC-C3N4 is the most promising bifunctional OER/ORR electrocatalyst with synergistic effects in several electrochemical devices. Keywords: Doped C3N4 catalyst, Density functional calculations, OER, ORR, Fuel cell


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Introduction In recent years, oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) have attracted substantial interest with regard to renewable energy conversion and storage devices. Research on efficient catalysts for OER/ORR and water splitting is ongoing and seeks to resolve the environmental concerns and resource depletion involved in the development of chemical-electrical energy conversion and storage technologies such as fuel cells, rechargeable metal-air batteries and supercapacitors.1-7 Among these energy devices, fuel cells efficiently convert chemical potential energy from fuel into electric energy with high electrical efficiency and high power density; however, this requires a favorable catalyst for practical application with fast kinetics.8-11 Considerable progress has been made recently on the development of catalysts to facilitate OER/ORR occurring at the cathode. These reactions play an important role in regenerative fuel cells and dominate their overall performance. Understanding the OER/ORR mechanisms of various catalysts could provide design guidelines for material and process development, as well as facilitating the discovery of new catalysts. Generally, OER/ORR can proceed in Langmuir-Hinshelwood (LH) or Eley-Rideal (ER) mechanisms.12 The LH mechanism comprises all reactive intermediates on the surface while the ER mechanism includes species from the electrolyte that react with the surface



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intermediate. Despite the controversy over the mechanism, ER mechanism is generally accepted with lower reaction energy barrier than that of LH mechanism,13 and many researchers have conducted theoretical studies on OER/ORR based on the ER mechanism. However, there are two feasible reaction pathways in ER mechanism, two-step pathway and four-step pathway depending on the relative stability of 𝑂∗ and 𝑂𝑂𝐻 ∗ intermediates generated after the adsorption of 𝑂$ on the catalyst.14 In this work, we sought to describe the detailed reaction pathway of the OER/ORR as well as proposing solutions for the determination of the preferred reaction pathway on the ER mechanism. Among the available catalysts, Pt-, Ir-, and Ru-based catalysts15-18 exhibit outstanding OER/ORR catalytic activity. However, the high associated cost is one of the main drawbacks19-23 to their practical use, despite noble-metal materials being the best catalysts for fuel cell systems. In this regard, an efficient OER/ORR catalyst that is low cost and highly abundant is needed. Research efforts have sought to reduce or replace noble-metal catalysts in both on the theoretical and experimental fields.24-28 In particular, various carbon-based materials have been extensively studied because they have unique advantages for designated catalysis due to their tunable molecular structures, abundance and strong tolerance to acid/alkaline environments. Recent studies have revealed that graphene,29-30 graphite,31-32 vertically-aligned nitrogen-doped carbon


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nanotubes (VA-NCNTs),4 heteroatom-doped CNTs,33 and nitrogen-doped graphene sheets (Ngraphene)34 have excellent catalytic performance. The presence of N in N-doped graphene leads to more chemically active sites, a high density of defects and high electrochemical activity. Due to these enhanced electronic properties, N-doped catalysts in the C network are attractive for a wide range of applications, including as metal-free catalysts for OER/ORR in fuel cell systems. Recently, graphitic carbon nitride (g-C3N4) with N-rich including both graphitic and pyridinic N moieties is a promising catalyst due to its competitiveness over a wide range of electrocatalyst processes,35-39 despite pure g-C3N4 itself being inert with regard to OER/ORR activity. Here, we attempted to enhance the catalytic activity of g-C3N4 by introducing dopants such as P, S or PS into the g-C3N4 matrix, which is an effective way to manipulate electronic structure and electrochemical properties.40-43 Thus, we sought to describe the theoretical structure-activity relationship in C3N4-based electrocatalysts for OER/ORR based on a thorough understanding of the effects of dopants. We explored the causes of variation in OER/ORR performance with respect to the type of dopant by comparing the electronic and geometric effects of heteroatomdoped C3N4 structures (XY-C3N4, where X and Y indicate the dopant and doping site on C3N4, respectively). In addition, we revealed that the outstanding performance of PCSC-C3N4 is attributed to the synergistic effects between electronic and geometric factors.



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Computational Details All ab initio calculations were performed with the Vienna Ab initio Simulation Package (VASP 5.4.1).44-47 Ab initio calculations were carried out using the projector augmented wave (PAW) method48-49 with a generalized gradient approximation based on the Perdew-BurkeErzerhof (PBE) exchange-correlation functional.50-51 Integration in the Brilliouin zone was performed on the basis of the Monkhorst-Pack scheme using a Γ centered 10×10×1 k-point mesh in each primitive lattice vector of the reciprocal space for geometric optimization and density of states (DOS). A plane-wave cutoff energy of 500 eV was used. Lattice constants and internal atomic positions were fully optimized by the spin polarization calculation until the residual forces were less than 0.04 eV/Å. The information about the optimized unit cell of XY-C3N4 (Table S1, Supporting Information). The computational models were built in a hexagonal unit cell repeated along a and b directions with a vacuum region of up to 15 Å along the c direction in order to exclude the mirror interactions.


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Results and Discussions A. Formation energy (𝑬𝒇 ) of heteroatom-doped C3N4 We explored the favorable doping sites on C3N4 by calculating the formation energies (𝐸( ) of heteroatom-doped C3N4 structures, XY-C3N4, where X=P, S, or PS and Y= C or N, in order to investigate their OER/ORR activities, as shown in Figure 1. The calculated relative 𝐸( and the most stable structures are illustrated in Table S2, Supporting Information. The results show that 𝐸( has different tendencies depending on the doping site (C or N) and dopant (P or S). In the case of a C doping site, there are two distinguishable sites, bridging (CA site) and fusing points (CB site) connecting two hexagonal rings. Because of the size effect of dopant, XY-C3N4 structures are usually subjected to out-of-plane structural deformation in order to reduce the strain induced by the larger size of the dopants. Therefore, it can be expected that P and S dopants prefer the doping site that reduces the strain. Compared to the fusing point of a CB site, the CA site can afford to relax the strain because it is prone to out-of-plane deformation with structural flexibility. The calculated 𝐸( clearly shows that P prefers the CA site with lower 𝐸( . However, S induces chemical bond breakage at the CA site. Therefore, we considered the preferred doping sites as CA and CB for P and S doping, respectively. In addition, P and S co-doped PCSC-C3N4 has the same doping sites of CA and CB for P and S, respectively, as shown in Figure 1(c). On



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the other hand, there are three unique N doping sites, center (NC site), corner (NB site), and bridging (NA site) points, in the C3N4 moiety. The calculated 𝐸( reveals that both P and S prefer the NB site, which can be explained based on the structural flexibility, as discussed for the C doping site. However, both P and S cannot be substituted at the NB sites simultaneously due to overloaded strain. Instead, P and S co-doped PNSN-C3N4 is more stable when P and S are substituted at NA and NB sites, respectively, as shown in Figure 1(d). The case with C and N mixed doping sites is much more stable when P is substituted for C and S is substituted for N, especially at the CA and NB sites as shown in Figure 1(e), which are favorable sites for out-ofplane deformation in order to reduce the strain induced by the larger dopants. In this work, we considered the most stable XY-C3N4 structures, PCA-C3N4, PNB-C3N4, SCB-C3N4, SNB-C3N4, PCASCB-C3N4, PNASNB-C3N4, and PCASNB-C3N4, for the investigation of OER/ORR bifunctional electrocatalytic activities and their reaction mechanisms, as shown in Figures 1(c)-(e).

B. OER/ORR reaction pathways Given the well-established structures, we attempt to estimate the catalytic activities of the XY-C3N4 by sequentially calculating the Gibbs free energies for the elementary steps of OER/ORR based on the ER mechanism. The generally acceptable OER mechanism is the fourelectron associative mechanism.52 The four elementary steps are described as follows:


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𝑂𝐻 ) + ∗ → 𝑂𝐻 ∗ + 𝑒 )

(1)

𝑂𝐻 ∗ + 𝑂𝐻 ) → 𝑂∗ + 𝐻$ 𝑂(𝑙 ) + 𝑒 )

(2)

O∗ + 𝑂𝐻 ) → 𝑂𝑂𝐻∗ + 𝑒 )

(3)

OOH ∗ + 𝑂𝐻 ) → ∗ + 𝑂$ (𝑔) + 𝐻$ 𝑂(𝑙 ) + 𝑒 )

(4)

where * represents the active site, and 𝑂𝐻 ∗ , 𝑂∗ , and 𝑂𝑂𝐻 ∗ are adsorbed intermediates. In contrast to OER, ORR can proceed either by a two-step or four-step pathways depending on the relative stability of 𝑂∗ and 𝑂𝑂𝐻 ∗ intermediates generated after the adsorption of 𝑂$ on the catalyst. The two-step pathway is summarized using the following elementary steps in an alkaline environment: 𝑂$∗ + 𝐻$ 𝑂(𝑙 ) + 2𝑒 ) → 𝑂∗ + 2𝑂𝐻)

(5)

𝑂∗ + 2𝑂𝐻) + 𝐻$ 𝑂(𝑙 ) + 2𝑒 ) → 4𝑂𝐻 )

(6)

whereas the four-step pathway has following elementary steps: 𝑂$∗ + 𝐻$ 𝑂(𝑙 ) + 𝑒 ) → 𝑂𝑂𝐻 ∗ + 𝑂𝐻)

(7)

𝑂𝑂𝐻∗ + 𝑒 ) → 𝑂∗ + 𝑂𝐻 )

(8)

𝑂∗ + 𝐻$ 𝑂(𝑙 ) + 𝑒 ) → 𝑂𝐻 ∗ + 𝑂𝐻 )

(9)

𝑂𝐻 ∗ + 𝑒 ) → ∗ +𝑂𝐻 )

(10)



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Looking at the elementary reaction steps of ORR, each reaction pathway passes through a different intermediate at the first elementary step after the adsorption of 𝑂$ on the catalyst, 𝑂 ∗ in Eq. (6) and 𝑂𝑂𝐻 ∗ in Eq.(8). Therefore, the first reaction free energy ∆𝐺9 can be used as an index to determine the ORR pathway. So, we investigated the reaction free energy ∆G of each elementary step and compared the ∆𝐺9 values of both reaction pathways to determine which is the more favorable ORR pathway on the XYC3N4. For each step, the reaction free energy ∆G is given by the expression, ∆𝐺 = ∆𝐸 + ∆𝑍𝑃𝐸 − 𝑇∆𝑆, considering zero point energy (ZPE) corrections and entropic contributions (TS). To obtain ZPE contribution in the free energy expression, the vibrational frequencies of adsorbed species such as 𝑂 ∗ , 𝑂𝐻 ∗ , and 𝑂𝑂𝐻 ∗ were calculated with the fixed XY-C3N4 and we took the standard entropies from thermodynamic tables for gas phase molecules. The calculated ∆G values are listed in Tables S6-S8, Supporting Information, and ZPE, TS and frequencies of adsorbed species are listed in Tables S4-S5, Supporting Information. The calculated results clearly show that two-step pathway is preferable to four-step pathway with more negative ∆𝐺9 values, as shown in Figure 2. Therefore, it is worth noticing that ORR follows the two-step pathway compared to fourstep pathway of OER on XY-C3N4 catalysts based on the ER mechanism.


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C. OER/ORR catalytic activity: Free energy diagrams (FEDs) and overpotential (𝛈) From the calculated ∆G, we can deduce an important parameter of electrocatalytic activity, the magnitude of the potential-determining step (G BCD/BDD ) in consecutive reaction steps. This is the specific reaction step with the largest ∆G in the OER/ORR elementary reaction steps, that is, the concluding step to achieve a downhill reaction in the free energy diagram (FED) with increasing potential, as shown in Figure 3: GBCD = max [∆𝐺9J , ∆𝐺$J , ∆𝐺KJ , ∆𝐺LJ ] GBDD = max [∆𝐺9J , ∆𝐺$J ]

(11) (12)

The theoretical overpotential at standard conditions is then given by Equation (13) in alkaline conditions:

η BCD/BDD = OG BCD/BDD /𝑒P − 0.402 V

(13)

The theoretical overpotential (η) represents the relative stability of the intermediates between initial O$ and final HO) states and can be calculated by applying standard density functional theory (DFT) in combination with the computational standard hydrogen electrode (SHE) model. Because the equilibrium reduction potential of an oxygen molecule is 0.402 V in alkaline media, the chemical potential difference between O$ and HO) states should be 1.608 V, and all intermediate states should ideally have the same chemical potential with initial and final states



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under the equilibrium potential, U=0.402 V. However, the actual catalytic behaviors deviate from the ideal case due to correlation with binding energies of the intermediates. Therefore, most catalysts require overpotential (η) in order to achieve an overall downhill reaction. Consequently, the η of OER/ORR is an important indicator of the catalytic activities of a catalyst, and a lower η indicates a thermodynamically superior catalyst. So, we calculated the η for each active site on XY-C3N4 systems and determined the minimum η for OER/ORR in each structure. We constructed FEDs under different electrode potential U using the calculated reaction free energies ∆G listed in Tables S6-S8, Supporting Information. Figures 4 and 5 show the FEDs of representative models, SN-C3N4, PC-C3N4, and PCSC-C3N4, having the best OER and ORR performance among the investigated models of PCA-C3N4, PNB-C3N4, SCB-C3N4, SNB-C3N4, PCASCB-C3N4, PNASNB-C3N4, and PCASNB-C3N4. Figure 4 (a) shows the volcano plot of OER at all possible active sites on XY-C3N4, which represents the apparent catalytic activity with correlation between −𝜂BCD and ∆𝐺B∗ − ∆𝐺BU ∗ . This theoretical analysis reveals that the SN-C3N4, PC-C3N4, and PCSC-C3N4 structures have minimum 𝜂BCD for each S-, P- and PS-doped structures with 0.77, 0.48, and 0.42 V, respectively. The 𝜂BCD value of PCSC-C3N4 is comparable to those of the best conventional catalysts (~0.42 V for OER on RuO2).52 Figures 4 (b)-(d) show the detailed analysis of FEDs of the best OER performing models, SN-C3N4@C6, PC-C3N4@N3, and PCSC-


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C3N4@N6, at most active sites of each C6, N3 and N6 on the surface of XY-C3N4. Most elementary steps in OER have an uphill reaction at 0.00 V and at equilibrium potential of 0.402 V. Analyzing the FEDs at the equilibrium potential of 0.402 V, we can define the rate determination step as the 𝑂∗ formation from 𝑂𝐻 ∗ on SN-C3N4 and PC-C3N4 and the formation of 𝑂𝑂𝐻∗ from 𝑂∗ on PCSC-C3N4, as shown in Figure 4(a). Therefore, the ∆G at the rate determination step, 0.77, 0.48, and 0.42 V, become the 𝜂BCD for facilitating the OER as a spontaneously downhill reaction. By applying the increased electrode potentials of 1.17, 0.88, and 0.82 V, all elementary reactions can be downhill reactions for spontaneous OER. In the case of ORR, we considered the 𝑂$ adsorption free energy (∆𝐺BV∗ ) as well as the 𝜂BDD to determine the ORR catalytic activity in Figures 5(a)-(d), because ∆𝐺BV∗ and ηBDD are highly correlated to determine ORR activity. If the 𝑂$ adsorption reaction is very difficult to achieve as an initiation step in ORR, that material cannot have ORR activity, even though it can have a possibility to have a very small 𝜂BDD . Therefore, we defined a new indicator of ORR catalytic activity as figure of merit (FOM), which is defined by 𝐹𝑂𝑀 = −O𝜂BDD + ∆𝐺BV∗ P. Like 𝜂BDD , a lower FOM presents a thermodynamically superior catalyst. Using the FOM, we constructed a volcano plot of ORR at all possible active sites on XY-C3N4 in order to compare the ORR catalytic activity, as shown in Figure 5(a), where the SN-C3N4, PC-C3N4, and PCSC-C3N4



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structures have minimum 𝜂BDD of 0.51, 0.40, and 0.27 V, respectively, which is lower than that of the best catalysts identified theoretically (~0.45 V for ORR on Pt).53 The catalytic activity trend is in good agreement with the experimental result.54 However, if we only consider the 𝜂BDD without the ∆𝐺BV∗ , the OOR activity trend is in discord with the experimental result (Figure S1 and Table S9, Supporting Information). Therefore, we would like to emphasize the importance of ∆𝐺BV∗ in ORR activity. Figures 5 (b)-(d) show the detailed analysis of FEDs of the best ORR performing models, SN-C3N4@CN1, PC-C3N4@N1, and PCSC-C3N4@N2, at most active sites on the surface of XY-C3N4. Based on the same analysis of FEDs of OER, we can determine the rate determination step in ORR, as shown in Figure 5(a) and the 𝜂BDD . Our results reveal that the catalytic activity of both OER and ORR have the same order of PCSC-C3N4 > PC-C3N4 > SN-C3N4, with PCSC-C3N4 showing the best bifunctional performance with competitive η BCD/BDD of 0.42 V and 0.27 V compared to conventional Pt and RuO2 catalysts, respectively. Therefore, It is worth mentioning that P,S co-doping synergistically improved the catalytic activities of OER and ORR.


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D. Geometric and electronic doping effects on OER/ORR activity The synergistic effect of P,S co-doping can be explained based on the geometric and electronic effects of heteroatoms, as described in previous studies.55 Considering the atomic size of heteroatoms, the relatively larger S and P dopants can cause structural deformation of XY-C3N4, which improves the OER and ORR activity by enhancing the stability of intermediates with increasing p orbital character of active sites adjacent to the dopant from sp2 to sp3 hybridization. Compared to sp2 hybridized orbitals on pure C3N4, sp3 character of active sites on XY-C3N4 are more suitable for forming chemical bonds with intermediate species. Therefore, in out-of-plane structures, the intermediates prefer to bind to sp3 hybridized active sites. In addition, we investigated density of state (DOS) of pure C3N4 and XY-C3N4 (X=P, S, or PS and Y=N) to determine at what condition the OER and ORR exhibit outstanding performance. It can be expected that electron-rich heteroatom doping will induce electronic structure changes from a non-metallic doping effect to a metallic doping effect due to a downward band shift. Looking at the electronic structures of XY-C3N4 (X=P, S, or PS and Y=C or N), we preferentially analyzed the DOS of XY-C3N4 with retention of its planar structure to determine the electronic effect of the dopant, which shows a metallic property by shifting the DOS down, as shown in Figures 6(b)-(d) compared to Figure 6(a).



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Moreover, to verify the relationship between geometric and electronic structures of XYC3N4, we intentionally changed the structures of XY-C3N4 from in-plane to out-of-plane to increase the activity for binding intermediate species by increasing the sp3 character of active sites on XY-C3N4, as shown in Figures 6(e)-(g). These calculations clearly show that structural deformation can produce two remarkable features of DOS to maintain the metallic property or incur a non-metallic property depending on the doping site of XYC3N4. When out-of-plane deformation is applied in the in-plane PC-C3N4 and SN-C3N4, DOS tends to retain the metallic property. However, in the case of PN-C3N4 and SC-C3N4, the tendency of DOS changes from metallic to non-metallic. Interestingly, these tendencies of DOS depending on doping site are consistent with OER and ORR activity, where PC-C3N4 and SN-C3N4 maintaining a metallic property exhibited better catalytic activity than PN-C3N4 and SC-C3N4 with a non-metallic property (see Table S9). P,S codoped PCSC-C3N4 demonstrates a similar phenomenon in DOS to that of PC-C3N4 and SNC3N4; as a result, PCSC-C3N4 shows the best OER and ORR activity by maintaining a metallic property despite the presence of out-of-plane deformation. Consequently, it can be emphasized that there is close correlation between the electronic/ geometric structure and OER/ORR catalytic activities, and the best bifunctional OER/ORR catalytic activity


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of P,S co-doped PCSC-C3N4 is attributed to a synergistic effect between the electronic and geometric effects.

Conclusion We systematically investigated a key factor determining the preferred reaction pathway in OER/ORR as well as the relationship of structure-electrocatalytic activity of the OER/ORR for XY-C3N4, where X and Y are dopant and doping site on C3N4, respectively. The generally accepted ER mechanism of OER/ORR is re-evaluated by comparing the stability of intermediates governing the reactions, where we found that the OER and ORR respectively follows four-step and two-step reaction pathway. We also elucidated the importance of the 𝑂$ adsorption free energy (∆𝐺BV∗ ) in ORR activity. Considering the ∆𝐺BV∗ with a FOM, 𝐹𝑂𝑀 = −O𝜂BDD + ∆𝐺BV∗ P, we successfully represented the ORR activity of XY-C3N4. Moreover, we demonstrated that P and S co-doped C3N4 shows outstanding OER/ORR activity with a synergistic effect of geometric and electronic effects by generating structural deformation and modulating the electronic structure with heteroatom dopants. This synergistic effect of PCSC-C3N4 exhibits the minimum theoretical OER overpotential (𝜂BCD ), which was estimated to be 0.42 V; while the minimum overpotential for ORR (𝜂BDD ) was calculated to be 0.27 V, much lower than those of the best catalysts



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identified theoretically (~0.45 V for ORR on Pt). Therefore, our theoretical investigations suggest that the synergistic effect of geometric and electronic effects plays an important role in OER/ORR bifunctional catalytic activities of XY-C3N4 systems. This understanding of the structure-activity relationship can facilitate development of new highly efficient electrocatalytic materials.

Acknowledgments. This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2015R1C1A1A02036670). This work was also supported by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2017-C3-0032).

References 1. Yang, X. W.; Cheng, C.; Wang, Y. F.; Qiu, L.; Li, D., Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science 2013, 341 (6145), 534537. 2. Li, L.; Manthiram, A., Long-Life, High-Voltage Acidic Zn-Air Batteries. Advanced Energy Materials 2016, 6 (5), 1502054. 3. Su, D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.; Paraknowitsch, J.; Schlogl, R., Metal-free heterogeneous catalysis for sustainable chemistry. ChemSusChem 2010, 3 (2), 169180.


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4. Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M., Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323 (5915), 760-764. 5. Qu, L. T.; Liu, Y.; Baek, J. B.; Dai, L. M., Nitrogen-Doped Graphene as Efficient MetalFree Electrocatalyst for Oxygen Reduction in Fuel Cells. Acs Nano 2010, 4 (3), 1321-1326. 6. Li, Y.; Dai, H., Recent advances in zinc-air batteries. Chem Soc Rev 2014, 43 (15), 52575275. 7. Lee, D. U.; Fu, J.; Park, M. G.; Liu, H.; Ghorbani Kashkooli, A.; Chen, Z., SelfAssembled NiO/Ni(OH)2 Nanoflakes as Active Material for High-Power and High-Energy Hybrid Rechargeable Battery. Nano Lett 2016, 16 (3), 1794-1802. 8. Scott Nasha; Fabrice Andrieuxa; Richard Dawsona*; Hugh Sutherlandb, G. L., Direct Fuel Oxidation Alkaline Fuel Cells; The Kinetics of Borohydride Oxidation. VOL. 41, 2014, 247252. 9. Durst, J.; Simon, C.; Hasche, F.; Gasteiger, H. A., Hydrogen Oxidation and Evolution Reaction Kinetics on Carbon Supported Pt, Ir, Rh, and Pd Electrocatalysts in Acidic Media. J Electrochem Soc 2015, 162 (1), F190-F203. 10. Njodzefon, J. C.; Graves, C. R.; Mogensen, M. B.; Weber, A.; Hjelm, J., Kinetic Studies on State of the Art Solid Oxide Cells: A Comparison between Hydrogen/Steam and Reformate Fuels. J Electrochem Soc 2016, 163 (13), F1451-F1462. 11. Rheinlander, P. J.; Herranz, J.; Durst, J.; Gasteiger, H. A., Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure. J Electrochem Soc 2014, 161 (14), F1448-F1457. 12. Zhang, P.; Xiao, B. B.; Hou, X. L.; Zhu, Y. F.; Jiang, Q., Layered SiC Sheets: A Potential Catalyst for Oxygen Reduction Reaction. Sci Rep-Uk 2014, 4. 13. Keith, J. A.; Jerkiewicz, G.; Jacob, T., Theoretical investigations of the oxygen reduction reaction on Pt(111). Chemphyschem 2010, 11 (13), 2779-2794. 14. Dai, L.; Xue, Y.; Qu, L.; Choi, H. J.; Baek, J. B., Metal-free catalysts for oxygen reduction reaction. Chem Rev 2015, 115 (11), 4823-4892. 15. Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J., Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal Sci Technol 2014, 4 (11), 3800-3821. 16. 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 (8), 1765-1772.



Page 20 of 30

17. Lim, D. H.; Wilcox, J., Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles. J Phys Chem C 2012, 116 (5), 36533660. 18. Wu, J. B.; Yang, H., Platinum-Based Oxygen Reduction Electrocatalysts. Accounts Chem Res 2013, 46 (8), 1848-1857. 19. Duan, L. L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. C., A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II. Nat Chem 2012, 4 (5), 418-423. 20. Youngblood, W. J.; Lee, S. H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E., Photoassisted Overall Water Splitting in a Visible Light-Absorbing Dye-Sensitized Photoelectrochemical Cell. J Am Chem Soc 2009, 131 (3), 926-927. 21. Sartorel, A.; Carraro, M.; Scorrano, G.; De Zorzi, R.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M., Polyoxometalate embedding of a tetraruthenium(IV)-oxo-core by template-directed metalation of [gamma-SIW10O36](8-): A totally inorganic oxygen-evolving catalyst. J Am Chem Soc 2008, 130 (15), 5006-5007. 22. Tong, L. P.; Duan, L. L.; Xu, Y. H.; Privalov, T.; Sun, L. C., Structural Modifications of Mononuclear Ruthenium Complexes: A Combined Experimental and Theoretical Study on the Kinetics of Ruthenium-Catalyzed Water Oxidation. Angew Chem Int Edit 2011, 50 (2), 445-449. 23. Joya, K. S.; Subbaiyan, N. K.; D'Souza, F.; de Groot, H. J. M., Surface-Immobilized Single-Site Iridium Complexes for Electrocatalytic Water Splitting. Angew Chem Int Edit 2012, 51 (38), 9601-9605. 24. Ikeda, T.; Boero, M.; Huang, S. F.; Terakura, K.; Oshima, M.; Ozaki, J., Carbon alloy catalysts: Active sites for oxygen reduction reaction. J Phys Chem C 2008, 112 (38), 1470614709. 25. Yu, D. S.; Zhang, Q.; Dai, L. M., Highly Efficient Metal-Free Growth of Nitrogen-Doped Single-Walled Carbon Nanotubes on Plasma-Etched Substrates for Oxygen Reduction. J Am Chem Soc 2010, 132 (43), 15127-15129. 26. Zhang, J. T.; Zhao, Z. H.; Xia, Z. H.; Dai, L. M., A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotechnol 2015, 10 (5), 444-452. 27. Liu, R. L.; Wu, D. Q.; Feng, X. L.; Mullen, K., Nitrogen-Doped Ordered Mesoporous Graphitic Arrays with High Electrocatalytic Activity for Oxygen Reduction. Angew Chem Int Edit 2010, 49 (14), 2565-2569. 28. Huang, Z.; Zhou, H. H.; Yang, W. J.; Fu, C. P.; Chen, L.; Kuang, Y. F., Three-


Page 21 of 30 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


Dimensional Hierarchical Porous Nitrogen and Sulfur-Codoped Graphene Nanosheets for Oxygen Reduction in Both Alkaline and Acidic Media. Chemcatchem 2017, 9 (6), 987-996. 29. Li, Y. F.; Li, M.; Jiang, L. Q.; Lin, L.; Cui, L. L.; He, X. Q., Advanced oxygen reduction reaction catalyst based on nitrogen and sulfur co-doped graphene in alkaline medium. Phys Chem Chem Phys 2014, 16 (42), 23196-23205. 30. Wang, X. Q.; Lee, J. S.; Zhu, Q.; Liu, J.; Wang, Y.; Dai, S., Ammonia-Treated Ordered Mesoporous Carbons as Catalytic Materials for Oxygen Reduction Reaction. Chem Mater 2010, 22 (7), 2178-2180. 31. Sidik, R. A.; Anderson, A. B.; Subramanian, N. P.; Kumaraguru, S. P.; Popov, B. N., O-2 reduction on graphite and nitrogen-doped graphite: Experiment and theory. J Phys Chem B 2006, 110 (4), 1787-1793. 32. Liu, Z. W.; Peng, F.; Wang, H. J.; Yu, H.; Zheng, W. X.; Yang, J. A., Phosphorus-Doped Graphite Layers with High Electrocatalytic Activity for the O-2 Reduction in an Alkaline Medium. Angew Chem Int Edit 2011, 50 (14), 3257-3261. 33. Yang, L. J.; Jiang, S. J.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X. Z.; Wu, Q.; Ma, J.; Ma, Y. W.; Hu, Z., Boron-Doped Carbon Nanotubes as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction. Angew Chem Int Edit 2011, 50 (31), 7132-7135. 34. Li, M. T.; Zhang, L. P.; Xu, Q.; Niu, J. B.; Xia, Z. H., N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. J Catal 2014, 314, 66-72. 35. Yu, W. L.; Xu, D. F.; Peng, T. Y., Enhanced photocatalytic activity of g-C3N4 for selective CO2 reduction to CH3OH via facile coupling of ZnO: a direct Z-scheme mechanism. J Mater Chem A 2015, 3 (39), 19936-19947. 36. Yuan, Y. W.; Zhang, L. L.; Xing, J.; Utama, M. I. B.; Lu, X.; Du, K. Z.; Li, Y. M.; Hu, X.; Wang, S. J.; Genc, A.; Dunin-Borkowski, R.; Arbiol, J.; Xiong, Q. H., High-yield synthesis and optical properties of g-C3N4. Nanoscale 2015, 7 (29), 12343-12350. 37. Zheng, Y.; Jiao, Y.; Zhu, Y. H.; Cai, Q. R.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S. Z., Molecule-Level g-C3N4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions. J Am Chem Soc 2017, 139 (9), 3336-3339. 38. Hu, E. L.; Ning, J. Q.; He, B.; Li, Z. P.; Zheng, C. C.; Zhong, Y. J.; Zhang, Z. Y.; Hu, Y., Unusual formation of tetragonal microstructures from nitrogen-doped carbon nanocapsules with cobalt nanocores as a bi-functional oxygen electrocatalyst. J Mater Chem A 2017, 5 (5), 22712279. 39. Zheng, Q.; Durkin, D. P.; Elenewski, J. E.; Sun, Y. X.; Banek, N. A.; Hua, L. K.; Chen, H.



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N.; Wagner, M. J.; Zhang, W.; Shuai, D. M., Visible-Light-Responsive Graphitic Carbon Nitride: Rational Design and Photocatalytic Applications for Water Treatment. Environ Sci Technol 2016, 50 (23), 12938-12948. 40. Song, L.; Liu, Z.; Reddy, A. L. M.; Narayanan, N. T.; Taha-Tijerina, J.; Peng, J.; Gao, G. H.; Lou, J.; Vajtai, R.; Ajayan, P. M., Binary and Ternary Atomic Layers Built from Carbon, Boron, and Nitrogen. Adv Mater 2012, 24 (36), 4878-4895. 41. Shim, Y.; Han, J.; Sa, Y. J.; Lee, S.; Choi, K.; Oh, J.; Kim, S.; Joo, S. H.; Park, S., Electrocatalytic performances of heteroatom-containing functionalities in N-doped reduced graphene oxides. J Ind Eng Chem 2016, 42, 149-156. 42. Putri, L. K.; Ong, W. J.; Chang, W. S.; Chai, S. P., Heteroatom doped graphene in photocatalysis: A review. Appl Surf Sci 2015, 358, 2-14. 43. Wang, X. W.; Sun, G. Z.; Routh, P.; Kim, D. H.; Huang, W.; Chen, P., Heteroatom-doped graphene materials: syntheses, properties and applications. Chemical Society Reviews 2014, 43 (20), 7067-7098. 44. Kresse, G.; Hafner, J., Ab-Initio Molecular-Dynamics for Open-Shell Transition-Metals. Phys Rev B 1993, 48 (17), 13115-13118. 45. Kresse, G.; Hafner, J., Ab-Initio Molecular-Dynamics Simulation of the Liquid-Metal Amorphous-Semiconductor Transition in Germanium. Phys Rev B 1994, 49 (20), 14251-14269. 46. Kresse, G.; Furthmuller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp Mater Sci 1996, 6 (1), 15-50. 47. Kresse, G.; Furthmuller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B 1996, 54 (16), 11169-11186. 48. Kresse, G.; Joubert, D., From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B 1999, 59 (3), 1758-1775. 49. Blochl, P. E., Projector Augmented-Wave Method. Phys Rev B 1994, 50 (24), 1795317979. 50. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys Rev Lett 1996, 77 (18), 3865-3868. 51. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple (vol 77, pg 3865, 1996). Phys Rev Lett 1997, 78 (7), 1396-1396. 52. Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Norskov, J. K.; Rossmeisl, J., Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. Chemcatchem 2011, 3 (7), 1159-1165. 53. Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.;


Page 23 of 30 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


Jonsson, H., Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J Phys Chem B 2004, 108 (46), 17886-17892. 54. Shinde, S. S.; Lee, C. H.; Sami, A.; Kim, D. H.; Lee, S. U.; Lee, J. H., Scalable 3-D Carbon Nitride Sponge as an Efficient Metal-Free Bifunctional Oxygen Electrocatalyst for Rechargeable Zn-Air Batteries. Acs Nano 2017, 11 (1), 347-357. 55. Lee, C. H.; Jun, B.; Lee, S. U., Theoretical evaluation of the structure–activity relationship in graphene-based electrocatalysts for hydrogen evolution reactions. RSC Adv. 2017, 7 (43), 27033-27039.



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For Table of Contents Use Only

We systematically investigated the relationship between structure and electrocatalytic activity of heteroatom-doped carbon nitride (XY-C3N4, where X and Y indicate the dopant (P or S) and doping site on C3N4, respectively) for a metal-free bifunctional electrocatalyst.


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Figure 1. Structure of pure-C3N4 and XY-C3N4 structures. (a) Alphabetic characters and (b) Arabic numbers indicate substitutional doping sites and active sites for OER/ORR, respectively. (c) and (d) represent XC-C3N4 and XN-C3N4 structures (X= P and S), respectively. (e) PCSN-C3N4 structure.



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Figure 2. The first reaction free energy (∆𝐺# ) of the four-step pathway, 𝑂%∗ + 𝐻% 𝑂(𝑙 ) + 𝑒 . → 𝑂𝑂𝐻∗ + 𝑂𝐻 . , and the two-step pathway, 𝑂%∗ + 𝐻% 𝑂(𝑙 ) + 4𝑒 . → 𝑂∗ + 2𝑂𝐻 . + 2𝑒 . , for (a) PC-C3N4, SC-C3N4 and PCSC-C3N4 (b) PN-C3N4, SN-C3N4 and PNSN-C3N4 depending on various active sites.


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Figure 3. Free energy diagrams (FEDs) of (a) four-step associative OER pathway and (b) twostep ORR pathway at zero potential (U = 0.00 V), at equilibrium reduction potential (U = 0.402 V) and at overpotential (U = 0.402 V + η V).



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Figure 4. (a) The volcano plots of OER at all possible active sites on XY-C3N4. (b)-(d) the free energy diagrams (FEDs) of SN-C3N4, PC-C3N4 and PCSC-C3N4 structures having the best catalytic activity. (The order of OER activity is well agreement with the experimental results, PS-C3N4 > P-C3N4 > S- C3N4, in Ref 54)


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Figure 5. (a) The volcano plots of ORR at all possible active sites on XY-C3N4. (b)-(d) the free energy diagrams (FEDs) of SN-C3N4, PC-C3N4 and PCSC-C3N4 catalysts showing the best catalytic activity. (The order of ORR activity is well agreement with the experimental results, PS-C3N4 > P-C3N4 > S- C3N4, in Ref 54)



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Figure 6. The density of states (DOSs) and structures of (a) the pure C3N4, (b)-(d) the XY-C3N4 keeping planar structure for only electronic effect, and (e)-(g) the XY-C3N4 with out-of-plane deformation for both electronic and geometric effects, where inset number means the degree of intentional deformation and the green circles indicate the N atoms considered sp3 hybridization character.


Metal-Free Oxygen Evolution and Oxygen Reduction Reaction

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