Hexagonal Boron Nitride/Reduced Graphene Oxide Heterostructures as Promising Metal‐Free Electrocatalysts for Oxygen Evolution Reaction Driven by Boron Radicals

Developing highly efficient earth‐abundant alternatives to traditional noble metal catalysts is essential for clean and sustainable energy‐conversion and energy‐storage technologies, yet still challenging in limited active sites and weak resistance to electrochemical corrosion. Herein, density‐functional theory calculations demonstrate that hexagonal boron nitride (h‐BN), albeit often being considered inert, can generate boron‐active radicals at defective sites by forming heterogeneous structures with graphene‐containing point vacancies, leading to a substantial electron delocalization and charge transfer, indicating a superior catalytic activity. Experimentally, the van der Waals heterostructure is rationally designed with h‐BN nanosheets (BNNs) anchored on reduced graphene oxide (rGO) as strongly coupled composite catalysts. Despite the poor conductivity in BN and lower catalytic activity in rGO, the created heterostructures demonstrate unexpected, improved oxygen evolution reaction (OER) activity with excellent stability in alkaline electrolyte. Qualitative analysis of the valence band offset and theoretical calculation reveal that the formation of heterostructures can significantly drive the electron transfer between C and B atoms near the vacancies across the interface and cause a half‐metallic property of BN, decreasing the free energy barrier of four‐electron OER kinetics. Herein, the synthetic schemes of h‐BNNs are guided as highly active metal‐free OER electrocatalysts.

nanoparticles as heterogeneous electrocatalysts, [14][15][16] but seldom as catalysts itself.Very limited reports showed that h-BN can catalyze oxygen reduction reaction by wrapping into reduced graphene oxide (rGO) and CNTs, [17,18] in which the interfacial charge transfer arising from the van der Waals (vdW) interactions between the graphitic planes has been responsible for the electrocatalytic performance.Furthermore, boron radicals at defects and edges of exfoliated BN nanosheets (BNNs) have been recently identified as active species in h-BN, that can promote the catalytic process. [19]However, unlike metal catalysts, further understanding toward reaction intermediate energetics is highly required to reconcile the mechanistic interpretations of nonmetal BN alternatives on these electrochemical systems.
Herein, we conduct density-functional theory (DFT) calculations to demonstrate that the inert h-BN can generate boron active radicals at nitrogen vacancy sites by forming heterogeneous structures with defective graphene, leading to a substantial electron delocalization and charge transfer.This immediately suggests to experimentally design and construct h-BN/graphene vdW heterostructures with chemically exfoliated h-BNNs anchored on large-area rGO as strongly coupled composite OER catalysts.The formation of vacancy defects at interfacial heterostructures provides a unique chemical coordination environment and electronic interaction to boost an unexpected, improved OER activity in alkaline electrolyte, albeit poor conductivity in h-BN and lower catalytic activity in rGO.The combined theoretical and experimental studies reveal that the formation of heterostructures can significantly drive the electron transfer between C and B atoms near the vacancies across the interface and cause a half-metallic property of BN, therefore decreasing the free energy barrier of four-electron OER kinetics.In addition, the enhanced π bonding in the framework can greatly improve the stability of the resultant catalysts and promote their electron donor-acceptor properties.This work aims to provide theoretical and experimental basis for modulating the OER kinetics in metal-free heterostructure catalysts by controlling the interface environment at hybrid structure for stabilizing the OER intermediates and eventually achieving high OER activities.

Structure and Catalytic Mechanism of G/BNNs Heterostructures
Edge sites and topological defects are unavoidable in BN nanomaterials; however, the radicals at edge are greatly inhibited as compared to surface. [19]Our prior studies showed that the boron and nitrogen vacancies are inherent topological defects in chemically exfoliated BNNs and can be substantially produced during the sample synthesis or afterward, [20] which are generally hypothesized to be radicals capable of electrocatalytic activity.Thus, we first perform DFT calculations on a stacking vdW heterostructure of graphene and h-BN monolayers (denoted as G/BNNs) with different atomic models to understand the origin of catalytic activity.Single nitrogen (boron) vacancy (V N /V B ) is created by removing a single N (B) atom from a supercell of 4 Â 4 h-BN monolayer.In contrast, carbon vacancy (V C ) is mostly observed in rGO, which is created by removing a single C atom from a graphene atomic layer.The atomic models of G/BNNs stacking bilayer consisting of graphene with V C on h-BN with V N and graphene with V C on h-BN with V B are illustrated in Figure 1a after optimization.The interlayer model of the heterojunction can realize the polarization electric field which is neither strong nor weak, and promote the transfer between electrons, thus improving the electrocatalytic process. [21,22]For OER, the ability of adsorption/desorption of oxygen intermediates (OH Ã , O Ã , and OOH Ã ) is the key to high activity.As suggested by the calculated optimized absorption location, the C atoms close to V C in graphene are the preferential binding sites for the oxygen intermediates when interacting with the B and N atoms close to V N in h-BN; whereas the G/BNNs heterostructure with h-BN-containing V B is unstable.As a result, the spin-resolved projected density of states (DOS) distributions and band structures of G/BNNs heterostructure with h-BN-containing V N are computed to explore the conductivity, which is vital for the charge transfer required to promote catalytic activity.The calculated DOS (Figure 1b) reveal that the G/BNNs heterostructure exhibits a bandgap of 1.84 eV due to the introduction of the occupied states induced by C and N vacancies to the bandgap.Near the Fermi level, the electronic state of uncoordinated C atom participates more with the spin-down states; whereas, the spin-up states vanish at the Fermi level.The G/BNNs hybrid thus possesses a half-metallic property, [23] which greatly favors the charge transfer at interface.Due to the spin polarization of electrons, an impurity level of 0.26 eV is formed in the energy band as shown in Figure 1c.Further evidence is presented in the local electronic redistributions as shown in Figure 1d.As clearly observed, an electron transfer occurs from the B atoms close to V N in BN slab to C atoms adjacent V C in graphene, suggesting a greater electron donor capacity of B radicals in this heterostructure.More specific, the charge redistribution is preferentially located toward the C atom, which is nearest to the B radical.
Figure 1e,f shows the charge distribution and spin-polarized density of G/BNNs heterostructure.As previously reported, the pure h-BN monolayer containing a single V N was calculated to be 1 μB. [19]However, when forming the heterostructure of G/BNNs, the B atoms close to V N defect center are polarized antiferromagnetically, becoming nonmagnetic (À0.01 μB).The spin density mainly locates on the three nearest neighboring C atoms at V C sites in graphene layer, which exhibits a magnetic moment of 0.37 μB due to the interfacial bonding between C from graphene and the B atom around V N .Prior studies on N-doping carbonbased catalysts showed that higher electronegativity of nitrogen over carbon can impart a partial charge transfer that is beneficial to the adsorption and subsequent desorption of the intermediates involved in the reaction. [24]In our case, the significant electron transfer occurs at around the C and B atoms near the vacancies across the stacking interface, which facilitated the modulation of the electronic structures of both B-N and C-C networks and induce free electrons.
The DFT calculation further demonstrates the most specific active sites in accordance with the four-electron OER reaction in alkaline solutions ( Several potential atomic sites are screened through the interaction with OH Ã , O Ã , OOH Ã , and O 2 Ã (the asterisk denotes the adsorption site), among which three nearest neighboring C atoms around V C in graphene above B atoms around V N in h-BN are the possible absorption sites for the intermediates (denoted as A1, A2, and A3) as shown in Figure 2a.A1 is the nearest position to the underneath B radical in the h-BN monolayer, A2, and A3 are farer.Figure 2b-d illustrates the corresponding reaction process taking A1 as example.The Gibbs free energy change (ΔG) of every elemental step is calculated by using the standard hydrogen electrode model proposed by Nørskov et al. [25] The reaction profiles of the OER at 0 and 1.23 V for G/BNNs following the suggested reaction pathways are present in Figure 2e and S1 in Supporting Information.The free energy of the rate-determining step (RDS) determines the limiting reaction barrier, which can be used to evaluate the catalytic activity.For the site A1, the RDS is the oxidation of H 2 O to OH Ã with lower limiting barrier of 2.48 and 1.25 eV at U = 0 and 1.23 V, respectively.For A2 site, the RDS is the oxidation of O Ã to OOH Ã ; whereas the A3 site also shows a similar with A2 site RDS at the formation of *OOH (Figure S1, Supporting Information).Furthermore, the calculated overpotential μ shows the smallest overpotential (μ = 1.25 V) for A1 among all these reaction sites (Figure 2f ).Thus, the coupling of carbon atom at A1 site and boron radicals near V N has a substantial impact on determining the OER activity in G/BNNs heterostructure-containing V C in graphene and V N in BN.
As control experiments, we also calculate the OER reaction profile of a G/BNNs stacking heterostructure consisting only a V C -containing graphene monolayer on a perfect h-BN monolayer.As Figure S2 in Supporting Information demonstrates, the RDS is the oxidation of O Ã to OOH Ã with limiting barrier of 2.23 V.As is evident, the reaction barrier of G/BNNs without V N defects in h-BN monolayer is higher than that of the heterostructurecontaining V N .That is, only containing V C in graphene, without V N in h-BN, cannot fulfill the role of promoting the OER activity.
Thus, the overall results highlight the critical role of the B atoms together with N vacancy in the OER kinetics of G/BNNs composites.The ab initio molecular dynamics simulations at a temperature of 300 K are conducted to verify the thermal stability of this G/BNNs heterostructure.It is worth noting that after 10 000 fs simulations, the geometric configuration of this G/BNNs heterostructure is stable (Figure S3, Supporting Information).After 10 000 fs simulation, the geometric configuration of the composite structure, and there is no fluctuation or detachment of each atom.The total energy fluctuates within a constant value.In other words, it is experimentally viable to construct the vacancy-containing h-BN in a heterogeneous structure with graphitic graphene.

Heterostructure Synthesis and Characterization
The G/BNNs heterostructure composites were synthesized using a one-step hydrothermal method as described in Experimental Section.The microstructure of the as-prepared heterostructures was first studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM).Unlike the previous report of BN/graphene hybrid, which mostly exhibits B/N codoped graphene planes [26] and/or BCN clusters, [27] BNNs are well dispersed and wrapped within the large-area graphene atomic sheets (Figure 3a,b).The typical size of BNNs is in the range of hundreds of nanometers in diameter and several nanometers in thickness; whereas, rGO is in larger size of 30-50 μm as the substrate for BNNs.In contrast to the original smooth surface (Figure S4, Supporting Information), which is not conducive to form catalytic sites, [28] rGO layers become more corrugated after hybridizing with BNNs.[31] In addition, the entire contact of BNNs surfaces with rGO is expected to provide more interactive sites for the subsequent OER reaction.
The structures of the as-synthesized G/BNNs heterostructures were also revealed by X-Ray diffraction (XRD), Fourier transformed infrared (FTIR), and Raman.The (002), (101), and (004) planes of h-BN (JCPDS Card 34-0421) clearly indicate the presence of BNNs within rGO sheets in the as-synthesized heterostructures, neither homogenous BCN hybridizing clusters nor BN-doped carbon nanomaterials (Figure S5, Supporting Information).A slight asymmetric broadening in FTIR peak (Figure S6, Supporting Information) is due to the formation of defects in BNNs resultant from the hydrothermal process. [32]s previously reported, hydrothermal reaction process not only removes oxygen-containing functional groups from chemically exfoliated graphene [33] or BNNs, [34] but also introduces a large number of defects.The formation of the defects is also confirmed by electron paramagnetic resonance measurements (Figure S7, Supporting Information).Raman spectra show characteristic D and G vibrational modes in G/BNNs heterostructures (Figure 3c), in which the D band at 1354 cm À1 demonstrates an intermediate shifting as compared with that in individual rGO (1351 cm À1 ) and BNNs (1361 cm À1 ).This is due to the hybridizing of BN and rGO in the heterostructure.Generally, D band originates from the phonon scattering at the regional boundary caused by sp 3 defective sites, while the G band arises from the plane sp 2 C-C or B-N stretching vibration. [35]The intensity ratio I D /I G , a measure of the defect or defect-induced charge density in carbon materials, [36] is increased from 1.03 in rGO to 1.34 in G/BNNs heterostructure.This clearly implies that defect-induced charge transfer occurs in the hetero-interface. [28]Such charge transfer can cause localized strains in the graphene lattice, resulting in an increased I D /I G ratio for a hybrid construction.Thus, the defects in G/BNNs are pivotal to drive interfacial charge transfer in the heterostructures to improve the catalytic performance.
The elemental distribution of carbon, nitrogen, and boron is further confirmed by energy-dispersive X-ray spectroscopy elemental mapping measurements as shown in Figure 3d-f and S8 in Supporting Information, revealing the configuration of a BN flake on a large-area C substrate in the heterostructures.This is further supported by TEM results, in which several BNNs homogenously emerge throughout the whole graphene layers (Figure 3g).The high-resolution TEM image in Figure 3h shows a lattice spacing of 0.25 nm, which matches (100) crystal plane of h-BN lattice.The hexagonal diffraction spots and diffraction rings in the selected area electron diffraction pattern (inset of Figure 3i) further verify the single crystal nature of BNNs on rGO substrate as compared with the individual rGO and BNNs, respectively (Figure S9, Supporting Information).In addition, the marked region in Figure 3i displays defective areas, primarily from the formation of vacancies, such as V N and V B , due to the preparation process of BNNs. [37,38]The presence of large amounts of defects should be the most active region for adsorption/desorption of the OER intermediates.Nevertheless, the vdW interfacial interaction of h-BN and rGO is revealed by X-ray photoelectron spectroscopy (XPS) (Figure S10, Supporting Information).Different from forming the chemical bonds, both B 1s and N 1s core-levels are shifted toward higher binding energies after the formation of G/BNNs heterostructure, reflecting that the Fermi-level shifts toward the conduction band (CB) as a result of the interfacial charge transfer. [39]

OER Activity of G/BNNs Heterostructures
The electrocatalysis performance of OER on the G/BNNs heterostructures was evaluated using a three-electrode system under O 2 -saturated 0.1 M KOH at room temperature (25 °C).The G/BNNs heterostructure samples with different quantities of BNNs to rGO from 1:1 to 1:7 in preparation were all investigated.Meanwhile, the individual components (rGO and BNNs) catalyst were also measured as controlled experiment.Linear sweep voltammetry curves were obtained at a scan rate of 5 mV s À1 to reduce the capacitive current after activation of repeated cyclic voltammogram scans and the results are shown in Figure 4a.An overpotential at 10 mA cm À2 was used as an important criterion to evaluate the OER activity. [40]The pure BNNs show almost neglectable catalytic activity due to its electronic insulating nature.For the pure rGO, an overpotential of 530 mV is required to obtain the current density of 10 mA cm À2 and the current density at 1.6 V RHE is 3.7 mA cm À2 .After hybridizing with BNNs, the G/BNNs with mass ratio of 1:3 exhibits the lowest overpotential of 420 mV at 10 mA cm À2 , while the current density at 1.6 V RHE is improved to 6.4 mA cm À2 (Figure S11, Supporting Information).Those values are comparable to those of metal-free carbon-related OER catalysts in experiments, as BCN/C 60 catalysts (390 mV) and the benchmark RuO 2 catalysts (410 mV) in 0.5 M NaOH, [41] and Pt-containing OER catalysts in theory (%0.4 V) [42] (Table S1, Supporting Information).The Tafel slopes in Figure 4b further illustrate the unexpected catalytic activity in the G/BNNs heterostructure.The lowest value of 279 mV decade À1 is obtained for the heterostructure 1:3 among pure BNNs, rGO and all the other G/BNNs.This result strongly suggests that the G/BNNs 1:3 activates water-oxidation reaction kinetics, [43] which is further supported by the Nyquist plots shown in Figure 4c.The charge-transfer resistance of G/BNNs1:3 is smallest, suggestive of fastest charge-transfer process among all the samples.The subsequent increase of BNNs content in G/BNNs heterostructures leads to a suppression of OER activity.This is mainly resultant from the restricted interfacial interaction due to the reduced laminar surface between BNNs and rGO since large amount of BNNs agglomerate together when BNNs content increases up 1:7 (Figure S12, Supporting Information).Concomitant with the aforementioned DFT calculations, the experimental results suggest that boron radicals at defective sites in G/BNNs interfaces generate the most active sites for the OER.In addition, the electrochemical active surface area and intrinsic activity of the catalyst were also studied (Figure S13, Supporting Information), suggesting an improved catalytic activity of G/BNNs 1:3 as compared to other catalysts.Furthermore, the OER stability is another important factor to evaluate the electrocatalysts.Figure 4d illustrates the potential of G/BNNs hybrid composites at a constant current density of 10 mA cm À2 , in which the potential remains unchanged for 10 h.In essence, the interfacial charge transfer between graphitic B-N and C-C hybrid, that can break the charge neutrality to benefit for O 2 adsorption, [44] plays a critical role for G/BNNs heterostructures in both of the activity and stability of OER process.In addition, the vacancies and defects in G/BNNs are also viable in the OER reaction, as the same heterostructure (G/BNNs 1:3) without vacancies and defects shows lower OER activity (Figure S14, Information).

Interfacial Charge Transfer at G/BNNs Heterostructures
To further corroborate the important role of interfacial charge transfer in OER electrocatalysis, we further conducted the small binding energy range scan of XPS-valence band spectral (VBS) analysis and the results are shown in Figure 5, S15, and S16 in the Supporting Information.The work function Φ can be estimated by the following relationship: Φ ¼ ΔV þ φ, where φ is the work function of the XPS analyzer (4.55 eV).ΔV is contact potential difference of the sample, which can be determined by the distance between two inflection points of the VBS curves, as shown in Figure 5a-c.Also, the valence band (VB) potential can be also calculated by the empirical relationship VB ¼ φ þ ΔV À 4.44. [45]The work function of BNNs, rGO, and G/BNNs 1:3 is calculated to be 6, 7.26, and 7.48 V versus NHE, respectively.Thus, rGO has a smaller work function but a higher (E F ), whereas BNNs has a larger work function and a lower E F .When BNNs and rGO form hybrid structures, the charge balance at the interface will be disturbed, thus leading to an electron transfer from a higher E F of rGO to a lower E F of BNNs until the E F tends to equilibrate, which is consistent with the charge density distribution calculations.As illustrated in Figure 5d, the variation in work function accelerates the transfer of the electrons from the CB of BNNs to the valance band left on the surface of rGO, and an electron accumulation layer is built within the interface of the heterostructure.The estimated VB edge is 1.65 and 2.46 V versus NHE for rGO and BNNs, respectively.This gives a significantly large potential barrier of %0.81 V versus NHE at the band edges of G/BNNs, which will greatly promote the redox ability of heterostructure.

Conclusions
In summary, we constructed a G/BNNs vdW heterostructure with h-BNNs anchored on large-area rGO substrate as highly efficient metal-free OER catalysts.The combination of theoretical calculations and the experiments reveal that origin of the enhanced catalytic activity includes the position of electron transfer, the exposed interfacial area and the boron radicals at the vdW interfaces.By forming the heterogeneous structure, the charge rearrangement under electron delocalization of carbon atoms toward boron radicals at defective sites can generate significant charge transfer, leading to a half-metallic property of h-BN and thus improved OER electrocatalytic performances in G/BNNs.The minimum OER overpotential of 420 mV at a current density of 10 mA cm À2 can be achieved, which is comparable to those of non-precious carbon-related catalysts.Moreover, the catalysts showed excellent OER stability and durability.Significantly, the existence of nitrogen vacancies in BNNs plays a critical role to provide highly active and stable reactive sites of B and C atoms in G/BNNs catalysts.The unexpected OER catalytic performance can be attributed to the strong electronic coupling across the stacked rGO and BNNs layers, in which a fast charge transfer from B atoms near nitrogen vacancies of BN to C atoms in rGO.Our work not only identifies the importance of boron radicals together with topological vacancies being responsible for the activity of h-BN, but also opens up the possibility for achieving inexpensive, highly efficient metal-free electrocatalysts for a broad range of energy conversion and storage applications.

Experimental Section
Sample Preparation: The BNNs was directly peeled off from the h-BN bulk powder (99.5%, Alfa Aesar) by using an acid solution in one step as described previously. [46]The resultant BNNs generally consists of large numbers of point vacancies and remains intact in scale. [37]Similarly, graphene oxide (GO) was prepared by the modified Hummers method. [35]O suspension (20 mL, 1 mg mL À1 ) was mixed with different amounts of BNNs powders (20, 60, 100 and 140 mg), then were ultrasonicated briefly.Finally, the mixture of GO and BNNs was employed in a hydrothermal route to get G/BNNs composites, where the reaction was conducted at 180 °C for 12 h.The final product was freeze-dried.
Characterizations: SEM measurements were obtained from Philips FEI Quanta Magellan 400 equipment.TEM was performed using a JEOL JEM-2200FS.XRD was performed using a Cu Kα radiation of Bruker D8.FTIR spectra were obtained on Thermo Fisher Scientific Nicolet iS20.XPS (ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA) was performed with Al-Kα radiation (hν ¼ 1486.6 eV) in a detection chamber with a pressure of 2.6 Â 10 À9 mbar.
Electrochemical Measurements: Electrochemical measurements were performed with a three electrodes cell configuration on a PARSTAT 3000A workstation (Ametek, USA) under ambient conditions.A platinum sheet was used as the counter electrode while Hg/HgO was used as the reference.The working electrode was a 1 Â 1 cm 2 nickel foam.The tested electrolyte uses 0.1 mol L À1 KOH solution.In a typical electrode preparation, 5 mg of synthesized catalyst was dispersed in 1 mL aqueous solution (750 μL deionized water and 250 μL isopropyl alcohol) and then ultrasonically dispersed in 20 μL Nafion aqueous solution.Next, 4 μL of catalyst ink dispersion was transferred onto a nickel foam.Potentials reported in this study were all quoted against the reversible hydrogen electrode (RHE) using equation E RHE ¼ E Hg=HgO þ 0.098 þ 0.059 Â pH, where E RHE is the potential calibrated against RHE and E Hg=HgO is the potential measured against the Hg/HgO reference electrode.
DFT Calculations: The theoretical calculations were performed using the Vienna ab initio simulation package code.The projector augmented wave method was used to describe the ion-electron interaction.The electronic exchange and correlation were approximation by generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional with the cutoff energy of 600 eV.To solve the problem that PBE cannot accurately describe the weak interaction, DFT-D3 scheme was applied.In this case, a supercell of 4 Â 4 was chosen.The vacuum region was set 20 Å to make sure that no interaction between each heterojunction.The atomic relaxation and electronic properties calculation with Monkhorst-Pack method were carried out 3 Â 3 Â 1 and 4 Â 4 Â 1 k-point mesh for Brillouin zone.For geometry optimization, all atoms were fully relaxed until the energies and residual forces on each atom converged to 10 À5 eV and 0.01 eV Å À1 , respectively.Spin-polarization was considered in all our calculations.According to hydrogen electrode model, the adsorption free energy for adsorbed species in OER, including OH Ã , O Ã and OOH Ã , can be expressed by the following equation: ΔG ads ¼ ΔE ads þ ΔE ZPE À TΔS.ΔE ads is the adsorption energy change of adsorbates, ΔE ZPE is the zeropoint energy calculated from the vibrational frequencies, ΔS is the entropy change, and T is the system temperature (298.15K).Generally, in alkaline media, the OER reaction mechanism can be written as four-electron change mechanism below the list Step 2∶ OH Step 3∶ O Step 4∶ OOH where * presents an adsorption site on the catalyst, and OH Ã , O Ã , and OOH Ã denote the corresponding absorbed intermediates.The free energy of reactions (from Step 1 to Step 4) can be calculated using aforementioned equations ΔG 4 ¼ 4.92 eV À ΔG 1 À ΔG 2 À ΔG 3 (8)   The ΔG for intermediates of each step is considered as a reasonable description for the difficulty of OER.The theoretical overpotential η for OER can be calculated using the following equations: G OER ¼ max ðΔG 1 , ΔG 2 , ΔG 3 , ΔG 4 Þ, η OER ¼ G OER À 1.23 V.

Figure 1 .
Figure 1.a) The top and side views of the corresponding energetically optimized configurations; b) partial density of states with different elements; c) representing band structure, the black line is spun up, the red line is spun down; and d) corresponding electron charge density distributions.Yellow and cyan regions represent the electron accumulation and depletion, respectively.The isosurface is AE0.002 e Å À3 .e) The charge distribution and f ) spinpolarized density for G/hexagonal boron nitride (h-BN) nanosheets (BNNs) in the 3D isosurface version with the value of 0.02 e Å À3 .In (f ), spin up and spin down are represented in red and yellow, respectively.

Figure 2 .
Figure 2. Electrochemical performances of G/BNNs with vacancies in oxygen evolution reaction (OER).a) Density-functional theory (DFT) computations on the free energy of the reaction pathways; b-e) the corresponding reactive path of OER; f ) and volcano plots for OER of different reaction sites A1, A2, and A3.

Figure 4 .
Figure 4. Electrocatalytic characterization of G/BNNs, BNNs, and rGO in O 2 -saturated 0.1 mol L À1 KOH.a) Linear sweep voltammetry (LSV) curves (scan rate 5 mV s À1 ) normalized to the geometric area of the electrodes; b) the corresponding Tafel plots; c) Nyquist plots of electrochemical impedance spectra; and d) galvanostatic stability test of G/BNNs at a current density of 10 mA cm À2 .