Hybrid Molybdenum Carbide/Heteroatom-Doped Carbon Electrocatalyst for Advanced Oxygen Evolution Reaction in Hydrogen Production

Hydrogen energy is one of the key technologies that can help to prevent global warming. A water electrolysis process can be used to produce hydrogen, in which hydrogen is produced at one electrode of the electrochemical cell, and oxygen is produced at the other electrode. On the other hand, the oxygen evolution reaction (OER) requires multiple reaction steps and precious-metal-based catalysts (e.g., Ru/C, Ir/C, RuO2, and IrO2) as electrocatalysts to improve the reaction rate. Their high cost and limited supply, however, limit their applications to the mass production of hydrogen. In this study, boron, nitrogen-doped carbon incorporated with molybdenum carbide (MoC-BN/C) was synthesized to replace the precious-metal-based catalysts in the OER. B, N-doped carbon with nanosized molybdenum nanoparticles was fabricated by plasma engineering. The synthesized catalysts were heat-treated at 600, 700, and 800 °C in nitrogen for one hour to enhance the conductivity. The best MoC-BN/C electrocatalysts (heated at 800 °C) exhibited superior OER catalytic activity: 1.498 V (vs. RHE) and 1.550 V at a current density of 10 and 100 mA/cm2, respectively. The hybrid electrocatalysts even outperformed the noble electrocatalyst (5 wt.% Ru/C) with higher stability. Therefore, the hybrid electrocatalyst can replace expensive precious-metal-based catalysts for the upcoming hydrogen economy.


Introduction
To prevent global warming, cars using hydrogen fuel cells, an eco-friendly technology, are expected to be on the market in the near future [1][2][3]. The development of technology for hydrogen fuel cells related directly to hydrogen fuel cell cars is very important, but the infrastructure for hydrogen production is needed. Currently, hydrogen is produced from the hydrocarbon reforming process, which is not a completely eco-friendly method and is undesirable from a long-term perspective [4]. On the other hand, water electrolysis, which produces hydrogen directly from water, is the most eco-friendly and desirable method [5]. In water electrolysis, the kinetics of the oxygen evolution  Using the Brunauer-Emmett-Teller (BET) method, the surface properties of the catalysts were investigated and are represented in Figure 2b,c and Table 1. As can be seen in the SEM and TEM images of Figure 1, all porosity in the catalysts occurred from interparticle voids. The N 2 adsorption-desorption isotherms of all samples showed Type-IV isotherms with H3 hysteresis loops. The hysteresis loops of high P/P 0 indicate that capillary condensation takes place in the mesopores [37]. The pore size distribution plots confirmed that most of the pores were made up of mesopores and macropores. With increasing heat-treatment temperature, the BET surface area increased, and 800 MoC-BN/C had the largest BET surface area with 295.16 m 2 /g. In addition, the total pore volume and average Catalysts 2020, 10, 1290 4 of 12 pore diameter decreased with increasing temperature due to the instability of grain structure and the pulverization of the crystal nucleus [38,39]. Using the Brunauer-Emmett-Teller (BET) method, the surface properties of the catalysts were investigated and are represented in Figure 2b,c and Table 1. As can be seen in the SEM and TEM images of Figure 1, all porosity in the catalysts occurred from interparticle voids. The N2 adsorptiondesorption isotherms of all samples showed Type-IV isotherms with H3 hysteresis loops. The hysteresis loops of high P/P0 indicate that capillary condensation takes place in the mesopores [37]. The pore size distribution plots confirmed that most of the pores were made up of mesopores and macropores. With increasing heat-treatment temperature, the BET surface area increased, and 800 MoC-BN/C had the largest BET surface area with 295.16 m 2 /g. In addition, the total pore volume and average pore diameter decreased with increasing temperature due to the instability of grain structure and the pulverization of the crystal nucleus [38,39]. The chemical composition of 800 MoC-BN/C was identified by X-ray photoelectron spectroscopy (XPS) (Figure 3). Before the deconvolution of each peak, the XPS spectra were calibrated using the C 1s peak (~284.6 eV). Figure 3a shows that the C 1s was composed of four types of bonding: C-C (~284.6 eV), C-N (~285.4 eV), C-O (~286.7 eV), and C = O (~289.2 eV) [40]. In the C 1s spectrum, the C-C bonding was the most dominant peak. The N 1s spectrum was deconvoluted into pyridinic N (~399.0 eV), pyrrolic N (~400.4 eV), graphitic N (~401.7 eV), and oxidized N (~403.0 eV) [41,42]. These various bonding types were fabricated using quinoline as the precursor in plasma engineering. The B 1s spectrum was deconvoluted simply into two peaks: B-C (~189.0 eV) and B-N (~190.6 eV) [43,44]. In the B 1s spectrum, the B-C peak was higher than the B-N peak with showing the peak area ratio B-C: B-N = 1.47: 1. This means that most of the boron atoms were bonded with carbon atoms, but the number of boron atoms bonded with nitrogen atoms was also not little. Molybdenum can have various oxidation states. The Mo 3d peak could be deconvoluted into four peaks: Mo 0 (~228.1 eV and~231.6 eV), Mo 3+ (~229.1 eV and~232.9 eV), Mo 4+ (~230.1 eV and~234.9 eV), and Mo 6+ (~232.3 eV and~235.9 eV) [45,46]. The presence of especially high metallic molybdenum (Mo 0 ) in 800 MoC-BN/C would be contributed to the high OER catalytic activity [23].  The chemical composition of 800 MoC-BN/C was identified by X-ray photoelectron spectroscopy (XPS) (Figure 3). Before the deconvolution of each peak, the XPS spectra were calibrated using the C 1s peak (~284.6 eV). Figure 3a shows that the C 1s was composed of four types of bonding: C-C (~284.6 eV), C-N (~285.4 eV), C-O (~286.7 eV), and C = O (~289.2 eV) [40]. In the C 1s spectrum, the C-C bonding was the most dominant peak. The N 1s spectrum was deconvoluted into pyridinic N (~399.0 eV), pyrrolic N (~400.4 eV), graphitic N (~401.7 eV), and oxidized N (~403.0 eV) [41,42]. These various bonding types were fabricated using quinoline as the precursor in plasma engineering. The B 1s spectrum was deconvoluted simply into two peaks: B-C (~189.0 eV) and B-N (~190.6 eV) [43,44]. In the B 1s spectrum, the B-C peak was higher than the B-N peak with showing the peak area ratio B-C: B-N = 1.47: 1. This means that most of the boron atoms were bonded with carbon atoms, but the number of boron atoms bonded with nitrogen atoms was also not little. Molybdenum can have various oxidation states. The Mo 3d peak could be deconvoluted into four peaks: Mo 0 (~228.1 eV and~231.6 eV), Mo 3+ (~229.1 eV and~232.9 eV), Mo 4+ (~230.1 eV and~234.9 eV), and Mo 6+ (~232.3 eV and~235.9 eV) [45,46]. The presence of especially high metallic molybdenum (Mo 0 ) in 800 MoC-BN/C would be contributed to the high OER catalytic activity [23].

Electrochemical Performances for the OER
The linear sweep voltammetry (LSV) method was used to evaluate the electrochemical catalysis activities in the OER (Figure 4a,b). 800 BN/C was fabricated by the same conditions with 800 MoC-BN/C except for using graphite electrodes instead of molybdenum electrodes during plasma engineering. From the comparison between 800 BN/C and 800 MoC-BN/C, it was confirmed that the addition of molybdenum carbides (MoC) into heteroatom-doped BN/C can boost the OER activity.
The OER mechanism in alkaline media is considered as follows: where M represents a catalyst surface and * indicates an active site on the surface. In general, the OER process starts with the adsorption of the OH-molecule on the catalyst. Based on DFT calculation, the B atom in the B, N-doped carbon matrix exhibits the more negative adsorption energy of OH* molecules and served as the major active site [21]. From our results, as shown in Figure 4a, the incorporation of MoC further enhances the OER activity of B, N-doped carbon, which might be accounted for further lowering the OH* adsorption energy via the synergic effect between MoC and B active sites. At a current density of 10 mA/cm 2 , 5 wt.% Ru/C of the benchmark for the OER showed the lowest overpotential with 267 mV, but the difference with 800 MoC-BN/C was only 1 mV. Even at 50 and 100 mA/cm 2 , 800 MoC-BN/C had the lowest overpotential with 305 mV and 320 mV, respectively. In addition, the overpotentials of the MoC-BN/C catalysts decreased with increasing heat-treatment temperature and showed lower overpotentials than 5 wt.% Ru/C except for 10 mA/cm 2 . This was attributed to the enhanced electroconductivity by desorbing the hydrogen from

Electrochemical Performances for the OER
The linear sweep voltammetry (LSV) method was used to evaluate the electrochemical catalysis activities in the OER (Figure 4a,b). 800 BN/C was fabricated by the same conditions with 800 MoC-BN/C except for using graphite electrodes instead of molybdenum electrodes during plasma engineering. From the comparison between 800 BN/C and 800 MoC-BN/C, it was confirmed that the addition of molybdenum carbides (MoC) into heteroatom-doped BN/C can boost the OER activity.
The OER mechanism in alkaline media is considered as follows: where M represents a catalyst surface and * indicates an active site on the surface. In general, the OER process starts with the adsorption of the OH-molecule on the catalyst. Based on DFT calculation, the B atom in the B, N-doped carbon matrix exhibits the more negative adsorption energy of OH* molecules and served as the major active site [21]. From our results, as shown in Figure 4a, the incorporation of MoC further enhances the OER activity of B, N-doped carbon, which might be accounted for further lowering the OH* adsorption energy via the synergic effect between MoC and B active sites. At a current density of 10 mA/cm 2 , 5 wt.% Ru/C of the benchmark for the OER showed the lowest overpotential with 267 mV, but the difference with 800 MoC-BN/C was only 1 mV. Even at 50 and 100 mA/cm 2 , 800 MoC-BN/C had the lowest overpotential with 305 mV and 320 mV, respectively. In addition, the overpotentials of the MoC-BN/C catalysts decreased with increasing heat-treatment temperature and showed lower overpotentials than 5 wt.% Ru/C except for 10 mA/cm 2 . This was attributed to the enhanced electroconductivity by desorbing the hydrogen from the carbon lattice and forming of sp 2 -bonded carbon [47]. The mass activity of 800 MoC-BN/C was 5.46 mA/mg at 1.53 V (vs. RHE) and showed the highest mass activity compared with other catalysts. The mass activity of 600 and 700 MoC-BN/C showed 4.14 and 4.49 mA/mg, and it was higher than that of 5 wt.% Ru/C (3.12 mA/mg). The kinetic properties during the OER process were identified through a Tafel plot (Figure 4c). The 800 MoC-BN/C showed the lowest Tafel slope value, 50.7 mV/dec. This means that the lowest overpotential is needed to achieve the required current density at 800 MoC-BN/C among the catalysts. This linear tendency would be related to the increasing BET surface area with heat-treatment temperature.
The kinetic properties during the OER process were identified through a Tafel plot (Figure 4c). The 800 MoC-BN/C showed the lowest Tafel slope value, 50.7 mV/dec. This means that the lowest overpotential is needed to achieve the required current density at 800 MoC-BN/C among the catalysts. This linear tendency would be related to the increasing BET surface area with heattreatment temperature.
The cyclic voltammetry (CV) method with variable scan rates was carried out to identify the electrochemically active surface area (ECSA) of the MoC-BN/C catalysts in a non-faradaic region 0.94~1.04 V (vs. RHE) ( Figure S3). ECSA was compared simply with Cdl (electrochemical double-layer capacitance), which was calculated by dividing the linear slope of the capacitive current versus scan rate by two [48]. The Cdl values of each electrocatalyst were 36.89, 38.84, and 46.22 mF/cm 2 for 600, 700, and 800 MoC-BN/C, respectively. This linear tendency would be related to the increasing BET surface area with heat-treatment temperature [49] and was attributed to the enhanced probability of the occurrence of OER. capacitance), which was calculated by dividing the linear slope of the capacitive current versus scan rate by two [48]. The C dl values of each electrocatalyst were 36.89, 38.84, and 46.22 mF/cm 2 for 600, 700, and 800 MoC-BN/C, respectively. This linear tendency would be related to the increasing BET surface area with heat-treatment temperature [49] and was attributed to the enhanced probability of the occurrence of OER.
The charge transfer resistance (R ct ) of the MoC-BN/C catalysts was measured using the electrochemical impedance spectroscopy (EIS) method with the Nyquist plot at 1.580 V (vs. RHE), in which the OER was proceeded (Figure 4e). The Nyquist plots were fitted using a simple equivalent circuit, as shown in the figure [50]. Each semicircle is presented to the R ct during the OER process. The R ct value of 700 and 800 MoC-BN/C was the same with 0.54 Ω. In the case of 600 MoC-BN/C, the R ct was 0.60 Ω.
The durability of 800 MoC-BN/C catalyst, which has the best activity among the synthesized catalysts, was tested using the chronopotentiometry (CP) method at 10 mA/cm 2 for 20,000 s (Figure 4f). At 10 s of CP, the potential was 1.535 V (vs. RHE). After 20,000 s, the potential increased by only 9 mV. Even in the LSV curves, after CP, a slightly lower overpotential was observed at a high current density.
By comparing the SEM images of 800 MoC-BN/C before ( Figure 1c) and after OER (Figure 5a), it was confirmed that there was a little agglomeration, but the morphology remained almost unchanged. This showed that the catalyst has excellent stability. For comparison of the crystal structure change after the durability test, XRD analysis was conducted with the CP tested 800 MoC-BN/C powder. There were several peaks except for the original 800 MoC-BN/C peaks due to slurry components, electrolyte, and nickel foam. The XRD peaks at 24.2 • , 30.0 • , and 33.9 • were attributed to the (110), (111), and (200) planes for potassium of 1M KOH electrolyte, respectively. In addition, the KO 2 peak at 31.3 • mostly results from the electrolyte. The Ni peak at 44.4 • occurred from nickel foam used as an electrode. The XRD pattern also showed other peaks aside from 800 MoC-BN/C peaks, but from maintaining 800 MoC-BN/C peak shape, it could be seen that there is little structural change in the catalyst after the OER.  The charge transfer resistance (Rct) of the MoC-BN/C catalysts was measured using the electrochemical impedance spectroscopy (EIS) method with the Nyquist plot at 1.580 V (vs. RHE), in which the OER was proceeded (Figure 4e). The Nyquist plots were fitted using a simple equivalent circuit, as shown in the figure [50]. Each semicircle is presented to the Rct during the OER process. The Rct value of 700 and 800 MoC-BN/C was the same with 0.54 Ω. In the case of 600 MoC-BN/C, the Rct was 0.60 Ω.
The durability of 800 MoC-BN/C catalyst, which has the best activity among the synthesized catalysts, was tested using the chronopotentiometry (CP) method at 10 mA/cm 2 for 20,000 s ( Figure  4f). At 10 s of CP, the potential was 1.535 V (vs. RHE). After 20,000 s, the potential increased by only 9 mV. Even in the LSV curves, after CP, a slightly lower overpotential was observed at a high current density.
By comparing the SEM images of 800 MoC-BN/C before ( Figure 1c) and after OER (Figure 5a), it was confirmed that there was a little agglomeration, but the morphology remained almost unchanged. This showed that the catalyst has excellent stability. For comparison of the crystal structure change after the durability test, XRD analysis was conducted with the CP tested 800 MoC-BN/C powder. There were several peaks except for the original 800 MoC-BN/C peaks due to slurry components, electrolyte, and nickel foam. The XRD peaks at 24.2°, 30.0°, and 33.9° were attributed to the (110), (111), and (200) planes for potassium of 1M KOH electrolyte, respectively. In addition, the KO2 peak at 31.3° mostly results from the electrolyte. The Ni peak at 44.4° occurred from nickel foam used as an electrode. The XRD pattern also showed other peaks aside from 800 MoC-BN/C peaks, but from maintaining 800 MoC-BN/C peak shape, it could be seen that there is little structural change in the catalyst after the OER.

Synthesis of MoC-BN/C by Plasma Engineering
As precursors of the plasma engineering, quinoline (purity > 95 %, Junsei Chemical, Tokyo, Japan) and boric acid (purity > 99.5 %, Junsei Chemical, Tokyo, Japan) were mixed using a homogenizer (Ultrasonic wave PZT vibrator, KSC-80, Korea Process Technology, Seoul, South Korea) for 3 h at 20 V. The boric acid was dissolved in 50 mL quinoline for 10 mM concentration. Molybdenum wire (Nilaco, Tokyo, Japan, 1.5 mm diameter) was used for the solution plasma electrodes. The plasma was discharged for 30 min at 4 kV at 50 kHz, and a pulse width of 0.8 μs to synthesize the MoC-BN/C catalysts ( Figure 6). After discharge, the solution was filtered with a

Synthesis of MoC-BN/C by Plasma Engineering
As precursors of the plasma engineering, quinoline (purity > 95 %, Junsei Chemical, Tokyo, Japan) and boric acid (purity > 99.5 %, Junsei Chemical, Tokyo, Japan) were mixed using a homogenizer (Ultrasonic wave PZT vibrator, KSC-80, Korea Process Technology, Seoul, South Korea) for 3 h at 20 V. The boric acid was dissolved in 50 mL quinoline for 10 mM concentration. Molybdenum wire (Nilaco, Tokyo, Japan, 1.5 mm diameter) was used for the solution plasma electrodes. The plasma was discharged for 30 min at 4 kV at 50 kHz, and a pulse width of 0.8 µs to synthesize the MoC-BN/C catalysts ( Figure 6). After discharge, the solution was filtered with a polytetrafluoroethylene filter paper (diameter 55 mm, pore size 0.45 µm), and the filtered sample was dried in an oven for one day at Catalysts 2020, 10, 1290 8 of 12 80 • C. The dried sample was ground to a powder, and the sample was heated at 600, 700, and 800 • C for 1 h in nitrogen gas to improve the electrical conductivity.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 12 polytetrafluoroethylene filter paper (diameter 55 mm, pore size 0.45 μm), and the filtered sample was dried in an oven for one day at 80 °C. The dried sample was ground to a powder, and the sample was heated at 600, 700, and 800 °C for 1 h in nitrogen gas to improve the electrical conductivity.

Material Characterization
Scanning electron microscopy (SEM, Supra 25, Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM, Talos F200X, FEI, Hillsboro, OR, USA) were used to observe the morphology of the fabricated powder. In TEM analysis, energy dispersive X-ray spectroscopy (EDS) was accompanied to analyze the chemical composition. X-ray diffraction (XRD, Xpert 3, Malvern Panalytical, Malvern, UK) was used to determine the crystal structure. The surface area, total pore volume, and pore size distribution were analyzed using the Brunauer-Emmett-Teller (BET, nanoPOROSITY-XQ, MiraeSI, Gwangju, South Korea) method. Chemical bonding and composition of the powder were characterized by X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos Analytical, Manchester, UK).

Electrochemical Measurement
A potentiostat/galvanostat (VSP, Bio-Logic, Grenoble, France) was used to evaluate the electrochemical properties of the synthesized catalysts. The electrochemical test cell consisted of three electrodes. A Hg/HgO electrode (1M NaOH) and a spiral Pt wire electrode were used for the reference electrode and counter electrode, respectively. The working electrode was nickel foam (1 cm 2 ) that had been coated with a slurry prepared by mixing 8 mg of the heat-treated MoC-BN/C powder, 1 mg of carbon nanotubes (Jeno tube 9, JEIO, Ansan, South Korea), 1 mg of poly(vinylidene fluoride) (average Mw~534,000 by GPC, Sigma-Aldrich, St. Louis, MO, USA), and 100 μL of 1-methyl-2-pyrrolidone (purity > 99.5 %, Samchun Chemicals, Seoul, South Korea). To compare the electrochemical property of MoC-BN/C catalysts, 5 wt.% Ru/C (Ruthenium on carbon-extent of labeling: 5 wt.% loading, Sigma-Aldrich, St. Louis, MO, USA) was used as a comparative catalyst, and the same amount with MoC-BN/C powder was loaded in order to make the slurry. Before the electrochemical measurements, 1M KOH solution was saturated with N2 gas and used as the electrolyte. Linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV/s to examine the OER activity of the electrochemical catalyst. From the LSV curve, the Tafel slope was calculated to compare the kinetics of the catalysts in the OER. The double-layer capacitance (Cdl) was calculated by cyclic voltammetry (CV) at various scan rates (2,4,6,8, and 10 mV/s) in a non-faradaic region (0.94~1.04 V vs. RHE) to investigate the electrochemical surface area (ECSA) of the synthesized catalyst [51]. Electrochemical impedance spectroscopy (EIS) was conducted in potentiostatic mode at 1.580 V (vs. RHE), and the data was plotted on a Nyquist plot. The plotted semicircle is denoted as the charge-transfer resistance (Rct) [52]. The stability of the catalyst was analyzed by chronopotentiometry (CP) at a current density of 10

Material Characterization
Scanning electron microscopy (SEM, Supra 25, Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM, Talos F200X, FEI, Hillsboro, OR, USA) were used to observe the morphology of the fabricated powder. In TEM analysis, energy dispersive X-ray spectroscopy (EDS) was accompanied to analyze the chemical composition. X-ray diffraction (XRD, Xpert 3, Malvern Panalytical, Malvern, UK) was used to determine the crystal structure. The surface area, total pore volume, and pore size distribution were analyzed using the Brunauer-Emmett-Teller (BET, nanoPOROSITY-XQ, MiraeSI, Gwangju, South Korea) method. Chemical bonding and composition of the powder were characterized by X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos Analytical, Manchester, UK).

Electrochemical Measurement
A potentiostat/galvanostat (VSP, Bio-Logic, Grenoble, France) was used to evaluate the electrochemical properties of the synthesized catalysts. The electrochemical test cell consisted of three electrodes. A Hg/HgO electrode (1M NaOH) and a spiral Pt wire electrode were used for the reference electrode and counter electrode, respectively. The working electrode was nickel foam (1 cm 2 ) that had been coated with a slurry prepared by mixing 8 mg of the heat-treated MoC-BN/C powder, 1 mg of carbon nanotubes (Jeno tube 9, JEIO, Ansan, South Korea), 1 mg of poly(vinylidene fluoride) (average Mw~534,000 by GPC, Sigma-Aldrich, St. Louis, MO, USA), and 100 µL of 1-methyl-2-pyrrolidone (purity > 99.5 %, Samchun Chemicals, Seoul, South Korea). To compare the electrochemical property of MoC-BN/C catalysts, 5 wt.% Ru/C (Ruthenium on carbon-extent of labeling: 5 wt.% loading, Sigma-Aldrich, St. Louis, MO, USA) was used as a comparative catalyst, and the same amount with MoC-BN/C powder was loaded in order to make the slurry. Before the electrochemical measurements, 1M KOH solution was saturated with N 2 gas and used as the electrolyte. Linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV/s to examine the OER activity of the electrochemical catalyst. From the LSV curve, the Tafel slope was calculated to compare the kinetics of the catalysts in the OER. The double-layer capacitance (C dl ) was calculated by cyclic voltammetry (CV) at various scan rates (2, 4, 6, 8, and 10 mV/s) in a non-faradaic region (0.94~1.04 V vs. RHE) to investigate the electrochemical surface area (ECSA) of the synthesized catalyst [51]. Electrochemical impedance spectroscopy (EIS) was conducted in potentiostatic mode at 1.580 V (vs. RHE), and the data was plotted on a Nyquist plot. The plotted semicircle is denoted as the charge-transfer resistance (R ct ) [52]. The stability of the catalyst was analyzed by chronopotentiometry (CP) at a current density of 10 mA/cm 2 for 20,000 s. Before and after CP, LSV was performed to identify the change in catalytic activity. All potential values were converted to the reversible hydrogen electrode (RHE) with E RHE = E Hg/HgO + (0.059 × pH) + E o Hg/HgO .

Conclusions
In summary, a hybrid molybdenum carbide/heteroatom-doped carbon electrocatalyst was fabricated successfully by a simple plasma engineering. To enhance the electroconductivity of the synthesized catalysts, heat-treatments were conducted at 600, 700, and 800 • C. The catalytic performance increased with increasing heat-treatment temperature. The 800 MoC-BN/C catalyst heat-treated at 800 • C showed the best catalytic performance in the OER. Although the overpotential of 800 MoC-BN/C (268 mV) was slightly higher than that of 5 wt.% Ru/C (267 mV), at a high current density (100 mA/cm 2 ), the 800 MoC-BN/C (320 mV) surpassed 5 wt.% Ru/C (337 mV). In the CP test, 800 MoC-BN/C showed remarkable durability after 20,000 s at 10 mA/cm 2 . Overall, 800 MoC-BN/C is an efficient and high-performed electrocatalyst for the OER.