Vanadium-Doped FeBP Microsphere Croissant for Significantly Enhanced Bi-Functional HER and OER Electrocatalyst

Ultra-fine hydrogen produced by electrochemical water splitting without carbon emission is a high-density energy carrier, which could gradually substitute the usage of traditional fossil fuels. The development of high-performance electrocatalysts at affordable costs is one of the major research priorities in order to achieve the large-scale implementation of a green hydrogen supply chain. In this work, the development of a vanadium-doped FeBP (V-FeBP) microsphere croissant (MSC) electrocatalyst is demonstrated to exhibit efficient bi-functional water splitting for the first time. The FeBP MSC electrode is synthesized by a hydrothermal approach along with the systematic control of growth parameters such as precursor concentration, reaction duration, reaction temperature and post-annealing, etc. Then, the heteroatom doping of vanadium is performed on the best FeBP MSC by a simple soaking approach. The best optimized V-FeBP MSC demonstrates the low HER and OER overpotentials of 52 and 180 mV at 50 mA/cm2 in 1 M KOH in a three-electrode system. In addition, the two-electrode system, i.e., V-FeBP || V-FeBP, demonstrates a comparable water-splitting performance to the benchmark electrodes of Pt/C || RuO2 in 1 M KOH. Similarly, exceptional performance is also observed in natural sea water. The 3D MSC flower-like structure provides a very high surface area that favors rapid mass/electron-transport pathways, which improves the electrocatalytic activity. Further, the V-FeBP electrode is examined in different pH solutions and in terms of its stability under industrial operational conditions at 60 °C in 6 M KOH, and it shows excellent stability.


Introduction
Hydrogen is an efficient green-energy resource with its high gravimetric energy density and carbon-free nature. Hydrogen has emerged as a promising substitution for fossil fuels, which can then gradually decrease climate change and global warming [1][2][3][4][5]. Hydrogen also offers excellent transportability and is convenient to store in a compressed gas and liquid form, much like natural gas and oil. Currently, noble-metal-based electrocatalysts such as Pt/Pd and RuO 2 /IrO 2 are the benchmark electrodes for water splitting. However, the practical production of ultra-fine hydrogen by water electrolysis is hindered due to the limited availability of these elements and high costs [6][7][8]. The development of highly active electrocatalysts at an affordable cost remains to be one of the major research priorities for the green hydrogen supply chain.

Electrochemical Characterization
The 3-electrode (3-E) electrochemical characterizations of the FeBP and V-FeBP electrodes were performed with the target electrode as a working electrode, Pt plate as a counter electrode and Ag/AgCl as a reference in an electrochemical workstation (Wizmac, Daejeon, Korea). The reversible hydrogen electrode (RHE) potential (E) was based on the following relation for the HER and OER: E [V vs. RHE] = E + 0.059 × pH + 0.197 (Ag/AgCl). The polarization curves were obtained using linear-sweep voltammetry (LSV) at a scan rate of 5 mV/s between 0.2 and −0.6 V for the HER and 1.2 and 2.2 V for the OER in 1 M KOH. No iR compensation was adapted in any of the electrochemical characterizations and the data were plotted as received. The electrochemical impedance spectroscopy (EIS) was measured in the range of 100 kHz to 0.1 Hz at the voltage corresponding to 10 mA/cm 2 vs. RHE for the HER and OER catalytic turnover region with an amplitude of 5 mV as shown in Figure S4. Cyclic voltammetry (CV) was performed at different scan rates ranging from 40 to 180 mV/s in a non-faradic region between 0.1 and 0.3 V for the HER and 1.04 and 1.14 V for the OER. From the CV plots, the anodic and cathodic currents were obtained at specific potentials for HER and OER. The electrochemical double-layer capacitance (C dl ) plots were obtained based on ∆J = (J a − J c )/2 as shown in Figures S5 and S6. The slope of the C dl plot was used to estimate the electrochemical surface-active area (ECSA) in Figure S7. The different C dl values for the HER and OER reactions suggest different reaction processes. In addition, the 3-E and 2-E water-splitting performances were measured in different pH waters using 1 M KOH (alkaline), 0.5 M H 2 SO 4 (acidic), and 1 M PBS (neutral). The natural sea water was collected from the Yellow Sea (Incheon, Korea) and river water was obtained from the Han River (Seoul, Korea).

Results and Discussion
In this work, the FeBP electrodes were firstly optimized, and the vanadium (V) doping was optimized on the best FeBP. Firstly, the Fe concentration (FeN 3 O 9 ·9H 2 O) variation was performed between 0.1 and 3 mM for the FeBP electrode optimization as shown in Figure 1. Generally, the microspherical structures were fabricated as seen in Figure 1a-d and larger-scale images can be found in Figure S8. The microspheres were constructed with highly dense layers of croissant-like structures as seen in Figure 1(a-1-d-1). Thus, it was named 'microsphere croissant (MSC)' for the various layers of croissant bread. Along with the increased FeN 3 O 9 ·9H 2 O concentration, the density of MSCs was gradually increased as seen Figures 1a-d and S8. The size of the MSC was up to 20~30 µm. The MSC morphology with the layer-like structures can be largely advantageous for catalytic reactions due to the significantly increased surface area, allowing effective ion access and reactions [37]. The formation of the highly layered 3D structure of Fe-B-P electrocatalyst can be described as below in Equations (1)-(3).
The overall deposition process is: For the fabrication of the FeBP electrode, all the precursors were taken as shown in Equation (1). The Fe, B and P were formed by the corresponding precursors as seen in Equations (2) (3). The NaH 2 PO 2 yields the H 2 PO 2 − complex compound in Equation (4). Then, NH 4 F breaks down into NH4 + and F − in Equation (5). The generated NH 4 + ions can help to stabilize the pH in the solution and the highly electronegative F − ions help to form H bonds, which can increase the solution conductivity and increase the reaction speed in the ionic solution. Further, CO(NH 2 ) 2 (urea) splits into 2NH 3 and CO 2 ↑ in Equation (6). In this process, the generated 2NH 3 reacts with the water molecules to produce two ammonium (NH 4 + ) and two hydroxyl (OH − ) ions in Equation (6). As discussed, the ammonium and hydroxyl ions can also boost the solution conductivity and reaction speed. Finally, the possible fabrication reaction can be described as shown in Equation (7), where the main precursors take part in the formation of FeBP. The boric acid and the formation of HF during the reaction can react with water (HF + H 2 O → H 3 O + F − ) and helps to form more hydronium ions. During the reaction, the formation of hydronium ions induces the formation of bubbles. The bubble formation helps in layered crystal growth that offers a highly electrochemically active surface area. Figure 1e presents the At.% plots of Fe, B and P in the Fe concentration variation set. The At.% showed a gradually increased incorporation of Fe atoms with the increased Fe molarity. B was also more incorporated. However, P showed a gradually decreased incorporation, perhaps due to the high affinity of Fe and B. Additional full-range EDS spectra are provided in Figure S9. Figure 1f shows the Raman spectra of FeBP MSCs with the characteristic peaks at 172, 242, 269, 549, and 936 cm −1 . The Raman contour plots are shown in Figure 1(f-1-f-3). The highest Raman peaks demonstrated by the 1 mM Fe indicates the highest local crystallinity of FeBP MSC structures. Raman intensity was decreased for the other Fe concentrations. The highly local-crystalline FeBP MSC with microsheets can provide faster electron transfer and increased intrinsic electrocatalytic activity by lowering charge-transfer resistance [13]. In terms of electrochemical performance, the HER and OER LSV curves of the FeBP MSC electrodes are provided in Figure 1g,i with the corresponding overpotential values in Figure 1(g-1,i-1). The HER reaction in an alkaline medium can be described by the Volmer, Heyrovsky and Tafel steps, where the metal active sites can react with the H 2 O and generate a metal-hydride bond to produce H 2 [35,38] The Volmer reaction is the production of M-H*, followed by the Heyrovsky step. The Tafel steps explain the whole process of producing H 2 . In the water electrolysis process, the HER is a crucial half-reaction to produce hydrogen at the cathode through a two-electron transfer process with the generation of hydroxyl (OH − ) ions. In contrast, the OER entails four-proton-electron transfer reactions at the anodic metallic atomic sites [39]. OH − + * → HO* + e − , HO* + OH − → O* + H 2 O + e − , O* + OH − → HOO* + e − , HOO* + OH − → * + O 2 (g) + H 2 O + e − . Starting from the hydroxyl (OH − ) generated from the HER, O 2 is evolved through the protonation of HOO* coupled with the regeneration of 2H 2 O at the active sites. In general, the electrical current splits the water molecules into hydrogen and oxygen in alkaline water electrolysis in the presence of metal (M) sites [39]. In the HER and OER reactions, the strong M-H* and M-OH bindings are the key components of the catalytic surface and thus, the strong binding nature of H atoms and hydroxyl ions with a large surface area is important in water electrolysis. The FeBP MSC with the 1 mM Fe demonstrated the best HER and OER performances with the lowest overpotential of 105 and 220 mV at 50 mA/cm 2 , as summarized in Figure 1(g-1,i-1). The 1 mM Fe demonstrated the highest double-layer capacitance (C dl ) values of 1.8 and 2.1 mF/cm 2 for the HER and OER, as seen in Figure 1h,j, which suggests the largest electrochemical surface area (ECSA) of the 1 mM Fe. The improved performance of the FeBP MSC can be attributed to the improved local-crystalline quality and the balance between the ternary Fe, B and P elements with the MSC morphology, which can boost the catalytic activity in an alkaline environment [40]. The MSC structure formed with the appropriate number of Fe, B and P atoms can offer rich active sites for the H and OH − groups, and such a hierarchical structure can benefit the high reaction rate due to the large electrochemical surface area and the acceleration of charge transfer [16]. In addition, the P and B groups can act as electron donors to the d-orbitals of transition metals in the FeBP system, resulting in a high electron concentration of the Fe atoms, which can lower the reaction barriers for the H 2 O and OH − [33].
In addition to the Fe concentration variation (related data Figures S5-S10), the 100 • C reaction temperature (related data Figures S11-S14), 20 mM urea (related data Figures S15 and S16), 30% B and 70% P (FeB 30 P 70 ) (related data Figures S17-S21), and 100 • C post-annealing treatment (related data Figures S22-S26) were found to offer the best optimized performance. Figure S19 shows the XRD patterns of FeBP, FeB and, FeP. The two common peaks at 44.4 and 51.8 • correspond to the (111) and (200) planes of the nickel substrate in the XRD patterns [41]. Generally, the FeBP showed broader peaks with a lower intensity as compared with the FeP and FeB in Figure S19, which could be due to the increase in the short-range polycrystalline phases of FeBP. Generally, the polycrystalline phase can indicate a low electron transfer and high resistance. Thus, a lower electrochemical performance can be expected. However, a recent study showed that the short-range polycrystalline or amorphous phases can be beneficial to the improved electrochemical performance in water electrolysis [42,43]. The polycrystalline phase can offer abundant active sites and higher intrinsic electrochemical activity due to the structural flexibility and stability of the electrocatalysts.  Figure S35 without any change in the morphology. An adequate amount of V incorporation can induce the water-dissociation capacity and can decrease the energy barrier and reduce the impedance of charge transfer. In addition, post-annealing at an appropriate temperature can improve the crystallinity of electrodes by the reduction in point and line defects with the thermal diffusion of atoms [44,45]. In terms of the 2nd post-annealing duration optimization, the 15 min duration showed the best result in Figures S36-S38. Along with post-annealing at various temperatures for 15 min, the 50~100 • C samples showed similar morphologies before and after the annealing in Figure 2(a-b-2). However, the high-temperature-annealed samples showed a slight deformation of croissant layers at 150 • C and more deformation at 200 • C in Figure 2(c-2-d-2). The temperature of 200 • C also had a much lower density of the microsphere croissant (MSC) in Figure 2d. Excess diffusion energy at a high temperature can damage crystallinity due to defect formation and can separate the MSC from the NF during the annealing process. Further, the Raman analyses demonstrated the best intensity with the 50 • C sample, as clearly seen in Figure 2(e-e-3). It clearly demonstrates that the 50 • C sample had better crystallinity, which helps to obtain stable electrochemical activity. In addition, the 50 • C sample demonstrated uniform distributions of Fe L, B K, P K, and V L peaks, indicating the even diffusion of vanadium into the FeBP matrix, as shown by the EDS maps and line profiles in Figure 2(f-f-4,g). In addition to the Fe concentration variation (related data Figures S5-S10), the 100 °C reaction temperature (related data Figures S11-S14), 20 mM urea (related data Figures S15 and S16), 30% B and 70% P (FeB30P70) (related data Figures S17-S21), and 100 °C postannealing treatment (related data Figures S22-S26) were found to offer the best optimized     Figure 3a,e, the V-FeBP electrode annealed at 50 • C demonstrated the best HER and OER performances, and the performance gradually became worse with the increased temperature. The 50 • C sample demonstrated the lowest overpotentials of 52 mV and 210 mV at 50 mA/cm 2 for the HER and OER, as summarized in Figure 3(a-1,e-1). The bar plots in Figure 3(a-1,e-1) clearly show the overpotential values, which followed the sequence of 50 < 100 < 150 < 200 • C. The improved HER and OER performances could be due to the reduced lattice defects and better electrocatalytic activity following the appropriate heat treatment for an appropriate duration [37]. After the V doping and post-annealing optimization, the surface structure of the electrode can reorganize, and thus can introduce more active sites on the catalytic surface [46]. Additional active sites can speed up the electrochemical HER and OER reaction processes by increasing the conductivity to obtain better HER and OER performances. The V doping of the FeBP can largely improve the conductivity and electron density to enhance the electrocatalytic reaction. The addition of V can tune the electronic structure and activate more active sites. The partial electron transfer is possible from the V 2+ to Fe 2+ ions, which might help to improve the adsorption capacity of hydrogen protons and hydroxyl groups and improve the HER and OER processes [47]. The HER and OER Tafel analyses are shown in Figure 3b,f. The Tafel slopes can be acquired from the linear range of the HER and OER curves as shown in Figure 3b,f. The Tafel slope values in Figure 3(b-1,f-1) indicate the degree of the reaction and charge-transfer rates. The lower slope values indicate a higher electron transfer and thus a greater reaction rate. The 50 • C sample demonstrated the lowest HER and OER Tafel slope values of 98 and 72 mV/dec, as summarized in Figure 3(b-1,f-1). The HER and OER EIS measurements were performed to understand the transport characteristics of the V-FeBP electrodes. The HER and OER EIS were measured at different overpotential voltages based on the fixed current of 20 mA/cm 2 for the consistency between samples. The EIS measurements showed different R ct values at different voltages around the turnover region, as seen in Figure S4 [1]. The higher voltage application showed smaller R ct values and vice versa. In both the HER and OER EIS plots, the charge-transfer resistance (R ct ) was gradually decreased with the lower annealing temperatures, and the V-FeBP electrode annealed at 50 • C demonstrated the lowest (R ct ) of 25.3 and 26.4 Ω for the HER and OER EIS, which indicates that the 50 • C sample demonstrated the lower conductivity and outstanding charge-transport characteristics [1]. Further, the double-layer capacitance (C dl ) measurements based on the CV plots indicated the highest electrochemical active surface area of the 50 • C sample with 1.84 and 1.95 mF/cm 2 in Figure 3d,h. After doping, the electrochemical surface area of the V-FeBP MSC was significantly increased, indicating a higher electrochemical activity of electrode. The CV curves and anodic and cathodic current densities are provided in Figures S40 and S41.     Figures S42 and S43. Overall, the V-FeBP and benchmark electrodes demonstrated quite stable operations in alkaline, acidic, and neutral waters in Figure 4a-f. At the same time, the benchmark electrodes demonstrated better HER and OER performances in all three solutions, as clearly seen in Figure 4(a-1-f-1). Both V-FeBP and the benchmark electrodes demonstrated similar trends in terms of performance with the overpotentials in alkaline < acidic < neutral waters, indicating that both electrode configurations demonstrated the best performances in 1 M KOH water. The higher performance in KOH can be attributed to the high electrochemical conductivity due to the ionization of OH − [48]. KOH can offer high current density and electrode stability. In the electrochemical reaction process, the cation K + plays a crucial role in lowering the activation barrier for the dissociation of H 2 O into OH − + H + + e − . KOH dissociates into K + and OH − in water and H 2 O can be dissociated more easily into OH − and H + [49]. The lower HER and OER performances in the acidic solution could be due to the slow reaction rate with the electrode degradation in the low-pH water [50]. Similarly, the lowest performances in the neutral media could be due to the low ion migration in PBS solution, which could have resulted in the lowest kinetics during the HER and OER operations [3]. The lack of hydrogen protons or hydroxyl ions can obstruct the mass transport and cause extra energy consumption to dissociate water molecules under neutral conditions [51]. In short, the V-FeBP demonstrated good electrochemical performances with all the optimizations in terms of the LSV, Tafel, EIS, C dl , TOF and stability. This could be due to the good balance between the V, Fe, B and P components and the good crystalline quality, along with the unique microsphere croissant (MSC) morphology as discussed. Additionally, the HER and OER steady-state current observations were performed by the comparison of the LSV and CA currents in a 3-E system in Figures S44 and S45. This was to show the stability of the electrodes at different current densities [52]. The V-FeBP annealed at 50 • C demonstrated stable operations at various voltages as summarized in Figures S44 and S45, indicating a good stability of the V-FeBP electrode. One thing to notice here is that the V-FeBP achieved a comparable OER result in 1 M KOH in Figure 5d, indicating that the 2-E operation of V-FeBP electrodes can largely benefit from the good OER performance. Figure 5 shows the 2-E electrochemical performance of V-FeBP and benchmark electrodes in alkaline, acidic, and neutral media and the stability test. In the 2-E configuration, the Pt/C RuO 2 were used as the cathode and anode, and two V-FeBP electrodes were adapted as bi-functional electrodes, i.e., V-FeBP V-FeBP. Generally, the 2-E watersplitting performance trend was similar to the 3-E, i.e., alkaline < acidic < neutral waters, in Figure 5a-c. The specific overpotentials at 50 and 1500 mA/cm 2 are summarized in Figure 5(a-1-c-1). The overpotentials were 1.46 and 1.48 V at 50 mA/cm 2 and then reached 2.34 and 2.49 V at 1500 mA/cm 2 in 1 M KOH for the Pt/C RuO 2 and V-FeBP V-FeBP in Figure 5(a-1). The overpotentials were 1.49 and 1.51 V at 50 mA/cm 2 and 2.53 and 2.86 V at 1500 mA/cm 2 in 0.5 M H 2 SO 4 in Figure 5(b-1). Similarly, the overpotentials were 1.51 and 1.56 V at 50 mA/cm 2 and 2.76 and 3.68 V at 1500 mA/cm 2 in 1 M PBS in Figure 5(c-1). The benchmark configuration demonstrated better water-splitting performances over the V-FeBP V-FeBP configuration due to the superior intrinsic electrochemical properties of Pt/C and RuO 2 for the HER and OER operations. Notably, the bi-functional configuration of V-FeBP demonstrated 2.18 V as compared with 2.06 V of the Pt/C RuO 2 at 1000 mA/cm 2 as identified in Figure 5a, which is a quite comparable performance to the benchmarks. This indicates that V-FeBP V-FeBP can demonstrate a compatible watersplitting performance as compared with the Pt/C RuO 2 in 1 M KOH water, with the costs of the electrode materials being several orders less.  Figure 5 shows the 2-E electrochemical performance of V-FeBP and benchmark electrodes in alkaline, acidic, and neutral media and the stability test. In the 2-E configuration, the Pt/C ‖ RuO2 were used as the cathode and anode, and two V-FeBP electrodes were adapted as bi-functional electrodes, i.e., V-FeBP ‖ V-FeBP. Generally, the 2-E water-splitting performance trend was similar to the 3-E, i.e., alkaline < acidic < neutral waters, in  Pt/C and RuO2 for the HER and OER operations. Notably, the bi-functional configuration of V-FeBP demonstrated 2.18 V as compared with 2.06 V of the Pt/C ‖ RuO2 at 1000 mA/cm 2 as identified in Figure 5a, which is a quite comparable performance to the benchmarks. This indicates that V-FeBP ‖ V-FeBP can demonstrate a compatible water-splitting performance as compared with the Pt/C ‖ RuO2 in 1 M KOH water, with the costs of the electrode materials being several orders less.  The 2-E performance of V-FeBP V-FeBP and Pt/C RuO 2 in natural sea and river waters are shown in Figure 5d. The V-FeBP V-FeBP demonstrated a comparable overpotential of 1.63 V at 50 mA/cm 2 as compared to the 1.65 V of Pt/C RuO 2 in sea water. The river water showed a very low current for both electrode configurations. The sea water generally demonstrated a better performance due to the presence of numerous Na + and Cl − ions, which can increase the conductivity in the water, and thus the water-splitting performance can be improved. Meanwhile, the river water also includes various kinds of ion species such as Ca + , Mg + , Br − , HCO 3 − , SiO 2 , SO 4 − , Cl − , F − , etc. [53]. These anions and cations in the river water can slow down the reaction process and lower the overall current density. While the elemental compositions in both sea and river waters are similar, the majority of the ionic species in sea water ions are Na + and Cl − (over 90%) and HCO 3 − , Ca + , SiO 2 , SO 4 − constitute over 90% of the ionic species in river waters. In addition, the V-FeBP V-FeBP demonstrated a slightly improved water-splitting performance in 6 M KOH at 60 • C as compared to the 1 M KOH at 25 • C as seen in Figure 5e. The overpotential values are shown in Figure 5(e-1). The V-FeBP V-FeBP demonstrated quite a stable current in 1 M KOH at 25 • C and in 6 M KOH at 60 • C at 1000 mA/cm 2 in Figure 5f,g, which indicates a good stability of V-FeBP in industrial water-splitting conditions. The stability test at the high current of 1000 mA/cm 2 for 12 h did not show any significant difference, but there was a slightly increasing trend, likely due to the oxidation of metallic atoms and surface modifications, as shown in Figure 5f. Similarly, the chronoamperometry test did not show any degradation in the harsh industrial condition of 6 M KOH, indicating the excellent stability of V-FeBP, as shown in Figure 5g. The V-FeBP V-FeBP also demonstrated excellent repeatability after 1000 cycles in 1 M KOH, as shown in Figure 5h. The two-electrode activity after 1000 cycles showed a very negligible difference in performance, which clearly shows that the V-FeBP has good repeatability after a long operation. In addition, the HER and OER turnover frequency (TOF) of the post-annealing temperature variation set of the V-FeBP electrocatalysts was evaluated for the vanadium and iron active sites at 150 mV/cm 2 , as shown in Figure S46. The TOF indicates the number of H 2 and O 2 molecules generated per atomic site per unit of time at the turnover. The TOF can be used to indicate the intrinsic water-splitting activity of each catalytic atomic active site under a specified reaction condition [38,54]. As summarized in Figure S46, the V-FeBP annealed at 50 • C demonstrated the highest HER and OER TOF values of 3.32 and 2.10 site −1 s −1 . In addition, the 2-E LSV and CA comparison of V-FeBP V-FeBP is shown in Figure S47, and the steady-state LSV and CA currents showed minor differences, as summarized in Figure S47c,d, indicating a good stability and stable operations at various voltages. The comparison of the two-electrode performance with the state-of-the-art Fe-based electrodes and transition-metal-based electrodes at 50 mA/cm 2 in 1 M KOH are shown in Figures S48  and S49 and Table 1 and Table S1. The V-FeBP was 2nd in the overpotential comparisons. Further, the three-electrode comparison with the state-of-the-art transition-metal-based electrodes at 10 mA/cm 2 in 1 M KOH is summarized in Figure S50 and Table S2. Again, the V-FeBP was one of the best.

Conclusions
A unique microsphere croissant (MSC) configuration of a V-FeBP electrode was demonstrated on a form of bare nickel substrate. The FeBP MSC was first optimized in terms of various synthesis parameters, and then the vanadium doping was further optimized.
Generally, the well-balanced F-B-P elements showed better electrochemical performances over the FeB and FeP. The post-annealing played an important role in improving the crystallinity of FeBP MSCs. Overall, the V-FeBP electrode demonstrated quite a comparable performance as compared with the benchmark electrodes with the low overpotential of 52 and 210 mV at 50 mA/cm 2 for the HER and OER in a three-electrode configuration in 1 M KOH. The V-FeBP || V-FeBP also demonstrated a comparable overpotential of 1.48 V at mA/cm 2 as compared with the PtC || RuO 2 . This clearly indicates that V-FeBP can offer a compatible water-splitting performance in 1 M KOH water. In addition, the V-FeBP MSC demonstrated excellent stability and repeatability under industrial water-splitting conditions. This study presents an efficient approach based on the combination of the transition metal Fe combined with the non-metallic elements B and P, and the heteroatom doping of V, which can offer an alternative option for large-scale water electrolysis.  Table 1 in the main text, Figure S49. Comparison of 2-electrode performance with the all state-of-art electrodes at the current density of 50 mA/cm 2 in 1 M KOH. Related to Table S1, Figure S50 Table S2, Table S1. Comparison of 2-electrode performance with the state-of-art transition metal-based electrodes at density of 50 mA/cm 2 in 1 M KOH, Table  S2. Comparison of 3-electrode performance with the state-of-art transition metal-based electrodes at density of 10 mA/cm 2 in 1 M KOH. The references

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.