Efficiently Enhancing Electrocatalytic Activity of α-MnO 2 Nanorods / N-Doped Ketjenblack Carbon for Oxygen Reduction Reaction and Oxygen Evolution Reaction Using Facile Regulated Hydrothermal Treatment

Scalable, low-cost and highly efficient catalysis of oxygen electrocatalytic reactions (ORR/OER) are required for the rapid development of clean and renewable energy conversion/storage technologies. Herein, two types of α-MnO2 nanorods were prepared under hydrothermal treatment at 150 ◦C for 0.5 h (MnO2-150-0.5) or 120 ◦C for 12 h (MnO2-120-12), then supported on N-doped ketjenblack carbon (N-KB) as bi-functional ORR/OER catalysts. Their electrocatalytic activities toward ORR and OER were investigated systematically. As a result, MnO2-150-0.5/N-KB displays superior ORR catalytic activity, with much more positive half-wave potential and much larger limiting current density (0.76 V and 6.0 mA cm−2), comparable to those of 20 wt. % Pt/C (0.82 V and 5.10 mA cm−2). MnO2-150-0.5/N-KB also shows high electron transfer number (3.86~3.97) and low yield of peroxides (1–7%) during ORR process in the whole potential range of 0–1.0 V (vs. RHE). Meanwhile, the MnO2-150-0.5/N-KB also exhibits better OER activity with low overpotential, comparable to IrO2/N-KB. The excellent electrocatalytic activity of MnO2-150-0.5/N-KB can be attributed to the synergistic effect, relatively smaller size, higher amount of Mn3+, and low charge transfer resistance. This work offers a new strategy for scalable preparation of more efficient and cost-effective α-MnO2 bi-functional oxygen catalysts.


Introduction
Efficient catalysts for oxygen electrocatalytic reactions (ORR/OER) are required for the rapid development of clean and renewable energy conversion/storage systems, including fuel cells and metal/air batteries.As is well known, precious metals and their alloys are considered to be the most efficient commercial ORR catalysts while ruthenium and iridium oxides are still the most commonly used OER catalysts [1,2].However, their high cost and rarity have become the major hurdles in large-scale commercial application [3].Hence, it is necessary to develop cost-effective and highly efficient alternative electrocatalysts for ORR and OER.
To pursue superior catalytic activity, the influence factors of MnO 2 on the ORR/OER activity have been extensively investigated.First, the electrocatalytic activities of MnO 2 nanostructures are highly related to their crystalline phase.It was reported that the catalytic performance of nano-MnO 2 increased in order of β-< λ-< γ-< α-≈ δ-MnO 2 [21,22].Numerous reports confirmed the α-MnO 2 possess high catalytic activity for ORR [15,16,23,24] and OER [16,19], owing to the edge and corner shared [ MnO 6 ] octahedral forming a 2 × 2 tunnel structure with charge-balancing ions.The improved OER electrocatalytic performance of α-MnO 2 is mainly due to its abundant di-µ-oxo bridges [10].Secondly, the ORR electrocatalytic performance of nano-MnO 2 strongly depends on its morphologies.Cheng and coworkers investigated systematically the effect of the particle size and morphology of nano-MnO 2 -based catalyst on ORR activity [15].The results demonstrate that nanostructure α-MnO 2 is superior to counterpart microstructures with higher oxygen reduction potential and larger current density [15].The ORR activity increases when particle size decreases because of large surface area and numerous surface defects [10].Moreover, 1-dimensional α-MnO 2 nanowires exhibit better catalytic performance than 3-dimensional α-MnO 2 nanospheres with similar surface area [10].In another report, α-MnO 2 nanorods/nanotubes also show higher ORR activity compared to the core-corona spheres δ-MnO 2 [24].Finally, the electrocatalytic activity is also affected by the amount of Mn 3+ in MnO 2 .It is believed that Mn 3+ is favorable for ORR [25] and OER [26] because of single e g occupation.Therefore, much attention has been paid to increasing the content of Mn 3+ .For example, heat treatment is proved to be an effective way to increase the amount of Mn 3+ because heat treatment in Ar and Air can cause oxygen nonstoichiometry [27].It was also reported that hydrogenation [28] and cations doping [29,30] also increase the amount of Mn 3+ in MnO 2 .Furthermore, the amount of Mn 3+ can be tuned by adjusting the synthesis procedure appropriately.Generally, the preparation of MnO 2 is either by the oxidation of Mn 2+ , or by the reduction of Mn 7+ .Chen et al. compared nano-MnO 2 prepared by three types of potassium salt oxidants with different reduction potentials, and confirmed that the reaction rate is directly dependent on the redox potential of oxidant or reductant used [31].Inspired by Chen's work, two types of α-MnO 2 (namely dandelion-like and urchin-like) were prepared by two redox reactions with different redox potential, and the ORR/OER activities were compared systemically [32].The results demonstrated that dandelion-like MnO 2 was of higher electrocatalytic performance than the urchinlike one, attributing to its relatively larger BET and electrochemical active surface area, higher ratio of Mn 3+ /Mn 4+ , and low charge transfer resistance [32].
Activated by the above findings, we proposed a facile regulated hydrothermal method to prepared α-MnO 2 nanorods (α-MnO 2 NRs) of small particle size and high content Mn 3+ , aiming to improve the ORR/OER activity.The α-MnO 2 NRs were prepared from the oxidation of a MnSO 4 precursor by KMnO 4 during various hydrothermal reaction temperatures and times.It is believed that the fast reaction rate could increase disorder and produce defects with more Mn 3+ in MnO 2 .Based on this, we hypothesize that raising the temperature and shortening the reaction time will lead to greater increase defects and Mn 3+ .To further improve the ORR/OER, N-doped ketjenblack carbon (N-KB) was used as catalyst support to construct α-MnO 2 NRs/N-KB bi-functional oxygen catalysts.As expected, MnO 2 -150-0.5 (hydrothermal treatment at 150 • C for 0.5 h) /N-KB exhibits much better ORR/OER activity than that of MnO 2 -120-12 (hydrothermal treatment at 120 • C for 12 h) /N-KB due to its smaller size has higher Mn 3+ /Mn 4+ ratio.This work offers a new strategy on scalable preparation of more efficient and cost-effective α-MnO 2 bi-functional oxygen catalysts for ORR/OER.

Morphological Characterization
The surface morphologies of as-prepared MnO 2 NR samples were characterized by scanning electron microscope (SEM).The SEM images of MnO 2 -150-0.5 (a, b, and c) and MnO 2 -120-12 (d, e, and f) are shown in Figure 1.The surface morphologies of MnO 2 NRs prepared with various hydrothermal treatment temperature as well as time is quite different.Irregular rhombic morphology consisting of uniform nanorod-like structures are observed on the MnO 2 -150-0.5.In contrast, the MnO 2 -120-12 samples possess typical amorphous structures.Moreover, the average diameters of MnO 2 -150-0.5 and MnO 2 -120-12 are 17 nm and 33 nm approximately.Obviously, MnO 2 -150-0.5 has smaller size than those of MnO 2 -120-12.It is believed that higher reacting temperature causes the faster reaction rate, and the faster reaction rate causes more Mn 3+ content in MnO 2 (seen in Table 1).In addition, the shorter reacting time gives less chance for particles to aggregate, leading to smaller particle size.The decrease in the particle size to the nanometer scale can increase the surface-to-volume ratio.As result, the MnO 2 -150-0.5 has much larger specific surface area, which will provide more catalytic activity sites and energetically facilitate the electrocatalytic activity for ORR/OER.The detained microstructure of MnO 2 -150-0.5 was further characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) (Figure 2).As shown in Figure 2a,b, the typical nanorod-like structure is presented on the MnO 2 -150-0.5 with an average diameter of ca.20 nm, which is in accordance with the SEM results.In Figure 2c,d, the distances between adjacent lattice fringes are 0.694 nm and 0.491 nm, attributed to (1 1 0) and (2 0 0) planes of α-MnO 2 NRs respectively.

X-ray Diffraction (XRD) Pattern
The crystal phases of MnO 2 -150-0.5 and MnO 2 -120-12 are determined using XRD (Figure 3).These two samples provide sharp and narrow peaks suggesting they are crystalline.Furthermore, the diffraction peaks of these two samples are clearly indexed into the pure tetragonal phase of α-MnO 2 (JCPDS card PDF file no.44-0141) [32].No other peaks are observed in the XRD patterns, demonstrating high purity and crystallinity of these two samples.The nearly identical XRD patterns confirmed that the difference in ORR activity is not because of different crystalline phase existence.

ORR Catalytic Activities
To compare the ORR catalytic activities of the as-prepared two samples, linear sweep voltammetry (LSV) curves were recorded at a rotating rate of 1600 rpm (Figure 5).As can be seen, MnO 2 -150-0.5/N-KBexhibits slightly better ORR catalytic activity compared to that of MnO 2 -120-12/N-KB, with more positive half-wave potential and relative higher limiting current.It can be explained by the facts that MnO 2 -150-0.5 have higher amount of Mn 3+ and smaller particle sizes.Owing to the single electron occupation in σ*-orbital (e g ), the Mn 3+ are found to be favors for ORR and OER in previous reports [26].Hence, more content of Mn 3+ in MnO 2 , better electro-catalytic performance [32].On the other hand, small particle size tends to have large electrochemical active surface area, which also enhance the electrocatalytic activity.Hereafter, the oxygen electrocatalytic activities for MnO 2 -150-0.5 was further investigated.Evidently, KB samples show very poor ORR activity, with low negative half-wave potential and current density.After doping nitrogen into KB, N-KB catalytic exhibits much better ORR activity, which is most probably due to the more catalytic active sites for N-doped samples [34].Pure MnO 2 -150-0.5 also shows poor ORR activity, which is due to its poor electric conductivity.As expected, MnO 2 -150-0.5/N-KBdisplays the superior ORR catalytic activity, with much more positive half-wave potential and much higher limiting current density (0.76 V and 6.0 mA•cm −2 ), comparable to that of Pt/C (0.82 V and 5.10 mA•cm −2 ).This phenomenon can be explained by the following reasons.Firstly, the superior ORR activity is mainly due the synergistic effect between α-MnO 2 nanorods and N-KB.The MnO 2 /KB materials not only have the inherited advantages from the component materials (excellent catalytic performance for MnO 2 nanorods and high electric conductivity for N-KB), but also improve ORR activity due to synergetic effect.Moreover, the presence of intrinsically abundant di-µ-oxo bridges in α-MnO 2 nanorods greatly accelerate the ORR [9,12,16,32,35].Finally, its small particle sizes and high content of Mn 3+ species in its inner matrix also contributed [12,26,32,36].
Figure 6b shows CV curves of MnO 2 -150-0.5/N-KB in O 2 -saturated (black solid line) and Ar-saturated (red dotted line) 0.1 M KOH solution.There is no visible OPR peak when the electrolyte is saturated with Ar.In contrast, well-defined ORR peak is observed in the O 2 -saturated 0.1 M KOH solution, demonstrating the excellent electrocatalytic activity for ORR.More interestingly, all CV curves (both Ar and O 2 saturated) appear strong Mn 3+ /Mn 4+ redox peaks [32,37] in the potential range of 0.96 to 1.16V.This phenomenon agrees well with previous reports [32,38].The α-MnO 2 nanorods underwent a reduction from Mn 4+ to Mn 3+ in the surfaces of solid phase before the ORR process.Then, the Mn 3+ had its one σ* electron transferred to O-O π* orbital and converted back into Mn 4+ [32].Therefore, higher Mn 3+ content results in more available adsorption sites for O 2 , which eventually improves the electrocatalytic activity for ORR [26,29,32,38].
The LSV curves of MnO 2 -150-0.5/N-KBat different rotating rates were also measured to assess the kinetics of ORR.As shown in Figure 6c, the limiting current densities increase with the raising of the rotating rates, attributing to minimize the concentration polarization due to the shortened diffusion distance at high rotating rates [39].The curves inserted in Figure 6c exhibits the Koutechy-Levich (K-L) plots obtained from these LSV curves from 400 to 1600 rpm.The K-L plots are almost parallel with each other, suggesting similar electron transfer numbers for ORR at various potentials [40].The electron transferred numbers (n) are further calculated from the slope of the K-L plots.The n is around 3.95, which suggests MnO 2 -150-0.5/N-KBfavors a 4e transfer oxygen reduction mechanism [39,40].
The ORR kinetics was further assessed by Tafer slope (b), electron transfer number (n) and peroxide yields (H 2 O 2 %).As depicted in Figure 6d, the Tafel slope of MnO 2 -150-0.5/N-KB is 97.7 mV per decade, which is close to the one of Pt/C (~82 mV per decade) in the reference obtained at the same experimental condition [34].This suggests MnO 2 -150-0.5/N-KBhas similar kinetic behavior with the commercial catalyst 20 wt.% Pt/C.To further verify the ORR mechanism, the rotating-ring-disk electrode (RRDE) technique was performed to measure electron transfer number (n) and peroxide yields (H 2 O 2 %).As illustrated in Figure 6e, the n values of MnO 2 -150-0.5/N-KB(~3.86 to 3.97) are close to 4.0 in the whole potential range of 0~1.0 V (vs.RHE), further confirming 4e transfer oxygen reduction mechanism for MnO 2 -150-0.5/N-KB.It is noteworthy that the baseline has been subtracted to avoid overestimate the electron transfer number.Moreover, the percentage of peroxide yields (H 2 O 2 %) during the oxygen reduction is very low (about 1~7%) in the potential range of 0~1.0 V (vs.RHE).All these findings confirm a 4e oxygen reduction mechanism for ORR.
To further investigate the role of ion and charge transfer in the ORR, EIS of MnO 2 -150-0.5/N-KBand MnO 2 -120-12/N-KB were record in O 2 -saturated 0.1 M KOH solution at 0.165 V (vs.RHE).The Nyquist plots are shown in Figure 8, in which the EIS data have been fitted according to the equivalent circuit (inset of Figure 8).The equivalent circuit consists of R s , R f , R ct , C and W o represents uncompensated solution resistance, intrinsic resistance of the catalyst, charge transfer resistance, capacitance of double layer and Warburg impedance (relating to diffusion impedance) respectively.As listed in Table 2, the charge transfer resistance (R ct ) of these two samples comparable (49.47 Ω and 48.32 Ω respectively).The effect of R ct on the ORR activities is almost same for these two samples.Besides, the catalytic performance of nano-MnO 2 increased in order of β-< λ-< γ-< α-≈ δ-MnO 2 [21,22].In a previous report, α-MnO 2 nanorods/nanotubes also show higher ORR activity compared to the core-corona spheres δ-MnO 2 [24].It is believed that the α-MnO 2 nanorods-based bifunctional catalysts have high ORR activity.For these reasons, the ORR activity of MnO 2 -150-0.5/KB is a litter higher than that of MnO 2 -120-12/N-KB (Figure 5).The difference between MnO 2 -120-12/N-KB and MnO 2 -150-0.5/KBonly arises from the particle size and Mn 3+/4+ ratio.

OER Activities
To further compare their electrocatalytic performances, OER activities of the two as-prepared samples (namely MnO 2 -150-0.5/N-KBand MnO 2 -120-12/N-KB) were tested by RDE experiments at a scan rate of 10 mV•s −1 .Generally, OER activities are judged by the potential at the current density of 10 mA•cm −2 .As illustrated in Figure 9, MnO 2 -150-0.5/N-KB(1.83 V) is shift 110 mV more negative than MnO 2 -120-12/N-KB (1.94 V), which means the MnO 2 -15-0.5/N-KBcatalyze OER at lower overpotential than MnO 2 -120-12/N-KB sample.In other words, the MnO 2 -150-0.5/N-KBexhibits much better OER kinetic behavior than MnO 2 -120-12/N-KB.Moreover, the overpotential of MnO 2 -15-0.5/N-KB is close to the standard catalyst IrO 2 /N-KB with the same loading of catalysts.Similar to the ORR, the higher Mn 3+ content [26] and smaller size of α-MnO 2 nanorods also plays a vital role on the OER.The ionic and charge transfer play an important role in OER [10].Thus, the electrochemical impedance spectroscopy (EIS) was recorded to give deep insights into the OER process.The Nyquist plots are shown in Figure 10, in which the EIS data have been fitted according to the equivalent circuit (inset of Figure 10).The equivalent circuit consists of R s , R f, R ct , C and CPE represents uncompensated solution resistance, intrinsic resistance of the catalyst, charge transfer resistance, capacitance of catalyst and constant phase element of double layer, respectively.All the fitting parameters are listed in Table 3.The R s relating to the uncompensated solution of these two samples is comparable (59.13 Ω and 61.35 Ω, respectively).The R ct relates to the reaction kinetics, MnO 2 -150-0.5/N-KBexhibits lower charge transfer resistance (227.7 Ω) than that of MnO 2 -120-12/N-KB (310.2Ω,).Note it is in good accordance with the OER performance.

Durability of Electrocatalysts
The electrocatalytic durability is a major concern in practical applications.The stability of MnO 2 -150-0.5 and MnO 2 -120-12 for ORR was evaluated by the half-wave potential decay (∆E 1/2 ) before and after the accelerated durability test (ADT).The ADT was performed by subjecting catalyst to 5000 cycles from 0 to 1.0 (vs.RHE) in O 2 -saturated 0.1 M KOH solution at room temperature with a scan rate of 100 mV s −1 .As shown in Figure 11a, the half-wave potential of MnO 2 -150-0.5/N-KBexhibits a negative shift of ~37 mV after 5000 cycles, a litter larger than that of 20% Pt/C (Figure 11c, ~22 mV) and less than that of MnO 2 -120-12/N-KB (Figure 11b, ~49 mV).The results reveal that the durability of MnO 2 -150-0.5/N-KB is better than that of MnO 2 -120-12/N-KB.However, the durability of MnO 2 -150-0.5/N-KBstill need to be improved.

Preparation of α-MnO 2 Nanorods
The α-MnO 2 nanorods (α-MnO 2 NRs) were prepared from the oxidation of a MnSO 4 precursor by KMnO 4 during hydrothermal treatment.Typically, 2.03 g of manganese sulfate monohydrate (MnSO 4 •H 2 O) and 1.26 g of potassium permanganate (KMnO 4 ) were added into 80 mL of deionized water and agitated 5 min to form a homogenous aqueous solution.Then, 4 mL concentrated hydrochloric acid (37%) was added into the above solution under vigorous agitating for 5 min.The resulting solution was to a 100 mL Teflon-lined stainless steel autoclave, then hydrothermal treated at 150 • C for 30 min or 120 • C for 12 h.Afterwards the autoclave was taken out and naturally cooled to room temperature.Finally, the as-obtained product was vacuum filtrated with a 0.15 µm pore sized filter membrane, then dried overnight at 80 • C for further use.The obtained samples are denoted in the format: MnO 2 -hydrothermal treatment temperature-hydrothermal treatment time.For example, MnO 2 -150-0.5 denotes a sample prepared by hydrothermal treatment at 150 • C for 0.5 h.

Preparation of N-Doped Ketjenblack Carbon
To begin, 0.2 g of ketjenblack carbon (KB) and 1.2 g of melamine were dispersed in 80 mL deionized water under ultrasonication for 30 min.Then, the resulting solution was sealed into a 100 mL Teflon-lined stainless steel autoclave and heated at 120 • C for 24 h.After it was cooled down to room temperature naturally, the as-obtained product was vacuum filtrated using a filter membrane of 0.15 µm pore size and dried overnight at 80 • C.After carefully ground by agate mortar for more than 10 min, the resulting powder was transferred to a piece of porcelain boat, which was then covered with another piece of porcelain boat and further wrapped by copper foil.The treated porcelain boat was placed into a tube furnace and then heated to 650 • C for 2 h at a heating rate of 5 • C min −1 in argon flow.After that, it was naturally cooled down to room temperature, and hereafter the as-prepared samples were denoted as N-KB.

Electrochemical Measurements
The electrochemical experiments were performed on CHI760E electrochemical workstation (Shanghai Chenhua Inc., Shanghai, China) using three-electrode assemble in O 2 -saturated 0.1 M KOH.Double fluid boundary Ag/AgCl electrode and platinum wire worked as the reference electrode and auxiliary electrode, separately.The work electrodes prepared according to the following procedures.2 mg of as-prepared α-MnO 2 NRs and 4 mg of N-KB were firstly dispersed in 950 µL anhydrous ethanol by ultrasonication for 20 min.Then 50 µL of Nafion solution (5 wt.%) was added into α-MnO 2 NRs/N-KB dispersion and continuously ultrasonicated for 20 min to get a homogeneous catalytic ink.Catalytic inks of 20 wt.% Pt/C and IrO 2 /N-KB were also prepared to fabricate the standard catalysts for ORR and OER respectively.6 mg of commercial 20 wt.% Pt/C (Johnson Matthey) were used to fabricate the Pt/C catalytic ink with the similar method.The preparations of KB, N-KB and pure α-MnO 2 catalytic inks are all the same with the Pt/C.Catalytic inks of IrO 2 /N-KB contains 2 mg IrO 2 and 4 mg N-KB.Finally, 8 µL of as-prepared catalytic ink were loaded onto the surface of glassy carbon disk electrode to obtain a mass loading of 0.2446 mg cm −2 .
Prior to the electrochemical tests, high-purity O 2 was purged in the electrolyte for 30 min.The LSV was measured from 0.2 to −1.0 V (vs.Ag/AgCl) at a scan rate of 10 mV•s −1 with a rotation rate of 1600 rpm.The cyclic voltammetry (CV) was measured at a scan rate of 100 mV•s −1 .OER LSV measurements were carried out at a scan rate of 10 mV•s −1 at the potential window of 0.2-1.0V (vs.Ag/AgCl).The EIS were scanned in the frequency range of 10 5 -0.1 Hz at −0.2 V or 0.7 V (vs.Ag/AgCl) with the amplitude of 5 mV in O 2 -saturated 0.1 M KOH solution [12,21,22].
All potentials were finally converted to the values versus reversible hydrogen electrode (RHE) according to the Nernst equation given in Equation ( 1).
where E RHE is the applied potential vs. RHE; E Ag/AgCl represents the applied potential versus Ag/AgCl reference electrode and E 0 Ag/AgCl (0.2046 V at 25 • C) is the standard electrode potential of the Ag/AgCl electrode.
The number of electron transferred (n) was estimated according to the Koutecky-Levich equation given as follows [39,41,42]: The kinetic current density (j K ) for Tafel plots was determined according to the following equation: [34] where j, j L and j K are the recorded current density, the diffusion-limiting and kinetic current density, respectively.ω is the electrode rotating rates (rad•s −1 ), n means the transferred electron number, F is the Faraday constant (96,485 C•mol −1 ).C 0 (1.2 × 10 −3 mol•L −1 ) and D 0 (1.9 × 10 −5 cm 2 •s −1 ) correspond to the bulk concentration and diffusion coefficient of O 2 in 0.1 mol•L −1 , respectively.The v represents the kinematic viscosity of the electrolyte (0.01 cm 2 •s −1 ).
To further reveal the ORR mechanism, the peroxide yields (H 2 O 2 %) and the electron transfer number were calculated, respectively, using Equations ( 5) and (6) as follows [34]:

Conclusions
In this work, two α-MnO 2 nanorod samples were prepared from the oxidation of a MnSO 4 precursor by KMnO 4 under various hydrothermal reaction temperatures and time.Then, nanorod-like α-MnO 2 composited with N-doped ketjenblack carbon is obtained as electrocatalyst for ORR and OER.The MnO 2 -150-0.5 sample shows smaller sizes than that of MnO 2 -120-12 samples.Moreover, the higher ratio of Mn 3+ /Mn 4+ for MnO 2 -150-0.5 samples are confirmed by Mn 2p XPS spectra and Mn 3+ /Mn 4+ redox peaks in CV curves.As a result, MnO 2 -150-0.5/N-KBdisplays the superior ORR catalytic activity, with much more positive half-wave potential and much higher limiting current density (0.76 V and 6.0 mA•cm −2 ), comparable to that of Pt/C (0.82 V and 5.10 mA•cm −2 respectively).MnO 2 -150-0.5/N-KBalso shows high electron transfer number and low peroxide yield during the ORR process in the whole potential range of 0-1.0 V.The OER activities of these two samples were also compared.The MnO 2 -150-0.5/N-KBexhibits better OER activity with low overpotential and charge transfer resistance, comparable to IrO 2 /N-KB.All the results confirm that the smaller size, higher amount of Mn 3+ and low charge transfer resistance favors for ORR and OER.To conclude, MnO 2 -150-0.5/N-KBshows outstanding merit such as low cost, high efficient, facile fabrication and is expected to apply in alkaline fuel cells and metal/air batteries as a promising alternative catalyst.This report offers a new strategy for scalable preparation of more efficient and cost-effective α-MnO 2 for ORR and OER catalysts by easily regulated hydrothermal method.

Figure 7 .
Figure 7. LSV curves of various catalysts on RDE were recorded in Ar-saturated 0.1 M KOH solution at a scan rate of 10 mV•s −1 with a rotation rate of 1600 rpm.

Figure 11 .
Figure 11.LSV curves of MnO 2 -150-0.5 (a), MnO 2 -120-12 (b) and Pt/C (c) before and after the accelerated durability test (ADT).The ADT was performed by subjecting catalyst to 5000 circles from 0.57 to 0.82 V (vs.RHE) in O 2 -saturated 0.1 M KOH solution at room temperature at a scan rate of 100 mV s −1 .The baselines had been subtracted.

H 2 O 2
(%) = 200 × I r /N I d + I r /N (5) n = 4 × I d I d + I r /N(6)where I d , I r and N are the disk current, ring current and the collection efficiency, respectively.The empirical collection efficiency of Pt ring in RRDE experiments was 0.37 using [Fe(CN) 6 ] 3−/4− couple[34,42].

Table 2 .
Component values of the fitted equivalent circuit based on the ORR Nyquist plots.

Table 3 .
Component values of the fitted equivalent circuit based on the Nyquist plots.