Hexagonal Boron Nitride Supported Crystalline Manganese Oxide

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Hexagonal Boron Nitride Supported Crystalline Manganese Oxide Nanorods/Carbon: A Tunable Nanocomposite Catalyst for Dioxygen Electroreduction Indrajit M. Patil, Anita Swami, Rohit Chavan, Moorthi Lokanathan, and Bhalchandra A. Kakade ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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ACS Sustainable Chemistry & Engineering

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Hexagonal Boron Nitride Supported Crystalline Manganese Oxide Nanorods/Carbon: A

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Tunable Nanocomposite Catalyst for Dioxygen Electroreduction

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Indrajit M. Patil,†‡ Anita Swami,‡ Rohit Chavan,¶ Moorthi Lokanathan† ¥ and Bhalchandra

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Kakade†‡*

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†SRM

Research Institute, SRM Institute of Science & Technology, Kattankulathur603 203, Chennai (India)

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‡Department

of Chemistry, SRM Institute of Science & Technology - 603 203, Chennai (India)

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¶Polymer

Chonnam National University, Gwangju - 500757, South Korea.

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Energy Materials Laboratory, School of Applied Chemical Engineering,

¥Department

of Physics, SRM Institute of Science & Technology - 603 203,

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Chennai (India)

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Fax: (+91) 44-2745 6702; Tel: (+91) 44-2741 7920 *Corresponding

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author E-mail: [email protected]

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Abstract

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Dioxygen reduction is a key step in low temperature fuel cell catalysis research and ultimately

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of sustainable energy conversion technology. Herein, we report a simple strategy to design a

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cost-effective electrocatalyst comprising MnO2 nanorods on hexagonal boron nitride (h-BN)

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and their composite with high surface area carbon by chemical method. The optimized

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nanocomposite catalyst (MnBN/C-75) exhibits a substantial higher onset potential (Eonset = 0.9

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V) and limiting kinetic current density (JL = 5.6 mA cm-2) during oxygen reduction reaction

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(ORR) compared to other reported h-BN based metal supported or metal-free electrocatalysts.

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Moreover, this catalyst shows ~ 4-electron transfer pathway with low peroxide (HO2-)

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intermediate yield during electroreduction of oxygen, indicating a single step, first order

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kinetics as commercial Pt/C catalyst. Besides, the mass activity of 222 mA mg-1 calculated at

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0.6 V for MnBN/C-75 catalyst is ~21 times higher than that of MnBN (10.4 mA mg-1) and

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slightly lower than Pt/C (239 mA mg-1 @0.9V). Importantly, MnBN/C-75 nanocomposite

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reveals smaller deviation in half-wave potential (ΔE1/2 = 18 mV) compared to Pt/C catalyst

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(ΔE1/2 = 50 mV) even after 5k potential cycling under similar conditions. The relatively lower

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ionic diffusion and charge transfer resistance at electrode/electrolyte interface by MnBN/C-75

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electrode support to our claim regarding higher electrocatalytic activity. Thus the presence of 1 ACS Paragon Plus Environment

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Mn3+ ions in the form of MnOOH (during composite formation) along with both h-BN support

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and KB carbon at the electrode surface contributes immensely in boosting the electrocatalytic

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activity. Thus, it could be a promising electrocatalyst, if employed in cathode compartment of

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low temperature fuel cells to lead faster ORR kinetics.

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Keywords: hexagonal boron nitride, manganese oxide, ketjenblack carbon, oxygen

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electroreduction, fuel cell

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Introduction

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At present scenario of increasing energy demand along with depletion in the traditionally used

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fossil fuels, and an alarming increase in the pollution; need of alternative energy source with

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no or minimal effect on the environment is the need of the hour. Fuel cells are a promising

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technology for clean and efficient electrical power generation, in which chemical energy gets

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converted into electrical energy through electrochemical reaction.1,2 Usually, today's fuel cells

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use expensive platinum (Pt)-based nanoparticles as catalysts to accelerate the kinetics of more

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sluggish cathodic oxygen reduction reaction (ORR).2-11 Besides, implementation of low

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temperature fuel cell (LTFC) technology is being hindered due to shortage and poor stability

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of Pt-based catalysts under operating conditions.3,12 Hence, the systematic design and

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developing a cost-effective electrocatalyst for LTFCs and commercializing them has been

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extensively ongoing research around the world.1,13-16 Numerous non-precious materials, for

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instance hetero-atom doped carbon/graphene oxide, carbon nanotubes, transition metals and

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oxides, and their composites have been suggested as alternative electrocatalysts for the ORR.17-

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distinct properties.25-30 On the other hand, two-dimensional hexagonal boron nitride (h-BN)

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sheets, a structural analogue of graphene has also been emerging in various catalytic fields due

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to its excellent thermal and chemical stability and unique electronic and optical properties.31,32

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However, wide band gap energy (ca. 5-6 eV) of h-BN is offering a major obstacle for its

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widespread adoption in energy conversion processes.33,34 Nevertheless, recently we have

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shown a crucial role of h-BN via nanocomposite catalyst with carbonaceous materials for

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ORR.35,36

Further, graphene supported electrocatalyst has great impact on ORR activity due to its

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Besides, a density functional theory (DFT) study implies that transition metal supported

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h-BN surface can provide catalytically active sites for tunable ORR performance.37,38 Among

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them, manganese oxides (MnOx) have drawn particular attention because of their unimpeded 2 ACS Paragon Plus Environment

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abundance, low cost, minimum environmental impact and moderate electrocatalytic activity in

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alkaline media.39-43 Whereas, Chen et al. have reported that the catalytic activity of MnO2-based

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nanostructures strongly depend on crystal structure as well as morphology.43 Till today, there

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exists a big divergence in the understanding regarding the active crystallographic phase for

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ORR.43-45 Moreover, Lee et al. have studied the carbon supported MnOx as catalyst material for

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ORR in Zn-air battery and concluded that the synergetic effect between them contributes to

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enhance the activity.40 However, the overall ORR activity of MnOx or MnOx/C based catalysts

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is inferior as compared to state-of-the-art Pt/C catalyst.40,46-50 Further, it has been reported that

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kinetics of ORR on MnOx surfaces mostly followed 2 or mixed 2 and 4 electron transfer

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processes, which is unfavourable for ORR.51,52 Thus, by recognizing the above promising

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demand to develop MnOx based electrocatalyst (having low ORR overpotential with preferably

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4-electron transfer process), h-BN supported MnOx based nanocomposite catalyst has been

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designed for first time to enhance the kinetics of dioxygen reduction. Since, both h-BN and

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MnOx show low electrical conductivity,32,40 a less expensive and commercially available ketjen

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black carbon (KBC) has been used as conductive additive and of course to minimize the cost

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of technology.

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Herein, we report a simple strategy to design a cost-effective electrocatalyst comprising

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a composite of MnO2 nanorods on h-BN and carbon. Crystallographic structure and

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morphology of obtained composite nanomaterials were studied by X-ray diffraction (XRD)

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and transmission electron microscopy (TEM) respectively. At last, the catalyst was tested for

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ORR under alkaline condition at room temperature for its electrocatalytic activity. The

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optimized sample (MnBN/C-75) showed significant catalytic activity towards ORR with an

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onset potential of 0.9 V versus RHE and limiting kinetic current density of 5.6 mA cm-2 at 1600

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rpm. Besides, the crucial role of MnOx towards enhancing kinetics of ORR via weak van der

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Waals interaction with h-BN sheets and influence of conductive carbon material has been

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discussed in details.

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Experimental

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Materials

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All the chemicals and solvents were used without any further purification. Hexagonal boron

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nitride powder (h-BN) having particle size ~1 µm and potassium hydroxide (KOH) pellets were

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procured from Sigma Aldrich. Other chemicals and solvents such as, potassium permanganate

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(KMnO4), sulphuric acid (H2SO4) and sodium nitrate (NaNO3) were purchased from Rankem

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chemicals, India. Hydrogen peroxide (H2O2, 30% w/v) was procured from Fisher scientific. A 3 ACS Paragon Plus Environment


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commercial Pt/C catalyst was purchased from Fuel Cell Technologies Inc. USA. Millipore

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water (18.2 MΩ) was used throughout all the experiments and electrochemical property

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measurements.

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Synthesis of MnOx and h-BN based composite nanomaterial (MnBN/C)

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Initially, as received h-BN powder (0.2 g) was suspended in concentrated H2SO4 for a period

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of 30 mins and then 0.1 g NaNO3 was added with continuous stirring for 15 mins. A weighed

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quantity (0.6 g) of KMnO4 was added to above mixture and the temperature of this exothermic

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reaction was controlled by ice bath. The reaction mixture was stirred at 35 °C for 30 minutes

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and then 150 mL de-ionized (DI) water was slowly added into the system. To reduce residual

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KMnO4, an appropriate amount of 3% H2O2 solution was added dropwise into the reaction

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mixture until no bubbles were observed. Upon H2O2 treatment, the above suspension turned

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brown in color. The obtained suspension was centrifuged and washed with DI water; resulting

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in brown colored powder after drying at 60 oC and was designated as MnBN. Finally, MnBN

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catalyst was supported on ketjenblack carbon (KBC) hydrothermally at 180 oC. The MnBN

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catalyst mixed with different amounts of KBC viz. 10 wt%, 33 wt%, 50 wt%, 66 wt%, 75 wt%

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and 90 wt% and were denoted as MnBN/C-10, MnBN/C-33, MnBN/C-50, MnBN/C-66,

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MnBN/C-75 and MnBN/C-90 respectively. For better comparison, α-MnO2 nanorods have

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been synthesized separately and MnO2/C-75 composite was prepared under the similar

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conditions as that of MnBN/C-75, without addition of BN powder.

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Physical Characterization

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The MnBN/C catalyst was characterized using various spectroscopic and microscopic

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techniques. The morphological studies of prepared samples were carried out using field

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emission scanning electron microscope (FE-SEM; S-4700, Hitachi) and TEM on FEI Tecnai

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G2 20 Twin microscope operated at 200 kV. Powder XRD patterns were recorded on a X’pert

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pro diffractometer, PANalytical using Cu K radiation (= 1.5406 Å, 40 kV, 40 mA) in the 2θ

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range of 5-900 with the scan rate of 2o min-1. X-ray photoelectron spectroscopy (XPS)

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measurements were done with a Thermo K-5 Alpha XPS instrument at a pressure better than

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1×10-9 Torr with a pass energy of 50 eV, electron take-off angle of 60o, and an overall resolution

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of ~1 eV using monochromatic Al Kα (source, hν = 1486.6 eV). The spectra were fitted using

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a combined polynomial and Shirley type background function.

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Electrochemical Measurements

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All the electrochemical measurements were conducted on a CHI760E workstation (CH

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Instruments, Inc., USA) in an aqueous 0.1 M KOH solution at room temperature. The rotating

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disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements were carried out

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in a homebuilt electrochemical cell to evaluate a catalyst performance. A platinum (Pt) wire

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and Ag/AgCl (sat. KCl) were used as counter and reference electrode respectively. Prior to

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catalyst loading on 4 mm glassy carbon (GC) disk electrode, surface of GC electrode was

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cleaned using 0.05 mm alumina powder slurry and DI water. The catalyst ink was prepared

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using our previously reported method35 and working electrode was coated with the catalyst

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layer, yielding a catalyst loading of 0.16 mg cm-2. For comparison, Pt/C (20 wt% on carbon

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support) modified electrode was also prepared with actual Pt-loading of 31.8 μg cm-2. Cyclic

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voltammetry (CV) measurements were carried out in N2- and O2-saturated electrolyte and

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linear sweep voltammetry (LSV) measurements were performed at various rotation speeds with

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a sweep rate of 10 mV s-1 under O2-saturated 0.1 M KOH solution. Finally, the stability of the

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catalyst was studied by scanning the working electrode between -0.4 to 0 V vs. Ag/AgCl (using

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a standard protocol of triangular potential vs. time curve with 16 s for a single cycle) at the

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scan rate 50 mV s-1 up to 5,000 durability cycles. RRDE measurements and calculations of

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number of electron transfer were carried out by using our previously reported method (see ESI

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for more details).36 Finally, the electrode potential was calibrated and converted with respect

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to the reversible hydrogen electrode (RHE) using E

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complete details of calibration study is given in supporting information.

(RHE)

= E

(Ag/AgCl)

+ 0.973 V.53,54 The

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Results and Discussion

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The overall synthetic strategy for the preparation of MnBN/C composite nanomaterial has been

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illustrated in Scheme 1. Initially, the composite of h-BN and manganese dioxide (MnBN) was

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obtained by the procedure discussed in experimental section and subsequently high surface

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area carbon was mixed with it using one step hydrothermal method. The crystal structure of

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obtained composite nanomaterial was examined through powder XRD. XRD patterns of bare

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h-BN and as-synthesized MnBN samples are shown in Figure 1(a). The well-defined

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diffraction peaks in the XRD pattern of as-prepared MnBN sample can be readily indexed to

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α-MnO2 using the standard JCPDS file no. 44-0141. It is important to note that, the most intense

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peak at around 2θ = 26.7o corresponds to the (002) plane of h-BN. Further, the bare h-BN shows

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pronounced XRD peaks at various 2θ positions and the data is in good agreement with the file

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no. 034-0421.35,55 The XRD pattern of bare KBC (hereafter referred as KB carbon) reflects 5 ACS Paragon Plus Environment


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two broad peaks at around 24.8o and 43.3o corresponding to the (002) and (101) planes of

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amorphous carbon material (Fig. S2 in ESI).56,57 Importantly, two noticeable changes were

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observed in case of the XRD pattern of MnBN/C-75 (as clearly seen in Fig. 1b). Firstly,

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MnBN/C-75 composite exhibits distinct diffraction pattern than parent (MnBN), indicating

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obvious changes in the crystal structure of MnO2. i.e. from manganese dioxide (MnIVO2) to

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manganese oxide hydroxide (Manganite; MnIIIOOH), which is confirmed by ICSD no. 01-088-

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0649. The formation of MnO(OH) is very well understood by following chemical reaction (Eq.

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1). During hydrothermal treatment of MnBN with KBC, Mn+4 (MnO2) reduce partially to Mn+3

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(MnOOH) due to reducing abilities of KBC, which is clearly seen in their respective XRD

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patterns (Fig. 1b and inset). 𝑀𝑛𝑂2 + 𝐻2𝑂 + 𝑒 ― → MnOOH + 𝑂𝐻 -

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(1)

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Furthermore, the well defined and most intense diffraction peak at 2θ = 26.1o corresponding to

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(-111) plane of MnO(OH) phase was observed with the interlayer spacing (d) of 0.34 nm

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(obtained from Bragg’s law). Secondly, the broader diffraction peak corresponding to the KB

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carbon has been completely vanished after composite formation, suggesting KB carbon

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becomes more amorphous under this synthetic condition (as clearly seen in Fig. 1b and its

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inset). Such conspicuous changes occurred during the synthesis of MnBN/C-75 catalyst mainly

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contribute to enhance the electrokinetics of ORR (details are given in later text). Further, the

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crystal structure of as prepared MnO2 sample was confirmed through XRD (Fig. S3 in ESI).

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Moreover, XRD pattern of MnO2/C-75 composite exhibits structural changes from α-MnO2 to

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manganese oxide hydroxide (MnOOH). The diffraction pattern of MnO2/C-75 composite

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matches with standard ICSD card no. 00-018-0805, suggesting the formation of MnOOH (Fig.

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S3).

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The morphology of as-prepared MnBN/C-75 nanocomposite was studied by FE-SEM

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and TEM analysis. Accordingly, Fig. S4 shows the presence of MnOOH nanorods in contact

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with h-BN surfaces thanks to initial seed growth of MnO2 on h-BN. Also, a homogeneous

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distribution of carbon has clearly been observed through out the MnBN/C-75 composite (Fig.

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S4). Importantly, the existence of B, N, C, Mn and O elements in the MnBN/C-75

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nanocomposite was confirmed by SEM elemental mapping (shown in Fig. S5) that also reveals

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an even carbon distribution in the composite. Additionally, the energy-dispersive X-ray

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analysis spectrum (EDS) of MnBN/C-75 composite has been shown in Fig. S6, confirming the

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presence and relative compositions of B, N, C, Mn, O etc. elements. The comparative SEM

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images of various samples are illustrated in Fig. S7 (see ESI). As shown in figure, the structural 6 ACS Paragon Plus Environment

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changes rendered the variation in the morphology of MnOOH (in case of MnBN/C-75

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composite) with enhancement in the aspect ratio of the nanorods perhaps due to Ostwald

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ripening process. On the other hand, as prepared MnO2 nanorods and its composite shows

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insignificant changes in their morphologies (clearly seen in Fig. S7).

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TEM image in Fig. 2(a) reveals disk shaped particles of pristine h-BN of size ˂ 500 nm.

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Further, the uniform distribution of MnO2 nanorods (NRs) on h-BN sheets was clearly observed

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in low resolution TEM (Fig. 2b). It is important to note that, the growth of MnO2 NRs especially

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was observed on h-BN surface and such phenomenon has been firstly observed in the present

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study. However, the growth mechanism on h-BN surface is still unclear, though it could be

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believed that a highly crystalline h-BN would play an important role in growth of MnO2 NRs.

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Figure 2(c) shows the low resolution TEM of MnBN/C-75 composite nanomaterial with an

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average diameter of NRs estimated to be 12 ± 2 nm.. As shown in Fig. 2(d), MnO(OH) NRs

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show strong interaction with h-BN particles perhaps due to seeded growth of MnO2 nanorods

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on highly crystalline substrate of h-BN. It is also important to note that such growth of MnO2

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nanorods has not been observed during oxidation of graphite powder. A comparatively easy

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oxidation of graphitic layers (than h-BN) in acidic KMnO4 solution leads to formation of stable

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salts of Mn (Mn+2, Mn+4, Mn+5), but the growth of MnOx like species might be hindered due

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to the continuous oxidation of C-C bond. However, feeble oxidation of h-BN (does not undergo

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cycloaddition due to absence of conjugation) in presence of Mn+2 renders the formation of

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MnO2 nanorods on h-BN surface. Thus, such an interface between h-BN and MnO2 could

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furnish a platform for more dioxygen adsorption followed by electroreduction.

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The chemical composition and surface electronic states of as-prepared MnBN and

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MnBN/C-75 nanocomposite catalysts were investigated by XPS analysis. Accordingly, Fig. 3

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shows the deconvoluted core level XP spectra of MnBN nanocomposite and the data has been

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compared to that of MnBN/C-75 nanocomposite (shown in Fig. S8 and S9). The XP spectra

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reveal various peaks, which can be attributed to the core levels of Mn 2p, B 1s, N 1s and O 1s

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(Fig. 3). As shown in Fig. 3(a), Mn2p core level spectrum can be split into two peaks at 642.2

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and 653.6 eV corresponding to the spin-orbit coupling components of Mn 2p3/2 and Mn 2p1/2

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respectively (for both Mn3+ and Mn4+ states). Further, the spin energy separation between the

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Mn 2p3/2 and Mn 2p1/2 is observed to be 11.4 eV indicating dominant Mn4+ state due to MnO2

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in the composites (also validated using XRD studies).42 However, as seen in Fig. 3(a), the

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surface of the MnO2 nanorods are dominated with Mn3+ states (formation of MnOOH) due to

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slight Mn4+ to Mn3+ reduction. Further, the deconvoluted peaks at binding energies (BEs) of

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642/653.4 eV and 643.7/654.8 eV due to spin-orbit pair of Mn 2p core level have been assigned 7 ACS Paragon Plus Environment


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to Mn3+ and Mn4+species respectively, revealing presence of Mn3+ species at the surface.58-60

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In addition, the peak at 648 eV was assigned to a shake-up satellite peak.52 The B 1s spectrum

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in Fig. 3(b), was deconvoluted into two peaks at 190.3 eV (major) and 191.6 eV, which can be

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assigned to B-N and B-O bonding respectively.24,55,61 Additionally, N 1s spectrum can be fitted

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into two peaks located at 397.9 and 398.9 eV corresponding to the N-B and N-O linkages

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respectively in the resultant composite (Fig. 3c).55 Of course, the presence of both B-O as well

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as N-O bonding in XP spectra of MnBN sample is quite expected as per our experimental

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conditions. The O 1s spectrum can be deconvoluted into four components and peak at 529.5

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eV can be assigned to oxygen bonded to metal atom (Mn-O-Mn). Further, the appearance of

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additional two peaks at 531.5 and 532.4 eV reveals the presence of O-B and O-N linkages

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respectively, and peak at 533 eV has been attributed to the presence of hydrated molecule (as

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clearly seen in Fig. 3d). Additionally, the chemical environment of MnBN/C-75 and MnO2/C-

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75 composites has also been studied by high resolution XP spectra (Fig. S8). As shown in the

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Fig. S8(a), MnBN/C-75 composite reveals the spin-orbit coupling spectrum of Mn 2p core with

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binding energy of 642.3/653.5 and 643.9/654.9 eV corresponds to the Mn3+ and Mn4+ state

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respectively. Obviously, the Mn3+ is dominant oxidation state in MnBN/C-75 composite, which

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is in good agreement with crystal structure (MnOOH) using XRD study. Notably, XP spectrum

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of MnO2/C-75 composite (for Mn2p core) shows relatively similar features as that of MnBN/C-

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75 (as clearly seen in Fig. S8(d)). The comparative binding energies of various elements present

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in all composites are summarized in Table S1 (in ESI). Additionally, both the B1s and N1s

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spectra of MnBN/C-75 sample show two peaks in their core level spectrum, which can be

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assigned to B-N and their respective oxides (Fig. S9). Moreover, the O1s spectrum of both

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MnBN/C-75 and MnO2/C-75 composites can be deconvoluted into four components (Fig.

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S8(b, e)). The peaks at lower BEs (~ 530 eV) corresponding to the typical metal-oxygen bond.

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The peak at 531.3 and 532.6 eV in case of MnBN/C-75 composite was assigned to B-O and N-

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O linkages respectively. Whereas, peak at higher BEs corresponding to the -OH linkage or

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presence of hydrated molecule. Importantly, C1s spectrum can be split into two peaks, major

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at 284.5 eV corresponding to the C-C linkage and ~286 eV has been assigned to C-O, which is

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quite expected as per our experimental conditions (Fig. S8(c, f)).

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Electrocatalytic activity and durability

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To assess the ORR activity under alkaline conditions, we performed the CV and RDE

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measurements for all MnBN/C catalysts at room temperature. Figure 4(a) shows the 8 ACS Paragon Plus Environment

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comparative CV curves under N2- and O2-saturated 0.1 M KOH solution for most active

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MnBN/C-75 catalyst. As shown in the figure, MnBN/C-75 catalyst exhibits a pronounced and

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sharp cathodic ORR peak at around 0.7 V vs RHE when the solution is saturated with oxygen,

4

confirming the electrocatalytic activity towards ORR. The high ORR activity and kinetic

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current information are also obtained from its ORR polarization curve with respect to onset

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potential (Eonset), half-wave potential (E1/2) and diffusion-limiting current density (JL). Figure

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4(b) illustrates a significant Eonset of 0.9 V vs RHE with substantially higher JL of 5.6 mA cm-2

8

at 1600 rpm under O2-saturated 0.1 M KOH solution. Additionally, LSVcurves show that the

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‘JL’ increases with increase in rotation speed of the electrode, because of the shortened

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diffusion layer distance at higher rotation speeds.35,62,63 To get a further insight into the

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dioxygen electroreduction of composite nanomaterials, the kinetics of the reaction was studied

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using RDE and RRDE techniques. A Koutecky-Levich (K-L) approach provides kinetic

13

information in the form of the number of electrons transferred (n) per oxygen molecule during

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electroreduction of oxygen. Accordingly, Fig. 4(c) shows the K-L plot (J-1 vs. ω-1/2) and the

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number of electrons transferred to the MnBN/C-75 electrode was calculated using the K-L

16

equation:62 1

17

18 19

1

1

𝐼 = 𝐼𝑘 + 𝐼𝑑 =

Where,

1

1

(2)

𝐼𝑘 + 𝐵.𝜔1/2

𝐼𝑘 =

(𝐼 × 𝐼𝐿)

(3)

(𝐼𝐿 - 𝐼)

-1/6𝐶 𝐵 = 0.62𝑛𝐹𝐴𝐷2/3 𝑂2 𝑂2 𝜈

(4)

20

Where, ‘I’ is the measured current, ‘Ik’ is kinetic limiting current, ‘Id’ is diffusion limiting

21

current, ‘𝜔’ is the rotation rate of RDE, and ‘B’ is K-L slope which is determined by using K-

22

L plot. ‘n’ is the number of electrons transferred during ORR process, ‘F’ is the Faraday

23

constant (96485 C/mol), ‘ν’ is the kinematic viscosity of the 0.1 M KOH electrolyte (0.01

24

cm2/s), ‘DO2’ is the diffusion coefficient of oxygen molecule (1.9×10-5 cm2/s), ‘CO2’ is the bulk

25

concentration of oxygen (1.2×10-6 mol/lit.) and ‘A’ is the geometrical surface area of RDE.64

26

Usually for classic Pt-based catalyst,the direct 4-electron transfer oxygen electroreduction is a

27

more favourable pathway (Eq. 5) than initially a 2-electron electrochemical reduction of O2

28

molecule into peroxide (HO2-) followed by instantaneous decomposition of HO2- via another

29

2-electron (Eq. 6-7) for the ORR.65,66

30

𝑂2 + 2𝐻2𝑂 + 4𝑒 ― → 4𝑂𝐻 ―

(5) 9

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1

𝑂2 + 𝐻2𝑂 + 2𝑒 ― → 𝐻𝑂2― + 𝑂𝐻 ―

(6)

2

𝐻𝑂2― + 𝐻2𝑂 + 2𝑒 ― → 3𝑂𝐻 ―

(7)

3

The ‘n’ calculated from the slope of the K-L plot is 3.9 and 4.0 in case of MnBN/C-75 and

4

commercial Pt/C catalyst (obtained from Fig. S10 in ESI) respectively, indicating that

5

MnBN/C-75 catalyst favours a Pt-like direct 4-electron transfer pathway for dioxygen

6

reduction. Further, RRDE experiment was carried out to confirm the ‘n’ value during ORR, by

7

particularly detecting the intermediate such as peroxide (HO2-) species generated during the

8

ORR process.67 According to the inset of Fig. 4(d), the ring current (Jr) has been very inferior

9

compared to disk current (Jd) for MnBN/C-75 sample. It also shows a peroxide yield of less

10

than 15% over the potential range of 0.1 to 0.8 V versus RHE indicating an average ‘n’ value

11

of 3.9 (as clearly seen in Fig. 4d).

12

We have performed the comparative ORR measurements for various composite

13

catalysts to gain insight into the origin of such high electrocatalytic activity. As shown in Fig.

14

5(a), except MnBN/C-75 catalyst (with one-step ORR kinetics), all other samples exhibit non-

15

linear ORR polarization curves in the diffusion-limiting current region indicating 2 or mixed 2

16

to 4-electron transfer for O2-reduction process. Importantly, MnBN/C-75 composite reveals

17

the most positive onset potential (the potential at which the catalyzed ORR current starts to

18

appear) than all other MnBN/C based composite catalyst. Though the onset potential of

19

MnBN/C-75 catalyst is slightly inferior to Pt/C catalysts, its limiting current density is much

20

closer to Pt/C under similar experimental conditions (Fig. S11). Besides, the E1/2 value of

21

composite catalyst shows almost 100 mV positive shift after carbon support, while the

22

performance decreases at higher carbon content than optimal composition (i.e. 75 wt%), which

23

is clearly seen in Fig. 5a and 5c. Since, MnOx and pristine h-BN show poor electrical

24

conductivities, a slower charge transport from electrode to O2 molecule could be envisaged

25

during electrocatalysis. Hence, electrochemical impedance spectroscopy (EIS) measurements

26

were carried out to get insight of various processes involved at the electrode/electrolyte

27

interface during ORR. Accordingly, Nyquist plot was obtained using AC amplitude of 5 mV

28

in the frequency range of 100 kHz to 100 mHz with a 0 V bias potential as depicted in Fig. 5b.

29

As expected, h-BN shows overall highest resistance due to its inherent insulating behavior. On

30

the other hand, MnBN/C-75 electrode exhibits steeper slope at low frequency region as

31

compared to other samples, indicating minimum ionic diffusion resistance from electrolyte to

32

the electrode surface during ORR process (as clearly seen in the inset of Fig. 5b). Moreover, 10 ACS Paragon Plus Environment

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1

the smaller diameter of a partial semicircle in the high frequency region of MnBN/C-75 sample

2

suggests a relatively lower charge transfer resistance (Rct) at the electrode/electrolyte

3

interface.68,69 The additional feature of MnBN/C-75 electrode curve has been analyzed based

4

on a simple Randles equivalent circuit model, depicted in Fig. S12. It is important to note that,

5

the Warburg impedance (W) and constant phase element (CPE) have been modeled mainly due

6

to the mixed kinetic and diffusion controlled polarization processes. The impact of KB carbon

7

on the MnBN catalyst to improve the ORR activity and kinetics was also investigated by

8

varying the carbon loading. Figure 5(c) reveals a remarkable trend in positive Eonset potential

9

with respect to carbon loading and a dramatic decrease beyond optimal (i.e. 75 wt%) carbon

10

loading. This study implies that, the charge transfer resistance of MnBN electrode decreases

11

with an increase in carbon content. Whereas, decrease in the ORR response of the MnBN/C-

12

90 electrode may be due to blocking of possible active sites of MnBN nanocomposite by KB

13

carbon. This trend has been observed for three independent experiments. Further, relatively

14

smaller Tafel slope of 122 mV dec-1 for MnBN/C-75 catalysts than that of MnBN/C-90 (127

15

mV dec-1) and other nanocomposite catalysts has been seen, suggesting a better ORR

16

electroreduction kinetics by MnBN/C-75.

17

The better ORR performance of optimized composite nanomaterial was further

18

confirmed by comparing its mass activity (Im) and kinetic limiting current density (Jk) with

19

state-of-the-art Pt/C catalyst. Figure 6(a) reveals the histogram of comparative mass activity

20

(after normalizing the kinetic current, Ik, by the total mass of active materials) values calculated

21

at 0.6 V. Accordingly, a MnBN/C-75 composite catalyst (222 mA mg-1) exhibits the ~31.7,

22

17.5 and 21.3 times higher mass activity than that of pristine h-BN (7 mA mg-1), bare KBC

23

(12.7 mA mg-1) and as-synthesized MnBN sample (10.4 mA mg-1) respectively. Importantly,

24

MnBN/C-75 catalyst demonstrates highest Jk value of 7.1 mA cm-2, signifying faster ORR

25

kinetics among all composite nanomaterial catalysts (as clearly seen in Fig. 6a). It is worth

26

noting that MnBN/C-75 not only shows enhanced ORR kinetics compared to other composite

27

nanomaterials, but also exhibits comparable mass activity (Im) and kinetic current density (Jk)

28

similar to that of commercial Pt/C catalyst (Fig. 6b). Further, MnBN/C-75 nanocomposite

29

shows ‘n’ and ‘JL’ value of 3.9 and 5.5 mA cm-2 @ 0.2 V respectively, which are very close to

30

that of Pt/C catalyst (4.0 and 5.7 mA cm-2) under similar experimental conditions (clearly seen

31

in Fig. 6b-c and its inset). Overall ORR activity, kinetics and stability of electrocatalyst are few

32

important parameters to define a better catalyst. Therefore, the stability tests were conducted

33

by continuous CV measurements in 0.1 M KOH with the sweep rate of 50 mV s-1 up to 5k 11 ACS Paragon Plus Environment


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1

durability cycles. The MnBN/C-75 composite displays much better stability with a small

2

deviation in its E1/2 (18 mV) compared with state-of-the-art Pt/C catalyst (50 mV) after 5k

3

durable cycles under alkaline conditions (Fig. 6b-c).

4

Moreover, Pt or Pt-alloy nanoparticles shows methanol cross-over effects at the cathode

5

compartment in direct methanol fuel cells (DMFCs) resulting into the decrease in catalytic

6

response. However, MnBN/C-75 composite catalyst exhibits negligible changes in

7

amperometric I-t (current versus time) curve after addition of 1 M methanol. On the other hand,

8

the Pt/C shows appreciable response in the I-t curve due to the addition of methanol, indicating

9

enormous cross-over effects, which would accelerate poisoning effects. (as seen in Fig. S14).

10

Thus, the obtained MnBN/C nanocomposite demonstrates enhanced electrochemical activity

11

and significant durability with first order reaction kinetics towards dioxygen electroreduction.

12

Hence, it could be a promising electrocatalyst at cathode compartment in alkaline fuel cells to

13

lead fast ORR kinetics.

14 15

Influence of MnOx and h-BN composite nanomaterials on ORR kinetics

16

It should be noted that the electrocatalytic performance of MnOx based catalyst depends on its

17

crystallographic structure and oxygen vacancies. Further, Roche et al. and a few others have

18

reported that, ORR activity on MnOx surface usually increases in the following order: β-MnO2

19

< λ-MnO2 < γ-MnO2 < α-MnO2 ≈ δ-MnO2.43,44,58 Moreover, MnOOH phase is more favourable

20

for oxygen electroreduction among the other MnOx species (Mn2O3, Mn3O4 and Mn5O8),

21

reported by Ohsaka and his co-workers.70 Usually, during electroreduction of oxygen, surface

22

protonation of MnO2 takes place that leads to the formation of MnIIIOOH. In the present study,

23

the formation of such crystallographic phase has already been confirmed through XRD analysis

24

(as discussed earlier). Thus, it is believed that the enhanced kinetics on MnOx surface could be

25

attributed to the higher content of Mn3+ in MnOx species. It has also been reported that, a more

26

contribution of Mn3+ in MnOx species can lead to a better catalytic performance, due to the

27

single electron occupation in the σ*-orbital (eg) of Mn3+.58,71 However, there exists an

28

uncertainty about the dioxygen reduction kinetics on MnOx surfaces because of the complex

29

chemistry of the Mn species. Therefore, to the best of our knowledge the ORR pathway on

30

MnOx surface follows sequential steps as:43,65,58

31 32

2𝑀𝑛𝑂𝑂𝐻 + 𝑂2→ 2MnOOH… (𝑂2) ads 2𝑀𝑛𝑂𝑂𝐻…(𝑂2)𝑎𝑑𝑠 + 𝑒 - → 𝑀𝑛𝑂𝑂𝐻…(𝑂)𝑎𝑑𝑠 +𝑀𝑛𝑂2 + 𝑂𝐻 12 ACS Paragon Plus Environment

(8) (9)

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𝑀𝑛𝑂𝑂𝐻…(𝑂)𝑎𝑑𝑠 + 𝑒 - → 𝑀𝑛𝑂2 + 𝑂𝐻 -

1

(10)

2 3

In order to elucidate the influence of h-BN on efficiency of MnBN/C-75 catalyst for

4

ORR, we have carried out controlled electrochemical measurements for various catalysts along

5

with their composites. MnO2/C-75 (MnO2 supported on KB carbon) and h-BN/C-75 (h-BN

6

supported KB carbon) were also prepared for their comparative electrochemical catalytic

7

studies. The overall comparative ORR response for all these catalysts have been illustrated in

8

Fig. S15. Importantly, pristine h-BN, MnO2, MnBN and carbon black, exhibiting non-linear

9

ORR behavior in the diffusion-limiting current region indicating 2 or mixed 2 to 4-electron

10

transfers for O2-reduction process. Interestingly, all these catalysts show promising behavior

11

(Eonset, E1/2 etc.) with one-step ORR kinetics, upon composite formation with carbon black (as

12

clearly seen in Fig. 5a and Fig. S15). Notably, MnBN/C-75 composite catalyst demonstrates

13

the most promising behavior with greater ORR stability under alkaline condition (Fig. 5a, 6b

14

and Fig. S14). Though, manganese in MnO2/C-75 composite catalyst exhibits similar structure

15

as that in MnBN/C-75 composite, MnO2/C-75 catalyst exhibits significant loss (shift of ΔE1/2

16

= 31 mV @2000 cycles) in its ORR catalytic activity during electrochemical cycling test (as

17

clearly seen in Fig. S16). On the other hand, based on DFT study, Koitz et al. suggested that

18

metal-supported h-BN shows tunable ORR performance.72 However, there is no chemical

19

interaction between -B, -N and Mn/MnOx, though we belive that van der Waals interactions

20

among h-BN sheets and MnOx nanorods provide ideal three phase boundries for O2 adsorption.

21

Additionally, XPS analysis confirmed that the B-O and N-O bondings on BN sheet, which may

22

introduce defects in perfect h-BN lattice. Thus, the physical interaction between h-BN and

23

metal/metal oxides along with defects in BN crystal lattice contributed towards more O2

24

adsoption followed by electroreduction. Further, the presence of Mn3+ ions in the form of

25

MnOOH (during composite formation) along with both h-BN support and KB carbon at the

26

electrode surface (through the synergetic effect between them) contributes immensely in

27

boosting the electrocatalytic activity. Therefore, it has been suggested that, h-BN plays a

28

crucial role in oxygen electrokinetics as well as overall stability of MnBN/C nanocomposite

29

catalyst.

30 31 32

Conclusion



1

In summary, we have shown that optimized MnBN/C nanocomposite reveals enhanced

2

electrokinetics towards dioxygen reduction under alkaline media. A single step growth and

3

assembly of MnO2 nanorods on h-BN surface has been achieved, which showed an interesting

4

oxygen electroreduction property after composite formation with KB carbon. It is found that,

5

MnBN/C catalyst exhibits a mixed kinetic and diffusion controlled polarization processes

6

during ORR. Importantly, this catalyst exhibited a substantial higher Eonset of 0.9 V and JL of

7

5.6 mA cm-2 with first order reaction kinetics for ORR. The presence of Mn3+ ions in the form

8

of MnOOH along with both h-BN support and KB carbon at the electrode surface (through the

9

synergetic effect between them) contributed in boosting the electrocatalytic activity. Hence,

10

we believe that it could be a promising candidate (low cost Noble metal-free electrocatalyst)

11

to lead faster ORR kinetics in low temperature fuel cells as well as metal air-batteries.

12 13

Acknowledgement

14

The authors thank the Department of Science and Technology, Science and Engineering

15

Research Board (DST-SERB-EMR/2016/005888), India for financial support. Authors also

16

acknowledge DST-FIST (Fund for Improvement of S & T infrastructure; SR/FST/CST-

17

266/2015(c)) and SRM Institute of Science & Technology for providing infrastructure and

18

facilities like HR-TEM.

19 20

Supporting Information

21

The Electronic Supplementary Information (ESI) is available free of charge on the ACS

22

Publications website. Calibration curve, XRD pattern, comparative SEM with elemental maps,

23

XPS, K-L plot, EIS curves for MnBN/C-75, ORR activity comparison, MOR study and

24

stability response of MnO2/C-75 sample are available in supporting information.

25 26 27

Author Information

28

Email address: 1) [email protected] (B. Kakade*)

29

2) [email protected] (I. Patil)

30

3) [email protected] (A. Swami)

31

4) [email protected] (L. Moorthi) 5) [email protected] (R. Chavan)

32 33

References 14 ACS Paragon Plus Environment

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1

Figures and Figure captions

2

Scheme 1

3 4 5

Scheme 1: Schematic illustration of synthesis of MnBN/C nanocomposite catalyst.

6 7 8



1

Figure 1

2 3

Figure 1. (a) XRD pattern of as-synthesized MnBN sample with pristine h-BN; and (b)

4

comparative XRD profiles of pristine h-BN, MnBN and MnBN/C-75 composite nanomaterial,

5

whereas inset shows the difference in peak positions of (002) plane of h-BN.

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1


Figure 2

2 3

Figure 2. TEM image of (a) pristine h-BN;(b) as-synthesized MnBN sample; (c) and (d) low

4

and high resolution TEM images of MnBN/C-75 composite nanomaterial respectively.

5



Figure 3

1 2 3 4

5 6

Figure 3. Deconvolution XP spectra for (a) Mn 2p; (b) B 1s; (c) N 1s and (d) O 1s core levels

7

of MnBN sample. Raw dataare denoted by blue circles and a solid pink lines represent fitted

8

spectra.

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Figure 4

1

2 3

Figure 4. (a) CV profile of MnBN/C-75 nanocomposite catalyst in N2 (red line) and O2 (blue

4

line)-saturated 0.1 M KOH at a scan rate of 20 mV s-1; (b) LSV curves of MnBN/C-75 with

5

different rotating speed at the scan rate 10 mV s-1 (geo: geometric current density); (c)

6

Koutecky-Levich plot obtained from LSV curves given in figure (b); and (d) The extent of

7

peroxide yield and number of electrons transferred (n) of MnBN/C-75 catalyst,whereas inset

8

shows RRDE test of ORR under O2-saturated 0.1 M KOH at a scan rate and rotation rate of 10

9

mV s-1 and 1600 rpm respectively.

10



1

Figure 5

2 3

Figure 5. (a) ORR response for bare h-BN, KB carbon, MnBN and MnBN/C-75 electrodes at

4

1600 rpm; and (b) corresponding Nyquist plots of all the catalysts, whereas inset shows EIS

5

curves at high frequency region; (c) comparative ORR polarization curves of various MnBN/C

6

composite catalyst at 1600 rpm under O2-saturated 0.1 M KOH electrolyte; (d) Tafel-plots for

7

(i) MnBN/C-10, (ii) MnBN/C-33, (iii) MnBN/C-50, (iv) MnBN/C-66, (v) MnBN/C-90 and (vi)

8

MnBN/C-75 at low current density region.

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1


Figure 6

2 3

Figure 6. (a) The electrochemical activity in terms of mass activity and kinetic current density

4

(Jk) obtained from comparative LSV curves given in figure 5(a,c) and K-L plot in Fig. S10 and

5

S13 respectively; comparative stability performance of (b) MnBN/C-75 and (c) Pt/C catalyst,

6

before and after 5k durable cycles under O2-saturated 0.1 M KOH (sweep rate and rotation

7

speed of 10 mV s-1and 1600 rpm respectively are employed in both cases); whereas inset of

8

figure (b) reveals the activity comparison of MnBN/C-75 and Pt/C catalyst along with number

9

of electron transfer (n) involved in ORR process.

10



1 2

TOC Graphics

3 4 5 6 7 8 9 10 11 12

A crucial role of h-BN for boosting ORR activity and stability of MnBN/C nanocomposite

13

catalyst


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Hexagonal Boron Nitride Supported Crystalline Manganese Oxide

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