Nanoscale Carbon Modified Î±-MnO2 Nanowires: Highly Active and

Subscriber access provided by READING UNIV

Article 2

Nanoscale Carbon Modified #-MnO Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content Julian A. Vigil, Timothy N. Lambert, Jonathon William Duay, Collin J. Delker, Thomas E. Beechem, and Brian S. Swartzentruber ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16576 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Nanoscale Carbon Modified α-MnO2 Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content Julian A. Vigil, † Timothy N. Lambert, *,† Jonathon Duay, † Collin J. Delker, ‡ Thomas E. Beechem, § and Brian S. Swartzentruber ‡ †

Department of Materials, Devices & Energy Technologies,

‡

Nanostructure Physics & Center for Integrated Nanotechnologies,

§

Nanoscale Sciences Department,

Sandia National Laboratories; Albuquerque, New Mexico, USA, 87185.

ABSTRACT:

Carbon-coated α-MnO2 nanowires (C-MnO2 NWs) were prepared from α-MnO2 NWs by a twostep sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (< 5 nm, ≤ 1.2 wt. % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly five orders of magnitude (α-MnO2, 3.2 x10-6 S cm-1; C-MnO2, 0.52 S cm-1) and an increase in surface Mn3+ (average oxidation state: α-MnO2, 3.88; C-MnO2, 3.66) while suppressing a phase change to Mn3O4 at

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 42

high temperature. The enhanced physical and electronic properties of the C-MnO2 NWs – enriched surface Mn3+ and high conductivity – are manifested in the electrocatalytic activity toward the oxygen reduction reaction (ORR), where a 13-fold increase in specific activity (αMnO2, 0.13 A m-2; C-MnO2, 1.70 A m-2) and six-fold decrease in charge transfer resistance (αMnO2, 6.2 kΩ; C-MnO2, 0.9 kΩ) were observed relative to the precursor α-MnO2 NWs. The CMnO2 NWs comprised of ~99 wt. % MnO2 and ~1 wt. % carbon coating also demonstrated an ORR onset potential within 20 mV of commercial 20% Pt/C and chronoamperometric current/stability equal to or greater than 20% Pt/C at high overpotential (0.4 V vs. RHE) and high temperature (60 °C) with no additional conductive carbon.

KEYWORDS: manganese oxide, nanowires, carbon coating, composite, oxygen reduction reaction, electrocatalysis, alkaline media

INTRODUCTION: Fuel cells and metal-air batteries are promising electrochemical technologies for supporting increased grid penetration of intermittent renewable power (solar and wind).1-3 For these devices that utilize ambient (or pure) O2 as a fuel for the cathodic discharge reaction, the kinetic limitations of the oxygen reduction reaction (ORR) must be addressed to increase device capacity and deliver electricity at a competitive cost.4-6 Thus, electrocatalysts are needed to catalyze the ORR at low overpotential with preference to the 4 e– pathway, and must meet elemental abundance and durability constraints that currently limit the commercialization of Ptbased catalysts and devices.5, 7 Owing to unfilled d-orbitals and reversible redox transitions near


2

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the thermodynamic potential of the ORR, transition metal oxides (TMOs) represent one class of active and abundant electrocatalyst materials in alkaline electrolyte.2 Manganese is extremely abundant and a range of manganese oxide (MnOx) crystal structures exist built from [MnO6] octahedral subunits comprising Mn in the 3+ and/or 4+ valence state. The octahedral Mn3+ center (d4) has eg electron occupancy of 1, and is therefore required (based on the proposed mechanism of the ORR on MnOx 8-10) for fast OH– displacement and O2 adsorption in the rate-limiting step. Thus, Mn3O4 spinel (hausmannite) and Mn2O3 (bixbyite) structures with octahedral Mn3+ subunits have shown high electrocatalytic activity toward the ORR.11 Despite constituting octahedral Mn4+ in a (theoretical) pristine state, MnO2 structures also demonstrate high ORR activity, in some cases exceeding that of Mn3O4 and Mn2O3 in comparative studies (e.g. MnO2 ≈ MnOOH > Mn2O3 > Mn3O412 and MnO2 > Mn3O4 > MnO13). The activity is rationalized by partial Mn3+ content in octahedral sites from oxygen vacancy, partial reduction, and charge balance with structural cations and water molecules in characteristic MnO2 tunnel (pyrolusite, ramsdellite, hollandite, etc.) and layered (birnessite) crystal structures.14-15 In addition, the surface Mn4+/Mn3+ redox transition in alkaline solution occurs at ~0.9 V vs. RHE, enabling O2 adsorption at low ORR overpotential.10 Recently, our group elucidated the role of Mn3+ in α-MnO2 nanowire (NW) electrocatalysts via substitutional cation doping and surface characterization using X-ray photoelectron spectroscopy (XPS), where overall ORR activity correlates with increased surface Mn3+ content and increased covalent character of the structural Mn-O bonds.9, 16 Ryabova et al. also reported a comprehensive electrochemical study of B-site Mn perovskites, MnO2, MnOOH, Mn2O3, and Mn3O4, and identified the potential of the surface Mn4+/Mn3+ redox transition as the sole descriptor of specific ORR activity.10 With these developments in understanding the


3


Page 4 of 42

mechanism of ORR activity mediated by the Mn3+/Mn4+ couple, notable improvements in the ORR activity of MnOx have primarily been achieved by (i) controlling the surface chemistry by manipulating valence,9-10,

12, 16-17

Mn-O bond character,9 oxygen vacancy,18 etc. and (ii)

addressing the prohibitive electrical resistivity of MnOx by blending or in situ growth with carbon.19-22 In the first approach (i), modifications are intrinsic to the MnOx surface and often result in changes to the crystal and/or electronic structure that enhance ORR activity and electron conduction simultaneously. Li et al. showed that oxygen deficiency in β-MnO2 results in improved overlap of the Mn HOMO with the O LUMO for ORR charge transfer and a change in the band structure resulting in increased electronic conductivity.18 In the latter approach (ii), blends with carbon and hybrid structures with MnOx formed in situ on the outer surface of carbon materials (e.g. graphene) have shown enhancement by dispersing active sites in a conductive network and boosting overall conductivity.21,

23-24

Yu et al. reported a MnO2

nanofilm/N-doped graphene sphere composite composed of 84.9 at. % C with a half-wave ORR potential within 10 mV of 20% Pt/C and a Zn-air cell with peak power of 82 mW cm-2.21 Furthermore, Dai and co-workers have reported on a series of materials that follow a general strategy of targeting strongly coupled inorganic/nanocarbon hybrids for ORR electrocatalysis.25 Among these, Mn3O4/Nano-C and MnCo2O4/N-rmGO hybrids have shown high activity and stability toward the ORR, compared to physical inorganic/nanocarbon mixtures as well as commercial Pt/C, owing to strong coupling evidenced by decreased ORR charge transfer resistance (by EIS) and strong M-O-C (M = Mn, Co) interfacial bonding and preference toward Mn3+ formation identified by X-ray absorption spectroscopy.25


4

Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Despite these successes, methods are still needed to increase electrocatalytic activity using cheap and abundant precursors, “green” synthetic methods, and with limited carbon content to maximize the active MnOx mass loading. The “carbon-coating” approach has received interest in the charge storage field, as carbon-coating TMOs has consistently demonstrated improved stability and charge-discharge cycling properties for Li-S and Na-S battery cathodes,2627

supercapacitor electrodes,28-29 and Li-intercalation electrodes.29-32 Carbon-coating has also

proven effective in stabilizing and increasing the activity of metal and metal phosphide electrocatalysts for water splitting.33-37 For ORR electrocatalysis, carbon-coating has primarily been employed on materials comprising metals, phosphides, and sulfides.35, 38-40 There are only a few reports to date on the intentional encapsulation of TMO structures within carbon materials for the ORR, specifically BaMnO3,41 Mn3O4,42 and Fe3O4.43 Here, we report on the synthesis and characterization of carbon-coated α-MnO2 NWs that demonstrate increased surface Mn3+ content, single-nanowire conductivity, and specific ORR activity owing to a thin, porous, low mass-percentage amorphous carbon coating (< 5 nm, ≤ 1.2 wt. % C) derived from sucrose. The carbon coating process, and the resulting retention of the α-MnO2 crystal structure after pyrolysis, is notable from a synthetic standpoint considering a partial phase change from αMnO2 to Mn3O4 (that is detrimental to ORR activity) is observed in the absence of a carbon coating.

EXPERIMENTAL SECTION Materials: MnSO4·H2O (Alfa Aesar), KMnO4 (Sigma Aldrich), sucrose (ultra-pure, MP Biomedicals), Nafion solution (5 wt.% in alcohol, Sigma Aldrich), isopropanol (99%, Pharmco-


5


Page 6 of 42

Aaper), ethanol (200 proof, Pharmco-Aaper) and commercial benchmark electrocatalyst powder 20% Pt/C (E-TEKSM) were used as-received without modification or purification. Synthesis of α-MnO2 NWs: α-MnO2 nanowires (NWs) were prepared hydrothermally by a similar method to that previously reported by our group.9, 16, 19 KMnO4 (2.78 g, 17.6 mmol) was added to 95 mL of DI H2O and allowed to stir for approximately 10 min MnSO4·H2O (1.11 g, 6.6 mmol) was added to the above solution and allowed to stir until the salts were fully dissolved. The dark solution was transferred to a 125 mL capacity Teflon-lined stainless steel autoclave, and placed in an oven at 140 °C. The temperature was maintained for 120 h, at which point the autoclaves were removed and allowed to cool naturally to room temperature. The product αMnO2 NWs were collected by centrifugation and washed four times with DI H2O and four times with EtOH. After isolation by rotary evaporation, the product was dried overnight in a vacuum oven at 60 °C (typical yield ~1 g). Synthesis of C-MnO2 NWs, Csucrose, and MnxOy NWs: Carbon-coating of the α-MnO2 NWs was achieved by a similar approach to that used for BaMnO3.41 Sucrose was dissolved in 20 vol. % DI H2O/ 80 vol. % EtOH (95 wt. % α-MnO2: 5 wt. % sucrose). The remaining specified total mass of α-MnO2 NWs was added to the solution and allowed to stir overnight to form a homogeneous brown dispersion. The dispersion was then heated to ~80 °C under vigorous stirring to slowly drive off the solvent. The resulting solid was transferred to a vacuum oven and heated to 60 °C overnight. The solid was collected and transferred to an alumina crucible and heated in a tube furnace under N2-flow (100 mL min-1) to 800 °C (~3.2 °C min-1) for 1 h. The product C-MnO2 NWs were collected after cooling naturally under N2-flow. The “control” materials were prepared under identical heating conditions, beginning with either un-coated αMnO2 NWs (to form, hereafter, MnxOy NWs) or sucrose (to form, hereafter, Csucrose).


6

Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Powder X-ray diffraction (PXRD): Catalyst powders were dispersed in EtOH and drop-cast on zero-background Si PXRD plates. Patterns were collected using a PANalytical X’Pert PRO powder diffractometer connected to the X’Pert Data Collector software suite. Ten min scans were run from 10 – 80 degrees 2θ with no rotation and a source power of 45 kV and 40 mA. The patterns were analyzed using MDI Jade 9 software with the ICPDD database. Scanning/high resolution transmission electron microscopy (STEM/HRTEM): Imaging and X-ray microanalysis of C-MnO2 NWs and MnxOy NWs were done using a FEI Company Titan G2 80200 operated at 200kV, equipped with a spherical aberration corrector on the probe-forming optics and four silicon-drift X-ray detectors. Both bright-field and high-angle annular dark field images were acquired with a sub-0.136 nm probe with a current of 200pA and convergence angle of 18 mrad. TEM, selected-area diffraction (SAD), STEM, and X-ray microanalysis data were also acquired with an FEI Company Tecnai F30-ST operated at 300kV. The X-ray spectral image (full spectrum from each pixel in an array) data were analyzed with Sandia’s Automated eXpert Spectral Image (AXSIA) multivariate statistical analysis software44-45 to both filter noise and extract the relevant chemical components with no a priori input as to what elements may be present. SAD patterns were analyzed to extract interplanar spacing values for comparison to data from the Powder Diffraction Files (PDF) of the relevant chemical phases. X-ray photoelectron spectroscopy (XPS): α-MnO2, C-MnO2, and MnxOy NWs were loaded on to carbon tape prior to analysis. Spectra were collected using a Kratos AXIS Ultra DLD photoelectron spectrometer with a monochromatic Al Kα (1486.7 eV) source. The analysis area was an elliptical spot size of 300 x 700 microns. Several locations on each sample were analyzed to obtain a representative sampling. Survey spectra were recorded at pressures less than 5 x 10-9 Torr with 80 eV pass energy, 500 meV step sizes, and 100 ms dwell times. High


7


Page 8 of 42

resolution spectra were collected with a 20 eV pass energy, 50 meV step sizes, and 100 ms dwell times. Data processing was performed with CasaXPS Version 2.3.15. High-resolution corelevel peaks were compared by normalizing counts for each respective core-level. Raman Scattering Spectroscopy: Raman measurements were performed utilizing a WiTec alpha300R Raman microscope with 2.5mW of 532 nm laser light using a 55X/0.5 NA objective. All measurements were performed with a 600 l/mm grating resulting in a spectral resolution of ~1 cm-1. Line scans were taken over a 50 mm distance with a spectrum acquired every 2 µm. Spectral response showed no significant spatial dependence allowing for use of the average response shown in Figure S1, in which the raw output was smoothed using a Savitzky-Golay filter owing to the low signal to noise of the response. Major features are observed at energies of 181, 385, 577, and 637 cm-1 that are in line with that expected for α-MnO2.46 No response was found in the 1300-1600 cm-1 window where sp2 carbons manifest. Iodometric Titrations: ~10-15 mg of α-MnO2 or C-MnO2 NWs were dissolved in 10 mL of 6 M HCl. Then, ~0.3 g of KI was added, followed by 1 mL of starch indicator solution (1% in water). The solution was then immediately titrated with ~0.1 M Na2S2O3 that was previously standardized against a 0.01667 (1/60) M KIO3 standardized solution. The solution was titrated to a faint yellow color. Elemental Analysis: ~5 mg of α-MnO2 or C-MnO2 NWs were dissolved in 3 mL of concentrated HNO3 (60% w/v). The solution was then diluted to a mass of 100 g by DI H2O. The resulting solution was diluted once more by adding 1 g of solution to 9 g of 2% HNO3 to make a final concentration of 5 ppm α-MnO2 or C-MnO2 NWs. A Perkin Elmer NexIon 350D ICP-MS instrument was then used to analyze this solution to obtain the Mn atomic content of the


8

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials. Using this result along with that obtained from the iodometric titrations, an average oxidation state (AOS) for the α-MnO2 and C-MnO2 NWs was calculated using Eq. 1.

AOS =

!"#$ !% & '#( )*$$ %(!) +,-. /

∗4

(1)

Carbon percentage was deduced by subtracting the initial mass of the C-MnO2 NWs by the masses of the constituent elements and molecules calculated through ICP-MS and iodometric titration measurements with the assumption that any extraneous mass was due to carbon. Thermogravimetric Analysis – Differential Scanning Calorimetry (TGA-DSC): Alumina cups were cleaned by stirring in hot aqua regia for ~1 h, followed by heating to 1000 °C in an air furnace for 2 h. ~10 mg of dry α-MnO2, C-MnO2, or MnxOy NW powder was transferred to a clean, dry alumina cup and loaded into the instrument alongside an empty alumina background cup. A TA Instruments SDT Q600 TGA/DSC instrument with UHP Ar feed gas at a flow rate of 100 mL min-1 was used for the measurements. The heating profile followed the parameters of the pyrolysis step reported above, and the sample was then air cooled to room temperature. The theoretical carbon content in the C-MnO2 NW product after pyrolysis was calculated assuming each component in the 95 wt. % α-MnO2: 5 wt. % sucrose precursor mixture does not affect the mass loss of the other component. Individually, sucrose retained 21.0 wt. % and the α-MnO2 NWs retained 87.7 wt. % during a typical pyrolysis step ramp to 800 °C. Hence, a precursor mixture pyrolyzed with an initial basis of 100 mass units (95 units of α-MnO2: 5 units of sucrose) would be expected to retain a total mass of (95 x 0.877 units of α-MnO2) + (5 x 0.210 units of Csucrose) = (83.3 units of α-MnO2) + (1.05 units of Csucrose) = 84.4 units. The resulting C-


9


Page 10 of 42

MnO2 NW product with an overall mass retention of 84.4 wt. % product would therefore theoretically contain (1.05 / 84.4) x 100 wt. % = 1.2 wt. % carbon. Brunauer–Emmett–Teller Surface Area (BET SA): At least 100 mg of dry α-MnO2 or C-MnO2 NW powder was transferred to a clean, dry quartz tube with a filler rod and isothermal jacket. Prior to analysis, samples were degassed for 20 h at 100 ºC. The tube was transferred to the analysis port and submerged in a bath of LN2 to maintain a temperature of 77 K throughout the analysis. A Micromeritics ASAPTM 2020 instrument was used with UHP N2 as the analysis gas. Results were analyzed in the Micromeritics software suite according to the BET method in the P/Po range from 0.1 to 0.2. Single Nanowire Conductivity: ~2.5 mg of α-MnO2 or C-MnO2 NWs were added to 3 mL of IPA, then sonicated in a bath sonicator for 15 min to form a suspension. The α-MnO2 NW suspension was further diluted by adding 150 µL of the suspension to 4 mL of IPA and sonicated again in a bath sonicator for 15 min For the C-MnO2 NWs, probe sonication was needed to separate the material into discrete nanowires; otherwise, only bundles of nanowires were obtained. Thus, the C-MnO2 NW suspension was diluted to 300 mL with IPA and sonicated with a probe sonicator (2 s on, 9.9 s off for a total of 2.5 min sonication). ~50 µL of each suspension was drop-cast onto Si/SiO2 prefabricated substrates. An optical microscope was then used to image the nanowires relative to the alignment marks on the substrate. Note: The contrast of the nanowire in an optical image is not due to light reflecting off the nanowire itself, but a “shadow” or diffraction effect due to the combination of nanowire and oxide layer it is sitting on. KLayout, a semiconductor layout design drawing program, was used to design a contact pattern for each individual nanowire. A 90 s ozone plasma treatment was then used to promote adhesion of the photoresist. The photoresist consisted of two layers for an improved lift-off profile. The first


10

Page 11 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer, EL9 [9% copolymer, methyl methacrylate acid (MMA), and methacrylic acid (MAA) in ethyl lactate], was applied by spin-coating at 4000 rpm for 30 s resulting in a ~300 nm thickness. After a 90 s bake at 180° C, the second layer, PMMA 950 K-A2 [2% poly(methylmethacrylate) in anisole], was applied by spin-coating at 4000 rpm for 30 s resulting in a ~50 nm thickness. This was followed by another bake at 180°C for 90 s. Electron beam lithography (EBL) was performed using an FEI NovaNanoSEM instrument with Nanometer Pattern Generation System software. The contact pattern features were written with an area dose of 325 µC/cm2. The grid alignment marks on the substrate were used to ensure the accurate placement of the electrode contacts and contact pads. The pattern was then developed for 75 s using a 1:3 methyl isobutyl ketone (MIBK): IPA developer followed by a 15 s rinse in IPA. Next, 5 nm of Ti followed by 150 nm of Au was evaporated onto the EBL patterned substrate. Lift-off of the remaining photoresist was the performed by soaking the platform in acetone overnight leaving behind only the written contacts and nanowires. A four-terminal measurement was then performed using an Agilent Technologies B1500 semiconductor device analyzer. The voltage was swept between 0 and 20 V on the outer contacts to apply a current while the voltage was monitored between the inner contacts. The slope of the corresponding I-V curve after the contacts have been fully activated, eliminating the diode junction,47 was used to report the resistance values. Representative I-V curves demonstrating the voltage range where the resistance values were extracted can be found in Figure S2. Using these resistance values and the length and diameter of the nanowires obtained by SEM, the conductivity of the wires could be calculated using Eq. 2, σ=

"

(2)

3( 4


11


Page 12 of 42

where σ is the conductivity, l is the length of the nanowire, R is the measured resistance, and r is the radius of the nanowire. Here, we assume the nanowire is cylindrical; however, the nanowires have a more rectangular and/or square cross-sectional morphology, consistent with other reports.48-49 Assuming the diameter measured here is actually one side of a square would result in potential errors of up to 27% in the cross-sectional area as calculated above; however, the estimated conductivity here varies over 5 orders of magnitude thus reducing the relative magnitude of this error. Electrochemical Characterization: α-MnO2, C-MnO2, MnxOy, Csucrose and commercial 20% Pt/C catalyst films were drop-cast onto rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) working electrodes from 2.5 mg mL-1 inks. In a typical ink, 2.5 mg catalyst powder, 0.75 mL DI H2O, 0.2 mL i-PrOH and 0.05 mL 5 wt. % Nafion solution were combined and sonicated for > 20 min prior to drop-casting. 10 µL of ink was cast on 0.2475 cm2 glassy carbon RRDE electrodes (Pine Research Instrumentation) and 5 µL of ink was cast on 0.0707 cm2 glassy carbon RDE electrodes (Bioanalytical Systems, Inc.), resulting in a loading of 0.101 mg cm-2 or 0.177 mg cm-2, respectively, for all catalysts. Catalyst films were tested in 0.1 M KOH electrolyte that was purged and blanketed with O2 for ORR experiments or Ar for backgroundcurrent experiments. RDE experiments were carried out using a RDE-2 three-electrode cell with a rotating working electrode (Bioanalytical Systems, Inc.), controlled by a VersaSTAT 4 potentiostat (Princeton Applied Research) and the VersaStudio software suite. The glassy carbon/catalyst film rotating working electrode was accompanied by a Ag/AgCl (3 M NaCl) reference electrode and Pt coil counter electrode.‡ Linear scanning voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) methods were used to assess the catalysts’ ORR activity. LSV


12

Page 13 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


scans were collected scanning from 0.1 V to -0.7 V vs. Ag/AgCl at a scan rate of 5 mV s-1 at rotation rates of 500, 900, 1600, 2500 and 3600 RPM. Potentiostatic EIS spectra were collected at 10 mV RMS at the half-wave potential of the obtained LSV curves for each respective catalyst with no electrode rotation, by scanning the frequency from 105 to 10-2 Hz. High-temperature ORR studies were conducted in a modified jacketed cell at 60 °C with 1 M NaOH electrolyte under constant O2 purging with no electrode rotation. Potentiostatic experiments were sustained at 0.72 V vs. RHE for 10 min and then 0.42 V vs. RHE for 1 h. Absolute diffusion-limited current was taken from the LSV curve at a potential of 0.4 V vs. RHE and used to calculated diffusion-limited geometric current density (jgeo), active mass activity (jm), and specific current density (js) using the geometric electrode area, active mass loading, and BET SA, respectively. The corresponding error was calculated using the proportional standard deviation of jgeo values at 2500 RPM for α-MnO2 (9 %), C-MnO2 (5 %) and 20% Pt/C (10%), as well as the standard deviation in BET SA for js. RRDE experiments were carried out using a MSR rotating disk stand three-electrode cell controlled by a WaveDriver 20 bipotentiostat (Pine Research Instrumentation) with the AfterMath software suite. A glassy carbon disk/Au ring working electrode was used (exclusively) to ensure that hydroperoxy anion oxidation was diffusion-limited on the ring electrode for ORR mechanism analysis.50 Collection efficiency was determined for each catalystcoated electrode using the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple on an electrode identical to that used for ORR LSV studies. The cell also included a SCE (sat. KCl) reference electrode and Pt wire counter electrode. Independent studies utilizing a carbon counter electrode were also performed to alleviate any concerns of Pt contamination of the working electrode.‡ LSV scans were collected scanning the disk from 0.2 V to -0.8 V vs. SCE at a rate of 5 mV s-1 while holding


13


Page 14 of 42

the holding the ring electrode at 0.3 V vs. SCE. Percent peroxide generation (%P) and electron transfer number (n) values were calculated using the measured disk (oxygen) reduction current, and collection efficiency-normalized ring (hydroperoxy oxidation) current.9

RESULTS & DISCUSSION α-MnO2 NWs were synthesized hydrothermally as reported previously,9, 16, 19 and modified by a two-step sucrose coating and pyrolysis method41 to yield carbon-coated α-MnO2 NWs (hereafter, C-MnO2 NWs). Briefly, KMnO4 and MnSO4·H2O were dissolved in DI H2O in approximately a 3:1 mole ratio and heated in a Teflon-lined stainless steel autoclave at 140 °C for 120 h. The αMnO2 NW product was then dispersed in 20 vol. % DI H2O/ 80 vol. % EtOH with sucrose in a 95 wt. % α-MnO2: 5 wt. % sucrose ratio. The dispersion was stirred overnight, heated until a dry powder was formed, dried under high-vacuum overnight, and finally pyrolized at 800 °C for 1 h under flowing N2. The “control” materials were prepared under identical heating conditions using either un-coated α-MnO2 NWs (to form MnxOy NWs) or sucrose (to form Csucrose). Scanning electron microscope (SEM) images of the product C-MnO2 NWs are provided in Figure 1a and Figure S3. The bulk powder was imaged as-synthesized, and NWs are generally observed in large aggregates (Figure 1a, Figure S3a) or isolated bundles (Figure S3b) with a wide distribution of nanowire length, ranging from 300 nm to 4 µm. The C-MnO2 NWs were found to have an average length of 1.2 ± 0.7 µm (25 wires), with an average diameter of 0.10 ± 0.03 µm. The average length and diameter of the precursor α-MnO2 NWs (Figure S3c) are 2.0 ± 0.6 µm and 0.06 ± 0.02 µm (11 wires), respectively.


14

Page 15 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) SEM image of bulk C-MnO2 NWs; (b) HRTEM image of a C-MnO2 NW, index αMnO2 PDF #00-004-0141; (c) HRTEM image of a C-MnO2 NW and corresponding EDS elemental mapping for Mn and C (inset); (d) HRTEM image of a MnxOy NW showing lattice fringes with 4.9 Å spacing; (e) PXRD patterns of the bulk NW powders; (f) Representative highresolution XPS spectra in the Mn 3s binding energy region with peak fitting and ∆E(Mn 3s) splitting energy values.

High-resolution transmission electron microscope (HRTEM) images of the C-MnO2 (Figure 1b, 1c) and MnxOy (Figure 1d) NWs are also provided. Lattice fringes with interplanar spacing of 6.9 Å observed for the C-MnO2 NWs (Figure 1b) correspond to the (110) planes of the cryptomelane phase of MnO2 (α-MnO2 with K+ ions occupying the open tunnels; PDF #00-


15


Page 16 of 42

004-0141), consistent with phase retention from the α-MnO2 NWs despite the pyrolysis step at 800 °C. Raman scattering bands (Figure S1) and powder X-ray diffraction (PXRD, Figure 1e) patterns also correspond with retention of the α-MnO2 phase in the C-MnO2 NWs.46 Figure 1b shows the termination of the α-MnO2 lattice and continuation of an irregular carbon layer varying in thickness from ca. 2 – 3 nm. Electron diffraction from graphitic carbon was not observed, suggesting the carbon layer is amorphous. HRTEM with energy-dispersive spectroscopy (EDS) was used to image and map the elements in the crystal and coating (Figure 1c). In addition to being detected on the surface of the nanowire, C is detected beyond the boundary of Mn (corresponding to the end of the α-MnO2 lattice), confirming the presence of a carbon coating. The HRTEM image of a control MnxOy NW (Figure 1d) shows lattice fringes with 4.9 Å spacing. Due to the overlap of interplanar spacing for the {200} planes of α-MnO2 and the {101} planes of Mn3O4 a phase cannot be assigned for the HRTEM image, however PXRD confirms the presence of both α-MnO2 and Mn3O4 in the MnxOy NWs (Figure 1e). Thus, the carbon coating formation from sucrose serves as a semi-protective layer that suppresses a partial phase transformation from α-MnO2 to Mn3O4 on the time scale of the reaction. PXRD patterns for the α-MnO2, C-MnO2 and MnxOy NWs are shown in Figure 1e. The α-MnO2 NW pattern is indexed to the cryptomelane phase of α-MnO2 (PDF #00-004-0141), as previously reported.9,

16, 19

The reflections in the C-MnO2 NW pattern are also assigned to

cryptomelane α-MnO2, but deviate in a non-uniform manner to lower angles with increasing two-theta, which is indicative of an expansion of the unit cell. The preferred orientation characteristic of hydrothermally grown α-MnO2 with an intense (110) reflection19,

51-52

is not

observed for the C-MnO2 NWs, which is not unexpected after crystalline restructuring during the high-temperature pyrolysis step (Figure 1e).

After correction for sample alignment, cell


16

Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


refinement yielded unit cell parameters of (a, c) = (10.008 Å, 2.909 Å) for the C-MnO2 NW pattern, an expansion of 0.223 Å in the a-direction, 0.046 Å in the c-direction, and 17.3 Å3 in unit cell volume relative to the literature (PDF #00-004-0141). The absence of a (002) reflection from graphitic carbon in the C-MnO2 NW pattern at 2θ ≈ 24 - 25 ° is consistent with an amorphous state of the carbon coating. Additionally, Raman bands characteristic of graphitic carbon were not observed, in line with amorphization of the carbon and a net increase in sp3 carbon that occurs with this process.53 The PXRD pattern for the MnxOy NWs is indexed to a mixed phase of primarily Hausmannite Mn3O4 (PDF #01-075-1560), as well as the α-MnO2 (PDF #00-004-0141) phase evidenced by preferred orientation of the {110} planes (Figure 1e). Surface area (SA) was calculated from Brunauer–Emmett–Teller (BET) theory and N2 adsorption-desorption experiments. The BET SA of the C-MnO2 NWs was lower than the precursor α-MnO2 NWs: 11.3 m2 g-1 and 73.6 m2 g-1, respectively (Figure S4). Average pore diameter and volume also decreased from 13.4 nm to 10.3 nm and 0.306 cm3 g-1 to 0.0291 cm3 g1

, respectively, from α-MnO2 to C-MnO2. Mass change and heat flow were monitored during the

pyrolysis step using thermogravimetric analysis and differential scanning calorimetry (TGADSC). Individually, sucrose retained 21.0 wt. % and the α-MnO2 NWs retained 87.7 wt. % during a typical pyrolysis step ramp to 800 °C (Figure S5). Pyrolysis of the sucrose-coated αMnO2 NW precursor (to form the C-MnO2 NWs) resulted in 84.9 wt. % total mass retention for the C-MnO2 NW product, which is within 0.7% of the expected value, and calculated carbon content of 1.2 wt. %. Given that the carbon coating process results in MnO2 phase retention (O: Mn = 2 for an ideal MnO2), rather than formation on MnxOy with increasing Mn3O4 content (idealized O: Mn = 1.33), this 1.2 wt. % can be considered to be an upper bound of the carbon content in the C-MnO2 NWs. Hence, the C-MnO2 NWs contain ≤ 1.2 wt. % carbon.


17


Page 18 of 42

Using XPS measurements, Mn valence at the surface of the NW is readily approximated by the binding energy (BE) difference between the positions of the Mn 3s multiplet components.52 Full survey spectra for the C-MnO2 and MnxOy NWs are provided in Figure S6. Representative high-resolution XPS spectra in the Mn 3s BE region for the α-MnO2, C-MnO2, and MnxOy NWs are shown in Figure 1f. Average Mn 3s multiplet splitting energy [∆E(Mn 3s)] values of 4.51 ± 0.02 eV, 4.74 ± 0.08 eV, and 5.16 ± 0.01 eV were calculated for the α-MnO2, C-MnO2, and MnxOy NWs, respectively. Standard values of ∆E(Mn 3s) reported by Nelson et al. (4.5 eV for MnO2 and 5.4 eV for Mn2O3) are used to determine the relative concentration of surface Mn3+ and Mn4+ oxides.54 The surface Mn3+ content of the C-MnO2 NWs is markedly higher than the α-MnO2 NWs, and higher than that previously achieved by Cu- and Ni-doping.9, 16

Iodometric titration and elemental analysis methods were also used to quantify the bulk Mn AOS of the α-MnO2 and C-MnO2 NWs (see Experimental Section for details). The results are summarized in Table 1, showing a similar increase in concentration of Mn3+ and decrease in Mn average oxidation state (AOS) for the C-MnO2 NWs for the bulk. Assuming a linear dependence of ∆E(Mn 3s) on Mn AOS, the calculated Mn AOS at the surface via XPS is 3.73 ± 0.09 for the C-MnO2 NWs (compared to 3.66 ± 0.03 in the bulk obtained by titration, Table 1), 3.98 ± 0.02 for the α-MnO2 NWs (compared to 3.88 ± 0.05 in the bulk obtained by titration, Table 1), and 3.27 ± 0.01 for the MnxOy NWs. The results indicate a higher AOS at the surface of the nanowires than in the bulk, possibly due to slight surface oxidation caused by exposure to air. Elemental analysis results for carbon content also correspond well with the thermal decomposition analysis, indicating 0.67 wt. % C vs. ≤ 1.2 wt. % C, respectively.


18

Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of Mn Average Oxidation State for α-MnO2 and C-MnO2 NWs as Determined by XPS (Surface) vs. Iodometric Titration/Elemental Analysis (Bulk) Methods XPS surface XPS surface Mn Bulk Mn3+ (%) Bulk Mn AOS* ∆E(Mn 3s) (eV) AOS* α-MnO2 NWs

4.51 ± 0.02

3.98 ± 0.02

12

3.88 ± 0.05

C-MnO2 NWs

4.74 ± 0.08

3.73 ± 0.09

34

3.66 ± 0.03

*AOS = Average Oxidation State

Single-nanowire, four-point contact devices were fabricated and tested to compare the resistance and conductivity of the C-MnO2 NWs to the semiconducting α-MnO2 NWs. The reader is referred to the Experimental Section, and our recent work that first reported these devices and measurements,16 for details on the fabrication. The fabrication is summarized in Figure 2a. Briefly, NWs were dispersed in IPA and drop-cast onto patterned Si/SiO2 substrates (Figure 2a, 1), which were then spin-coated with a positive photoresist (Figure 2a, 2). Contacts were written on the patterns using EBL (Figure 2a, 3) and developed in MIBK/IPA (Figure 2a, 4), followed by deposition of Au/Ti (Figure 2a, 5) and final lift-off in an acetone soak (Figure 2a, 6). A SEM image of a representative C-MnO2 device is shown in Figure 2b. To address any possible particle aggregation or annealing from the thermal treatment, the C-MnO2 NWs were sonicated with an additional probe tip prior to fabrication to ensure single nanowires were isolated for devices (See Experimental Section).


19


Page 20 of 42

Figure 2. (a) Schematic representation of single-nanowire device preparation (Reproduced with permission from ref. 16. Copyright 2017 American Chemical Society.); (b) SEM image of a CMnO2 device in the nanowire-four-point contact region; (c) Average measured single-nanowire resistance and calculated conductivity (as compared to α-MnO2, NiMnO2, CuMnO2 – ref. 16).

The potential between the outer contacts was swept from 0 to 20 V for activation and the resulting current-voltage behavior of the inner contacts was measured. The voltage drop between the inner contacts was used to calculate the resistance of the NW according to Ohm’s Law (Figure 2a, 7). The average results from five C-MnO2 devices are shown in Figure 2c. Error bars represent the standard error of the mean. The C-MnO2 NWs are approximately five orders of magnitude less resistive than the α-MnO2 NWs,16 with average resistance of 3.16 x105 ± 8.63


20

Page 21 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x104 Ω. The conductivity of each NW was then calculated using tilt-corrected SEM measurements of the diameter of the NW in its respective device. The average diameter of the NWs as determined by SEM measurements of the devices are 83 ± 34 nm (α-MnO2 NWs) and 105 ± 26 nm (C-MnO2 NWs), which corresponds well with the average dimensions from bulk SEM images (see above). We note that the individual NWs used for devices are in the high range of length relative to the bulk (~ 4 µm) as these allowed for depositing the electrical contacts of the four-point device. Thus, these NWs may not represent a true random sample of the C-MnO2 NWs; however, the total NW length that was interrogated for the C-MnO2 and α-MnO2 NWs (i.e. the NW length between the two inner contacts of the device) were in fact quite similar, with average inner contact distances of 1.07 ± 0.01 µm (α-MnO2 NWs) and 1.03 ± 0.05 µm (C-MnO2 NWs). The C-MnO2 NWs are also approximately five orders of magnitude more conductive than the α-MnO2 NWs: 0.52 S cm-1 and 3.2 x10-6 S cm-1, respectively. The resistance and conductivity of Cu-α-MnO2 (2.9% Cu) and Ni-α-MnO2 (4.9% Ni) from our previous report are also shown in Figure 2c.16 The comparison indicates that carbon coating is a more effective approach to increasing NW conductivity than cation doping. Electrocatalytic activity toward the ORR was investigated on drop-cast catalyst films on a glassy carbon rotating disk electrode (RDE) in alkaline electrolyte (0.1 M KOH), with all catalysts being tested at identical mass loading of 0.177 mg cm-2. Figure 3a shows the linear scanning voltammetry (LSV) curves for the C-MnO2, α-MnO2, MnxOy, Csucrose and commercial 20% Pt/C (E-TEKSM) catalysts at 2500 RPM. The onset and half-wave potentials increase in the order of (onset): 0.68 V (Csucrose) < 0.75 V (MnxOy) < 0.88 (α-MnO2) ≈ 0.88 V (C-MnO2) < 0.9 V (20% Pt/C) and (half-wave): 0.55 V (Csucrose) < 0.62 V (MnxOy) < 0.72 V (α-MnO2) < 0.75 V (C-MnO2) < 0.81 V (20% Pt/C). The Csucrose, MnxOy, α-MnO2, C-MnO2, and 20% Pt/C catalysts


21


Page 22 of 42

attain diffusion-limited geometric current density (jgeo) of -1.4 mA cm-2, -1.7 mA cm-2, -1.7 mA cm-2, -3.4 mA cm-2, and -4.0 mA cm-2, respectively. In contrast to the report on carbon-coated BaMnO3, where a shift in the onset potential by 70 mV to lower overpotential is observed,41 the onset potential remains the same for the C-MnO2 NWs compared to the precursor α-MnO2 NWs. Given that the onset of ORR is governed by O2 adsorption on the MnOx surface (thus the Mn3+/Mn4+ redox), a change in the onset should (i) not be expected when the underlying α-MnO2 surface is largely retained, and (ii) be independent of the absence or presence of a conductive carbon. In fact, previous work shows the intrinsic onset potential of the ceramic is retained in the case of cation doping,9,

16, 19

physically blended TMO/conductive carbons,19,

55

and strongly

coupled TMO-C hybrids.25 However, the C-MnO2 NWs demonstrate a shift in half-wave potential by 30 mV, which is notably the region where maximum power could be extracted from a fuel cell.19 Recent reviews stress the importance in catalysis science to improve practices and benchmarking; specifically, reported electrocatalytic current should be normalized to SA or electrochemically active surface area (ECSA) and catalyst loading in addition to the electrode geometric area.56 To compare the ORR activity of the reported catalysts on the basis of mass and specific catalyst area, jgeo is converted to active mass activity (jm) using the catalyst mass loading and wt. % of active material and specific current density (js) using the catalyst mass loading and BET surface area of each catalyst (Figure 3b). Absolute diffusion-limited jm values for the αMnO2, C-MnO2, and 20% Pt/C catalysts are 9.6 A gMnO2-1, 19.4 A gMnO2-1, and 113 A gPt-1, respectively. Absolute diffusion-limited js values for the α-MnO2 and C-MnO2 catalysts are 0.13 A m-2 and 1.70 A m-2, respectively. Thus, the ORR activity of the C-MnO2 NWs is higher than the α-MnO2 NWs by a factor of two on the basis of mass and more than an order or magnitude


22

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the basis of specific catalyst area. Determination of the SA or ECSA for js of 20% Pt/C was not pursued here, however it has been reviewed elsewhere and is expected to be in the range of 0.8 – 8.6 A m-2 based on results in acidic electrolyte.57

Figure 3. (a) Background-subtracted RDE linear scanning voltammograms collected in 0.1 M KOH at 2500 RPM: catalytic currents normalized to geometric electrode area; (b) Absolute diffusion-limited geometric current density (jgeo), active mass activity (jm), and specific activity (js) for selected catalysts calculated from voltammograms; (c) EIS Nyquist plots obtained at constant applied half-wave potential of each catalyst in O2-sat. 0.1 M KOH with no rotation; (d) RRDE-derived ORR n values obtained in O2-sat. 0.1 M KOH at 2500 RPM; (e)


23


Page 24 of 42

Chronoamperometric ORR activity at 60 °C in O2-sat. 1 M NaOH with no rotation, 0.72 V vs. RHE (0-600 s) and 0.42 V vs. RHE (600-4200 s).

Electrochemical impedance spectroscopy (EIS) was used to determine the charge transfer resistance (RCT) toward the ORR at constant applied half-wave potential of each catalyst. The Nyquist Plots shown in Figure 3c were modeled by the Randles circuit with characteristic solution resistance, RCT, and double-layer capacitance. RCT decreases in the order of 6200 Ω (αMnO2) > 1780 Ω (MnxOy) > 920 Ω (C-MnO2) > 310 Ω (20% Pt/C), demonstrating that the carbon-coating leads to a less resistive electrocatalyst at low overpotential. The ORR pathway on the surface of each catalyst was characterized by quantifying percent peroxide generation (%P) and electron transfer number (n) using a rotating ring disk electrode (RRDE) with a catalyst loading of 0.101 mg cm-2, and the collection efficiency at the ring determined using the [Fe(CN)6]3-/4- couple on an identical catalyst-coated disk according to the recent suggestion by Zhou et al.50 The n value represents electrons transferred per O2 molecule and ranges from 2 ≤ n ≤ 4, where n = 4 corresponds to the preferred direct 4e- reduction of O2 to OH- and n = 2 corresponds to the 2e- reduction of O2 to HO2-. The generated HO2- can follow a serial pathway with a further 2e- reduction to OH-, giving rise to n values 2 < n < 4 (analogously 100% > %P > 0%), or catalytically decompose in a heterogeneous reaction on the surface to regenerate O2 (resulting in a pseudo-4e- ORR pathway). The relationship between applied disk electrode potential and n value is shown in Figure 3d; average n values for the four catalysts are 3.98 (20% Pt/C) > 3.84 (C-MnO2) > 3.75 (α-MnO2) > 3.6 (MnxOy). Due to the derivation of n and %P, the relationship between %P and applied electrode potential (Figure S7) is analogous to n, and


24

Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MnxOy (lowest n value) forms the highest proportion of peroxide product. There is a notable benefit to carbon-coating in average n value for the C-MnO2 NWs over the α-MnO2 NWs (Figure 3d), particularly in the observed reduction of percent peroxide generation at low overpotential (@ 0.85 V: 8% (C-MnO2) < 18% (α-MnO2), Figure S7). Considering the ORR rate-limiting step is proposed to occur at the Mn3+ site for all MnOx catalysts reported here (CMnO2, α-MnO2, MnxOy),8-10 the improved n value of the C-MnO2 NWs is credited to improved conductivity and Mn3+ content relative to the α-MnO2 NWs. An expected advantage from partially encapsulating the active MnO2 surface of the CMnO2 NW electrocatalyst is stability. To accelerate degradation, and model industrial fuel cell operations (commonly 1 M KOH or NaOH electrolyte at 60 – 80 °C), stability tests were performed in 1 M NaOH electrolyte at 60 °C at constant applied potential with no electrode rotation. Shown in Figure 3e, the test comprised two chronoamperometric segments: a ten min segment at 0.72 V (approximate half-wave region) and a 1 h segment at 0.42 V (high overpotential). The 20% Pt/C outperforms C-MnO2 in jgeo at low overpotential as expected from the LSV curves (jgeo = -0.78 mA cm-2 and -0.60 mA cm-2, respectively, after ten min); however, both catalysts are stable and maintain jgeo of -1.2 mA cm-2 for 1 h at high overpotential. The electrocatalytic ORR activity of the C-MnO2 NWs is proposed to originate primarily from the MnO2 surface and carbon-MnO2 interface (despite possible partial encapsulation and possible ORR activity from the carbon alone) based on analysis of: (i) the porosity of the nanoscale carbon, and (ii) the intrinsic ORR potentials of the C-MnO2 NWs compared to the precursor α-MnO2 NWs and control Csucrose. The 10.3 nm average pore diameter of the C-MnO2 NWs (Figure S4) demonstrates sufficient porosity in the nanoscale carbon coating for mass


25


Page 26 of 42

transfer of solvated O2 and ORR product ions between the bulk electrolyte and the catalytically active MnO2 surface. Analogous investigations of carbon-coated Fe3O4 nanorods43 (N-doped carbon coating: mesopores with diameter of 3.4 – 3.8 nm) and bimetallic PdCo nanoparticles39 (N-doped carbon coating: hierarchical micropores and mesopores with diameter of 0.6 nm and 4 nm, respectively) also identify the underlying metal or oxide nanostructure as the catalytically active surface with sufficient porosity for mass transfer. This was further confirmed by Hadidi et al., where the Fe3O4 cores of the nanorods were etched and the resulting carbon shells exhibited a significant decrease in ORR activity (71.8% decrease in jgeo, 0.15 V negative shift in onset). Here, the intrinsic ORR potentials of the C-MnO2 NWs (onset, 0.88 V; half-wave, 0.75 V) correspond with those of the precursor α-MnO2 NWs (onset, 0.88 V; half-wave, 0.72 V) and not the carbon control material Csucrose (onset, 0.68 V; half-wave, 0.55 V), confirming the ORR activity at low overpotential cannot be attributed to the carbon alone. The ORR steady-state jgeo for the Csucrose catalyst is also only 59 % of that for the C-MnO2 NWs, despite equal total mass loading and ~100 wt. % C content in Csucrose (compared to ≤ 1.2 wt. % C in the C-MnO2 NWs). This is in agreement with the work of Dai’s group on strongly coupled inorganic/nanocarbon hybrids, where the properties of a nanoscale carbon are systematically changed to improve the overall ORR activity of the hybrid (e.g., Mn3O4/Nano-C > Mn3O4/rGO) while the active site of the inorganic material (Mn3O4) remains constant.25 Further enhancements in the electrocatalytic ORR activity of the C-MnO2 NWs relative to the α-MnO2 NWs and previously reported Cu- and Ni-doped α-MnO2 NWs9, 16 are attributed to the enrichment of surface Mn3+ and increased single-nanowire conductivity. The increase in surface Mn3+ character is proposed to reduce the ORR overpotential by stabilizing O2 adsorbates and improving rates of electron transfer in the rate-limiting step of the mechanism,9-10, 16 while


26

Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the increased conductivity is expected to reduce charge transfer resistance and facilitate current distribution through the nanowire network and to the MnO2 surface, improving active site utilization.16, 25 While we expect that the Mn3+/Mn4+ composition largely dictates any change in intrinsic ORR activity,9,

16

it is worth noting that the changes in the crystal structure upon

conversion from the α-MnO2 NWs to C-MnO2 NWs could potentially improve ORR activity, including lattice strain, increased defect sites, oxygen vacancies, and crystallographic orientations.18,

58-59

The electrocatalytic ORR activity and stability of the C-MnO2 NWs

compared to 20% Pt/C is notable considering non-precious metal catalysts often require high mass loading and/or blending with a conductive carbon to achieve such competitive activity.10, 13, 19, 60

For example, blending Ni- and Cu-doped α-MnO2 NWs with graphene-like carbon has

previously shown to result in electrocatalysts with activity rivaling that of 20% Pt/C.19 However, the Cu-α-MnO2 and Ni-α-MnO2 NWs require 80 wt. % graphene (in contrast to the ≤ 1.2 wt. % C content of the C-MnO2 NWs presented here), which significantly reduces the active mass loading (to 20% TMO, vs. ≥ 98.8% TMO for the C-MnO2 NWs) and, potentially, the overall cost-effectiveness of the electrode.19 Table S1 compares the electrocatalytic ORR activity of the C-MnO2 NWs to other stateof-the-art carbon-coated electrocatalysts. Among the carbon-coated TMO materials, the C-MnO2 NWs demonstrate ORR activity highlighted by the most positive onset potential and within 40 mV of the most positive half-wave potential (Table S1). In addition, the desired high TMO/low C content of the C-MnO2 NWs is improved compared to the other reported carbon-coated oxide materials: ≤ 1.2 wt. % C for the C-MnO2 NWs reported here, 28.5 wt. % for the N-doped carboncoated Mn3O4 nanorods,42 and 65 wt. % (60 wt. % carbon black, 14.5 wt. % of the remaining 40 wt. % catalyst) for the N-doped carbon-coated Fe3O4 nanorods.43 The ORR potentials and


27


Page 28 of 42

overall activity of the C-MnO2 NWs are also competitive with highly active carbon-coated metals, sulfides, and phosphides despite lower mass loading and C content: 24.8 wt. % C for the porous carbon-coated cobalt sulfide nanocomposites38 and 88 at. % C for FeP embedded in N,Pdoped porous carbon nanosheets35 (Table S1).

CONCLUSIONS Carbon-coated α-MnO2 (C-MnO2) NWs were prepared from α-MnO2 using a two-step sucrose coating and pyrolysis method. This approach affords a thin, porous, low mass percentage amorphous carbon coating (< 5 nm, ≤ 1.2 wt. % C) on the surface of the NW, with retention of the most active phase α-MnO2 and the NW morphology. Coating and pyrolysis also leads to increased surface and bulk Mn3+ concentration, and decreased resistance (increased conductivity) of the individual NWs. The single-nanowire resistance of the C-MnO2 NWs are five orders of magnitude lower than the α-MnO2 NWs: 3.16 x105 and 3.79 x1010 Ω, respectively. Despite a lower BET SA relative to the α-MnO2 NWs, the C-MnO2 NWs demonstrate high activity toward the ORR with a 13-fold increase in diffusion-limited specific activity (α-MnO2, 0.13 A m-2; CMnO2, 1.70 A m-2) and six-fold decrease in charge transfer resistance (α-MnO2, 6.2 kΩ; CMnO2, 0.92 kΩ). The C-MnO2 NWs exhibit comparable activity and stability to commercial 20% Pt/C in high temperature chronoamperometric stability tests with equal mass loading and no additional conductive carbon additives. Such high activity from a TMO-C composite electrocatalyst with ~99% TMO content is unprecedented and the results presented here augur well for future studies into carbon-coated TMO electrocatalysts for oxygen reduction.


28

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. Raman spectrum of the C-MnO2 NWs; Representative I-V curves from the single-nanowire devices; Additional SEM and TEM images; N2 adsorption-desorption isotherms; TGA-DSC analysis; XPS survey spectra; RRDE peroxide percentage; Electrocatalytic ORR activity comparison table of state-of-the-art carbon-coated electrocatalysts; Additional ORR experiments and discussion addressing the use of a Pt counter electrode.

AUTHOR INFORMATION Corresponding Author *e-mail: [email protected]; phone: 1 505 284 6967 (T. N. L.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes ‡

ORR experiments were also performed using a graphite rod counter electrode due to concerns

raised during review about possible Pt involvement in the ORR. See Figure S8 and the accompanying discussion, which rules out ORR activity derived from Pt contamination at the working electrode.


29


Page 30 of 42

ACKNOWLEDGMENT This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Maria Kelly, Ruby Aidun, Anthony McDonald and Drs. Michael T. Brumbach, Paul G. Kotula and Mark A. Rodriguez are thanked for their technical assistance. We also thank Prof. Plamen Atanassov and his group members at the University of New Mexico for helpful discussions on high-temperature RDE studies.

REFERENCES: 1.

Wang, Z.-L.; Xu, D.; Xu, J.-J.; Zhang, X.-B. Oxygen electrocatalysts in metal-air

batteries: from aqueous to nonaqueous electrolytes. Chem. Soc. Rev. 2014, 43 (22), 7746-7786. 2.

He, Q.; Cairns, E. J. Review—Recent Progress in Electrocatalysts for Oxygen Reduction

Suitable for Alkaline Anion Exchange Membrane Fuel Cells. J. Electrochem. Soc. 2015, 162 (14), F1504-F1539. 3.

Nie, Y.; Li, L.; Wei, Z. Recent advancements in Pt and Pt-free catalysts for oxygen

reduction reaction. Chem. Soc. Rev. 2015, 44 (8), 2168-2201. 4.

Nguyen, T.; Spendelow, J.; Margolis, R. Levelized Costs of Electricity from CHP and

PV. https://www.hydrogen.energy.gov/pdfs/14003_lcoe_from_chp_and_pv.pdf.


30

Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.

Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.;

Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108 (46), 17886-17892. 6.

Ramaswamy, N.; Mukerjee, S. Fundamental Mechanistic Understanding of

Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid versus Alkaline Media. Adv. Phys. Chem. 2012, 2012, 491604. 7.

Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.;

Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.-i.; Iwashita, N. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107 (10), 39043951. 8.

Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-

Horn, Y. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal–air batteries. Nat. Chem. 2011, 3 (7), 546-550. 9.

Davis, D. J.; Lambert, T. N.; Vigil, J. A.; Rodriguez, M. A.; Brumbach, M. T.; Coker, E.

N.; Limmer, S. J. Role of Cu-Ion Doping in Cu-α-MnO2 Nanowire Electrocatalysts for the Oxygen Reduction Reaction. J. Phys. Chem. C 2014, 118 (31), 17342-17350. 10.

Ryabova, A. S.; Napolskiy, F. S.; Poux, T.; Istomin, S. Y.; Bonnefont, A.; Antipin, D.

M.; Baranchikov, A. Y.; Levin, E. E.; Abakumov, A. M.; Kéranguéven, G.; Antipov, E. V.; Tsirlina, G. A.; Savinova, E. R. Rationalizing the Influence of the Mn(IV)/Mn(III) Red-Ox Transition on the Electrocatalytic Activity of Manganese Oxides in the Oxygen Reduction Reaction. Electrochim. Acta 2016, 187, 161-172.


31


11.

Page 32 of 42

Stoerzinger, K. A.; Risch, M.; Han, B.; Shao-Horn, Y. Recent Insights into Manganese

Oxides in Catalyzing Oxygen Reduction Kinetics. ACS Catal. 2015, 5 (10), 6021-6031. 12.

Tang, Q.; Jiang, L.; Liu, J.; Wang, S.; Sun, G. Effect of Surface Manganese Valence of

Manganese Oxides on the Activity of the Oxygen Reduction Reaction in Alkaline Media. ACS Catal. 2014, 4 (2), 457-463. 13.

Lima, F. H. B.; Calegaro, M. L.; Ticianelli, E. A. Electrocatalytic activity of manganese

oxides prepared by thermal decomposition for oxygen reduction. Electrochim. Acta 2007, 52 (11), 3732-3738. 14.

Post, J. E. Manganese Oxide Minerals: Crystal Structures and Economic and

Environmental Significance. Proc. Natl. Acad. Sci. U.S.A. 1999, 96 (7), 3447-3454. 15.

Post, J. E.; Von Dreele, R. B.; Buseck, P. R. Symmetry and Cation Displacements in

Hollandites: Structure Refinements of Hollandite, Cryptomelane and Priderite. Acta Cryst. B 1982, 38, 1056-1065. 16.

Lambert, T. N.; Vigil, J. A.; White, S. E.; Delker, C. J.; Davis, D. J.; Kelly, M.;

Brumbach, M. T.; Rodriguez, M. A.; Swartzentruber, B. S. Understanding the Effects of Cationic Dopants on α-MnO2 Oxygen Reduction Reaction Electrocatalysis. J. Phys. Chem. C 2017, 121 (5), 2789-2797. 17.

Hao, J.; Liu, Y.; Shen, H.; Li, W.; Li, J.; Li, Y.; Chen, Q. Effect of Nickel-Ion Doping in

MnO2 Nanoneedles as Electrocatalyst for the Oxygen Reduction Reaction. J. Mater. Sci.: Mater. Electron. 2016, 27 (6), 6598-6605. 18.

Li, L.; Feng, X.; Nie, Y.; Chen, S.; Shi, F.; Xiong, K.; Ding, W.; Qi, X.; Hu, J.; Wei, Z.;

Wan, L.-J.; Xia, M. Insight into the Effect of Oxygen Vacancy Concentration on the Catalytic Performance of MnO2. ACS Catal. 2015, 5 (8), 4825-4832.


32

Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19.

Lambert, T. N.; Davis, D. J.; Lu, W.; Limmer, S. J.; Kotula, P. G.; Thuli, A.; Hungate,

M.; Ruan, G.; Jin, Z.; Tour, J. M. Graphene-Ni-α-MnO2 and -Cu-α-MnO2 nanowire blends as highly active non-precious metal catalysts for the oxygen reduction reaction. Chem. Commun. 2012, 48 (64), 7931-7933. 20.

Vigil, J. A.; Lambert, T. N.; Eldred, K. Electrodeposited MnOx/PEDOT Composite Thin

Films for the Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2015, 7 (41), 2274522750. 21.

Yu, Q.; Xu, J.; Wu, C.; Zhang, J.; Guan, L. MnO2 Nanofilms on Nitrogen-Doped Hollow

Graphene Spheres as a High-Performance Electrocatalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8 (51), 35264-35269. 22.

Vigil, J. A.; Lambert, T. N.; Kelly, M.; Aidun, R. Hybrid PEDOT/MnOx nanostructured

electrocatalysts for oxygen reduction. Mater. Chem. Front. 2017, 1, 1668-1675. 23.

Bag, S.; Roy, K.; Gopinath, C. S.; Raj, C. R. Facile Single-Step Synthesis of Nitrogen-

Doped Reduced Graphene Oxide-Mn3O4 Hybrid Functional Material for the Electrocatalytic Reduction of Oxygen. ACS Appl. Mater. Interfaces 2014, 6 (4), 2692-2699. 24.

Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grützke, S.; Weide, P.; Muhler, M.;

Schuhmann, W. MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped Carbon Matrix for High-Performance Bifunctional Oxygen Electrodes. Angew. Chem. Int. Ed. 2014, 53 (32), 8508-8512. 25.

Liang, Y.; Li, Y.; Wang, H.; Dai, H. Strongly Coupled Inorganic/Nanocarbon Hybrid

Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135 (6), 2013-2036.


33


26.

Page 34 of 42

Carter, R.; Oakes, L.; Douglas, A.; Muralidharan, N.; Cohn, A. P.; Pint, C. L. A Sugar-

Derived Room-Temperature Sodium Sulfur Battery with Long Term Cycling Stability. Nano Lett. 2017, 17 (3), 1863-1869. 27.

Li, Z.; Yin, L. Nitrogen-Doped MOF-Derived Micropores Carbon as Immobilizer for

Small Sulfur Molecules as a Cathode for Lithium Sulfur Batteries with Excellent Electrochemical Performance. ACS Appl. Mater. Interfaces 2015, 7 (7), 4029-4038. 28.

Zhao, Y.; Li, S.; Wang, C.-a. Manganous-Manganic Oxide@Carbon Core-Shell

Nanorods for Supercapacitors with High Cycle Retention. ECS J. Solid State Sci. Technol. 2016, 5 (2), M5-M11. 29.

Wang, T.; Peng, Z.; Wang, Y.; Tang, J.; Zheng, G. MnO Nanoparticle@Mesoporous

Carbon Composites Grown on Conducting Substrates Featuring High-performance Lithium-ion Battery, Supercapacitor and Sensor. Sci. Rep. 2013, 3, 2693. 30.

Wang, J.; Zhang, C.; Kang, F. Nitrogen-Enriched Porous Carbon Coating for Manganese

Oxide Nanostructures toward High-Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7 (17), 9185-9194. 31.

Li, X.; Xiong, S.; Li, J.; Liang, X.; Wang, J.; Bai, J.; Qian, Y. MnO@Carbon Core–Shell

Nanowires as Stable High-Performance Anodes for Lithium-Ion Batteries. Chem. - Eur. J. 2013, 19 (34), 11310-11319. 32.

Han, A. R.; Kim, T. W.; Park, D. H.; Hwang, S.-J.; Choy, J.-H. Soft Chemical

Dehydration Route to Carbon Coating of Metal Oxides: Its Application for Spinel Lithium Manganate. J. Phys. Chem. C 2007, 111 (30), 11347-11352.


34

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33.

Wang, J.; Gao, D.; Wang, G.; Miao, S.; Wu, H.; Li, J.; Bao, X. Cobalt nanoparticles

encapsulated in nitrogen-doped carbon as a bifunctional catalyst for water electrolysis. J. Mater. Chem. A 2014, 2 (47), 20067-20074. 34.

Ahn, S. H.; Manthiram, A. Direct growth of ternary Ni-Fe-P porous nanorods onto nickel

foam as a highly active, robust bi-functional electrocatalyst for overall water splitting. J. Mater. Chem. A 2017, 5 (6), 2496-2503. 35.

Zhang, R.; Zhang, C.; Chen, W. FeP embedded in N, P dual-doped porous carbon

nanosheets: an efficient and durable bifunctional catalyst for oxygen reduction and evolution reactions. J. Mater. Chem. A 2016, 4 (48), 18723-18729. 36.

Yu, J.; Li, Q.; Chen, N.; Xu, C.-Y.; Zhen, L.; Wu, J.; Dravid, V. P. Carbon-Coated Nickel

Phosphide Nanosheets as Efficient Dual-Electrocatalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces 2016, 8 (41), 27850-27858. 37.

Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon coated porous nickel

phosphides nanoplates for highly efficient oxygen evolution reaction. Energy Environ. Sci. 2016, 9 (4), 1246-1250. 38.

Bai, F.; Huang, H.; Hou, C.; Zhang, P. Porous carbon-coated cobalt sulfide

nanocomposites derived from metal organic frameworks (MOFs) as an advanced oxygen reduction electrocatalyst. New J. Chem. 2016, 40 (2), 1679-1684. 39.

Zhang, Z.; Liu, S.; Tian, X.; Wang, J.; Xu, P.; Xiao, F.; Wang, S. Facile synthesis of N-

doped porous carbon encapsulated bimetallic PdCo as a highly active and durable electrocatalyst for oxygen reduction and ethanol oxidation. J. Mater. Chem. A 2017, 5 (22), 10876-10884.


35


40.

Page 36 of 42

Varnell, J. A.; Tse, E. C. M.; Schulz, C. E.; Fister, T. T.; Haasch, R. T.; Timoshenko, J.;

Frenkel, A. I.; Gewirth, A. A. Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts. Nat. Commun. 2016, 7, 12582. 41.

Xu, Y.; Tsou, A.; Fu, Y.; Wang, J.; Tian, J.-H.; Yang, R. Carbon-Coated Perovskite

BaMnO3 Porous Nanorods with Enhanced Electrocatalytic Perporites for Oxygen Reduction and Oxygen Evolution. Electrochim. Acta 2015, 174, 551-556. 42.

Gao, S.; Geng, K. Facile construction of Mn3O4 nanorods coated by a layer of nitrogen-

doped carbon with high activity for oxygen reduction reaction. Nano Energy 2014, 6, 44-50. 43.

Hadidi, L.; Davari, E.; Ivey, D. G.; Veninot, J. G. C. Microwave-assisted synthesis and

prototype oxygen reduction electrocatalyst application of N-doped carbon-coated Fe3O4 nanorods. Nanotechnology 2017, 28 (9), 095707. 44.

Keenan, M. R.; Kotula, P. G. Optimal scaling of TOF-SIMS spectrum images prior to

multivariate statistical analysis. Appl. Surf. Sci. 2004, 231-232, 240-244. 45.

Kotula, P. G.; Keenan, M. R.; Michael, J. R. Automated analysis of EDS spectral images

in the SEM: A powerful new microanalysis technique. Microsc. Microanal. 2003, 9, 1-17. 46.

Gao, T.; Fjellvåg, H.; Norby, P. A comparison study on Raman scattering properties of α-

and β-MnO2. Anal. Chim. Acta 2009, 648 (2), 235-239. 47.

Schroder, D. K., Semiconductor Material and Device Characterization. 3 ed.; John Wiley

& Sons, Inc.: Hoboken, 2006. 48.

Tompsett, D. A.; Parker, S. C.; Islam, M. S. Surface properties of α -MnO2: relevance to

catalytic and supercapacitor behaviour. J. Mater. Chem. A 2014, 2 (37), 15509-15518.


36

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49.

Yuan, Y.; Wood, S. M.; He, K.; Yao, W.; Tompsett, D.; Lu, J.; Nie, A.; Islam, M. S.;

Shahbazian-Yassar, R. Atomistic Insights into the Oriented Attachment of Tunnel-Based Oxide Nanostructures. ACS Nano 2016, 10 (1), 539-548. 50.

Zhou, R.; Zheng, Y.; Jaroniec, M.; Qiao, S.-Z. Determination of the Electron Transfer

Number for the Oxygen Reduction Reaction: From Theory to Experiment. ACS Catal. 2016, 6 (7), 4720-4728. 51.

Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.; Shen, P. Facile Controlled

Synthesis of MnO2 Nanostructures of Novel Shapes and Their Application in Batteries. Inorganic Chemistry 2006, 45 (5), 2038-2044. 52.

Sinha, A. K.; Pradhan, M.; Pal, T. Morphological Evolution of Two-Dimensional MnO2

Nanosheets and Their Shape Transformation to One-Dimensional Ultralong MnO2 Nanowires for Robust Catalytic Activity. J. Phys. Chem. C 2013, 117 (45), 23976-23986. 53.

Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and

amorphous carbon. Phys. Rev. B 2000, 61 (20), 14095-14107. 54.

Nelson, A. J.; Reynolds, J. G.; Roos, J. W. Core-Level Satellites and Outer Core-Level

Multiplet Splitting in Mn Model Compounds. J. Vac. Sci. Technol., A 2000, 18 (4), 1072-1076. 55.

Cheng, F.; Su, Y.; Liang, J.; Tao, Z.; Chen, J. MnO2-Based Nanostructures as Catalysts

for Electrochemical Oxygen Reduction in Alkaline Media. Chem. Mater. 2010, 22 (3), 898-905. 56.

Bligaard, T.; Bullock, R. M.; Campbell, C. T.; Chen, J. G.; Gates, B. C.; Gorte, R. J.;

Jones, C. W.; Jones, W. D.; Kitchin, J. R.; Scott, S. L. Toward Benchmarking in Catalysis Science: Best Practices, Challenges, and Opportunities. ACS Catal. 2016, 6 (4), 2590-2602. 57.

Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for

Oxygen Reduction Reaction. Chem. Rev. 2016, 116 (6), 3594-3657.


37


58.

Page 38 of 42

Wang, H.; Xu, S.; Tsai, C.; Li, Y.; Liu, C.; Zhao, J.; Liu, Y.; Yuan, H.; Abild-Pedersen,

F.; Prinz, F. B.; Nørskov, J. K.; Cui, Y. Direct and continuous strain control of catalysts with tunable battery electrode materials. Science 2016, 354 (6315), 1031-1036. 59.

Zheng, H.; Modibedi, M.; Mathe, M.; Ozoemena, K. The thermal effect on the catalytic

activity of MnO2 (α, β, and γ) for oxygen reduction reaction. Mater. Today 2017, 4 (11), 1162411629. 60.

Chung, H. T.; Won, J. H.; Zelenay, P. Active and stable carbon nanotube/nanoparticle

composite electrocatalyst for oxygen reduction. Nat. Commun. 2013, 4, 1922.


38

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanoscale carbon modified α-MnO2 nanowires provide for highly active and stable oxygen reduction electrocatalysts with only ~1% carbon content. 29x10mm (300 x 300 DPI)



Figure 1. (a) SEM image of bulk C-MnO2 NWs; (b) HRTEM image of a C-MnO2 NW, index α-MnO2 PDF #00004-0141; (c) HRTEM image of a C-MnO2 NW and corresponding EDS elemental mapping for Mn and C (inset); (d) HRTEM image of a MnxOy NW showing lattice fringes with 4.9 Å spacing; (e) PXRD patterns; (f) Representative high-resolution XPS spectra in the Mn 3s binding energy region with peak fitting and ∆E(Mn 3s) splitting energy values. 98x54mm (300 x 300 DPI)


Page 40 of 42

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Schematic representation of single-nanowire device preparation (Reprinted with permission from J. Phys. Chem. C, 2017, 121 (5), pp 2789–2797. Copyright 2017 American Chemical Society); (b) SEM image of a C-MnO2 device in the nanowire-four-point contact region; (c) Average measured singlenanowire resistance and calculated conductivity (as compared to α-MnO2, NiMnO2, CuMnO2 – Ref. 16). 81x79mm (300 x 300 DPI)



Figure 3. (a) Background-subtracted RDE linear scanning voltammograms collected in 0.1 M KOH at 2500 RPM: catalytic currents normalized to geometric electrode area;; (b) Absolute diffusion-limited geometric current density (jgeo), active mass activity (jm), and specific activity (js) for selected catalysts calculated from voltammograms; (c) EIS Nyquist plots obtained at constant applied half-wave potential of each catalyst in O2-sat. 0.1 M KOH with no rotation; (d) RRDE-derived ORR n values obtained in O2-sat. 0.1 M KOH at 2500 RPM; (e) Chronoamperometric ORR activity at 60 °C in O2-sat. 1 M NaOH with no rotation, 0.72 V vs RHE (0-600 s) and 0.42 V vs RHE (600-4200 s). 113x72mm (300 x 300 DPI)


Page 42 of 42

Nanoscale Carbon Modified Î±-MnO2 Nanowires: Highly Active and

Recommend Documents