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Understanding the Effects of Cationic Dopants on #MnO Oxygen Reduction Reaction Electrocatalysis 2

Timothy N. Lambert, Julian A. Vigil, Suzanne E. White, Collin J. Delker, Danae J. Davis, Maria Kelly, Michael T. Brumbach, Mark A. Rodriguez, and Brian S. Swartzentruber J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11252 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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The Journal of Physical Chemistry

Understanding the Effects of Cationic Dopants on αMnO2 Oxygen Reduction Reaction Electrocatalysis

Timothy N. Lambert,1* Julian A. Vigil,1 Suzanne E. White,1 Collin J. Delker,2 Danae J. Davis,1 Maria Kelly,1 Michael T. Brumbach,3 Mark A. Rodriguez3 and Brian S. Swartzentruber2

1

Department of Materials, Devices, and Energy Technologies, 2 Nanostructure Physics & Center

for Integrated Nanotechnologies, 3 Materials Characterization & Performance; Sandia National Laboratories, Albuquerque, NM, USA, 87185.

*Corresponding author: Tel: 505 284 6967; E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT Nickel doped α-MnO2 nanowires (Ni-α-MnO2) were prepared with 3.4% or 4.9% Ni using a hydrothermal method. A comparison of the electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte versus that obtained with α-MnO2 or Cu-α-MnO2 is provided. In general, Ni-α-MnO2 (e.g. Ni-4.9%) had higher n values (n = 3.6), faster kinetics (k = 0.015 cm s-1) and lower charge transfer resistance (RCT = 2264 Ω @ half-wave) values, than MnO2 (n = 3.0, k = 0.006 cm s-1, RCT = 6104 Ω @ half-wave) or Cu-MnO2 (Cu-2.9%, n = 3.5, k = 0.015 cm s-1, RCT = 3412 Ω @ half-wave), and the overall activity for Ni-α-MnO2 trended with increasing Ni-content, i.e. Ni-4.9% > Ni-3.4%. As observed for Cu-α-MnO2, the increase in ORR activity correlates with the amount of Mn3+ at the surface of the Ni-α-MnO2 nanowire. Examining the activity for both Ni-α-MnO2 and Cu-α-MnO2 materials indicates that the Mn3+ at the surface of the electrocatalysts dictates the activity trends within the overall series. Single nanowire resistance measurements conducted on 47 nanowire devices (15 of α-MnO2, 16 of Cuα-MnO2-2.9% and 16 of Ni-α-MnO2-4.9%) demonstrated that Cu-doping leads to a slightly lower resistance value than Ni-doping, although both were considerably improved relative to the un-doped α-MnO2. The data also suggest that the ORR charge transfer resistance value, as determined by electrochemical impedance spectroscopy, is a better indicator of the cation-doping effect

on

ORR

catalysis

than

the

electrical

resistance

of

the

nanowire.


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INTRODUCTION Manganese oxides (MnOx) are an attractive class of electrocatalysts for the alkaline oxygen reduction reaction (ORR) due to their low cost, abundance and promising catalytic activity.1-5 Such catalysts are of interest in high capacity primary ceramic-air batteries6 and fuel cells,7 and could replace more expensive precious metal catalysts such as Pt/C.4 Strategies to increase the catalytic activity of MnOx have included both extrinsic and intrinsic modifications. Extrinsic modifications have largely been aimed at increasing the overall conductivity of a MnOx composite, as well as the general electrocatalytic surface availability, through effective dispersion of the MnOx electrocatalyst. Such approaches have included blending or hybridizing with conductive carbons (e.g. graphene-based materials, carbon nanotubes, etc.),8-12 metallic nanoparticles,13 and more recently with semiconducting polymers such as poly(3,4ethylenedioxythiophene).14-15 These composite materials have shown ORR activity comparable to commercial Pt/C catalysts, as well as other state-of-the-art non-precious metal electrocatalysts, including transition metal-carbon materials, heteroatom-doped carbon and graphene, and nonmanganese based metal oxides.16-20 The electrocatalysis of the ORR with MnOx is a surface mediated reaction that occurs at the three-phase boundary zone between O2 (gas), electrolyte (liquid) and surface atoms of the MnOx electrocatalyst (solid). Hence, intrinsic alterations to the MnOx phase, size, geometric shape, valence and electronic structure and conductivity (e.g. through oxygen vacancies, defects or metal ion doping) are all expected to have effect on the electrocatalysis.8, 13, 21-28 For example, αMnO2 exhibits high activity towards the ORR, and notably outperforms other phases of MnO2: α ~ δ > γ > λ > β according to Cao et al.28 The argument towards higher activity for α-MnO2 included the existence of larger 2 x 2 tunnels (d ~ 4.6 Å) on/within which ORR can occur, as


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opposed to the smaller 1 x 2 (γ-MnO2, d ~ 2.3 Å) and 1 x 1 (β-MnO2, d ~ 1.89 Å) tunnels for other phases.13 Trends of α- > β- > γ- according to Cheng et al.13 were also reported, whereby an increased conductivity of the β-MnO2 > γ-MnO2 led to increased ORR performance. Given an identical phase of α-MnO2, nanostructures (e.g. nanowires and nanospheres) are also known to be superior to microstructures based on surface area arguments.13 Metal ion doping is also a general strategy to improve electrocatalytic activity of MnOx.2, 24, 26

For example, our group and others have reported on the improvement in ORR activity of α-

MnO2 nanowires/nanorods upon doping with cations, such as Ni2+ and Cu2+.8,

21, 25, 27

The

substitutional doping of Ni- or Cu-ions into α-MnO2 provides superior catalytic activity to the already highly active α-MnO2, despite having lower surface areas. Furthermore, blending with graphene-like carbon (GLC) provided Ni-α-MnO2/GLC and Cu-α-MnO2/GLC carbon composites, with activity that rivaling that of Pt/C.8 Ni-doped α-MnO2 nanorods have exhibited superior peroxide decomposition,25 and also been examined for their oxygen evolution reaction (OER) behavior.27 Our recent, detailed structure activity studies on Cu-α-MnO2 elucidated the effect that Cu2+ has on the α-MnO2 nanowire, and help to explain its role in improving electrocatalytic activity.21 Briefly, analysis of the X-ray diffraction data indicated that Cu-doping leads to an increase in the number of crystalline edge defects (possible catalytic sites) due to a crystallite size that is roughly half that of the parent α-MnO2 nanowire. As determined from ∆Mn 3S splitting values obtained by X-ray photoelectron spectroscopy (XPS), Cu-doping also provides for a structure that stabilizes the Mn3+ valence at the surface of the nanowire. This is of importance, because the Mn3+/Mn4+ redox couple acts as the electrochemical mediator during ORR, analagously to observations for the B-site cation mediated catalysis in perovskite materials.19,

21, 29

The analysis of the electrocatalytic data suggested that while the ORR


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mechanism is not altered by the addition of Cu, the reaction kinetics became more rapid. This was attributed to an increased Mn-O covalency upon doping, as stronger Mn-O bonds are expected to promote the redox-driven exchange between O2 and the HO– on the axial eg orbital of the manganese ion.19, 21 Cu- or Ni-metal ion doping is also expected to increase the inherent conductivity of the αMnO2 nanowire,30 an attribute generally expected to improve catalytic activity, but often not measured.4 Herein, we report on the factors that lead to improved ORR electrocatalysis of αMnO2 with Ni-doping, and provide a comparison between Ni- and Cu-doped nanowires to determine the intrinsic characteristics most relevant to describing ORR activity. We also report on the fabrication of single nanowire devices and measurement of the electrical resistance of both Ni-α-MnO2 and Cu-α-MnO2 nanowires, and relate these values to ORR activity. Such measurements are required in order to understand the true role of metal ion dopants in nanostructured electrocatalysts and have not yet been reported for α-MnO2.

EXPERIMENTAL Preparation of M-α α-MnO2 (M = Ni, Cu) nanowires. α-MnO2, Ni-α-MnO2 and Cu-α-MnO2 nanowires were synthesized by a hydrothermal method as we have previously reported.8,

21

MnSO4·H2O (0.6 mmol) and KMnO4 (1.8 mmol) were added to 5 mL of DI H2O, independently, allowed to stir for ~ 5 minutes, and then combined. The combined aqueous solution was allowed to stir for an additional 5 minutes. The mixture was then added to a Teflon-lined stainless steel autoclave (Parr, 45 mL capacity) and placed in an oven at 140 °C for 120 hours. After cooling to room temperature, the product was collected, centrifuged, and washed four times with DI H2O


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and four times with EtOH. The washed, re-dispersed (in EtOH) α-MnO2 nanowires were isolated via rotary evaporation and dried under vacuum oven overnight. Cu-α-MnO2 was prepared in a 1 Mn2+:1 Cu2+ molar ratio by adding Cu(NO3)2·3H2O (0.6 mmol) to the MnSO4 solution in the first step above. Ni-α-MnO2 was similarly prepared in two ratios, 1 Mn2+:0.5 Ni2+ and 1 Mn2+:0.25 Ni2+, by adding Ni(NO3)2·6H2O (0.3 mmol, 0.15 mmol, respectively) in the first step above. Physiochemical characterization. Scanning electron microscopy (SEM): Nanowire powders were loaded directly onto carbon tape and sputtered with gold-palladium. Images were collected using a Zeiss Supra 55VP field emitter gun scanning electron microscope (FEGSEM). For images of the nanowire devices, an FEI NovaNanoSEM scanning electron microscope was used. This instrument was used to image devices and substrates at many stages of the fabrication, and to perform electron beam lithography, vide infra. X-ray diffraction (XRD): Nanowire powders were loaded directly onto holders from an alcoholic dispersion, which was allowed to dry at room temperature (RT). A Bruker D8 Advance X-ray Diffractometer was operated using the DIFFRAC plus XRD Commander software. Scans were performed from 10° to 80° 2θ at a rate of ~ 0.096° 2θ min-1. The patterns were analyzed using Jade 9.0 software and the International Centre for Diffraction Data (ICDD) database. X-ray photoelectron spectroscopy (XPS): Nanowire powders were loaded on carbon tape for 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


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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 resolution spectra were recorded

with a 20 eV pass energy, 50 meV step sizes, and 100 ms dwell times. Charge neutralization was used for all samples to reduce any potential differential charging effects. Data processing was performed with CasaXPS Version 2.3.15. High-resolution core-level peaks were compared by normalizing counts for each respective core-level. Single nanowire resistance measurements. A detailed schematic of the nanowire device fabrication and nanowire/substrate/contact interface is provided in Scheme S1 (ESI). Si substrates with an upper layer of insulating oxide at least 20 nm thick were used. A protective photoresist coating of at least 2 µm was spun over the topside of the wafer. The oxide was then removed from the bottom side of the wafer by etching in buffered oxide etch (BOE) for 2 minutes. The photoresist layer was removed by submerging the entire wafer in acetone and IPA, respectively, for approximately five minutes each. A grid of alignment marks corresponding to each substrate was drawn using a layout drawing program. A negative optical resist, AZ nLOF 5510, was spun over the surface to a thickness of ~ 900 nm. This resist was exposed using an optical mask of MJB83 mask aligner at 100 mJ cm-2. Following a 110 °C post-bake for 60 seconds, the sample was developed in an AZ300MIF developer for 60 seconds. 10 nm of Ti were deposited over the entire sample surface using an Inficon IC/5 Deposition Controller, followed by a minimum of 90 nm Au. Liftoff of the remaining photoresist was accomplished by soaking in acetone for at least 20 min. The substrate was finally cleaned by O2 plasma clean (5 min) at 75 W in a LFE Corp. PDS/PDE-301 Barrel Plasma Etcher to remove surface organics as needed. 2 mg of nanowire powder was suspended in 3 mL of IPA by sonication, and then further diluted in IPA by a factor of 160 – 320. After 15 minutes of sonication, dilute solutions of MnO2,


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CuMnO2-2.9 and NiMnO2-4.9 were allowed to sit for 5 minutes. 50 µL of each respective solution were dropped on to a square centimeter of the prepared substrate and allowed to evaporate at room temperature in air. Optical images of the nanowires in proximity to alignment marks were used to draw contact patterns to individual nanowires via a layout-drawing program. The substrates were plasma cleaned (3 min), vide supra, so that the photoresist would adhere well to the upper surface. A bilayer electron beam positive photoresist, whose bottom layer was to be more easily developed than the top, was applied. This step ensured a sidewall profile suitable for metal lift-off. Each layer was spin-coated at 4000 RPM for 30 s, and then baked on a hot plate at 180 °C for 90 s. The bottom layer, composed of EL11 (11% copolymer, methyl methacrylate (MMA) and methacrylic acid (MAA) in ethyl lactate), was ~ 500 nm thick. The upper layer, composed of 950K-A2 [2% poly(methyl methacrylate) in anisole], was ~ 50 nm thick. Electron beam lithography (EBL) was performed using an FEI NovaNanoSEM via the Nanometer Pattern Generation System (NPGS) program in a two-step process. Fine features were written at a beam current of ~ 50 – 100 pA. Large contact pads were written at a beam current of ~ 500 – 1000 pA. Rotation correction was employed and the grid of alignment marks on the substrate was used to ensure accurate placement of the contacts onto each nanowire. The entire pattern was then developed in a 1:3 methyl isobutyl ketone (MIBK):IPA solution for 75 s, then rinsed in IPA for 15 s. 140 – 170 nm gold was deposited over 10 nm titanium in an Inficon IC/5 Deposition Controller. Lift-off was accomplished by soaking in acetone (20 min), leaving behind the final device: the substrate with only nanowires and their respective Ti/Au contacts. An Agilent Technologies B1500 Semiconductor Device Analyzer with EasyExpert software was used to obtain resistance data using a 4-terminal measurement setup to eliminate contact


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resistance. The applied voltage was swept from 0 to 20 V as needed to activate devices. The current and voltage between the four points was measured, and the potential drop and current between the inner contacts was used to calculate the resistance of each individual nanowire. Electrocatalytic characterization. Oxygen reduction reaction activity was determined using a Bioanalytical Systems, Inc. RDE-2 Rotating Disk Electrode operated by a Versastat 4 potentiostat. The three-electrode cell comprised a glassy carbon (GC) rotating working electrode (A = 0.0788 cm2), Ag/AgCl (3 M NaCl) reference electrode and Pt coil counter electrode. 50 mL of electrolyte, 0.1 M KOH, was added to the cell and purged with UHP O2 (or N2 for background experiments) for 20 min prior to an experiment and blanketed throughout. 5 mg of the nanowire powder was added to a solution of 200 µL IPA and 300 µL Nafion solution (5 wt. % in light alcohols), and the ink was placed in a bath sonicator for 20 min. 5 µL of the ink was then dropcast on to the GC working electrode, which had been previously polished with 0.05 µm alumina slurry and rinsed with EtOH. The electrodes were allowed to sit and evaporate overnight. Commercial 20% Pt/Vulcan XC-72 (20% Pt/C) catalyst was purchased from E-Tek; 20% Pt/C inks and electrodes were prepared identically as described above to ensure equal mass loading characterization. Linear scanning voltammetry (LSV) scans were performed in 0.2 V to -0.6 V vs. Ag/AgCl. All potentials have been converted to the reversible hydrogen electrode (RHE) and hereafter all potentials are on the RHE scale, unless otherwise noted. In order to analyze the curves using Koutecky-Levich (K-L) theory, LSV scans were repeated at increasing scan rates of 500, 900 1600, 2500 and 3600 RPM. n values and rate constants were calculated according to reported conventions from the K-L plot, as discussed in the Results and Discussion. Onset potentials were determined using the tangential method, i.e. the onset potential is defined as the point of


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intersection of the LSV tangent at the half-wave potential with the zero-current line (potential axis). Electrochemical impedance spectroscopy (EIS) spectra were obtained at onset and halfwave potentials of each specific catalyst during potentiostatic experiments from 105 to 10-1 Hz. Chronoamperometry experiments to determine catalyst stability and MeOH tolerance were carried out at 0.65 V vs. RHE for 5400 seconds; MeOH was injected after 1800 seconds to achieve a final electrolyte composition of 3% MeOH in 0.1 M KOH.

RESULTS AND DISCUSSION Ni-α-MnO2 NWs with varying Ni-content were synthesized by a hydrothermal method, analogous to our earlier reports,8, 21 as depicted in Equation 1. ⋅ ( ) + ( ) ⋅ 6 ( ) + ( ) → − − () ()

(1)

Two Ni-α-MnO2 nanowire samples were prepared from an aqueous solution with initial molar ratios of 1:0.25 and 1:0.5 (Mn:Ni), at a fixed KMnO4:MnSO4 ratio, and compared to α-MnO2 and our most active α-Cu-MnO2 nanowire electrocatalysts.21 Elemental analysis via X-ray photoelectron spectroscopy (XPS) determined elemental surface Ni compositions of Ni-α-MnO2 at 0.29% Ni (1:0.25) and 0.54% Ni (1:0.5) while Cu-α-MnO2 had 0.33% Cu (1:0.25), 0.46% Cu (1:0.5) or 0.64% Cu (1:1).21 Acid digestion followed by inductively coupled plasma mass spectrometry (ICP-MS) analysis gave bulk Ni composition values of 3.4% (1:0.25) and 4.9% Ni (1:0.5), versus 1.3% (1:0.25), 2.4% (1:0.5) and 2.9% (1:1) for Cu-α-MnO2 (Table 1).21 In general, Ni-doping also led to nanowires with a lower surface area, pore size and pore volume than MnO2, Table 1 and Fig. S1 (ESI).8


10

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Table 1. XPS, ICP-MS, and Nitrogen Adsorption Analysis of As-Prepared α-MnO2, Ni-α-MnO2 and Cu-α-MnO2 Catalyst Powders. sample

MnO2 CuMnO2-2.9

initial Mn:M % M, reactant mole XPS ratio (M = Cu (M = Cu or or Ni) Ni) a N/A N/A a

% M, ICP-MS (M = Cu or Ni) N/A a

BET surface area (m2 g-1) 73.6

1 Mn: 1 Cu

0.64 ± 0.06 2.92 ± 83.8 Cu 0.05 Cu NiMnO2-3.4 1 Mn: 0.25 0.29 ± 0.05 3.41 ± 51.8 Ni Ni 0.02 Ni NiMnO2-4.9 1 Mn: 0.5 Ni 0.54 ± 0.11 4.88 ± 52.3 Ni 0.07 Ni a N/A = not applicable, no Ni/Cu present in α-MnO2 synthesis.

average pore size (nm)

pore volume (cm3 g-1)

13.4

0.31

11.6

0.24

4.60

0.12

8.56

0.22

Hereafter, for comparison purposes the nanowire catalysts will be generally referred to by their bulk dopant percentages, such as MnO2, NiMnO2-3.4, NiMnO2-4.9 and CuMnO2-2.9. Representative scanning electron microscopy (SEM) images for MnO2, CuMnO2-2.9, NiMnO23.4 and NiMnO2-4.9 provided in Fig. 1a – 1d demonstrate the fact that Ni- or Cu-doping has no appreciable effect on the resulting nanowire morphology. The average aspect ratio of the


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nanowires is approximately 20:1, with an average length of ~ 2.5 µm and diameter of ~ 125 nm. Uniform distribution of Ni in NiMnO2 nanowires has been confirmed by transmission electron microscopy (TEM) elemental analysis, as shown in Fig. S2 (ESI).8 X-ray diffraction (XRD) patterns provided in Fig. 1e are consistent with the formation of α-MnO2 (PDF #00-044-0141) although a more detailed analysis indicates ~4% of rock salt MnO (PDF #04-005-4310) is present, vide infra. These results are consistent with the substitutional doping of Ni- or Cu-ions into the α-MnO2 lattice. XPS was also used to analyze the surface characteristics of the prepared nanowires, as the Mn valence has an impact on ORR activity.4, 21, 29, 31 Ni-doping in Ni–MnOx nanoparticles has previously been suggested to stabilize the Mn3+/Mn4+ mediating species involved in the first step of the ORR.2 The degree of multiplet splitting of the Mn 3s 5S and 7S states is well known to correlate with Mn valence,32-33 specifically with the Mn3+:Mn4+ ratio. MnO2 exhibits the smallest ∆E Mn 3s at ~ 4.5 eV and the reported MnO2 nanowires correspond well with this expected value, with a ∆E Mn 3s of 4.51 eV (Fig. 1f).32-33 The Mn 3s spectra of MnO2, NiMnO2-3.4, NiMnO2-4.9 and CuMnO2-2.9 nanowires are provided in Fig. S3 (ESI) and the ∆E Mn 3s are summarized in Fig. 1f. The inclusion of Ni results in an approximately linear increase in the ∆E Mn 3s, equal to 4.57 eV for NiMnO2-3.4 and 4.64 eV for NiMnO2-4.9. For comparison purposes, the ∆E Mn 3s values for CuMnO2-1.3 (4.53 eV), CuMnO2-2.4 (4.56 eV) and CuMnO2-2.9 (4.62 eV) are also provided (Fig. 1f).21 Clearly, both Ni2+ and Cu2+ doping of the α-MnO2 nanowires leads to an increase in the Mn3+ content at the surface of the nanowire. The greater slope corresponding to Ni2+, observed in Fig. 1f, suggests that surface Ni2+ has a greater influence on the stabilization of Mn3+ than does Cu2+. This may be due to the fact that the Ni2+ ions (0.69 Å) are more similar in size to Mn3+ (0.645 Å) and Mn4+ ions (0.530 Å), than the larger Cu2+ ions


12

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(0.73 Å) and are incorporated into the structure more easily. This notion is supported by the observation that the bulk Ni composition is higher than Cu when comparing the starting Mn:(Ni, Cu) precursor ratio in nanowire samples (Table 1).21

Figure 1. (a-d) SEM images of MnO2, CuMnO2-2.9, NiMnO2-3.4 and NiMnO2-4.9 nanowires (respectively, scale bar = 1 µm); (e) XRD patterns of MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red, solid) indexed to α-MnO2 PDF# 00-44-0141; (f) XPS surface atomic % of metal dopant vs Mn 3s splitting for MnO2 (black), NiMnO2 (red)


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and CuMnO2 (blue). Error bars shown represent standard error of the mean. CuMnO2 values reprinted with permission from J. Phys. Chem. C, 2014, 118 (31), pp 17342–17350. Copyright 2014 American Chemical Society.

The effects of the Ni dopant on the unit cell parameters of the α-MnO2 lattice (Fig. S4, ESI) were also examined by XRD analysis, Table 2. Ni doping appears to have expanded the cell size in the a-axis (9.843 Å) for NiMnO2-4.9 when compared to the α-MnO2 nanowire (9.836 Å) phase or literature value (9.785 Å). This is similar to our previous observations with Cu-doping.21 However, the c-axis does not appear to change very much, again consistent with observations for Cu-α-MnO2 nanowires. The average bond length for the NiMnO2-3.4 (1.90 Å) and NiMnO2-4.9 (1.92 Å) samples showed shorter distances for bond lengths than for the Cu doping. The Mn-O bonding in the plane of the octahedra are essentially identical in length at ~1.89 Å; the apical oxygens show some expansion in Mn-O bond length, but to a lesser or equal degree than is observed for Cu (1.92 Å),21 consistent with the sizes of the cations. Because Ni does not display as large a Jahn-Teller distortion as Mn,34-35 doping of Ni on the Mn site reduces overall bond length variation and increases rigidity of the octahedra. The average value for the Mn-O bonds in the octahedra is almost exactly 1.9 Å and the standard deviation is small at ± 0.015 Å.

Table 2. Unit Cell Parameters and Crystallite Size of α-MnO2 Lattice in MnO2, Ni-α-MnO2 and Cu-α-MnO2 Catalyst Powders, as Determined by XRD. sample

a (Å)

c (Å)

volume (Å3)

crystallite size (nm)

MnO2

9.836 (3)

2.858 (1)

276.5

36


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CuMnO2-2.9

9.866 (5)

2.857 (1)

278.1

16

NiMnO2-3.4

9.826 (3)

2.857 (1)

275.9

28

NiMnO2-4.9

9.843 (1)

2.858 (2)

276.9

40

The sample displayed anisotropic peak broadening which manifest as broadening of peaks with a-axis dependence, as observed for Cu-doped samples. Hence, the determined average values of crystallite size [NiMnO2-3.4 (28 nm) and NiMnO2-4.9 (40 nm), as opposed to MnO2 (36 nm) and CuMnO2-2.9 (16 nm)], may not fully represent the significant aspect ratio present in the actual sample crystallites (which are also likely to be rod or needle-like in shape). Structurally, it appears that the stacking of the unit cell along the direction of the open channel shows the longer coherence length, and the shortened dimension of the crystallite is along the sheet direction of the structure. The Ni-3.4 sample also had a minor MnO (rock salt-like phase) phase present, at a low phase fraction (~ 4 wt. %). A significant content of K was refined in this structure as well where the K resided in the channels of the lattice. As Cu- and Ni-metal ion doping is expected to increase the conductivity of the α-MnO2 nanowire,30 an attribute generally expected to improve catalytic activity, the electrical resistance was determined by four-point measurements on the MnO2, CuMnO2-2.9 and NiMnO2-4.9 nanowire catalysts. The general sample drop coating, substrate preparation, electron beam lithography and the measurement process is depicted in Fig. 2a, with greater detail found in Scheme S1 (ESI). In a typical device preparation, nanowires were dispersed in IPA at a concentration between 2 and 4 µg mL-1 by bath sonication, then dropped on to patterned and cleaned Si/SiO2 substrates at a concentration of 50 µL cm-2 and allowed to dry in air (Fig. 2a, step 1). Optical images of the wires in proximity to substrate alignment marks were used to draw


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Page 16 of 37

contact patterns to an individual wire. The substrate was then cleaned with O2 plasma and spincoated with a bilayer electron beam positive photoresist (Fig. 2a, step 2). The patterns were next used to perform EBL to write the contacts 36 (Fig. 2a, step 3), with a current of 50 – 100 pA for fine features and 500 – 1000 pA for large contact pads. Final preparation of the device was achieved by development of the pattern in MIBK/IPA (Fig. 2a, step 4), deposition of 140 – 170 nm Au over 10 nm Ti (Fig. 2a, step 5), and lift-off by soaking in acetone (Fig. 2a, step 6), leaving behind the Si/SiO2 substrate with only the nanowire and its respective Ti/Au contacts now suitable for measurement (Fig. 2a, step 7). A SEM image of a full final device and the nanowire/contact interface region are shown on in Fig. 2b, left and right, respectively.


16

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Figure 2. (a) Schematic of the nanowire-drop coating/substrate preparation/EBL/measurement process in measuring single nanowire resistance; (b) SEM images of the total device and fourpoint contact region (left to right); (c) Histogram displaying the distribution of measured resistance values for MnO2 (black), NiMnO2-4.9 (red) and CuMnO2-2.9 (blue) nanowires across the range of 0 to 1012 Ω.

Four-point contact measurements were used in order to isolate the resistance of the nanowire from the resistance of the device and the metal-to-nanowire contacts, themselves.37 Contacts


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were placed on the wire with approximately 400 nm gaps between points 1 and 2, and 3 and 4 (Fig. 2b, right). The devices were measured by sweeping the electrical potential between outer contacts, 1 and 4, from 0 to 20 V in order to activate the device and monitor the current, while measuring the voltage drop between the inner contacts, 2 and 3. All measurements were made at room temperature in a dark box to prevent optical effects. The resistance of the nanowire was calculated using Ohm’s Law (Fig. 2a, step 7). We have chosen not to convert the measured resistance (R) to resistivity and conductivity, due to the uncertainty in cross-sectional shape and uniformity of the wires and measurements made via SEM. However, it can be assumed that the distribution would be proportional, as the nanowires chosen for devices had the same range of length and diameters. The results from the 47 devices (15 of MnO2, 16 of CuMnO2-2.9 and 16 of NiMnO2-4.9) are shown as a histogram in Fig. 2c. The MnO2 nanowires exhibit the highest resistance, with the maximum population of wires being measured on the order of 109 Ω. The average, median and standard error of the mean of the log(R) values for MnO2 are 8.87, 9.05 and 0.39, respectively. The NiMnO2-4.9 nanowire resistance distribution is bimodal (proposed to be due to some inhomogeneity of doping for the nanowires sampled); however, the highest population of wires was measured on the order of 105 Ω. The second local maximum in frequency of NiMnO2-4.9 devices was at 108 Ω. log(R) average, median and standard error of the mean values for the NiMnO2-4.9 nanowires were 6.40, 5.60 and 0.46, respectively. The CuMnO2-2.9 nanowires had the lowest resistance distribution with all of the devices measuring below 107 Ω, and the maximum population of resistance was 105 Ω. log(R) average, median and standard error of the mean values for the CuMnO2-2.9 nanowires were 5.06, 5.02 and 0.19, respectively. In summary, the average (and median resistance values) indicated that the electrocatalyst resistance trended as MnO2 > NiMnO2-4.9 > CuMnO2-2.9.


18

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ORR activity of NiMnO2-3.4 and NiMnO2-4.9 catalysts was then assessed using rotating disk electrode (RDE) linear scanning voltammetry (LSV). Briefly, catalysts powders were dispersed in IPA and Nafion binder solution, mixed by bath sonication and drop cast onto a glassy carbon rotating working electrode to form the active catalyst film, consistent with previous studies.13, 21 The three-electrode cell consisted of the working electrode with catalyst film, a Ag/AgCl reference electrode and a Pt wire counter electrode in 0.1 M KOH electrolyte.ǂ All potentials are adjusted and reported vs. the reversible hydrogen electrode (RHE). LSV scans were carried out from 1.0 V to 0.4 V at a scan rate of 5 mV s-1 and are provided in the Fig. S5 (ESI). Scans were conducted at rotation rates of 500, 900, 1600, 2500, 3600 RPM in order to evaluate the data using Koutecky-Levich analysis. Representative ORR LSV scans at 2500 RPM for NiMnO2-3.4 and NiMnO2-4.9 as compared to CuMnO2-2.9 and MnO2 are provided in Fig. 3a. Onset potentials calculated by the tangential method (see Experimental section) were nearly identical @ 0.88-0.89 V for the catalysts: MnO2 (0.889 V), NiMnO2-3.4 (0.879 V), NiMnO2-4.9 (0.891 V) and CuMnO2-2.9 (0.886 V). Further analysis of the onset region of the LSV curve also provided similar Tafel slope (b) values (61-75 mV dec-1): MnO2 (b = 67 mV dec-1), NiMnO2-3.4 (b = 68 mV dec-1), NiMnO2-4.9 (b = 61 mV dec-1) and CuMnO2-0.64 (b = 75 mV dec-1), Fig. S6 (ESI). However, the ability to produce the most diffusion-limiting geometric ORR current density (jgeo) trended as follows (Fig. 3a): MnO2 (-1.43 mA cm-2) < NiMnO2-3.4 (-1.75 mA cm-2) < CuMnO22.9 (-2.93 mA cm-2) < NiMnO2-4.9 (-3.11 mA cm-2). At equal mass loading, commercial 20% Pt/C demonstrates an onset potential of 0.90 V and terminal current density of -5.01 mA cm-2. Mass activity (mA mg-1) of the four catalysts trends in the same way as all catalysts was tested at identical mass loadings. More importantly, specific activity values can be calculated as an intrinsic ORR metric for each catalyst based on the measured BET surface area (Fig. 3b). While


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the trend in diffusion-limiting specific ORR current density (js) follows the same order as jgeo, it more effectively illustrates the increased activity due to Ni- vs Cu-doping in terms of an equalsurface area basis (intrinsic). Analysis of the current-rotation rate relationship (i-1 vs. ω-1/2) for multiple rotation rates (Fig. S5, ESI) using the Koutecky-Levich (K-L) equation (Equation 2) provided kinetic rate constant (k) values, and electron transfer numbers (n) as reported in Fig. 3c and Table 3. The K-L equation is shown in Equation 2, where i is current, iK is kineticallylimited current, iL is diffusion-limited current, n is the ORR electron transfer number, F is the Faraday constant, A is electrode area, k is the rate constant, C is the concentration of dissolved oxygen in the electrolyte, D is the diffusion coefficient of oxygen, ν is the kinematic viscosity, and ω is the rotation rate.


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Figure 3. (a) ORR LSV geometric current density (jgeo) curves of MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed), NiMnO2-4.9 (red, solid) and 20% Pt/C (grey, dotted) in 0.1 M KOH at 2500 RPM; (b) ORR LSV specific (relative to BET surface area) current density (js) curves of MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red,


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solid) in 0.1 M KOH at 2500 RPM; (c) K-L plots and calculated n values for MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red, solid), and the theoretical slopes corresponding to n = 2 and n = 4. (d) EIS spectra recorded at the half-wave ORR potential of MnO2 (black diamonds), CuMnO2-2.9 (blue triangles), NiMnO2-3.4 (red squares) and NiMnO2-4.9 (red circles); (e) Chronoamperometric response (0.65 V vs. RHE; difference vs. t = 0) of NiMnO2-4.9 (red, circle) and 20% Pt/C (grey, square) for 5400 seconds, with 3% MeOH injected at t = 1800 seconds.

=

!

+

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=

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+

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n values were increased upon doping for both Ni and Cu, with the higher n value trending with the % amount of dopant ion: MnO2 (n = 3.02) < CuMnO2-2.9 (n = 3.49) ~ NiMnO2-3.4 (n = 3.51) < NiMnO2-4.9 (n = 3.66). k values were found to trend in the order of 0.006 cm s-1 (MnO2) < 0.008 cm s-1 (NiMnO2-3.4) < 0.015 cm s-1 (CuMnO2-2.9) ~ 0.015 cm s-1 (NiMnO2-4.9), with the more covalent structures providing the fastest kinetics.15 Previous efforts have demonstrated that cation doping lowers the activation energy for peroxide decomposition, such that more effective peroxide decomposition is achieved with Ni-α-MnO2 nanowires than α-MnO2.27 However, our previously reported RRDE studies on α-MnO2 and Ni-α-MnO2 nanowires have already demonstrated that the ring/disk current for both α-MnO2 and Ni-α-MnO2 is less than 1% across the potential window of 0.54 – 0.84 V vs. RHE,8 indicating rapid peroxide disproportionation in the α-MnO2 nanowire structure, even prior to doping and an apparent 4electron reaction.


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Table 3. Electrochemical Impedance and ORR Activity Descriptors for α-MnO2, Ni-α-MnO2 and Cu-α-MnO2 Catalyst Powders, as a Function of Mn 3s Splitting Determined by XPS. sample

∆E, Mn 3s (eV)

MnO2

4.51 ± 0.02 4.57 ± 0.03 4.60 ± 0.02 4.64 ± 0.02

NiMnO2-3.4 CuMnO2-2.9 NiMnO2-4.9

Eonset (V vs. RHE) 0.89 ± 0.01 0.88 ± 0.02 0.89 ± 0.01 0.89 ± 0.01

Ehalf-wave (V vs. RHE) 0.68 ± 0.02 0.69 ± 0.03 0.70 ± 0.02 0.70 ± 0.02

Idiffusion-limiting (mA cm-2)

n (e - )

k (cm2 s-1)

RCT (Ω)

-1.43 ± 0.14

3.02 ± 0.22 3.51 ± 0.20 3.49 ± 0.05 3.66 ± 0.13

0.006 ± 0.001 0.008 ± 0.002 0.015 ± 0.009 0.015 ± 0.008

6104 ± 716 3724 ± 1406 3412 ± 987 2264 ± 1334

-1.75 ± 0.21 -2.93 ± 0.56 -3.11 ± 0.23

Electrochemical Impedance Spectroscopy (EIS) experiments (in O2-purged and blanketed 0.1 M KOH) were performed from 105 to 10-1 Hz at each catalyst’s ORR half-wave potential, and the results are shown in Fig. 3d. Modeling to the equivalent Randles circuit yielded charge transfer resistance (RCT) values of 6104 Ω (MnO2) > 3724 Ω (NiMnO2-3.4) > 3412 Ω (CuMnO22.9) > 2264 Ω (NiMnO2-4.9). RCT values calculated in the onset region of the ORR LSV yielded the same trend: 24598 Ω (MnO2) > 14467 Ω (NiMnO2-3.4) > 10321 Ω (CuMnO2-2.9) > 4909 Ω (NiMnO2-4.9), again exhibiting a trend that higher Mn3+ surface content leads to more effective electrocatalysis, Fig. 3e demonstrates the superior stability and tolerance to the presence of MeOH of the NiMnO2-4.9 catalyst, relative to 20% Pt/C. Overall consideration of the above metrics indicates the overall catalytic activity toward the ORR trends as MnO2 < NiMnO2-3.4 < CuMnO2-2.9 < NiMnO2-4.9, Table 3.


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Figure 4. Trend plots demonstrating the use of the Mn 3s splitting, as determined by XPS, as the main descriptor of ORR activity and relevant characteristics for α-MnO2 electrocatalysts. These points represent (from low to high ∆E, left to right) MnO2 (4.51 eV), NiMnO2-3.4 (4.57 eV), CuMnO2-2.9 (4.6 eV) and NiMnO2-4.9 (4.64 eV): (a) half-wave ORR potential (left) and diffusion-limiting specific ORR current density (right); (b) ORR n-value (left) and ORR kinetic rate constant (right); (c) EIS charge transfer resistance at half-wave ORR potential (left) and single nanowire resistance (right). Error bars shown represent standard error of the mean.

Fig. 4 shows the relationship between Mn 3s XPS splitting and important ORR activity and resistance parameters. As discussed previously, the Mn 3s splitting is a relative measure of the surface Mn valence available for catalysis; its importance lies in determining the abundance of surface Mn3+ and Mn4+ species, as the ORR rate-determining step is understood to be mediated by the Mn3+/Mn4+ redox couple.21 Larger Mn 3s splitting values corresponds to equal or better performance in the ORR and lower RCT values. This indicates that resistance, as measured by


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EIS, is a more appropriate predictor of ORR activity than single nanowire resistivity measurements. The result of Fig. 4 is significant in that it shows un-doped α-MnO2 and doped αMnO2, by two different metal ions (Cu or Ni), fit the trend and suggests that Mn 3s splitting could be a singular descriptor of ORR activity and charge transfer resistance regardless of the intrinsic modification used.

CONCLUSIONS In summary, the incorporation of Ni ions into α-MnO2 nanowires leads to a more effective electrocatalyst. Ni-α-MnO2 displayed a higher n value as well as faster kinetics and a lower charge transfer resistance than MnO2 and Cu-α-MnO2, despite having a lower surface area and larger crystallite size. The overall activity for Ni-α-MnO2 also trended with increasing Nicontent. Single nanowire resistance measurements indicated that both Ni-α-MnO2 (106.40 Ω) and Cu-α-MnO2 (105.06 Ω) are less resistive than α-MnO2 (108.87 Ω), with Cu-doped α-MnO2 being slightly less resistive than Ni-doped α-MnO2. The data indicates that ORR charge transfer resistance values, as determined by EIS, are a better indicator of cation doping effect on ORR catalysis than the electrical resistance of the nanowire. The data presented for Ni-α-MnO2, Cu-αMnO2 and α-MnO2 indicates that metal ion doping leads to an increased Mn3+ ion content at the surface of the nanowire, and the observed higher n values (MnO2, n = 3.02; CuMnO2-2.9, n = 3.49; NiMnO2-4.9, n = 3.66), faster kinetics (MnO2, k = 0.006 cm s-1; CuMnO2-2.9, k = 0.015 cm s-1; NiMnO2-4.9, k = 0.015 cm s-1) and lower charge transfer resistance values (MnO2, RCT = 6104 Ω; CuMnO2-2.9, RCT = 3412 Ω; NiMnO2-4.9, RCT = 2264 Ω) trend with this feature.


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SUPPORTING INFORMATION The following additional schematics and figures can be found in the supporting information: expanded single nanowire device fabrication schematic, adsorption isotherms, TEM EDS element maps, representative XPS Mn 3s spectra, model I4/m crystal, average ORR LSV curves and Tafel plots.

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-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC0494AL85000.

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TOC GRAPHIC


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Figure 1. (a-d) SEM images of MnO2, CuMnO2-2.9, NiMnO2-3.4 and NiMnO2-4.9 nanowires (respectively, scale bar = 1 µm); (e) XRD patterns of MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red, solid) indexed to α-MnO2 PDF# 00-44-0141; (f) XPS surface atomic % of metal dopant vs Mn 3s splitting for MnO2 (black), NiMnO2 (red) and CuMnO2 (blue). Error bars shown represent standard error of the mean. CuMnO2 values reprinted with permission from Ref. 15. Copyright 2014 American Chemical Society. %" 82x132mm (300 x 300 DPI)



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Figure 2. (a) Schematic of the nanowire-drop coating/substrate preparation/EBL/measurement process in measuring single nanowire resistance; (b) SEM images of the total device and four-point contact region (left to right); (c) Histogram displaying the distribution of measured resistance values for MnO2 (black), NiMnO24.9 (red) and CuMnO2-2.9 (blue) nanowires across the range of 0 to 1012 Ω. 82x120mm (300 x 300 DPI)


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Figure 3. (a) ORR LSV geometric current density (jgeo) curves of MnO2 (black), CuMnO2-2.9 (blue), NiMnO23.4 (red, dashed), NiMnO2-4.9 (red, solid) and 20% Pt/C (grey, dotted) in 0.1 M KOH at 2500 RPM; (b) ORR LSV specific (relative to BET surface area) current density (js) curves of MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red, solid) in 0.1 M KOH at 2500 RPM; (c) K-L plots and calculated n values for MnO2 (black), CuMnO2-2.9 (blue), NiMnO2-3.4 (red, dashed) and NiMnO2-4.9 (red, solid), and the theoretical slopes corresponding to n = 2 and n = 4. (d) EIS spectra recorded at the halfwave ORR potential of MnO2 (black diamonds), CuMnO2-2.9 (blue triangles), NiMnO2-3.4 (red squares) and NiMnO2-4.9 (red circles); (e) Chronoamperometric response (0.65 V vs. RHE; difference vs. t = 0) of NiMnO2-4.9 (red, circle) and 20% Pt/C (grey, square) for 5400 seconds, with 3% MeOH injected at t = 1800 seconds. 82x208mm (300 x 300 DPI)



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Figure 4. Trend plots demonstrating the use of the Mn 3s splitting, as determined by XPS, as the main descriptor of ORR activity and relevant characteristics for α-MnO2 electrocatalysts. These points represent (from low to high ∆E, left to right) MnO2 (4.51 eV), NiMnO2-3.4 (4.57 eV), CuMnO2-2.9 (4.6 eV) and NiMnO2-4.9 (4.64 eV): (a) half-wave ORR potential (left) and diffusion-limiting specific ORR current density (right); (b) ORR n-value (left) and ORR kinetic rate constant (right); (c) EIS charge transfer resistance at half-wave ORR potential (left) and single nanowire resistance (right). Error bars shown represent standard error of the mean. 177x69mm (300 x 300 DPI)



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Graphic for TOC 82x27mm (300 x 300 DPI)


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Understanding the Effects of Cationic Dopants on α ... - ACS Publications

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