Vertically Aligned and Surface Roughed Pt Nanostructured Wire Array

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Vertically Aligned and Surface Roughed Pt Nanostructured Wire Array as High Performance Electrocatalysts for Methanol Oxidation Chuqing Liu, Zhiyang Li, Peng Yu, Hsi-Wu Wong, and Zhiyong Gu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00680 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Vertically Aligned and Surface Roughed Pt Nanostructured Wire Array as High Performance Electrocatalysts for Methanol Oxidation

Chuqing Liu, Zhiyang Li, Peng Yu, Hsi-Wu Wong and Zhiyong Gu*

Department of Chemical Engineering University of Massachusetts Lowell One University Ave Lowell, MA 01854, USA

* Corresponding author: Phone: (978) 934-3540 Fax: (978) 934-3047 E-mail: [email protected]

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ACS Applied Energy Materials 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

ABSTRACT In this research, an enhanced electrocatalyst based on vertically aligned and surface roughed Pt nanostructured wire arrays (Pt NWAs) has been developed. Our results showed that Pt NWAs possessed extremely high electrochemical performance and efficiency towards methanol oxidation. The three-dimensional (3D) nanostructured wire array electrode was prepared with electrodeposition method using anodic aluminum oxide (AAO) membrane on silver substrate, after which the active array electrode was obtained by dissolving the AAO templates. Varying the electrodeposition current density and synthesis time led to controlled length and surface roughness of the Pt nanostructured wires. Scanning electron microcopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD) were employed to characterize the morphology, composition and crystal structure of the nanostructured wires. Cyclic voltammetry (CV) and chronoamperometry were used to analyze the performance and endurance of the wire array electrode. Electrochemical impedance spectroscopy (EIS) measurements provided insights in the electron transfer resistance between the electrode and analyte. Gas chromatography (GC) and UV-Vis spectroscopy were carried out to identify the presence of the possible products and the relative concentration change of methanol versus the numbers of cyclic voltammetry scanning. The electrochemical measurements showed only one dominant forward anodic peak and the absence of backward anodic peak. GC results showed that methanol concentration decreased linearly with the CV scanning cycles, and negligible amount of side products were detected, suggesting a complete methanol oxidation. This prepared electrode exhibited high electrochemical activity, high endurance, and low electron transfer resistance.

KEYWORDS: 3D Pt Nanostructured Wire Array; Rough Surface; Electrocatalyst; Methanol Oxidation; CO Tolerance; Fuel Cells

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1. INTRODUCTION In the past decade, methanol oxidation has been subjected to an increasing amount of studies due to its advantages for the direct methanol fuel cell (DMFC) applications to generate environmentally friendly clean energy

1-5

. Among many liquid organic fuels,

methanol has good reactivity at room temperature and high-energy density, with efficiency comparable with hydrogen-based fuel cell. Unlike hydrogen that can be explosive and hazardous, methanol is very stable and can be safely handled, stored and transported. In terms of its price, methanol is relatively inexpensive since it can be easily extracted and produced from fossil fuels or biomass fermentation 5. Comparing to larger alcohol molecules such as ethanol and isopropanol, methanol has higher selectivity in CO2 formation during electrochemical oxidation due to its small size, leading to a faster reaction rate with minimum amount of side products 5-6. Many electrocatalysts for methanol oxidation are based on alloying platinum (Pt) with other noble or non-noble metals, such as ruthenium (Ru), gold (Au), nickel (Ni), tin (Sn), tungsten (W), palladium (Pd) and silver (Ag), to increase the performance and endurance of the catalyst and lower the cost

1, 3, 6-8

. Traditionally, glass carbon electrode (GCE),

graphene sheets and carbon nanotubes have been used as major support materials for direct methanol oxidation

9-12

. The support materials are coated with the active catalytic

noble metals, such as Pt or Pt alloyed nanoparticles, to enhance the activity and durability of the catalyst

2, 13-16

. The structure modification and alloying processes of the catalyst

usually require multi-step preparations, sometimes under harsh environmental conditions, such as high temperature or high pressure, which are time consuming, energy inefficient and difficult to handle

1-2, 11-12, 17-18

. Attempts have been made to randomly disperse

electrocatalytic materials on the surface of a support electrode, such as glassy carbon electrodes

19

. However, the overlapped catalysts in two-dimensional (2D) structure and

the usage of Nafion could compromise its electrocatalytic activities due to the slower molecular diffusion rate and the accumulation of side products. In this work, one-step synthesis of three-dimensional (3D) Pt nanostructured wire array electrodes with rough surface was developed as the electrocatalyst for direct methanol

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oxidation reaction (MOR). The 3D array geometry coupled with variation of the surface roughness aims to significantly enlarge the surface area, to enhance the mass and electron transfer, and to facilitate the diffusion, increasing the overall activity of the electrocatalyst. The morphologies and elemental compositions of the Pt nanostructured wire arrays (Pt NWAs) were analyzed with the scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray powder diffraction (XRD). Electrochemical

testing

techniques

including

cyclic

voltammetry

(CV),

chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were employed to study the performance of the Pt NWA electrodes towards methanol oxidation. Finally, gas chromatography (GC) and UV-Vis spectroscopy were used to characterize possible reaction products for uncovering the reaction pathways during the methanol oxidation.

2. EXPERIMENTAL SECTION 2.1. Materials and reagents Anodic aluminum oxide (AAO) membrane was purchased from Whatman (Anodisc, 25 mm diameter, 200 nm pore diameter). Pt electroplating solution (Platinum TP RTU, Technic Inc.) was used as purchased without further purification. Methanol (CH3OH, 99.8%) was purchased from Acros Organics. 0.5 M sulfuric acid (H2SO4) solution was prepared by diluting 98 wt% H2SO4 (Acros Organics) with DI water, which was obtained from a Barnstead Nanopure Water Purification System. K3Fe(CN)6 (99%), KCl (99%), and formaldehyde (99%) were purchased from Sigma-Aldrich; K3Fe(CN)6 and KCl were diluted to 20 mM and 0.2 mM with DI water, respectively. Commercial Pt on carbon support (Pt/C, 10 wt%) and formic acid (99%) were purchased from Fisher Scientific. Nafion was purchased from Solutions Technology, Inc and diluted to 5% with ethanol. All reagents were of analytical grade. 2.2. Synthesis of Pt nanostructured wire arrays with different surface conditions The Pt nanostructured wire arrays were fabricated by electrodeposition method using the

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AAO membrane as the template

19-20

. One side of the AAO membrane was first coated

with a 200 nm thick Ag layer in a CHA 6 Pocket Electron Beam Evaporator (CHA’s Solution TM Process Development System, Fremont, CA, USA) at the rate of 0.1 nm/s to form a sealing and electrically conductive layer on AAO. The AAO with Ag substrate layer was connected to a copper (Cu) plate using conductive Cu tape, which was then connected to an O-ring joint with a glass container that was filled with the Pt electrolytic solution. The Pt nanostructured wires were synthesized by electrodeposition using the chronopotentiometry technique, which kept a constant electrodeposition current density over a chosen range of time. By varying the current densities, nanostructured wires with different surface roughness were synthesized 19-20. The parameters of the current densities with their corresponding surface roughness were 1.5 mA cm-2 for the smooth Pt NWA, 3.3 mA cm-2 for the semi-rough Pt NWA, and 5 mA cm-2 for the rough Pt NWA. Different synthesis times were selected to keep the nanostructured wires with the same length. The deposition time for each NWA were as follows: 3 hours for the smooth NWA, 1.11 hours for the semi-rough NWA and 0.5 hour for the rough NWA (Table S1). After the completion of the synthesis, the Pt deposited AAO membrane was dissolved in 1 M sodium hydroxide (NaOH) solution for 30 minutes to etch the AAO membrane while keeping the Pt NWAs with Ag layer attached on the Cu plate. Then, the Pt NWA-Ag-Cu was washed repeatedly with DI water for cleaning

20

. After drying in air, the Pt NWA-

Ag-Cu was glued to a larger Cu plate using Cu tape to form the working electrode. A non-conductive inert polymer was used to seal all the exposed area of Cu to ensure only the Pt NWA area was exposed as the final electrode for oxidation reaction. The whole process of the Pt NWA synthesis is shown in the Scheme 1.

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Scheme 1. Synthesis of Pt nanowire structured arrays with (a) smooth surface condition; (b) rough surface condition through electroplating method

2.3. Characterizations and electrochemical measurements The morphology and elemental composition of the Pt NWAs were characterized by a JEOL 7401F field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (JEOL, Peabody, MA, USA). The crystal structure and grain size of the Pt nanostructured wires were determined using Xray powder diffraction (XRD, Scintag PAD X with Cu Kα radiation). A three-electrode electrochemical cell was used to perform the electrochemical

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measurements. The prepared Pt NWA electrode was employed as the working electrode. Pt wire electrode (0.3 mm dia, 99.9%, Alfa Aesar) and Ag/AgCl electrode (Bioanalytical Systems, Inc.) were used as the counter and reference electrodes, respectively. Electrodeposition, cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) were carried out on a VersaSTAT3 electrochemical system (Princeton Applied Research, Oak Ridge, TN, USA). The EIS Nyquist curves were fitted with ZSimpWin. After different scanning cycles of CV, 800 µL of reacted solutions were mixed with 3 mL of dichloromethane (DCM) to extract the organic products. Once the mixing procedure was completed, two distinct layers formed with the aqueous layer on the top and the organic DCM layer at the bottom. One mL of the DCM solution was then taken using a syringe for gas chromatography (GC) analysis. The standard solution of reactant (methanol) and possible side products (formaldehyde and formic acid) were prepared in a similar manner and used as a control group, except without CV scans. For reaction product identification using GC, a Shimadzu GC2010 Plus GC equipped with a mass spectrometer (MS) was used. The samples were injected into the GC/MS system equipped with a Shimadzu SH-RXi-5Sil MS column (30 m). High purity helium (Airgas) was used as a carrier gas in the column with a constant flow rate of 91.1 mL/min. The inlet temperature was set at 280 °C. The programmed temperature regime for the GC oven was: start at 30 °C, hold for 3 minutes, ramp up to 110 °C at 3 °C/min, ramp up to 180 °C at 20 °C/min, then ramp up to 200 °C at 5 °C/min, finally ramp up to 280 °C at 20 °C/min. The temperature of the MS detector was set at 280 °C. The quantification of the reaction products with GC was achieved using a second analytical channel equipped with a flame ionization detector (FID). The samples were injected into the GC/FID system equipped with a Shimadzu Rxi-5ms column (15 m). The GC was programmed with the following inlet operating parameters: high purity helium carrier gas set at a constant flow pressure of 11.5 kPa, inlet temperature set at 285 °C, and carrier gas with direct injection mode. The detector temperature was set at 285 °C, with an air flow rate of 400 mL/min and a hydrogen gas flow rate of 40 mL/min. The GC oven was programmed with the following temperature regime: start at 32 °C, hold for 3

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minutes, ramp up to 110 °C at 3 °C /min, ramp to 180 °C at 20 °C /min, then ramp to 200 °C at 5 °C /min and finally ramp to 280 °C at 20 °C /min. In order to validate the methanol oxidation products, a Lambda 25 UV-Vis spectrometer (Perkin Elmer, MA, USA) was also used. The unreacted solution was used as blank (0.5 M CH3OH + 0.5 M H2SO4), and CO2 was injected into the reactor to form H2CO3 as the control that resulted from the complete oxidation pathway. 0.5 mM of formic acid (HCOOH) standard was also prepared as the reference of one of possible side products that might form during the partial oxidization of methanol. The reacted solution was compared with the control for its identity, and the trend of relative concentration change was analyzed against the number of CV cycles.

3. RESULTS AND DISCUSSION 3.1 Characterization of the Pt NWAs In this study, Pt nanostructured wire arrays (Pt NWAs) with different roughness were synthesized by varying the current density and the deposition time. Figure 1 (a-c) shows the SEM images for the morphology of the Pt NWAs that were synthesized with different parameters. The SEM image of the smooth Pt NWA prepared at low current density of 1 -2

mA cm is shown in Figure 1(a). The inset image showed a single nanostructured wire, indicating the smooth surface across the entire Pt nanostructured wire. When the current density increased to 4 mA cm-2, a semi-rough surface structure began to appear as shown in Figure 1(b). By examining the surface of a single semi-rough nanostructured wire, the top half was rough and the bottom half appeared to be smooth. Similarly by increasing the current density to 5 mA cm-2, the rough Pt nanostructured wire array was produced, as shown in Figure 1(c). The surface of the Pt nanostructured wires became very rough across the entire wire as shown in Figure 1(c) inset image. Based on the above observations, it can be concluded that electrodeposition with higher current density resulted in rougher surface of the Pt nanostructured wires.

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Figure 1. SEM images of (a) smooth Pt NWA; (b) semi-Rough Pt NWA; (c) rough Pt NWA. From the SEM images, each nanostructured wire had a length of about 2 µm and diameter of ~200 nm. The length of each Pt nanostructured wire was kept constant by adjusting the synthesis time for different electrodeposition currents. The array structures were observed in all three images (Figure 1 a-c), and the higher deposition current, the rougher surface of the Pt nanostructured wires. Below the nanostructured wire array, a layer of silver substrate (labeled as Ag) could be observed, which supported and kept all the nanostructured wires vertical. EDS mapping of the array and line mapping of a single wire were also performed, and the results further confirmed that the presence of Ag located mostly on the substrate, and not much on the Pt wire array. According to the SEM images, the vertically aligned 3D array structure would prevent the overlapping of the

9



nanostructured wires, which was a common problem in traditional 2D electrode modification methods21, and Nafion will not be necessary to be used. The 3D structure enabled each Pt nanostructured wire to fully contact with the analyte for maximal methanol oxidation rate

20

. Also, it was believed that the well-spaced vertical structure

and Nafion free characteristics of the Pt NWA electrode would further enhance the ion transport rate and perform more efficiently when compared to the 2D modified electrode with Nafion 19. Figure 2(a) depicts the EDS spectra of smooth, semi-rough and rough Pt NWAs. The Pt signals were from the Pt nanostructured wires, Ag signals came from the silver substrate, and Cu signals were from the copper plate that was used to hold the nanostructured wire array. Small C and O peaks possibly came from carbon and oxygen related impurities that were formed either on the Pt nanostructured wires or on the sliver substrate. No other metal peaks were observed in the EDS spectra, which demonstrated that the Pt NWAs were rather clean. The peak positions of the three EDS spectra were similar, which showed that different surface roughness of the Pt NWAs had no effect on the actual elemental composition of the nanostructured wires. Figure 2(b) shows the X-ray diffraction (XRD) diagrams for the smooth, semi-rough and rough Pt NWAs. Each of the figures consisted of five peaks in the scanned range, and all NWA electrodes had these peaks at the same locations. The approximate locations and corresponding indices were as following, 40.0° (111), 46.5° (200), 67.9° (220), 81.8° (311) and 86.1° (222). Since the indices (h k l) were either all even or all odd, it illustrated that the unit cells of the crystalline phase of these Pt nanostructured wires were identified as face-centered cubic (fcc), and it was consistent with other references 14, 19. In addition, there was a trend of increase in symmetry and sharpness of the peaks from smooth to semi-rough to rough Pt nanostructured wires. This indicated an increase in grain size as the deposition current density increased and the nanostructured wires became rougher 19. The average grain size was estimated to be 4.6, 7.4 and 9.6 nm for the smooth, semi-rough and rough Pt nanostructured wires, respectively, based on the line broadening of the X-ray peaks with the Scherrer’s equation22. These results indicated a linear correlation between the electrodeposition current density (correspondingly the

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roughness of the Pt nanostructured wires) and the nanostructured wire grain sizes7.

Figure 2. (a) EDS spectra and (b) XRD diagrams for the smooth, semi-rough and rough nanostructured wire arrays. 3.2 Electrochemical measurements Cyclic voltammetry results To investigate the relationship between the morphology and electrocatalytic activity of the Pt NWA, the electrochemically active surface area (ECSA) of the smooth (S), semi-

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rough (SR) and rough (R) Pt NWAs were first estimated using the Fe3+/Fe2+ redox couple method 23 in cyclic voltammetry (CV) with the scan rate of 0.05 V/s between the electric potential range (-0.2, 1V). The resulting curves were shown in Figure S1 of the Supporting Information. According to the Randles–Sevick equation, the ECSA of S, SR and R Pt NWA were calculated to be 0.25, 0.29 and 0.52 cm2, respectively. The performance of the prepared Pt nanostructured wire catalysts toward methanol electro-oxidation was evaluated by cyclic voltammetry (CV). CV plots of Pt NWAs with different roughness are depicted in Figure 3(a). The light green curve showed the CV results of the silver (Ag) thin-film as a control sample since Ag was used as the substrate for Pt NWA formation. The black, red and blue curves showed the CV results for the smooth, semi-rough, and rough Pt NWAs, respectively. The first anodic peak in the forward scans of all three nanostructured wire arrays was observed between 0.64 and 0.7 V, which indicated the formation of the most oxidized product CO2, along with possibly other less oxidized side products, including HCHO and HCOOH in trace levels 24. In many reports, a second oxidation peak during the backward scans was observed between 0.4 V and 0.5 V

3, 7-8, 13, 25-27

, which suggested the presence

of more partially oxidized, undesired side products (such as HCOOH and HCOH) due to the incomplete oxidation during the forward scan. In this study, no backward oxidation peak was observed in the 3D vertically aligned Pt NWA electrodes. High forward peak with the absence of this backward peak demonstrated a high catalytic efficiency and completeness of the forward oxidation reaction achieved by the Pt NWA electrode, which was highly desired.

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Figure 3. (a) CV plots of methanol oxidation with 0.5 M CH3OH and 0.5 M H2SO4 on Pt NWAs with various roughness with the inset showing the linear dependency of the maximum peak current density on surface roughness quantitatively represented by the deposition current density; (b) comparison of the CV curve obtained in this work from the rough Pt NWA with the one obtained from the tradition 2D GCE/Pt NW electrode19. During methanol oxidation, CO was produced as an intermediate that can often form a single bond (e.g. Pt-C=O) and binds tightly with the electrode surface, which poisons the electrode and lowers the overall energy conversion efficiency 3, 12. Consequently, the CO tolerance of the electrocatalyst can be determined indirectly from the ratio of the forward oxidation peak current (If) to the backward oxidation peak current (Ib), If/Ib 12. A low

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value of If/Ib usually indicates an inability to remove the excessive accumulation of CO and other carbon residual species from the electrode surface, which results in low CO tolerance and poor efficiency of the conversion from methanol to CO2 and partial oxidation of methanol. In contrast, a high value of If/Ib suggested an improvement and enhancement of the CO tolerance

12, 29

. Since in most references the forward and

backward anodic oxidation peaks have similar heights, the If/Ib ratio is usually between the values of 0.9 to 2.23

12, 30

. In this study, the backward oxidation peak was not

observed. Therefore, the If/Ib ratios were significantly larger than most of values observed in the literature 12, 30, which indicated that the 3D Pt NWA electrodes possessed high CO tolerance and great efficiency for the complete oxidation of methanol. The maximum peak heights of each Pt NWA electrodes were as followed: 211.35 mA cm-2 for the smooth Pt NWA, 491.61 mA cm-2 for the semi-rough Pt NWA, and 698.67 mA cm-2 for the rough Pt NWA, as shown in the inset image of Figure 3(a). The Ag thinfilm had a peak current of 57.16 mA cm-2, which was similar to the reported value of approximately 60 mA cm-2 for Ag nanoparticles coated on carbon nanotubes

31-32

. As

anticipated, this Ag control sample had almost negligible activity compared to the Pt NWA electrodes. Among the Pt NWA electrodes, the rough Pt NWA had the highest peak height, and the smooth NWA had the smallest peak height, with the semi-rough NWA in between. The results shown in Figure 3(a) indicated that when the Pt nanostructured wires became rougher and overall surface area increased, the reaction rate for methanol oxidation also increased. Similar correlation between the roughness of the Pt nanostructured wires and maximum peak current density was also illustrated in other references 19, 33. In Figure 3(b), the CV performance of the rough Pt NWA was compared to a previous work by Ruan et al, which studied rough Pt nanowires randomly dispersed on a glass carbon electrode (GCE)

19

. The forward oxidation peaks of both curves appeared within

similar potential range between 0.65 V and 0.70 V (red dotted lines on the curve). The activity of the major oxidation peak from the rough Pt NWA was 453 mA mg-1, which was 4.5 times higher than the peak height obtained from the 2D Pt nanowires on the GCE electrode (107 mA mg-1) in Ruan’s work. A backward oxidation peak at 0.45 V was also

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observed in Ruan’s results, which was common in many other electrocatalysts for methanol oxidation reactions

17, 26-29

. As discussed above, this backward oxidation peak

indicated that the electrocatalyst was not able to fully oxidize the methanol during the forward CV scan, and a second oxidation was required to react any partially oxidized species remaining on the electrode. The absence of the backward or second oxidation peak in our work indicated that the Pt NWA electrodes were more effective in oxidizing the methanol during the forward CV scan, resulting in greater performance when compared to previous reports

1, 13, 15, 18, 25, 34-37

. Additionally, the CV measurements of

commercial Pt/C (Pt nanoparticles on carbon support) on bare Cu plate or Ag coated Cu plate are shown in Figure S2. The electrochemical measurements showed an activity of 99 mA mg-1 (for Cu plate) and 117 mA mg-1 (for Ag coated Cu plate), respectively, indicating similar activity of different substrates; however, these values are much lower than that of the rough Pt NWA in this work, which is 453 mA mg-1. The results indicate that the vertical aligned 3D array structure attribute mostly for the activity improvement. To test the stability and durability of the Pt NWAs, multiple cycles of CV were carried out without the replacement of the methanol solution. Figure S3 shows the performance of the first 20 cycles; no noticeable current density decrease was observed, which indicates the stable electrochemical performance when there is large amount of methanol in the solution. For the smooth (S) Pt NWA, when the scans increased to 100 and 200 cycles, the activity decreased as shown in Figure 4(a) and the major anodic oxidation peak shifted slightly to the right. After 200 cycles, the original solution was replaced with new solution and another CV measurement was conducted, which showed almost 100% recovery. SEM image of the smooth Pt NWA after 200 cycles of CV is shown in Figure S4(a). The morphology of the smooth wires seemed not changed much after 200 cycles, which indicates that the activity decrease after many cycles was mostly due to the depletion of the methanol source. For the semi-rough (SR) and rough (R) Pt NWAs, similar activity decrease was observed after 100 and 200 cycles (Figure 4b-c). When the solutions were replaced with a new solution after 200 cycles, a small but noticeable peak current decrease was observed. SEM imaging of the semi-rough and rough Pt NWAs after 200 cycles of CV (Figure S4 b-c) showed that there was some aggregation for these wire arrays, especially the rough Pt NWA. These morphology changes may contribute to

15



the activity decrease after extended CV cycling. However, the overall durability, stability, and anti-poising performance of the prepared Pt NWAs are good.

Figure 4. Multi-cycle CV of (a) smooth, (b) semi-rough and (c) rough Pt NWAs for 100, 200 cycles and then a new CV cycle with fresh methanol solution.

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The CV plots of the three Pt NWA electrodes demonstrated a high catalytic activity such as high forward oxidization peak and small backward oxidation peak comparing to the literature results

18, 25, 27, 35

. Additionally, the rough Pt NWA electrode outperforms other

electrodes under similar reaction conditions, including the commercial Pt/C catalyst and other Pt-based alloy electrodes 7-8, 25, 27. The most noticeable observation for the Pt NWA electrode synthesized in this work was the absence of the backward oxidation peak during a single cycle of CV scans, which was observed in many other references. This indicated a more efficient oxidation reaction during the forward scan. To illustrate the high performance of the vertically aligned array structure electrocatalyst, the mass based current density of the rough Pt NWA obtained from the CV testing was compared against values from several other works studying electrodes with different geometries, but with similar chemical compositions (Pt or Pt based alloyed) (Figure 5). It could be observed that Pt-based alloys with Ru, Ni, or Fe did not achieve high performance compared to Pt nanowires. The geometry of the electrocatalysts seemed to play a more critical role in determining the overall performance. For instance, Pt nanoparticles (NPs) dispersed on graphene outperformed any of the Pt/Ru alloys on pure carbon electrode and also rough Pt nanowires on GCE electrodes. However, our rough Pt NWA significantly outperformed all electrode geometries listed due to its unique vertically aligned array structure.

17



Figure 5. Comparison mass activity of Pt NWA (this study) against other reference electrodes: non-uniform Pt NW 38, rough Pt NWs on GCE 19, P(ICA-co-EDOT)-Pt 39, Pt NPs on graphene 2, cubic Pt NPs/Graphene 40, PtRuFeNi/MWCNT 41, PtRu/C 42, Pt45Ru45Fe10/C 43, PtRuFe/MWCNT 41 and PtRuNi/C44. Chronoamperometric results Chronoamperometry (CA) was conducted at the major oxidation potential of 0.65 V versus Ag/AgCl reference electrode to evaluate the electrochemical catalyst performance over time. The current-time response at 0.65 V for 1800 s was shown in Figure 6. Consistent with the CV results, the methanol oxidation current densities of the rough Pt NWA was higher than those of semi-rough and smooth Pt NWAs at 0.65 V over the entire time period examined. The current density decayed the most for rough Pt NWA, and less rapid for the semi-rough and smooth Pt NWAs. The current density of each Pt

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NWA had all reached a constant value within 25 seconds, and these values are much higher than other published results 3, 8, 19, 25, 34-35. At the end of 1800 s, the current density of the rough NWA stayed at a current density approximate 200 mA cm-2, which was about 2 times higher than the semi-rough NWA (100 mA cm-2) and about 4 times higher than the smooth NWA (50 mA cm-2). This indicated that the rough NWA maintained a higher catalytic activity and stability over time.

Figure 6. Chronoamperometry of methanol oxidation on Pt NWA electrodes at 0.65V for 1800 s.

Electrochemical impedance spectroscopy (EIS) results Electrochemical impedance spectroscopy (EIS) was also carried out to understand how the structure of the nanostructured wire array electrodes affected the charge transfer properties and the type of reaction mode 45. Figure 7(a) showed the Nyquist plots of the smooth, semi-rough and rough Pt NWA electrodes at their major methanol oxidation potential, 0.65 V. EIS results from these Pt nanostructured wire electrodes shown a characteristic shape of a semicircle at 0.65 V. The diameter of the semicircle along the x-

19



axis has been considered as the charge transfer resistance (Rct) between the methanol solution and the electrode interface

15

. The Nyquist plots were semicircular shaped and

can be represented with a Randle equivalent circuit, as shown in Figure 7(b). In this equivalent circuit, Rs represents the Ohmic resistance of the bulk solution, Rct represents the charge/electron-transfer resistance, W or Zw is the Warburg impedance, which was related to the diffusion of ions to the electrode interface from the bulk solution, and Cdl was the double layer capacitance [also known as the constant phase element capacitance (CPE)] 46.

Figure 7. (a) Nyquist of smooth and semi-rough Pt NWA were fitted with the R(Q(RW)) circuit model (black and red dots); and rough Pt NWA was fitted with the R(Q(R)) circuit model (blue dots) at forward oxidation voltage, 0.65 V; (b) Randle equivalent circuit with the Warburg element for the Nyquist plot.

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The Nyquist plots were fitted using ZSimpWin to estimate the impedances of S, SR and R Pt NWA as shown in Figure 7(a) as the black, red and blue dots, which are 48.54 Ω, 39.40 Ω and 36.90 Ω, respectively. By comparing the fitted values and the horizontal diameter of the semi-circles, the smooth Pt NWA had the largest charge transfer resistance (Rct), and the rough Pt NWA had the smallest Rct; while the Rct of semi-rough NWA was in between. The rapid transport of charge carried can be achieved because rough NWA had higher surface area, better conductivity through the nanostructured wire 19, 33

, and more porosity of the nanostructured wires, which made it easier for the ions to

diffuse from the analyte to the surface of the electrode. These results indicated that the rough Pt NWA had the highest electrical conductance, which led to the fastest reaction rate among the three electrodes for methanol oxidation. The shape of the EIS curves also provided information of the controlling mode of the reactions. A semi-circular shaped EIS plot indicated a reaction-controlled process. When a 45 degrees line was observed, it indicated a diffusion-controlled reaction

47

and the

reaction rate would depend on how fast the ions diffused within the reaction medium, which was measured by the Warburg impedance, Zw. In a non-ideal solution, a combination of both kinetics and diffusion controlled processes was often observed, resulting in a hybrid of the semicircle and a linear line in the EIS plot. The EIS plot of the smooth Pt NWA appears to be dominated by a semicircular response at the high frequency region, indicative of kinetics controlled process. However, a linear line was observed at the low frequency region, indicating diffusion controlled process

48

. For

semi-rough NWA, the EIS curve consisted of a smaller semicircular shape at the very high frequency range and it became flattened at a low frequency range, which demonstrated a mostly kinetic controlled process. The EIS plot corresponding to the rough NWA depicted an even smaller semicircular curve, which shown that the rough NWA was predominately kinetics controlled. Overall, smooth, semi-rough and rough NWA displayed kinetics over diffusion controlled, which suggested the rate-limiting step was dominated by kinetics factors. It also shown that the Pt NWA electrode, leading to greater performance for methanol oxidation, carried out fast molecular transport and diffusion efficiently.

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Another form of the EIS plots, the Bode-magnitude plot, was often used to measure the relationship between the modulus of the absolute value of the electrical impedance, |Z|, and frequency, f root of

(Zim2

+

49-50

. By definition the modulus of the impedance is |Z| = the square

Zre2) 51.

Bode plots of the three Pt NWA electrodes are depicted in Figure

S5(a) and its logarithmic form is shown in Figure S5(b). In Figure S5(b), the curves of semi-rough and rough NWA were similar in shape that decreased quickly and approached a constant value; whereas smooth NWA shown a similarly decreased behavior. Overall, the |Z| values decreased as the frequency increased, even though the degrees of decreased are different for each electrode. The mixed circuit model in Figure 7(b) could explain this phenomenon. At low frequency less charged ions were transported and Cdl acted as a blockage, therefore these ions have to cross the circuit in a single path by overcoming Rct and Zw which created a large amount of overall impedance |Z|. As frequency increased and more ions were being transported, the charges will overcome Cdl eventually, which opened up another path within the circuit for charged to flow through. At the high frequency regime, some of the ions would be able to circumvent Rct and W and flow through the parallel path with Cdl, which resulted in reduction of the overall impedance |Z|. In Figure S5 (a & b), the height of the semi-rough and rough Pt NWAs in Bode plot depicted a significant less |Z| when comparing with Bode plot of the smooth Pt NWA, and rough Pt NWA’s curve illustrated the smallest overall |Z|. This shown the morphology and roughness of the Pt NWAs (correspondingly the surface to volume ratio) have a strong impact on the performance of the NWA electrodes by significantly increasing the overall electron transfer efficiency. The nanostructured wire samples with the roughest surface demonstrated the lowest electrical impedance, which suggested that the rough Pt NWA electrode had the best efficiency for electron transfer between solutions and the active surface of the electrode. The well-aligned vertical array structure allowed ions to diffuse easily throughout the nanostructured wires by minimizing the overall electrical resistance of the system, and the increase in nanostructured wire roughness further enhanced the overall performance.

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3.3 Possible reaction pathways Many mechanisms have been proposed for methanol oxidation, but the most widely used mechanism follows pathways of methanol adsorption and subsequent complete or partial oxidation 24, 52. Methanol is adsorbed on to the electrocatalyst surface to form an adsorbed intermediate (CHxOHad), which may undergo complete or partial oxidation. For complete oxidation, the adsorbed intermediate is oxidized to COad, which is further oxidized to CO2

53

. For partial oxidation, formaldehyde (HCHO) and formic acid (HCOOH) are

produced, which may eventually be oxidized into CO2. The partial oxidation pathway is less desirable, since the total number of electrons transferred during the MOR are significantly reduced, lowering the overall efficiency of MOR 53. In some cases reaction between HCOOH and CH3OH producing methyl formate (HCOOCH3) and H2O may take place 54. The reaction yields different side products dependent on several parameters, including methanol concentration, reaction temperature, and surface condition of the electrode

55

. In fact, the formation of carbon dioxide is found to be favored under high

temperature and rough electrode surfaces 55.

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Figure 8. Gas chromatography results: (a) reference sample [containing (reactant) methanol, (side products) formic acid and formaldehyde] vs. CV scan of MOR on rough Pt NWA 20 cycles; (b) integrated area under the curve of relative methanol concentration vs. number of CV scan cycles. To investigate the possible reaction pathways, gas chromatography (GC) was employed for quantitative product analysis after different cycles of CV scanning. As shown in Figure 8, a large peak and a small shoulder peak of solvent DCM were observed at t = 2.6 min and t = 2.49 min, respectively. Both peaks could be considered as backgrounds. The GC result of a reference sample containing methanol, formic acid, and

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formaldehyde is depicted in the bottom curve in Figure 8(a). It can be observed that methanol, formic acid and formaldehyde showed peaks around t = 2.08, 2.17 and 2.8 minutes, respectively. After CV scans of 1, 10 and 20 cycles, the reacted methanol solution only showed one dominant peak at 2.08 min (Figure 8a), and no formic acid and formaldehyde peaks were observed. The areas under the methanol peaks were integrated and plotted against the number of cycles as shown in Figure 8(b), which showed that the decrease of methanol concentration was inversely linearly dependent on the increase of number of cycles. From these results it could be concluded that methanol was consumed continuously as reaction proceeded, attributing to the anti-poisoning ability and good durability of the Pt NWA electrodes.

Figure 9. UV-Vis results: (a) real sample solution compared with carbonic acid and formic acid standard solution; (b) absorption peak values vs. number of cycles for rough (R), semi-rough (SR) and smooth (S) Pt NWA.

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To further analyze the methanol oxidation reaction, ultraviolet visible (UV-Vis) spectroscopy was employed to identify the possible products in the reacted samples. In Figure 9(a), a main absorption peak of the reacted solution was observed around 195 – 200 nm wavelength, which could be attributed to H2CO3 comparing well with the standard carbonic acid control solution instead of formic acid in 210 nm. H2CO3 was formed when the product CO2 reacted with the surrounding H2O molecule. It can also be observed that the absorption peak values increased with the number of CV scanning cycles on all the electrodes (rough, semi-rough, and smooth Pt NWAs), as shown in Figure 9(b), which was derived from Figure S6 in Supporting Information, suggesting that higher H2CO3 production was achieved with rougher Pt NWAs. According to the GC and UV-Vis results, it can be concluded that methanol was able to efficiently oxidize mostly into CO2 using the prepared 3D Pt nanostructured wire array electrode, and the complete oxidation of methanol was favorited over partial oxidation.

Figure 10. Possible schematic diagram of methanol and its oxidized products along with its reaction mechanism catalyzed by: (a) Pt NWs randomly dispersed on GCE; (b) Pt NWA electrode (this work).

Existing nanostructured wire electrode preparation methods normally involve a nanostructured wire dispersion onto GCE, which upon drying is sealed by using a

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conductive solution of Nafion, as shown in Figure 10(a). This electrode preparation method has many disadvantages. First, the overlapping between nanostructured wires (or nanoparticles) leaves very little space for reactant and product molecules to diffuse through, lowering the overall molecular transport rates. In addition, the electrons cannot more freely between the electrode and the bulk solution due to the poorly orientated and misaligned nanostructured wires or nanoparticles, resulting in an increase in overall electrical impedance and decrease in current density. Another problem of this electrode is the usage of the Nafion solution for sealing the electrocatalyst, which further lowers both the molecular and electron transport rates. In terms of the reaction mechanism, both the complete and partial oxidation pathway for methanol oxidation exist for this type of electrode arrangement (Figure 10a). The dominant reaction is the complete oxidation of methanol to the adsorbed intermediate, CHxOHad, which is then converted into carbon monoxide (COad) and generates the main product carbon dioxide (CO2), which can react with water to form carbonic acid (H2CO3). When the reaction proceeds, side products may also form, such as formaldehyde (HCHO) and formic acid (HCOOH) that result in the partial oxidation of methanol

24, 55

. The main reason for the formation of the side

products is likely due to tight space between the randomly dispersed electrocatalyst, which leads to the accumulation of the intermediates CHxOHad and shifts the reaction equilibrium towards the reactant side. Other than the formation of the side products, the overlapping structure also caused the accumulation of COad on the active surface, causing the electrodes to lose activities due to poisoning.

In this work the obstacles encountered by the prior electrode geometry have been overcome by the vertically aligned 3D nanostructured wire array structure (Figure 10(b)). First of all, the well-aligned and spaced vertical arrays allowed molecules to diffuse effectively between each nanostructured wire, which increased reaction rate. In addition, the electrons were able to efficiently transport throughout the cylindrical nanostructured wires due to the vertical aligned geometry. Since each nanostructured wires were fixed by the sliver substrate, the Nafion sealing solution was not necessary, which further increased the overall reaction rate. Other than the array structure the rough surfaces enlarged the active surface area and reactive sites, which further enhanced the overall

27



reaction performance. With these advantages, the rough Pt NWA was able to achieve complete oxidation of MOR to form CO2, which is confirmed by the GC and UV-Vis results of decrease in methanol concentration, increase in H2CO3 concentration, and the absence of side products. Our results suggest that the reaction products are continuously removed from the active sites and very little to no CO was accumulated (Figure 10(b) bottom). This prevented the Pt NWAs from being poisoned, which was observed in other Pt based electrode materials.

4. CONCLUSIONS In this work, an enhanced electrocatalyst electrode based on vertically aligned 3D Pt nanostructured wire array was developed for direct methanol oxidization reaction. Pt nanostructured wires with different surface roughness were fabricated into a 3D vertically aligned array structure, and the rough surface Pt nanostructured wire array showed significantly higher electrochemical catalyst performance. The cyclic voltammetry scanning showed one dominant oxidation peak compared with the negligible backward peak, indicating outstanding direct methanol oxidation efficiency. Multi-cycled CV scans, chronoamperometric and electrochemical impedance spectroscopy measurements showed great durability and low electron transfer resistance of these nanostructured wire array electrodes. The reaction products were characterized by gas chromatography and UV-Vis spectroscopy. The GC results showed linear decrease in methanol concentration as a function of CV scanning cycles and no presence of side products, which indicated a complete methanol oxidation, suggesting that the Pt NWAs had anti-poisoning ability. It was believed that both the 3D-aligned array structure and the rough surface improved efficient molecular and ion diffusion and maximized the total surface area of each available nanostructured wire. In addition, this electrode catalyst was able to sustain the desired reaction for a long period of time without either refreshing the electrode surface or replacing the solution. These results demonstrated that the Pt nanostructured wire array electrodes had extraordinary electrocatalyst efficiency, high performance, and long endurance for the direct methanol oxidation in acidic environment.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (provide by ACS) (Name of this paper provide by ACS) (PDF)

AUTHOR INFORMATION Corresponding Author *email: [email protected] *Phone: (978) 934-3540

ORCID Zhiyong Gu: 0000-0002-5613-4847 Hsi-Wu Wong: Chuqing Liu: 0000-0001-7843-746 Zhiyang Li: 0000-0002-6619-8465 Pen Yu: 0000-0001-6942-7131

Notes The authors declare no competing financial interest.

ACKNWLEDGMENTS We would like to thank Ethan Adams and Edward Fratto for carrying out some experiments.

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Vertically Aligned and Surface Roughed Pt Nanostructured Wire Array

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