Gallium Oxide Nanofibers for Hydrogen Evolution and Oxygen

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Gallium Oxide Nanofibers for Hydrogen Evolution and Oxygen Reduction Ashish Kakoria, Bandhana Devi, Abhishek Anand, Aditi Halder, Rik Rani Koner, and Sumit Sinha Ray ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01651 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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ACS Applied Nano Materials

Gallium Oxide Nanofibers for Hydrogen Evolution and Oxygen Reduction Ashish Kakoria‡†, Bandhana Devi§†, Abhishek Anand‡, Aditi Halder§, Rik Rani Koner‡, Sumit Sinha-Ray‡#* ‡School §School

of Engineering, Indian Institute of Technology Mandi, Mandi, HP-175005, India

of Basic Science, Indian Institute of Technology Mandi, Mandi, HP-175005, India

#Department

of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607-7022, USA

Abstract An effective, cost-efficient catalyst material that can replace platinum as electrode material in fuel cell has become the focus point of non-fossil fuel based alternate energy systems. Transition/post-transition metal oxide-based catalyst development is now the thrust area in the above-mentioned context for efficient energy conversions, especially oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, for the first time, application of electrospun beta-gallium oxide (-Ga2O3) nanofibers, a post-transition metal oxide, as an efficient bi-functional catalyst material is reported. These nanofibers are highly porous (specific surface area ~100-300 m2/g) and exhibit a mesoporous architecture (pore size ~1.5-2 nm) which facilitates better ion transport through the spongy morphology of individual nanofiber. The fabricated -Ga2O3 nanofibers performed at par with Pt/C catalyst, like for ORR the onset potential was 0.84 V (vs RHE) and for HER, although the onset potential was -0.34 V (vs RHE), the current density was visibly better than the latter catalyst. This catalyst also performed much better in methanol tolerance test and was near similar in current retention for 6 h, as measured in chronoamperometry. This performance was solely attributed by the large surface area and unique morphology presented by the material, via a rather simple fabrication technique, without addition of any dopant material. Keywords: Electrospinning; Nanofibers; Bi-functional catalyst; Oxygen reduction reaction; Hydrogen evolution reaction *Correspondence should be addressed to E-mail: [email protected] † Equal

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1. Introduction The development of non-fossil-fuel based alternative energy systems has gained enormous interest in both the academia and industry largely due to the global-warming which has become one of the key problem world-wide. In this context, the energy conversion technologies that include different types of electrochemical reactions such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have emerged as significant processes because of their high energy efficiency and clean power system.1-5In these techniques, electrode materials have played a promising role to develop high efficiency energy conversion systems which include fuel cells,6 metal-air batteries and water splitting devices7,8 to produce hydrogen and oxygen. Recently, extensive research efforts have been directed towards the development of electrode materials that may act as multifunctional materials and work in diverse systems.9,10 For example, HER is one of the important reactions in water splitting process that results in the production of hydrogen which can be used as clean fuel in proton exchange membrane-based fuel cell (PEMFC)for anodic reaction (hydrogen oxidation reaction). Likewise, ORR is a very important cathodic reaction in PEMFC where reduction of oxygen happens with formation of water. So, it is quite desirable to develop a highly active single electrode material for viable energy technology that integrate ORR, HER & OER. Currently, Pt and Ir/Ru based materials are used as state-of-the-art catalysts for HER, ORR and OER respectively. However, their high cost, low natural abundance and insufficient durability inhibit their wide applications in industry.11 Thus, considerable research efforts have been devoted to develop multifunctional electrocatalyst based on non-precious earth-abundant metals with improved properties. Generally, catalyst performance is greatly influenced by the active site density via either incorporation of dopant material as active element or interstitial defect or enhancing surface area, combined with structural porosity. In this endeavor, researchers are trying to develop metal oxide based catalysts doped with either carbonaceous elements or nitrogen or sulphur or metallic backbone like Pt/ Ni/ Mn etc. to enhance the conductivity and the electro-catalytic activity of towards both HER and ORR.12-18 However, the design of such metal oxides become a critical factor in attempts to harness the best of effects of catalysts, especially tailoring its specific surface area, porosity, architecture and several reports can be found where such oxides have been designed in a nano-tubular shape,19 3D foam-like structure20 and ordered mesoporous 2 ACS Paragon Plus Environment

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architecture21etc. Currently, electrospinning coupled with thermal treatment has been emerged as one of the versatile techniques which can be employed for large scale catalyst synthesis, due to its various advantages such as high specific surface area of electrospun nanofibers, controllable 3D architecture tuning high porosity and ability to blend additives in situ. So far, electrospinning has been efficiently used in fabrication of porous transition metal oxide nanofibers for both HER and ORR.22-27 In this work we are reporting the electrocatalytic activity of electrospun gallium oxide (Ga2O3) nanofibers as a bi-functional material for both HER and ORR. -Ga2O3 became dominant material in semiconducting industry, especially in power device applications, due to its fascinating wide bandgap (4.9 eV) with very high breakdown potential (~8MV/cm).28 Ga2O3 is also known for its abundant surface medium-strong Lewis acid sites which has significant effect on its catalytic behavior 29 and few reports are available demonstrating addition of gallium to Pt based catalyst for methanol oxidation,30 methanol oxidation coupled with oxygen reduction31 and ethanol oxidation32 reaction. However, the authors have not come across any such reports on electrospun -Ga2O3 nanofibers for bifunctional electrochemical catalysis particularly HER and ORR. Here, we have shown the effect of nano architecture and spongy porous like structure in enhancement of catalytic activity. 2. Experiments 2.1 Materials The polymer Polyacrylonitrile (PAN) (Mol wt.= 150 kDa), gallium nitrate hydrate (Ga (NO3)2.9H2O) [will be abbreviated as Ga-salt] and gallium oxide (Ga2O3) were obtained from Sigma-Aldrich. The requisite solvent for preparation of polymer and metal precursor solutions was N, N-dimethyl formamide (DMF), purchased from Alfa-Aesar. 2.2 Solution Preparation Three solutions of PAN and Ga-salt were prepared by mixing them in DMF. The polymer solutions and Ga-salt solutions were prepared separately. The proportion of polymer to Ga-salt, i.e. solid to solid in final mixture was kept at 1:0.25, 1:0.5, and 1: 0.75. In the final mixture, polymer concentration was always kept at 10 wt%. As mentioned, specific concentration of PAN was mixed with DMF at 80 oC using a magnetic stirrer for 8 h, while the requisite Ga-salt 3 ACS Paragon Plus Environment


solution in DMF was prepared by mixing both using a bath sonicator for 2 h. After the polymer solution was prepared, Ga-salt solution was mixed with the polymer solution and the mixture was kept for stirring at 80oC for 4 h. The three mixtures with different concentrations will be hereafter referred as PAN/Ga-25, PAN/Ga-50 and PAN/Ga-75, where the final polymer concentration is constant and only the salt amount varies. 2.3Electrospinning Electrospinning of all the three polymer solutions namely PAN/Ga-25, PAN/Ga-50 and PAN/Ga-75 was conducted at room temperature with the solutions being pumped at a constant rate of 0.6 mL/hr using a syringe pump (GenieTouchTM,Lucca Technologies) through an 18gauge needle connected to a 5 mL disposable syringe (Dispovan) and applied voltage of 10 kV. The electrospun nanofibers were collected on a home-made rotating drum collector rotating at 200 rpm. The gap between the needle tip and the collector was maintained at 20 cm. The schematic of the electrospinning set up and a representative image of the polymer/Ga-salt composite nanofiber mat are shown in Figure 1a. The nanofibers were collected for 8 h to obtain a sufficiently large mat (a physical scale of it is provided in Figure 1b), before the mats were taken to heat treatment.

Figure 1: Process schematic elucidating the synthesis and electrochemical testing of the catalysts: (a) electrospinning of PAN/Ga composite nanofibers, (b) an actual image of the 4 ACS Paragon Plus Environment

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collected nanofiber mat with a physical scale, followed by structural characterization of the (c) pre-calcined nanofibers and (d) post-calcined nanofibers after calcination at 1000 oC; finally (e) electrochemical testing in a three-electrode set up with working electrode being the post-calcined nanofibers deposited on glassy carbon electrode.

2.4 Heat Treatment The as-prepared nanofiber mats were dried at room temperature for 5 h before any heat treatment. Thereafter the mats were calcined in a muffle furnace (Jupiter scientific Co.) in air atmosphere. The mats were heated to 1000oC from room temperature with ramp rate of 5oC/min and were held at that temperature for 5 h. After that the mats were cooled down to room temperature inside the furnace at same atmosphere before taking the samples out. The collected samples were stored in an air-tight Petri dish, before conducting the requisite characterization. 2.5 Characterization All SEM images were obtained using Nova Nano SEM-450 field emission scanning electron microscope (FE-SEM, JFEI, S.E.A. PTE LTD.) after sputter coating the samples with Au to a thickness of 5 nm (Figure 1c-d). X-Ray diffraction pattern of pre-calcined and postcalcined samples were collected using Smart Lab 9kW rotating anode x-ray diffractometer o

(Rigaku Corporation) within 5 to 95 degrees using copper (Cu-K, λ=1.54 A ) radiation at 45 kV and 100 mA source. High resolution transmission electron microscope (HR-TEM) images of post-calcined samples were collected by FEI, HRTEM (FP 5022/22-Tecnai G2 20 S-TWIN) using a carbon grid by drop casting post-calcined samples on it. Thermo-gravimetric analysis (TGA) was conducted on all samples using a thermo gravimetric analyzer coupled with differential scanning calorimetry (DSC), STA 449 F1 Jupiter (NETZSCH Geratebau GmbH,) in O2 atmosphere at a ramp rate of 5oC/min. UV-visible spectrophotometer and diffuse reflectance spectroscopy were carried out using UV-Vis spectrophotometer-2450 (Perkin Elmer Lambda 750). Fourier transform infrared spectrometry of pre-calcined and post-calcined samples was conducted using K8002AA carry 660 FTIR (Agilent Technologies) and for examining the chemical compositions, X-ray photoelectron spectroscopy (XPS) (Prevac, Poland) was employed. The multipoint surface area from Brunauer–Emmett–Teller (BET) analysis, porosity 5 ACS Paragon Plus Environment


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and pore radius and volume from Barrett-Joyner-Halenda (BJH) adsorption isotherms of all the post-calcined samples were measured using Quanta chrome BET. 2.6 Electrochemical Measurements 2.6 (a) Catalyst ink preparation For electrochemical measurements PAN/Ga-25, PAN/Ga-50, PAN/Ga-75 refer only to post calcined samples and readers are advised to note for any further confusion. 4 mg of PAN/Ga sample (PAN/Ga-25, PAN/Ga-50, PAN/Ga-75), 1 mg of Vulcan carbon and 25 μL of nafion were added in 500 μL of isopropyl alcohol and then ultrasonicated for 30 minutes to get homogeneous solution. 15 μL of this ink was pipetted out on glassy carbon electrode for further cyclovoltammetry characterizations. The loading of catalyst on the electrode was 0.46 mg/cm2. 2.6 (b)Electrochemical Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER) measurementsThe electrocatalytic performance of all catalysts (PAN/Ga-25, PAN/Ga-50 and PAN/Ga75) was analyzed by cyclic voltammograms (CV), linear sweep voltammograms (LSV) and chronoamperometric test by using Auto Lab electrochemical workstation (302N). The electrochemical workstation is coupled with a rotating disk electrode (RDE, PINE instruments) system. All the experiments were performed in three electrode configuration, using platinum wire as a counter electrode, Ag/AgCl (3M KCl) as a reference electrode and glassy carbon electrode having 5 mm diameter as a working electrode. Rotating ring-disk electrode (RRDE) measurements were carried out using similar electrode having carbon disk in the centre and gold ring on the outer side (Figure 1e). The electrolyte used was 0.1M aqueous KOH solution. The potential against Ag/AgCl were converted into potential against reversible hydrogen electrode by using the following equation: E(RHE) = E(Ag/AgCl) + (0.197 +0.059 × pH)

(1)

3. Results and Discussion 3.1 Gallium oxide nanofiber Initially, both pre- and post-calcined nanofiber samples were characterized using FTIR spectra to evaluate the characteristic groups present in them [Figure 2a and S1]. Herein the FTIR 6 ACS Paragon Plus Environment

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spectra of pre-calcined PAN/Ga-75 samples are provided in Figure 2a (data shown in red color), where the post-calcined sample spectrum is shown in the inset (data shown in blue color). Precalcined sample reveals peaks at 537, 1046, 1348, 1452, 2244 and 2926 cm-1 signifying the presence of C=O, C-N (blending), symmetric blending of CH3 in CCH3, CH2 (blending), CN (stretching) and CH2 bonds in polymer characteristic toPAN.33,34After calcination the polymer peaks were completely absent (c.f. Figure 2a inset) which confirms that post-calcined samples did not contain any polymer. The peaks observed at wavenumbers of 442 cm−1 and 683 cm−1 indicates to GaO6 octahedra and GaO4 tetrahedra respectively of β-Ga2O3.35 Removal of polymer during heat treatment was further confirmed via thermogravimetric analysis [c.f. Figure 2b (i-iv)]. TGA data suggests 100%, 95%, 60% and 40% mass loss at 1000oC for pristine PAN nanofibers (i), PAN/Ga-25 (ii), PAN/GA-50 (iii), PAN/Ga-75 (iv) respectively. This mass loss can be directly correlated with increasing Ga-salt concentration. The mass loss peak was observed at 291oC for pure PAN and PAN/Ga-25 which signifies the melting of PAN, albeit the peak temperature shifted 2-3oC at higher side for increased gallium concentration. Literature suggests the high crystalline PAN melts at 317oC,36 however, enthalpy of fusion calculation from the DSC data indicates [Figure S2] that crystallinity of pristine PAN fibers (here) and PAN/Ga fibers varied between 2%- 17% and hence melting point shift within each other and from pre-PAN is not anomalous. For samples with metal precursor, the transformation from Ga(NO3)2.9H2O to intermediate product Ga(OH)3 at ~ 200oC can be found in literature, which is however masked along with polymer decomposition in Figure 2b (ii-iv).37 Furthermore beyond 200oC transformation from Ga(OH)3 to Ga2O3 can occur which is its  polymorph leading to -Ga2O3 at about 500oC which then eventually forms into -Ga2O3.38 Next, the post-calcined sample PAN/Ga-75 was characterized using UV-visible spectroscopy to examine the band gap [Figure 2c]. The oxide sample exhibits strong absorbance at 257 nm (in UV region). The band gap of the material (E) was calculated using Planck-Einstein equation, E  hc /  , where h is Planck’s constant ( 4.135 1015 eV-s), c is speed of light ( 3 108 m/s) and λ is wavelength (here 257 nm) and the band gap was found to be 4.83 eV, which is characteristic to β-Ga2O3.39 The UV-Visible spectra for other two samples are provided in Figure S3a, which are similar with PAN/Ga-75 post-calcined samples.

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Figure 2: (a) FTIR spectrum of pre-calcined PAN/Ga-75 nanofibers (data shown in red color), with post-calcined nanofiber FTIR spectrum in the inset (data shown in black color); (b) TGA plot of – (i) pristine PAN, (ii) PAN/Ga-25, (iii) PAN/Ga-50, (iv) PAN/Ga-75 fibers; (c) UV-vis spectrum of post-calcined PAN/Ga-75 nanofibers showing sharp absorbance at 257 nm, indicating to β-Ga2O3 ; (d) XRD pattern of pre- and post-calcined PAN/Ga-75 (aka β-Ga2O3) fibers denoted by blue and red lines, respectively. The post-calcined fiber’s XRD diffraction pattern is matched with JCPDS card no. 00-041-1103 (β-Ga2O3); (e) TEM image of post8 ACS Paragon Plus Environment

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calcined PAN/Ga-75 nanofiber with selective area electron diffraction (SAED) pattern identifying planes characteristic to β-Ga2O3, as matched with XRD (at bottom right inset) and lattice fringes exhibiting crystalline nature of the oxide sample (at top left inset). Pre-calcined electrospun nanofibers were further investigated using powder X-ray diffraction. The diffraction patterns of all the samples were identical and the characteristic peaks appeared at 2 of 14.7o and 22.3o [c.f. Figure 2d and S3]. The reported distinctive 2 values of pure PAN are 17o and 26o.40 The shift in the characteristic peaks between the reported and the obtained case is due to the interaction between Ga-salt and the electronegative –CN group present in the PAN. However, these peaks were completely absent in post-calcined samples, as the calcination of the samples were conducted at 1000oC in air atmosphere, leading to a complete removal of polymer content, as established in FTIR-TGA studies. Elemental dispersive spectroscopy (SEM-EDS) data also supports the complete removal of any carbonaceous element or nitrogen during calcination [c.f. Figure S4]. All the post-calcined samples exhibit similar diffraction patterns with perfect match between the characteristic peaks with 2 of 30.3o, 31.7o, 35.2o, 38.8o, 57.6o, 64.7ocharacteristic to (400), (-202), (111), (-311), (-313), (-712) respectively confirming the phase to be β-Ga2O3, as corroborated with JCPDS card 00-041-1103. A diffraction pattern of β-Ga2O3 from JCPDS card 00-041-1103 is also plotted along with postcalcined samples, which shows a perfect matching between the two. The particle size (D) of the oxide samples are calculated using Scherrer equation D  K  /  Cos  where, K is a o

dimensionless shape factor (here considered as 0.89), λ is the X-ray wavelength 1.540 A , β signifies to the full width at half maximum (FWHM)of the spectrum and θ is the Bragg angle in degrees.41 The average particle size measured from all the peaks was 12.13 nm, which was further explored in TEM images. It can be seen from TEM image provided in Figure 2e, that the oxide nanofiber, in this case it was β-Ga2O3 derived from calcination of PAN/Ga-75, is about 2-3 m long, albeit they can break into shorter fibers due to extreme probe-sonication while suspending the sample in ethanol before drop casting to TEM grid. However, the most interesting finding from TEM was the morphology of the individual nanofiber which can be described as 1-D arrangement of fine nanoparticles. The SAED image shows spots, provided in inset of Figure 2e, suggesting polycrystalline nature of the oxide sample.42The porous nature of the individual nanofiber is lucrative as a catalyst material and hence the overall architecture was 9 ACS Paragon Plus Environment


evaluated under SEM and porosity index was measured using BET. XPS data also confirms the synthesis of Ga2O3 in PAN/Ga samples. Ga 2p spectrum (Figure S5a) shows the presence of two symmetrical peaks at 1150 eV and 1118 eV which corresponds to Ga 2p1/2 and Ga 2p3/2, respectively. The O 1s peak (Figure S5b) at 537 eV can be assigned to oxygen in Ga2O3 lattice.43 The SEM images of pre-calcined and post-calcined nanofibers are shown in Figure 3. It can be seen that the pristine nanofibers are smooth, with no undulation. However, a contrasting pattern is visible from the SEM images (c.f. Figure 3a, 3b and 3c: pre-calcined nanofibers) and from the fiber size distributions provided in Figure S6. The distributions are reported from diameter measurements of 180 nanofibers from each nanofiber mat for statistical parity between each case. It can be seen that with increase in salt concentration i.e. from 25 wt.% to 50 wt.%, the nanofiber mean diameter in pre-calcined samples decreased and with further salt concentration increment, i.e. from 50 wt.% to 75 wt.%, there is an increase in diameter. The details are shown in Figure S6. This observation is however not peculiar as in several cases researchers have observed a decrease in diameter of electrospun nanofiber after addition of salts. Salt addition can significantly increase the solution’s conductivity and decrease the solution viscosity and surface tension, which in turn can aid in sufficient elongation of polymer jets, with reduction of bead formation in nanofibers.44,45 However, with more salt addition, like in this case from PAN/Ga-50 to PAN/Ga-75, the polymer solution viscosity can reduce quite effectively, which can increase mass flow rate 46 leading to an increment in fiber diameter.


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Figure 3: SEM images of pre and post-calcined nanofibers, where the left-hand side column indicates to the pre-calcined nanofibers and the right-hand side column indicates to the postcalcined nanofibers - (a) PAN/Ga-25, (b) PAN/Ga-50 and (c) PAN/Ga-75. The images in the inset show the zoomed in view of the nanofibers, where the scale bar embedded in them is 2 m and scale bar of the zoomed-out view is 5 m. Although pre-calcined nanofibers in all the cases are smooth and without any visible undulation on them, the nanofibers post calcination show different morphology, which is due to the Ga-salt concentration in the final solution, albeit the nanofiber diameter reduction has followed the same pattern as it has been seen for pre-calcined nanofibers. In case of PAN/Ga-25, the post-calcined nanofibers are more beaded and string like and instead of individual long nanofibers present in the mat, they are more prone to form conjugate after short intervals (c.f. Figure 3a: post-calcined nanofibers). It is to be noted that in the case of post-calcined nanofiber diameter measurements, only the visible single nanofibers were taken into consideration. The 11 ACS Paragon Plus Environment


beaded string like nanofiber formation is not entirely unexpected as during heat treatment the polymer rich fibers can fuse with each other and thereafter can allow sintering of fibers to render a final shape resembling to Figure 3a: post-calcined nanofibers. As the Ga-salt concentration increased to 50 wt.% more pronounced nanofibrous morphology in oxide nanofibers was seen. Significant reduction in nanofiber diameter elucidates the complete removal of polymer. The oxide fibers entailed (c.f. Figure 3b and 3c: post-calcined nanofibers) enhanced surface area than previous case (almost double), also corroborated by BET and BJH adsorption measurements, shown in Figure 4b. As it can be seen in Figure 3c, the post-calcined fibers are much bigger, due to their high Ga-salt concentration, as explained above. However, the nanofibers are seemingly combined as nano-ropes with shorter fiber length compared to PAN/Ga-50, which is nothing but the appearance of channel rich nanofibers. This is also well illustrated in Figure 2e (TEM image). As the salt concentration reached to a comparable concentration to the polymer, the Gasalt rich microfibrils tend to separate during heat treatment along with polymer evaporation. A similar phenomenon has also been reported earlier by Ji et at al.47 The developmental stages can be idealized following Figure 4-a where the above phenomenon is lucidly illustrated. This also helps in understanding the decrement in porosity in calcined PAN/Ga-75 compared to PAN/Ga50. However, due to the overall channel-rich porous spongy like morphology, the specific surface area has increased monotonously. PAN/Ga-75 post-calcined oxide nanofibers contain much finer pores as can be clearly seen in Figure 3c, with much larger surface area. The pores are however smaller than the previous two cases, because of the higher interaction between salts inside polymer fibrils, simply due to increased salt concentration, which in turn leads to favorable growth of oxide nuclei and their subsequent growth.


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Figure 4: (a) Schematic representation depicting the channel rich porous fiber manifestation during heat treatment and with increased Ga-salt and (b) Surface area and pore radius measurement from BET and BJH adsorption isotherm of post-calcined nanofibers, where the data point in black represents surface area and in blue represents the pore radius. Such porous fibril like structure is favorable for improvement in catalytic performance as they provide much higher surface area, abundant pores for effective catalytic reactions and channels for ions to transport. The meso-porous structures are highly desired for the fast mass transport of the reactant to achieve efficient reaction kinetics as available in several literatures.48,49 And hence it can be predicted that such nanoarchitecture will aid in electrochemical activity in this case as well, which is discussed in following section. 3.2 Electrocatalytic activity Here on and after, PAN/Ga-25, PAN/Ga-50, PAN/Ga-75 refer to calcined samples only. Cyclic voltammetry experiments were performed in N2 and O2 saturated 0.1 M KOH to evaluate the electrocatalytic activity of PAN/Ga samples. The presence of reduction peak in ORR region after O2 saturation and absence of any such peak in N2 saturated electrolyte indicates the ORR active nature of developed catalysts (Figure 5a). The ORR activity of PAN/Ga-75 in O2 saturated electrolyte is found to be higher than PAN/Ga-50 and PAN/Ga-25 catalysts (Figure 5b). The oxygen reduction properties of PAN/Ga samples were further analyzed via linear sweep voltammetry (LSV) in oxygen saturated 0.1 M KOH at increasing rotation speeds ranging from 100 to 2500 rpm (c.f. Figure 5c). The maximum diffusion limited current density for PAN/Ga-75 was observed as 2.4 mA/cm2 at 2500 rpm. The LSV analysis confirms that catalyst PAN/Ga-75 exhibits onset potential of 0.84 V (vs. RHE), whereas the same for PAN/Ga-50 and PAN/Ga-25 are 0.83 V and 0.80 V (vs. RHE) respectively, as seen from Figure 5-d. In the same image the comparative ORR data for the fabricated catalysts and 20 wt% commercial Pt/C is also shown at a rotation speed of 900 rpm. Among the three catalysts in discussion, PAN/Ga-75 has the highest current density. The enhancement in catalytic performance of PAN/Ga-75 compared to others towards ORR is attributed to higher surface area with smaller pore size (refer to Figure 4b). To explore the ORR kinetics, Koutecky-Levich (K-L) plots were calculated at different potentials by using the following equation:



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1  1 1   1 1  =  + =  + 1  J  JK JL   JK 2 Bω  

(2)

B=0.62nFD02 3 ν -1 6 C0

(3)

J K =nFKC0

(4)

Where, J is the measured current density, JK and JL are the kinetic and diffusion limited current densities,  is the rotating speed of the electrode in rad s-1, F is the Faraday constant (96,485 C mol-1), C0 is the bulk concentration of O2 in 0.1 M KOH (1.2 × 10-6mol cm-3), D0 is the oxygen diffusion coefficient (1.9 × 10-5 cm2s-1), ν is the kinematic viscosity of the electrolyte (0.01cm2 s-1) and K is the electron transfer rate constant. The K-L plots show near linearity which indicate the reaction follow first order kinetics (Figure 5e and Figure S7). The slopes are almost similar with gradients ranging from 2.4 to 3.0 at different potentials (0.2 to 0.6 V vs. RHE) representing the mixed electron transfer number. Further, to corroborate the electron transfer number RRDE experiments were conducted. ORR can follow either of these two mechanisms- (a) involving four electrons at once in which oxygen is directly reduced to OH- or (b) two step reduction of oxygen involving two electrons in the first step where first HO2- is formed as an intermediate product followed by further reduction of the species to OH- involving two more electrons. The RRDE data is shown in Figure 5f and Figure S8. There is an increase in ring current value with increasing rotation speed which is due to enhanced diffusion of oxygen at higher convection. The electron transfer number (n) was calculated by using the formula given below-

  4I d   n=    Id + Ir  N 

(5)

Here Id is the disk current, Ir is the ring current and N is current collecting efficiency of gold ring (0.37). The average electron transfer number value for PAN/Ga-75 sample was 3.1 within the voltage range of 0.6-0.2 V (vs RHE). This is in clear agreement with the n value obtained from


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K-L plot. Details of other samples are provided in Table S1, however their values were also of the order of 3. The plot for electron transfer number and H2O2 production value has been shown in Figure S9.

Electrocatalytic activity of PAN/Ga based catalysts was also compared with

commercially available Ga2O3 (Figure S10a). It can be seen from the Figure S10a, post-calcined PAN/Ga-75 is much superior in ORR activity than commercial gallium oxide powder. The onset potential of Ga2O3 powder for ORR is 0.74 V vs. RHE, whereas for PAN/Ga-75 was 0.84 V vs RHE



Figure 5: (a) Cyclic voltammetry (CV) data of PAN/Ga-75 in N2 and O2 saturated 0.1 M aqueous KOH; (b) CV data of PAN/Ga-75, PAN/Ga-50 and PAN/Ga-25 in O2 saturated 0.1 M aqueous KOH; (C) ORR data of PAN/Ga-75 sample at various rotation speed; (d) Comparative ORR data of PAN/Ga-75, PAN/Ga-50, PAN/Ga-25 and 20 wt.% Pt/C at 900 rpm; (e) K-L plot for PAN/Ga-75 and (f) Ring current data for PAN/Ga-75.


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Further, electrochemical impedance spectroscopic (EIS) measurements were performed for PAN/Ga-75 catalyst (refer Figure S11) and it was found that the charge transfer resistance was 40 kΩ. Durability of catalyst is another equally crucial factor for determining the long-term performance which was also investigated. The stability of PAN/Ga-75 catalyst was tested using Chronoamperometric experiment (Figure 6a) for 6 h and was also compared with commercial 20 wt.% Pt/C at 0.73 V (vs. RHE) in O2 saturated 0.1 M KOH. It has been noticed that initially the current retention for PAN/Ga-75 was more compared to commercial Pt/C up to 3h. As example, after 100 min the former retains 78% of current whereas the latter retains 74 %. beyond the crossover, after ~3 h, current retention of PAN/Ga-75 decayed quicker than Pt/C and an oscillatory nature could be observed. However, at the end of 6h, the current retained by both the catalysts were near similar (for PAN/Ga-75 ~57 % and for Pt/C ~60%). Chronoamperometry test was also performed for PAN/Ga-50 and PAN/Ga-25 under similar conditions [Figure S12a: comparative chronoamperometry data for all three catalysts] and PAN/Ga-75 catalyst was found to be more durable than the remaining two catalysts which is attributed to its high surface area and more pronounced nanoporous architecture than PAN/Ga-25 and PAN/Ga-50.. In order to evaluate the robustness of developed catalyst for practical applications methanol tolerance of PAN/Ga-75 (Figure 6b) and other catalysts [Figure S12b] was also investigated. Impressively, it has been observed that there was minimal current loss in PAN/Ga-75 catalyst as compared to Pt/C on addition of methanol to it (Figure 6 b).

Figure 6: (a) Chronoamperometric response in O2 saturated 0.1 M KOH and (b) Methanol tolerance test of PAN/Ga-75 and Pt/C.



Page 18 of 28

The electrocatalytic performance of PAN/Ga catalysts were tested for hydrogen evolution studies in N2 saturated 0.1 M KOH. The HER activity of the developed catalysts was measured at scan rate of 10 mV/s in potential window ranging from 0.0 to -2 V (vs. Ag/AgCl). The onset potential for PAN/ Ga-75, PAN/Ga-50, PAN/Ga-25 was found to be -0.34 V, -0.53 V and -0.6 V (vs. RHE) respectively (Figure 7a). It is important to note that increasing current density from 10 mA/cm2 to 25 mA/cm2, the gradient of overpotential to current density is about 10.8 mV/ (mA/cm2) for Pt/C and 4.5 mV/ (mA/cm2) for PAN/Ga-75. To gain further insight into the catalytic performance, Tafel slopes of all the catalysts were calculated using the following equation-

  a  b log J

(6)

where ɳ is the over-potential, b is the Tafel slope and J is the current density. It is well known that the value of Tafel slope has important influence on the HER as it determines the rate of reaction.50 Smaller slope value signifies faster HER rate. The calculated Tafel slope for PAN/Ga-75, PAN/Ga-50 and PAN/Ga-25 is 70 mV/dec, 73 mV/dec and 260 mV/dec respectively. This suggests that PAN/Ga catalyzed HER process follow the VolmerHeyrovsky mechanism.51 The lower Tafel slope for PAN/Ga-75 (70 mV/dec) as compared to PAN/Ga-50, PAN/Ga-25 and other metal-oxide based catalysts further confirms the better catalytic kinetics for PAN/Ga-75 than other metal oxide-based catalysts. Table 1: Comparison of Tafel slopes between some literature available data and this work Electrocatalyst

Electrolyte

Tafel slope (mV/dec)

Reference

NENU500

0.5 M H2SO4

96

[52]

Mo2C/MoO3

0.5 M H2SO4

48

[53]

MoS2/TiO2

0.5 M H2SO4

81

[54]

PdCo-CN

0.5 M H2SO4

31

[55]

Fe1.89Mo4.11O7/MoO2

1 M KOH

79

[56]

Ni-Mn3O4/NF

1 M KOH

110

[57]

c-CoSe2/CC

1 M KOH

85

[58]

Ni/NiO-CNT

1 M KOH

82

[59]


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Pt-Ni/C

0.1 M KOH

60

[60]

PAN/Ga-75

0.1 M KOH

70

This work

As it can be seen from Table 1, PAN/Ga-75 performs at par with other reported catalysts, even surpassing them at some instances. It is worth mentioning that, this catalyst didn’t involve any dopant material like N2, carbon components, transition metal or metal oxides as it is selfexplanatory from Table 1, which are known to be effective in terms of enhancement factors for catalysis reactions. Hence as a dopant free catalyst, PAN/Ga-75 has immense potential to act as an efficient HER catalyst. EIS measurement of PAN/Ga-75 catalyst for HER reveals the charge transfer resistance to be 550 Ω as shown in Figure S13 The chronoamperometric measurement for HER studies using PAN/Ga-75 sample was performed at 0.43 V (vs. RHE) and shows about 65% current retention after 4 h which again elucidates its efficient catalytic activity. The comparative chronoamperometry data with Pt/C catalyst has been given in Figure S14 which indicates the higher stability of PAN/Ga-75 catalyst.



Figure 7: (a) Compiled HER data for PAN/Ga-75, PAN/Ga-50, PAN/Ga-25 and Pt/C (b) Tafel plots for PAN/Ga-25, PAN/Ga-50, PAN/Ga-75 and Pt/C and (c) Chronoamperometry data for PAN/Ga-75 at -1.4 V (vs. Ag/AgCl). On the other hand, commercial gallium oxide powder shows negligible catalytic activity towards HER (Figure S10b), which is possible because of its minimal porous structure as seen from SEM image (Figure S15). 3.3 Mechanistic study Spongy porous nano-architecture with high surface area plays an important role for the excellent electrocatalytic performance, which has been reported in several literatures.61 Generally, the high surface area with porosity promote electrocatalytic activity by exposing catalytically active sites and also enable the fast transport reactant, ions etc. As seen from BET and SEM measurements, the emerged surface area with meso-porous architecture of PAN/Ga-75 catalyst facilitates its fast diffusion of electrolyte ions toward catalytic active sites resulting in enhanced electrocatalytic activity, even though -Ga2O3 is non-conductor at room temperature and the fabricated catalyst didn’t contain any carbonaceous or metallic or nitrogen dopant. However, Ga2O3 is well-known for its catalytic activity due to Lewis acidic nature of Ga3+ ion’s presence on the surface which facilitates the electrocatalytic transformation. The meso-porous nanoarchitecture of PAN/Ga-75 is more pronounced than the other two catalysts, as seen in SEM and BET studies, which directly affects their electrocatalytic activities. The SEM image in Figure S13 reveals the morphology of commercially available Ga2O3 powder, which shows no/minimal porosity on the surface. The lack of porous architecture eventually results in its negligible catalytic activity, both in ORR and HER. The comparative study in Figure S10 between PAN/Ga-75 nanofibers and Ga2O3 in terms of ORR and HER proves the effectivity of the porous channel rich architecture. 4. Conclusion In conclusion, a facile method for large scale production of tunable morphology controlled β-Ga2O3 nanoarchitectures have been developed through electrospinning followed by thermal treatment. This structural analysis revealed the spongy channel-rich architecture with pore size of ~ 2nm. Such morphology is known to be helpful in catalytic activities for better ion 20 ACS Paragon Plus Environment

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transport and accessible catalytic sites. The fabricated metal-oxide nanofibers performed as bifunctional catalyst for oxygen reduction and hydrogen evolution reaction. Out of three fabricated catalysts, PAN/Ga-75 showed best catalytic activity for ORR with onset potential 0.84V (vs RHE) with current density2.4 mA/cm2 at 2500 rpm. In the case of HER, the same sample showed onset potential of -0.34 V (vs RHE) with current density increment better than Pt/C [for increment of current density from 10 mA/cm2 to 25 mA/cm2, the gradient of overpotential to current density is about 10.8 mV/ (mA/cm2) for Pt/C and 4.5 mV/ (mA/cm2) for PAN/Ga-75]. The value of Tafel slope of this catalyst was 70mV/dec, which is comparable with many reported transition metal oxide-based catalysts, if not better. This catalyst is capable of exhibiting bi-catalytic activity without adding any hetero-atom doping and carbon additive. On the other hand practically featureless commercial Ga2O3, with no/minimal surface porosity, exhibits poor catalytic performance in comparison to porous PAN/Ga-75 nanofibers. The tunable structural morphology opens up new path for developing electrocatalyst from electrochemically inert material which will help to design potential substitution of Pt. Acknowledgment This work was done under the financial support received from DST-SERB, India (ECR/2017/000670) and IIT Mandi (IITM/SG/SSR/60). BD and AK acknowledge the scholarship from Ministry of Human Resource Development (MHRD), India. The support from the Advanced Materials Research Centre (AMRC), IIT Mandi for the sophisticated instrument facility is gratefully acknowledged. Conflict of Interest Authors declare no conflict of interest. Supporting Information Structural characterization details of the three catalysts including FTIR spectra, Uv-Vis spectra, EDS data, XPS spectra etc., along with their several electrochemical characterization details, like K-L plot, ring current, electron transfer number etc. and their comparison with commercially available -Ga2O3.



Page 22 of 28

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Gallium Oxide Nanofibers for Hydrogen Evolution and Oxygen

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