Microsoft Word - 19Venkatesan.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 80, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Eliseo Maria Ranzi, Rubens Maciel Filho Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-78-5; ISSN 2283-9216 Electrochemical Hydrogen Generation by Biomass Electrolysis with Pt_MoS2 Electrode Maria Sarnoa,b*, Eleonora Ponticorvob a Department of Physics “E.R. Caianiello”, University of Salerno, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy. b NanoMates, Research Centre for Nanomaterials and Nanotechnology at the University of Salerno, University of Salerno, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy. msarno@unisa.it Here we report the performance in lignocellulosic material oxidation of Pt_MoS2-based electrode. The prepared nanocomposite, synthesized according to a “wet chemistry” approach, was broadly characterized: SEM-EDS images and XRD spectrum indicate the formation of nanoparticles with a few nanometers size dispersed on MoS2 nanosheets. The sample was tested for the oxidation of lignocellulosic biomass in alkaline solutions. A very small overpotential is demonstrated (0.17 V) showing that such process could significantly reduce the cost of generating hydrogen reducing the energy required for the anodic oxidation process. 1. Introduction In recent years, hydrogen has been regarded as the best alternative to fossil fuels (Dincer and Acar, 2015; Cortright et al., 2002). Steam reforming is typically adopted for large scale H2 production (Heinzel et al., 2002). On the other hand, it requires temperatures higher than 600 °C and produces low purity hydrogen with a high concentration of CO and CO2. Therefore, additional separation units are needed to obtain pure H2. With water electrolysis, it’s possible to achieve large pure hydrogen production starting from readily accessible, cheap and renewable resources. In particular, in the case of abundant renewable energy availability, the excess of energy can be stored in the form of H2 via water electrolysis (Zeng and Zhang, 2010). The main water electrolysis disadvantage is the high electrical energy requirements; the electrolysis of organic molecules is a promising alternative for H2 generation since the minimum thermodynamic potential needed for the electro-oxidation of the organic molecules is about one order of magnitude lower than that for water electrolysis. Pure H2 can be produced by the electrolysis of methanol (Sapountzi et al., 2017; Sarno et al., 2019), ethanol (Gutiérrez-Guerra et al., 2015) and bioethanol (Caravaca et al., 2013) solutions. However, some alcohols are already considered as important bio-fuels. Therefore, their further transformation to H2 is unattractive for practical purposes. Lignin is one of the most available bio-polymers in nature (Mohana Roopan, 2017), its abundant amount as a natural product or by‐product from different processes, has led to its utilization as feedstock for chemicals compounds and biofuels production. Numerous approaches have been developed for lignin degradation: thermochemical, catalytic and enzymatic processes have been studied (Pandey and Kim, 2011); however, challenges still persist for the development of processes with high efficiency and low cost. Electrochemistry allows working under moderate conditions compared to conventional processes. To enhance the potential of biomass-derived molecules (such as lignocellulosic materials) as an H2 resource, this technology should be improved. Platinum is the best catalyst in numerous electrochemical processes, considering the high cost and the low abundance of Pt there is a significant need to develop new active and cheap electrocatalysts. Molybdenum disulfide (MoS2) (Li et al., 2018) has attracted great attention because of its high ability for accepting electrons and protons. This material exhibits excellent corrosion resistance compared to other transition metal composites and similar electronic properties of Pt-group metals. In this study, we present hydrogen production by the direct electrochemical oxidation of a lignocellulosic biomass. The combined use of different techniques was used to characterize the synthesized nanomaterial. The hydrogen yield per weight of biomass was determined in a batch electrochemical cell. Scanning Electron DOI: 10.3303/CET2080026 Paper Received: 16 December 2019; Revised: 22 February 2020; Accepted: 19 April 2020 Please cite this article as: Sarno M., Ponticorvo E., 2020, Electrochemical Hydrogen Generation by Biomass Electrolysis with Pt_mos2 Electrode, Chemical Engineering Transactions, 80, 151-156 DOI:10.3303/CET2080026 151 Microscopy, Energy-dispersive X-ray spectroscopy, X-ray diffraction, and thermogravimetric analysis were employed for nanomaterials characterization. 2. Experimental Section 2.1 Nanocomposite synthesis Platinum (III) acetylacetonate (0.5 mmol) and ammonium tetrathiomolybdate (2 mmol) were loaded into the reagent mixture, consisting of 20 ml of 1-octadecene, oleic acid (6 mmol), oleylammine (6 mmol) and 1,2- hexadecandiol (10 mmol) as reducing agent. The temperature was increased to 200°C for 2 h and then the mixture was heated up to 285°C for 1 h, in an inert ambient (Sarno et al., 2018; Sarno et al., 2016). After synthesis, Pt/MoS2 nanocomposite obtained was purified alternating ethanol and hexane washing and separating by centrifugation (Sarno and Ponticorvo, 2019). To remove the organic content, thermal treatment at 150 °C for 8h followed. 2.2 Characterization methods Scanning electron microscopy (SEM) was performed through the use of a LEO 1525 electron microscope and an energy dispersive X-ray (EDX) probe. Thermogravimetric analysis (TG-DTG) was carried out by using an SDTQ 600 Analyzer (TA Instruments) with a heating rate of 10 °C/min, from ambient temperature up to 1000 °C under airflow (Ciambelli et al., 2004; Sarno et al., 2012). Electrochemical characterization was carried out by means of Autolab PGSTAT302N potentiostat/galvanostat. To obtained the electrodes, 4 mg of synthesized nanocomposites were dispersed into 80 μl of a 5 wt% Nafion solution, 200 μl of ethanol and 800 μL of distilled water. The solution was partly deposited dropwise onto a Screen Printed Electrode (SPE) consisting of a Pt working electrode, a Pt counter electrode and a Ag reference electrode. Biomass (wood sawdust, particles size >10 μm 13.5 %, 2.6-10 μm 19.2 %, 0.1-2.6 μm 24.1 %, <0.1 μm 43.2 %) electrochemical conversion was evaluated in 1 M NaOH solution with a biomass concentration of 10 g/L. The electrocatalyst performance was evaluated also in the absence of biomass. Nitrogen was supplied continuously to the cell. The working electrode potential was then held constant for three hours, after which the solution was collected and analyzed via FT-IR (Vertex 70 apparatus (Bruker Corporation) by applying KBr technique) and UV-vis spectroscopy (Thermo-Scientific UV–vis Evolution Q60 spectrophotometer) to evaluate the modification of the biomass structure via electrochemical oxidation. 3. Results and discussion 3.1 Nanocomposite characterization Figure 1: XRD spectrum of Pt_MoS2 NPs In Figure 1 The X-ray diffraction pattern of Pt_MoS2 nanocomposite is shown. In particular, the typical peaks of platinum are visible, i.e. peaks at 39.2° (111), 45.1° (200), 67.3° (220), 82.1° (311) can be detected. The Pt crystalline size measured by Scherrer’s equation was found to be 2.5 ± 0.3 nm (Sarno et al., 2014). Furthermore, a very weak MoS2 nanosheets peak was observed at 58.9°, corresponding to (110) plane (Sarno 20 40 60 80 In te ns it y (a .u .) Degrees 2-theta 152 and Ponticorvo, 2017). The absence of (002) plane and the broadness of peak indicate that the obtained MoS2 is a single layer or few-layer graphene-like MoS2. Figure 2: SEM image of Pt_MoS2 NPs Figure 3: EDS maps of Pt_MoS2 NPs SEM measurements (Figure 2) were performed to evaluate the layers’ size of MoS2. The nanocomposite resulted constituted of layers varying from few micrometers to about twenty micrometers. Energy-dispersive X- ray spectroscopy (EDS) mapping with distributions of Mo, S, and Pt are shown in Figure 3. In this EDS mapping, Pt is found in the entire area indicating that Pt NPs are present everywhere within the observed window, as expected. Moreover, Mo and S are distributed in a similar manner in the sample. Figure 4: TG-DTG profile of Pt_MoS2 NPs The prepared electrode was characterized by the help of thermogravimetric analysis. During the thermal conversion of Pt_MoS2 in airflow, a decomposition-oxidation of the oleylamine/oleic acid organic chains covering the nanocomposite happens together with an SO2 release. After the sulfur/impurities oxidation, in the 153 temperature range 207–400 °C, at ~400 °C the oxidation of MoS2 to MoO3 with SO2 release starts. At 700 °C, a residue of 54% was observed, which is due to Pt and residual, non-sublimated, MoO3. 3.2 Lignocellulosic biomass electrolysis Figure 5: Ciclic voltammetry on Pt_MoS2 electrode in 1 M NaOH at 20 mV/s of scan rate. Figure 6: Ciclic voltammetry on Pt_MoS2 electrode in 1 M NaOH and 10 g/L of lignocellulosic material at increasing scan rate (20-500 mV/s). The prepared electrode was firstly characterized by cyclic voltammetry (CV) in 1 M NaOH solution in the absence of biomass (Figure 5). During the CV test no evident oxidation peaks are visible (i.e. no electrode oxidation/passivation), on the other hand, hydrogen evolution reaction (HER) occurs at the low overpotential of 0.56 V, it should be noted that the voltage was not adjusted according to the standard hydrogen electrode, thus the corrected overpotential is below to 0.2 V. In Figure 6 CV measurements with a biomass concentration of 10 g/L, at increasing scan rates, are shown. Oxidation peaks are clearly visible, attributed to electrochemical oxidation of biomass or its oxidation products or active intermediates, mediated by •OH radicals leading to removal of an electron from the biopolymers or its active intermediates (Wang et al., 2015). Figure 6 show also increments of the anodic and cathodic peaks at increased scan rates, shifts in the anodic direction of the oxidation peaks and shifts in the cathodic direction of the reduction peaks upon increasing scan rate can be also noted, suggesting that the reactions of biomass on the Pt_MoS2 electrode are irreversible (Sarno and Ponticorvo, 2019; Gowda and Nandibewoor, 2014). -1,2 -0,8 -0,4 0,0 0,4 -0,006 -0,003 0,000 0,003 0,006 C u rr e n t (A ) Potential (V) -1,2 -0,8 -0,4 0,0 0,4 -0,006 -0,003 0,000 0,003 0,006 C u rr e n t (A ) Potential (V) scan rate 154 Figure 7: Variation of oxidation peak vs. scan rate. Diffusion control of charge transfer processes involving biomass is demonstrated by observing the behavior of the anodic peak as a function of scan rate, as shown in Figure 7. Figure 8: Linear sweep voltammograms on Pt_MoS2 electrode in 1 M NaOH solution at increasing concentrations of lignocellulosic material (concentration range: 2-10 g/L), scan rate: 20 mV/s. The electrochemical behavior of the Pt_MoS2 electrode was also studied under biomass concentration increases: linear sweep voltammograms (LSVs) recorded at the concentrations range 2-10 g/L are shown in Figure 8. The oxidation peak at ~0.2 V is observed, which becomes more defined and with higher peak currents as the biomass concentration increases, showing that this process is biomass concentration dependent, and that higher biomass concentrations, close to the electrode surface, can increase the rate of reaction, which is a characteristic behavior of several electrochemical processes. 4. Conclusions In summary, a simple and efficient strategy was applied to synthesize a Pt_MoS2 based electrode. XRD, SEM, EDS and TG-DTG analysis confirmed the formation of the aforementioned nanocomposite structure. The prepared nanocomposite has been tested as electrochemical catalyst in order to oxidize ligncellulosic biomass in aqueous solutions, to give pure H2 at the cathode, at lower potentials than required for oxygen evolution (~ 0.8 V in alkaline media). Our results indicate that lignin oxidation is irreversible, forming oxidation products that likely further react via homogenous chemical reactions or additional heterogeneous charge transfer steps. 0,0 0,3 0,6 0,001 0,002 0,003 0,004 C u rr e n t (A ) Scan Rate ν 1/2 (V/s) 1/2 0,0 0,2 0,4 0,00000 0,00008 0,00016 0,00024 C u rr e n t (A ) Potential (V) 2-10 g/L of lignocellulosic material 155 Importantly, no oxidation/reduction processes of the electrocatalyst occurred during biomass oxidation process. 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