High-Purity V2O5 Nanosheets Synthesized from Gasification Waste

Jun 13, 2019 - Since other metal ions may also get rinsed out during the extraction ..... The autoclave was held at 180 °C for 6 h and cooled down na...
0 downloads 0 Views 6MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12474−12484

pubs.acs.org/journal/ascecg

High-Purity V2O5 Nanosheets Synthesized from Gasification Waste: Flexible Energy Storage Devices and Environmental Assessment He Li,† Hailin Tian,‡ Ting-Hsiang Chang,† Jianyi Zhang,† Shin Nuo Koh,† Xiaonan Wang,†,‡ Chi-Hwa Wang,†,‡ and Po-Yen Chen*,† †

Downloaded via UNIV OF GOTHENBURG on August 5, 2019 at 02:23:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore ‡ Environmental Research Institute, National University of Singapore, 1 Create Way, Singapore 138602, Singapore S Supporting Information *

ABSTRACT: Gasification waste, also known as carbon soot, is solid industrial waste from the bottom residual of an oil refinery and contains a substantial amount of toxic vanadium. In this work, we report an environmentally responsible pathway to harvest toxic vanadium from gasification waste, and the extracted vanadium can be utilized to synthesize high-purity V2O5 nanosheets for the fabrication of flexible, bendable, efficient supercapacitors. The carbonaceous waste was first rinsed with alkaline solution to leach out toxic vanadium. The vanadium-rich leachate was next utilized to synthesize high-quality V2O5 crystals with comparable purity (>98%) and crystallinity to commercial products. Two-dimensional V2O5 nanosheets were further crystallized by hydrothermal treatment for the fabrication of high-performance electrochemical electrodes. The V2O5 electrodes derived from gasification waste demonstrated similar specific capacitance (172 F g−1) to those from commercial V2O5 (173 F g−1). The waste-derived V2O5 nanosheets were further mixed with leached carbon nanoparticles for the fabrication of a symmetric, bendable, and flexible supercapacitor. The waste-derived V2O5 supercapacitor was able to be bent up to 160° and retained its specific capacitance. An environmental impact assessment was finally conducted to evaluate the environmental impacts of producing V2O5 crystals from gasification waste (in terms of the damage to human health, ecosystem diversity, and resource availability). The waste-derived approach was compared with traditional mining processes and showed a large improvement in all three endpoint damage categories. KEYWORDS: Waste management, Two-dimensional V2O5 nanosheets, Electrochemical energy storage, Flexible supercapacitors, Environmental impact assessment

■

INTRODUCTION Vanadium has become one of the widely used materials in electrochemical and electrochromic technologies due to its various oxidation states, especially in smart windows,1 supercapacitors,2−5 catalysts,6 lithium ion batteries,7,8 and flow batteries.9 In 2012, the global trade of vanadium products was around 81 thousand metric tons, and the vanadium demand is expected to increase due to the growing need of energy storage devices.10 The high-purity vanadium products, especially vanadium pentoxide (V2O5), show great promise as earth-abundant and cost-effective materials for the fabrication of high-performance electrochemical electrodes.11−14 However, there are health and environmental concerns regarding the current mining and refining procedures of vanadium ore for the synthesis of high-purity V2O5.15−18 In industry, “stone coal” is a term for the ore containing various metal oxides, and “vanadium ore” here represents the mineral with high vanadium concentration. A more specific term for a highconcentration vanadium mineral in stone coal is “roscoelite”. In general, roscoelite contains ca. 20% of carbonaceous © 2019 American Chemical Society

components, so a decarbonization and roasting process is normally required to effectively increase vanadium concentration and to turn all of the vanadium into V(V). The vanadium extraction from raw ore involves high-temperature roasting (decarbonization and oxidation at 850 °C) and multistage solvent extraction (e.g., acid rinsing, di(2-ethylhexyl) phosphate (D2EHPA), and tri-n-butyl phosphate (TBP) extraction).19 The processes generate greenhouse gases and discharge the wastewater rich in vanadium, aluminum, iron, and other heavy metal elements.20 High-cost industrial facilities are required to capture toxic dust and prevent the emission of harmful toxicants into the environment. If not managed properly, these toxic materials may induce serious genotoxicity,21−23 teratogenic effects,21 neurotoxicity,24 and respiratory toxicity to human health and ecosystems.25 Therefore, alternative vanadium sources that Received: April 12, 2019 Revised: June 7, 2019 Published: June 13, 2019 12474

DOI: 10.1021/acssuschemeng.9b02066 ACS Sustainable Chem. Eng. 2019, 7, 12474−12484

Research Article

ACS Sustainable Chemistry & Engineering are abundant, inexpensive, and greener would be highly required for the synthesis of high-purity vanadium compounds, ensuring the future manufacture of high-performance electrochemical devices in an environmentally responsible behavior. The vanadium composition in fossil fuel makes the residual of oil refinery a readily available vanadium resource. Because of the increasing demand for worldwide energy consumption, more petroleum is consumed in oil refinery plants, producing a large amount of carbonaceous waste that contains toxic vanadium compounds. According to the BP Statistical Review of World Energy 2018, Singapore consumed in excess of one million barrels of crude oil per day.26 After continuous distillation of crude oil, petroleum coke was sent to gasification plants to generate syngas. More than 30 tons of carbonaceous bottom ash in gasification plants (mainly carbon soot) are generated per day on Jurong Island, Singapore, and the carbon soot contains 0.8−1.5 wt % of vanadium. The current treatment is to combust the carbonaceous bottom ash, and the residual fly ash is then buried in a landfill; however, the combustion−landfill treatment produces a large amount of CO2 and risks the contamination of soil and water resources. The toxic vanadium in carbonaceous waste can be utilized as an alternative resource for the synthesis of high-value vanadium products. The benefits of reprocessing the carbon soot are 2-fold. First, the large quantity of toxic oil-refinery waste can be managed in an economic and environmentally responsible manner. Second, the extracted vanadium provides a readily available source for the synthesis of high-purity vanadium compounds without undergoing traditional mining processes that are environmentally harmful. Herein, we report an environmentally responsible pathway to extract toxic vanadium from industrial carbonaceous waste, and the extracted vanadium can be utilized to synthesize highpurity V2O5 nanosheets for the fabrication of flexible, bendable, and efficient supercapacitors. The environmental impacts of producing V2O5 crystals from gasification waste were evaluated through life cycle assessment using ReCiPe 2016 methodology in terms of three endpoint damage categories to emphasize its environmental significance.

Figure 1. Comparison of the process to synthesize V2O5 from vanadium ore and from gasification waste. (a) Commercial V2O5 from Sigma-Aldrich. (b) Waste-derived V2O5.

precipitate ammonium vanadate (NH4VO3), which can be thermally decomposed into V2O5 at 550 °C. Compared to the traditional V2O5 production, we proposed an alternative approach to extracting toxic vanadium from industrial waste for the synthesis of high-quality V2O5 crystals. Instead of following the combustion−landfill process, the carbon soot from gasification waste can serve as an alternative vanadium source. The waste-derived approach demonstrates a more straightforward pathway to minimize the emission of greenhouse gases and reduce the amount of industrial waste as well as avoid mining ore. The detailed synthesis of V2O5 crystals from gasification waste includes three major steps: (1) leaching out vanadium from carbon soot, (2) synthesizing NH4VO3 from the vanadium-rich leachate, and (3) converting NH4VO3 into V2O5 by undergoing high-temperature decomposition. First, we selected alkaline solution (here we used 1.0 M NaOH) to rinse vanadium out of the carbonaceous waste, due to the fact that vanadium is amphoteric and can dissolve in alkaline solution, yet other metallic components (e.g., Ni, Fe) stay insoluble (Table S1). The solid-to-solution ratio was kept at ca. 0.1 g mL−1. We further utilized a filter press to squeeze out the leachate, and the leachate was recycled to rinse multiple batches of carbon soot. As shown in Figure S1, the inductively coupled plasma atomic emission spectrometry (ICP-AES) results demonstrated that the leachate after 6 batches exhibited the vanadium concentration >5300 ppm and contained a trace of other metal ions (i.e., Ni, Fe). Next, the vanadium-rich leachate was mixed with NH4Cl in excess to obtain white powder precipitate. The X-ray powder diffraction (XRD) analysis in Figure S2 confirmed that the white precipitate was NH4VO3, an essential precursor for V2O5

■

RESULTS AND DISCUSSION Figure 1 demonstrates the flow diagram of two synthesis pathways for high-purity V2O5 (>98.0%), one of the widely used vanadium products in electrochemical energy storage applications. The conventional production begins with the mining of vanadium ore followed by high-temperature oxidation and multistage solvent extraction. The harvested ore contains roscoelite (vanadium oxidation states at III),18 so a roasting step is required for the synthesis of vanadium (V) products (e.g., V2O5). The roasting process at 850 °C may lead to the emission of toxic fumes (e.g., vanadium dust) and other air pollutants (e.g., sulfur oxides).18 The roasted products are further rinsed with multiple solvents to dissolve metal oxides and extract the vanadium (V) ions. In traditional processes, sulfuric acid (H2SO4) or organic solvents were used to extract vanadium from the roasted ore. Since other metal ions may also get rinsed out during the extraction processes, additional extraction processes are normally required by adjusting the pH value of leachate to separate vanadium exclusively. These multistage extraction processes generate large quantities of wastewater containing heavy metals.27−30 The final-stage solution containing vanadium ions is finally mixed with ammonium solution (e.g., ammonium chloride (NH4Cl)) to 12475

DOI: 10.1021/acssuschemeng.9b02066 ACS Sustainable Chem. Eng. 2019, 7, 12474−12484

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. (a) XRD patterns of w-V2O5 and c-V2O5. (b),(c) SEM image of w-V2O5. (d) XRD patterns of c- and w-V2O5 nanosheets. (e),(f) SEM images of w-V2O5 nanosheets. (g) TEM image of w-V2O5 nanosheets. (h) XPS spectrum of w-V2O5 nanosheets.

We further converted bulk V2O5 crystals into 2D V2O5 nanosheets via a simple hydrothermal treatment, and the ultrathin V2O5 nanosheets possess higher surface area for electrochemical energy storage applications. Both bulk w- and c-V2O5 crystals were first dispersed in 20% hydrogen peroxide (H2O2) solution. The V2O5 dispersion then underwent hydrothermal conversion at 180 °C for 6 h,32 and the bulk V2O5 was gradually transformed into ultrathin 2D nanosheets.33 The V2O5 nanosheets synthesized from two sources exhibited nearly identical XRD patterns with the representative peaks of (001), (003), (004), and (005) diffraction planes in accordance to the orthorhombic structure of V2O5 (JCPDS #40−1296) (Figure 2d).33 The SEM and transmission electron microscopy (TEM) images of w-V2O5 nanosheets were shown in Figure 2e−g, and the lateral dimension of V2O5 nanosheets was several hundred microns (inset of Figure 2f). The oxidation state of w-V2O5 nanosheets was confirmed by the XPS measurement (Figure 2h). The V 2p3/2 peak at 517.1 eV represented the exclusive presence of V(V),34 and the peak at 530 eV corresponded to the O 1s peak of V2O5.35,36 The Fourier-transform infrared spectroscopy (FTIR) spectra of w-

production. The NH4VO3 powders were further annealed at 550 °C for 2 h and decomposed into orange V2O5: 2NH4VO3 → V2O5 + 2NH3 + H 2O

(1)

The yield of waste-derived V2O5 (abbreviated as w-V2O5) was calculated to be 98.6%, which was similar to the reported value (98.5%) in the literature.18 The crystallinity of w-V2O5 was characterized by XRD and compared with commercial V2O5 product (abbreviated as c-V2O5) purchased from SigmaAldrich (#223794, >98%). As shown in Figure 2a, both wV2O5 and c-V2O5 exhibited the representative peaks of (001), (110), (101), (400), (310), and (411) diffraction planes in accordance to V2O5 (JCPDS #41−1426). As shown in the SEM images in Figure 2b,c, the w-V2O5 crystals presented the rod-like structures with an average length at 7 μm and an average diameter at 1 μm.31 We conducted ICP-AES analysis on w-V2O5, and the vanadium composition was found to be 55.0 wt %, close to the reported numbers (54.9−57.1 wt %) in the datasheet of c-V2O5. The purity of w-V2O5 was calculated to be around 98.2%. 12476

DOI: 10.1021/acssuschemeng.9b02066 ACS Sustainable Chem. Eng. 2019, 7, 12474−12484

Research Article

ACS Sustainable Chemistry & Engineering and c-V2O5 nanosheets were almost identical, and both nanosheets exhibited the peaks representing VO and V− O−V vibrations (Figure S3).37 The synthesis of w-V2O5 crystals was successfully demonstrated in different batches of carbon soot (Figure S4). After the toxic vanadium was leached out, the carbon soot was rinsed again by 1.0 M H2SO4 to remove the rest of metallic components (e.g., Fe, Ni). The result of XRF elemental analysis in Table 1 demonstrated that the carbon soot before

In Figure 3e, the Raman spectrum of leached carbon nanoparticles exhibited the characteristic bands at 1345 (D band) and 1585 cm−1 (G band),39,40 and the ratio of D and G band intensities was close to 1, implying the medium degree of graphitization of leached carbon nanoparticles. We expect the leached carbon nanoparticles are beneficial for multiple technologies, including conductive fillers, environmental sorbents, and light-to-heat converters, as the relevant works are ongoing. To demonstrate the potential applications of w-V2O5 nanosheets in electrochemical electrodes, the w-V2O5 nanosheets were mixed with conductive Super P and Nafion. The mixture was deposited on carbon papers for the fabrication of electrochemical electrodes. The electrode composed of wV2O5 nanosheets and Super P (abbreviated as w-V2O5-p) was used as a working electrode in a three-electrode system (with a Pt counter electrode, a Ag/AgCl reference electrode, and 0.5 M K2SO4 as electrolyte). A potential window (0.0−0.8 V) was chosen to avoid the dissolution of V2O5 nanosheets and the oxidation of water during the measurement.41 As a control, we also fabricated the electrode with c-V2O5 nanosheets and Super P (abbreviated as c-V2O5-p) as a working electrode. The cyclic voltammetry (CV) curves of w-V2O5-p and c-V2O5-p electrodes were compared in Figure 4a. Two CV curves were overlapped and exhibited two identical pairs of redox peaks at ∼0.25 V and ∼0.45 V representing the reversible two-step intercalation and deintercalation of K+ into/out of V2O5, respectively42

Table 1. Elemental Analysis of Carbon Soot before and after Leaching CHNS Elemental Analysis (wt %) C

H

N

S