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Energy harvest from organics degradation by twodimensional K+-intercalated manganese oxide Tianqi Li, Zhimi Hu, Xu Xiao, Huimin Yu, Liang Huang, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12160 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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ACS Applied Materials & Interfaces

Energy harvest from organics degradation by two-dimensional K+intercalated manganese oxide Tianqi Lia†, Zhimi Hua†, Xu Xiaoa†, Huimin Yua, Liang Huanga and Jun Zhoua* a

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, Hubei, P. R. China †

These authors contributed equally.

E-mail: [email protected]

Keywords: Energy harvest, two-dimensional material, MnO2, organic degradation, galvanic cell

Abstract Pollution treatment, the problem our world being deeply puzzled by, have required large amount of energy during enrichment and degradation. However, some pollutants, for an instance of organics in waste water, could offer energy but wasted. Here we report an energy harvesting galvanic cell, built by using a Pt foil as an anode and 2D K+-intercalated MnO2 as a cathode, which combines both dye degradation in waste water and the energy harvesting during the degradation process. Owing to the galvanic effect, this cell could accelerate the degradation rate and indicate the progress of degradation. Different kinds of organics could be degraded and produce energy in this cell with a stable open-circuit voltage (0.45 V). Magnification and imitation of this strategy offer a new chance to harvest the waste energy in other exothermic reaction.

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Introduction With the rapid consumption of fossil fuels, the energy shortage issue becomes one of the most urgent problems in modern society1-3. Additionally, the present conversion process of fossil-fuel energy to electricity such as thermal power generation is harmful to environment, and the commercial treatment of the environmental pollution requires extra energy which may aggravate the energy shortage2. For example, waste water treatment, one of the most important environmental treatments, accounts for amounts of energy consumption during enrichment and degradation4-6. However, the organics in waste water could offer energy which is wasted in form of heat during mineralization process7, 8. This kind of energy is a rich resource if we can collect it by developing cost-effective methods. Hence, it is of crucial importance to pursuit a new kind of energy harvesting system to handle the issues of both energy shortage and environmental pollution. The primary cell method with direct conversion from chemical energy to electric energy is thought to be a reasonable choice5, 6, 9. Based on this method, the energy wasted in water treatment could be harvested in form of electric energy by separating the oxidization of organics and oxygen reduction process10-13. Manganese oxide (MnO2) as a catalyst to mineralize organic waste water14-16 was displayed by following equation (1): MnO 2

Dye + O2 →CO2 + H2O

(1)

where the dye could be Congo red, Methyl blue, Acid red and other azo dyes. Without introducing H2O2 (free radical reaction) or ultraviolet lighting (photocatalytic reaction), the degradation of dyes is supposed to be a three-phase (MnO2/dye/O2) redox process on the surface of MnO2. The whole reaction could be simply separated to an anodic reaction (oxidation of dye) and a cathodic reaction (reduction of O2). Generally, the reduction of O2 can be expressed as17: H+ + O2+ e- → H2O EΘO2= 1.23 V vs. SHE

(2)

where EΘO2 is standard electrode potential of O2 (1.23 V vs. SHE). Owing to the high oxidizing property of MnO2 as following18: H+ + MnO2 +e- ⇄ Mn2+(surf) + H2O EΘMnO2= 1.229 V vs. SHE

(3)

the standard electrode potential of MnO2 (EΘMnO2) is 1.229 V vs. SHE, which is close to the EΘO2. Considering the overpotential (limitation of kinetics), the electrode potential of cathodic reaction should be lower than standard electrode potential and the electrode potential of anode should be higher than standard electrode potential. Consequently, MnO2 could be only used as


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a cathode electrode, otherwise the open-circuit voltage is lower than 0.001 V. To form the whole electronic circuit, the anodic reaction could be expressed as: Dye + H2O – e- → CO2 + H+

(4)

This reaction would occur on the surface of inert electrode through catalytic oxidation effect. Platinum (Pt) as an electron donor with high stability is often used as a catalyst for catalytic oxidation reaction19-21. Accordingly, the organics could be oxidized on the Pt anode via the above anodic reaction. Herein, we report a novel galvanic cell (GC) that combines both dye degradation in waste water and the energy harvesting during the degradation process. Based on the primary battery principle between MnO2 cathode and Pt anode, we obtain a stable open-circuit voltage and long-lasting short-circuit current from GC. With a means for electron migration whereby electrons move from the anode to the cathode through the external circuit, GC leads to the transfer from chemical energy to electric energy and thus could promote the degradation of dye. Experimental Section Synthesis of the 2D MnO2: In total, 5 g KNO3 powder was added into the crucible and transferred to the muffle furnace with a temperature of 380 °C for about 10 minutes. As the nitrate became the molten solution, 0.2 g manganese sulfate (MnSO4) power was added into the molten salt for 1 minute. Then, the product was moved out from muffle furnace and cooled to room temperature under ambient condition. Finally, the product was washed by DI water. Preparation of electrodes. 5 mg 2D MnO2 were dispersed in a solution of 800 µL isopropyl alcohol, 200 µL water and 20 µL 5 wt.% Nafion and dropped on a glassy carbon electrode (φ=3 mm) to achieve a loading of 250 µg/cm2. Characterization. All the electrochemical measurements were evaluated by CHI 660E electrochemical workstation in dark. The microstructural properties of electrode materials were characterized by X-ray diffraction using the Cu Kα radiation (λ = 1.5418 Å) (XRD, Philips X’ Pert Pro), field-emission SEM (FE-SEM, FEI Nova 450 Nano), TEM (HRTEM, TECNAI, Titan) and X-ray photoelectron spectroscope (XPS, AXIS-ULTRA DLD-600W). N2 adsorption-desorption isotherms were performed on a Micrometrics ASAP 2000. Absorption spectra were recorded with a UV-vis spectrophotometer (UV2550, SHIMADZU). The Total organic carbon (TOC) concentration in reaction solution was determined using a 3 Environment ACS Paragon Plus


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TOC analyser (TOC-VE, SHIMADZU). The concentrations of Congo red, methyl blue, acid red and their mixed dyes were determined with a UV-vis spectrophotometer, the wavelength range from 400 nm to 800 nm. Results and Discussion A typical GC was built in Congo red acid solution by using a Pt foil as an anode and the MnO2 as a cathode (Figure 1a, pH value ranging from 2 to 7). The MnO2 dispersion was dropped on the glassy carbon electrode mixed with 5% wt. Nafion as binder. By using molten salts method22, we have obtained two-dimensional (2D) K+-ion-intercalation MnO2 as shown in transmission electron microscope (TEM) and scanning electron microscope (SEM) images (Figure 1b and Figure S1). According to the selected area electron diffraction (SAED) images (inset, Figure 1b), the 2D MnO2 (K0.27MnO2 0.54H2O) is polycrystalline. The 2D MnO2 performed a high specific surface area of 117 m2/g (Figure S2a). According to X-ray diffraction pattern (Figure S3), the crystal structure of 2D MnO2 is the rhombohedral phase (K0.27MnO2∙0.54H2O, JCPDS Card no. 52-0556). The K ion could intercalate into the MnO2 layer to stabilize the 2D structure. Moreover, the as-synthesized 2D oxides exhibited high oxidizing property based on the multivalences of 2D MnO2 surface (Figure S4). Owing to the high catalytic property of MnO2 in acid solution23, the Congo red could be directly degraded under the catalysis of MnO2 in a simple redox process (Equation (1)) without GC (Figure 1c red curve, Figure S5a). However, for our GC, we find that the electrons could transfer much more easily with the assistant of external circuit by introducing the Pt foil to export the electrons, enabling the higher degradation rate of Congo red (Figure 1c black curve, Figure S5b). The degradation rate of Congo red in GC is close to the degradation rate by dispersed MnO2 sheets (Figure S6). The circuit load also influences the degradation rate of CR (Figure S7). With lots of hydrogen ions adsorbed on the surface, the electrode potential of 2D MnO2 (~ 0.9 V vs. Ag/AgCl) is much higher than Pt foil (~ 0.45 V vs. Ag/AgCl). Accordingly, as a primary battery, the open-circuit voltage of GC is 0.45 V that corresponding to the electrode potential difference between MnO2 and Pt, and was stable for more than 15 h (Figure 1d, black curve), which displays the stable catalytic effect of Pt and MnO2 in the acid Congo red solution instead of being etched by acid. The short-circuit current of GC kept falling off from ~ 1.6 µA to ~ 0.1 µA during more than 15 h measurement with low concentration Congo red solution (Figure 1d, red curve square symbol). However, we could obtain a long-lasting current in high concentration Congo red solution (Figure 1d, red curve triangular symbol), and inferred that the decline of current in low concentration is 4 Environment ACS Paragon Plus

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owing to the degradation of organics. All of the above results demonstrated that we could harvest the waste energy from the dye degradation process via GC. The effects on the performance of GC are summarized in Figure 2. In order to accurate study, all the measurements are conducted on low mass electrode materials (18 µg MnO2) in large volume electrolyte (12 mL 30 mg/L Congo red solution), which could assure the reaction rate is as slow as quasi-steady state and beneficial to the analysis. The output performances of GC including voltage (black curve, Figure 2a), current (red curve, Figure 2a) and power (Figure 2b) are measured for different load resistances, which indicated maximal short-circuit current of 20 µA/cm2 and open-circuit voltage of 0.45 V. The maximal output power is 1.5 µW/cm2 when the load resistance is ~ 3×104 Ω that equals to the internal resistance of GC. Figure 2c shows the effects of pH on the performance of GC. The electrode potential of MnO2 increases with the decrease of pH values (Figure 2a, black curve square symbol) owing to the higher adsorption rate of hydrogen ions on the surface of MnO2 in acid solution (H+ + MnO2 ⇄ MnOOH+(surf)). Meanwhile, the same adsorption/desorption rate of hydrogen ions on Pt surface24, 25 leads to the immovable electrode potential in solution with different pH values. Hence we could infer that the H+ ions mainly act on the MnO2 and then affect the short-circuit current of GC. The red curve in Figure 2c demonstrated that the short-circuit current of GC increased by adding acid (Figure S8). Generally, the lower pH value could offer more favorable adsorption of many compounds to the MnO2 surface. Moreover, the surface charge density of MnO2 increases along with the hydrogen ion concentration, resulting in lower over potential and higher reaction rate of oxygen reduction. For the sake of enhanced adsorption and higher catalytic activity of MnO2 for oxygen reduction, the electrons could transfer more easily at lower pH value, leading to higher short-circuit current of GC. The kinetics of GC could be inferred as ion diffusion-limited instead of surface-limited from Figure 2d and 2e. It is widely known that stirring could improve ion diffusion efficiently, so that the short-circuit current would change between stirring on and off (Figure 2d, red curve). Without Congo red, the short-circuit current of GC is close to 0 even though the GC is stirred continuously, which excluded the possibility that the energy came from stirring or other energy source (Figure 2d, black curve). The ion diffusion benefits not only from stirring but also from high concentration. When tuning the concentration of Congo red in solution from 0 mg/L to 35 mg/L with an increment of 5 mg/L by adding the high concentration Congo red, short-circuit current changed from ~ 0 to 1.7 µA step by step as shown in Figure 2e. Excluding the polarization current in 0 mg/L Congo red solution, the current is linearly 5 Environment ACS Paragon Plus


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related to the concentration of Congo red solution (Figure S9), which indicates the progress of degradation. In addition, we could also harvest energy in GC from other organic waste such as methyl blue and acid red (Figure 2f), which is owing to the degradation of these organics belongs to an exothermic catalytic oxidation process. As mentioned in Figure 2c, the open-circuit voltage and electrode potential are mainly based on the pH value, hence the potentials of MnO2 and Pt foil in different solutions are similar (Figure S10). However, short-circuit current are not identical in different dye solutions. The functional groups on organic dyes such as –NH2, N=N- and -NH- could be oxidized much more easily than ring-opening reaction26. With less content of such functional groups, the current in acid red is lower than others. In order to simulate real waste water, we mixed three kinds of dyes in solution. The simulated pollutants containing Congo red, methyl blue and acid red with same concentration could offer maximum current of ~ 0.9 µA (Figure 2f, blue curve) and be degraded at same time (Figure S11). We next discuss the mechanism of GC which includes substance participating in the reaction and the electrochemical processes on each electrode (Figure 3). Based on the whole MnO 2

reaction Dye + O2 →CO2 + H2O , we designed three steps to reveal the importance of O2 (Figure 3b). Step 1: we inputted N2 to remove the dissolved oxygen in solution, and the shortcircuit current was low without O2 involving into reaction. Step 2: after we turned to input O2 instead of N2, short-circuit current rose up soon. The sudden change of current could be explained that O2 dissolving rate is rapid and the sensitivity of MnO2 in reaction is high. Step 3: we removed the O2 and turned to input N2 into GC. The reaction could continue a while by using the dissolved oxygen in solution, then short-circuit current lowered down in several minutes. The degradation process of Congo red was consistent with the short-circuit current, which occurs apparently at step 2 with feeding O2 (Figure S12), demonstrating the degradation and energy harvesting took place simultaneously. In order to elucidate the half-reaction on each electrode, the three-electrode electrochemical measurements were performed, and the Pt foil or MnO2 was used as working electrode with Pt foil as counter electrode and Ag/AgCl electrode as reference (Figure 3c and d). There is a clear anodic peak in the linear sweep voltammetry (LSV) measurement on Pt electrode (working electrode) after adding Congo red (Figure 3c, red curve), which illustrated that the intermediate products were oxidative products of Congo red on the anode (Figure 3a, left part). Without Congo red, the Pt foil only performed a double electrode layer behavior in 6 Environment ACS Paragon Plus

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LSV measurement (Figure 3c, black curve). Analogous to the catalytic behavior of oxygen reduction reaction, MnO2 showed poor performance in acid solution (without Congo red) with large mixed kinetic- and diffusion-controlled zone27 (Figure 3d, black curve). Intriguingly, a cathodic peak appeared after adding Congo red (Figure 3d, red curve), and the onset potential didn’t change. We could infer that the Congo red or intermediate products introduce an extra reaction into the cathode electrode (Figure 3a, right part). For an instance, the intermediate MnO2

products could be reduced on the surface of MnO2 (e.g. –NO2 + e- + H+→ N2 + H2O and – MnO2

COOH + e- + H+→ CO2 + H2O ). Generally, the waste water could be decolorized through azo bond ruptured or auxochrome group oxidized. An ideal catalyst should not only decolorize waste water but also mineralize it, which could be indicted by total organic carbon (TOC) (Figure 3e). The degradation degree calculated from TOC (Figure 3e) is close to that calculated from UV-vis absorption spectrum (Figure 1c) which illustrates the degradation product in GC could be mineralized. The output of the devices can be scaled up simply through series connections of multiple devices or enlarge the area and mass loading. Connecting 5 GCs in series, we could get a stable ~ 2 V open-circuit voltage and a long-lasting short-circuit current changing from 1.7 µA to 0.8 µA during 3600 s measurement (Figure 4a). A 100 µF capacitor could also be charged to ~ 1.6 V by 5 GCs in series (Figure 4b). We also fabricated a large area carbon fabric electrode with MnO2 through dip coating method, in order to obtain the indication at progress of high-rate degradation. With large area (10 cm2) and mass loading (2.5 mg), shortcircuit current of GC is about 140 µA in 30 mg/L Congo red solution at the beginning of measurement (Figure S13a). Owing to the high rate degradation of Congo red, the degradation process finished in 5 minutes corresponding to the UV-vis spectrum (Figure S13b) and the current lower down close to ~ 0. The energy harvesting cell reported here provides routes to water treatment and energy harvesting. By introducing the Pt foil, we obtain a new galvanic cell that collecting electrons from exothermic reaction based on the catalysis of 2D MnO2. Owing to the galvanic effect, the degradation rate could be higher in the galvanic cell. From this cell, we could obtain a stable ~ 0.45 V open-circuit voltage and a ~ 20 µA/cm2 short-circuit current during degradation process of organic dye (30 mg/L). The GC is mainly limited by pH values, concentration of Congo red and O2. In detail, the lower pH value brings out higher activity of MnO2 and higher short-circuit current of GC. As reactants, both Congo red and O2 are



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necessary in the galvanic cell. Especially, the short-circuit current is linearly related to the concentration of Congo red, which could indicate the progress of degradation. Conclusion In conclusion, we have shown an energy harvesting galvanic cell based on 2D K+intercalated MnO2 with enhanced degradation of organic dye and harvesting energy in degradation process. This cell is built with the oxygen reduction on the MnO2 cathode electrode and the organics oxidization on the Pt anode electrode. Magnification of our cells according to the scaling laws as we have described is one of the main goals for future research. This ability to manipulate chemical reactions with introducing another half reaction offers a new chance to harvest the waste energy in other exothermic reaction field. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51322210, 51672097, 61434001, and 51602115), the National Program for Support of Topnotch Young Professionals and Director Fund of WNLO. We wish to thank Dr. L. Shen and Professor Y. Wang from Huazhong University of Science and Technology (HUST) for their help in TOC measurement. We also thank Prof. Y. Gogotsi from Drexel University for inspiration discussion. In addition, we wish to thank facility support from the Center for Nanoscale Characterization & Devices, WNLO of HUST and the Analytical and Testing Center of HUST. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. Characterization of 2D MnO2; UV-Vis absorption spectra of organics during degradation; other influence factors of degradation rate; large area electrodes of 2D MnO2 for GC. References (1) Boden, T. A.; Marland, G.; Andres, R. J. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S.

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Figure 1 | The energy harvesting galvanic cell based on the water treatment. a) Scheme of the galvanic cell. The 2D MnO2 mixed with 5% wt. Nafion was drop on the Φ3 mm carbon glass. The waste water included 30 mg/L Congo red was degraded between Pt sheet and 2D MnO2 with inputting O2. b) TEM image of 2D MnO2. c) The degradation rate of CR in GC or without Pt foil. The introduction of Pt foil could enhance the degradation rate. d) Open-circuit voltage (black curve) of the cell is maintained at ~ 0.45 V for more than 15 h. Short-circuit current (red curve square symbol) of GC in 30 mg/L Congo red solution from 1.6 µA to 0.1 µA corresponding to the degradation of Congo red. A long-lasting current could be obtained in high concentration Congo (300 mg/L) red solution (red curve triangular symbol).


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Figure 2 | The effect on the performance characteristics of the galvanic cell. a, b) Output voltage (black curve, a), current (red curve, a) and power (blue curve, b) of energy harvesting cell measured for different load resistances. Measurements were made ~ 10 min after introduction of Congo red. The plots indicate the internal resistance of the cell is ~ 3×104 Ω. c, the electrode potential (black curve) and short-circuit current (red curve) measured for different pH value. The potential of MnO2 changed with different pH value, and the potential of Pt sheet was stable in different pH solution. The short-circuit current of cell was increased by adding acid. d, the short-circuit current with stirring on and off (red curve). e, the shortcircuit current with adding Congo red into solution. The measurement was made after shorting two electrodes ~ 30 min. The high concentration Congo red was added into solution step by step resulting the concentration from 0 mg/L to 35 mg/L. f), the short-circuit current of cell in different solution (30 mg/L). The Mix dyes contented 10 mg/L Congo red, acid red and methyl blue.



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Figure 3 | Mechanism of the galvanic cell. (a) Processes involved in energy harvesting cell. The Congo red was oxidized on Pt sheet and the O2 was reduced on the MnO2. (b) ShortMnO2

circuit current inputting N2 or O2. Based on the whole reaction Dye+O2 ሱۛۛሮCO2 +H2 O, the short-circuit current is close to 0 with inputting N2 to remove O2. (c) and (d) The linear sweep voltammetry (LSV) of Pt sheet and MnO2 with or without Congo red at 20 mV/s. The Congo red was oxidized at about 0.65 V vs. Ag/AgCl on Pt foil. Without Congo red (d, black curve), MnO2 shows simple oxygen reduction reaction performance. The Congo red and intermediate products [X] could help the reduction of O2 on MnO2 at ~ 0.6 V vs. Ag/AgCl. (e) The total organic carbon after reaction. The final degradation product of organic carbon should be CO2.


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Figure 4 | The energy harvesting galvanic cell as an energy source or degradation indication. a) Open-circuit voltage (black curve) and short-circuit current (red curve) of 5 cells in series. Because of O2’s expending and organics’ degrading, the current fell off during testing. Inset: the open-circuit voltage measured by voltmeter. b) The charging curve of capacitor (100 µF) from 5 cells in series.



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Energy Harvest from Organics Degradation by Two ... - ACS Publications

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