Carbon Paper Composite

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Development of a Nanostructured #-MnO2/Carbon Paper Composite for Removal of Ni2+ / Mn2+ ions by Electrosorption Pengju Li, Yang Gui, and Daniel John Blackwood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02471 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

Development of a Nanostructured α-MnO2/Carbon Paper Composite for Removal of Ni2+ / Mn2+ ions by Electrosorption Pengju Li, Yang Gui, Daniel John Blackwood∗ Department of Materials Science and Engineering, National University of Singapore, Singapore 117574.

*Email: [email protected]

ABSTRACT Toxic metal ions, such as Ni2+ and Mn2+, in industrial waste streams are non-biodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than chemical / biological approaches without the high pressures needed for membranes. Here nanoneedle structured α-MnO2/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electro-adsorption and capacitive deionization removal of nickel and manganese ions from pseudo industrial waste streams. The specific performance of α-MnO2/CFP (16.4 mg Ni2+ per gram of active material) not only shows a great improve in comparison with its original CFP substrate (0.034 Ni2+ mg per gram), but is over six times that of activated carbon (2.5 mg Ni2+ per gram). The high performance of α-MnO2/CFP composite is attributed to its high surface area, desirable mesoporosity and pore size distribution that permits the further access of ions, and the property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn2+ ions are released due to partial reduction of the MnO2 when operated as a negative electrode. For the removal of Mn2+ ions an asymmetric arrangement, consisting of a MnO2/CFP positive electrode and an activated carbon negative electrode was employed. This arrangement reduced the Mn2+ concentration from 100 ppm to less than 2 ppm, a vast improvement over a systematical two activated carbon electrodes system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO2/CFP composite was maintained as a positive electrode it was completely stable. The technique was able to reduce both Ni2+ and Mn2+ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore.

Keywords: α-MnO2, Carbon Fiber Paper, Activated Carbon, Electrosorption, Nickel and Manganese Ions Removal

∗

Corresponding Author Email Address: [email protected] (D.J. Blackwood). 1

ACS Paragon Plus Environment

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1. INTRODUCTION The challenge of water security and environment pollution has becoming increasingly significant due to industrial contaminants and rapid population growth1,2. Toxic heavy metal ions in industrial waste streams, such as lead, nickel, cadmium and chromium, are non-biodegradable and can cause damage to the human body3. For example, nickel ions can cause gastrointestinal distress, pulmonary fibrosis, skin dermatitis, allergic sensitization and so on4-6. The major sources of nickel ion pollution in aqueous system come from galvanization, electroplating, mining and battery manufacturing7,8. Likewise, at high levels manganese has been associated with disorders of the nervous system, such as Parkinson’s and Huntington’s diseases9. The largest risk appears to be to welders or workers in steel production, who inhale of dust or fumes. However, Mn2+ ions easily bind in replacement of Ca2+/Mg2+ and ferrous/ferric ions and are known to inhibit ATP production within mitochondria10. In Singapore, the maximum levels of both metals that can be in trade effluent discharged into public sewers is 10 ppm.

To address the problem, several measures have been undertaken to remove the nickel and/or manganese ions such as adsorption11, chemical precipitation12, ion exchange13, nanofiltration14 and electrosorption15. Currently, electrochemical removal, in terms of capacitive deionization (CDI) and electrosorption, has aroused considerable attention for its competitive advantages such as low cost, low safe voltages and ease of control without generating large sludge volumes of biological approaches nor the application of high pressures needed with membranes16. Several electrode materials have previously been synthesized and investigated for CDI applications, such as carbon-based electrodes17 and metal oxide/carbon composites18,19. Although MnO2 is a favorable 2


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electrode material in supercapacitors due to its similarity and compatibility with carbon material, high capacitance, low price and easy preparation, the previous work using it for electrosorption mainly focused on removal of NaCl from saline water20-22, with few, if any, reported on MnO2/carbon materials applied to the electrochemical removal of nickel or manganese ions. Likewise, various MnO2 nanostructures and nanocomposites have been developed for the removal of heavy metal ions by direct chemical/physical adsorption23-25, but this technique lacks the operational control offered by electro-adsorption. However, porous cubic Fe–Mn oxide structures with high surface areas, with the Mn in a high valance state, have been shown to be a very efficient adsorbent material for the rapid removal of arsenic ions.26

Carbon based nanocomposites are also finding a range of applications in other fields. For example, Yin et al.27 developed Au cluster / reduced graphene oxide hybrids for catalysts for the oxygen reduction reaction. Later, Zhao et al.28,29 developed three-dimensional graphene aerogels decorated with Pt and PtRu nanoparticles for use in direct methanol and microbial fuel cells. These graphene aerogel composites were further developed by Khattak et al.30 for applications in solid-state supercapacitors, in which a hydrothermal method was used so that the noble metals could be replaced with Fe2O3 nanostructured particles. Yu et al.31 grew MnO2 nanosheet arrays on conductive carbon fibers for pseudocapacitor applications, which showed high stability over 3,000 charge / discharge cycles. Very recently, Gui & Blackwood32 demonstrated the potential of WO3 covered carbon cloth in desalination.

In this research, nanoneedle α-MnO2/carbon fiber paper (CFP) composites were synthesized and 3



used as electrodes for electrosorptive removal of nickel and manganese ions. In comparison to both pristine CPF and activated carbon (AC), the composite materials show significant improvement in terms of removal efficiency.

2. RESULTS AND DISCUSSION 2.1. Materials Fabrication and Characterization The MnO2/CFP composite structures were prepared via a hydrothermal reaction, adapting a procedure used by Chu et al.33 to grow MnO2 nanostructures. The sample obtained from 3-hour hydrothermal reaction is labelled as Mn-3h, while the other obtained from 12-hour process is labelled as Mn-12h. From a comparison of the weights of the CFP sheets before and after deposition it was estimated that the amounts of MnO2 deposited were 1.25 g and 0.54 g for the Mn-3h and Mn-12h samples, respectively. The observation that more MnO2 was deposited at the shorter time was unexpected, however it was noted that even after the shorter 3 hours deposition the reaction solution had become colorless, indicating that all the purple MnO4- ions had been reduced within that time. Furthermore, the amount of powder found at the base of the reaction vessel increased with time, suggesting some of the MnO2 became detached, possibly due to increasing stress in the deposit as its porosity increased. It will be shown later, by SEM and BET analysis, that the mean pore size increases with the duration of the hydrothermal reaction and it is likely that the larger pores caused increase of stress in the deposited MnO2 leading to more detachment after 12 hours than 3 hours.

Thermogravimetric Analysis (TGA) was conducted to confirm the weight percentage of the MnO2 on CFP for each sample, with the decomposition features being consistent with previous 4


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literature studies (Figure 1)34-36. The minor weight loss below 480 °C can be attributed to the evaporation of absorbed moisture and intercalated water molecules, along with possible partial decomposition to Mn5O8 that has been reported to start at 350 °C. At higher temperatures the MnO2 is decomposed to Mn2O3, which is reported to start at 483 °C, with any Mn5O8 decomposing at 550 °C, while further decomposition to Mn3O4 is not expected to occur until close to 1000 °C. Unfortunately, the majority of the weight loss observed at 500 °C – 650 °C range is due to the burning away of the CFP substrate, which barely starts at 600 °C but likely to occur at a lower temperature in the enriched oxygen atmosphere created by the decomposition of the MnO2, making full interpretation of the TGA data difficult. However, as the CFP is almost completely gone by 820 °C, the larger residual weight (expected to be Mn2O3) of the Mn-3h sample than that of Mn-12h sample is consistent with the former having a higher deposited weight of MnO2.

100

Weight percent (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


48.2 wt %

80 68.6 wt%

60

97.5 wt %

40

20

Mn-3h Mn-12h CFP

0 0

200

400

600

800

1000

Temperature (°C)

Figure 1. Thermogravimetric analysis of the Mn-3h (black solid line), Mn-12h (red dotted line) composites and a bare CFP substrate (blue dashed line).

Figure 2 shows SEM images of the prepared MnO2/CFP composites. It can be seen that the 5



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MnO2 deposits as a layer and spherical clusters on the carbon fibers, which under high magnification shows a structure akin to that of sea urchins with a large number of nanoneedles protruding from the central sphere. For the 3-h sample, the nanoneedles growing directly on the carbon fibers can also be seen (Figures 2a & 2b), and the detached particles collected from the bottom of the autoclave were also investigated by SEM (Figures S1a & 1b), showing the same morphology of sea urchin-like structures formed by nanoneedle clusters. Increasing the reaction time from 3 h to 12 h results an increase in the size of the deposits on the carbon fibers (Figure 2c & 2d), but does not change the overall morphology of the MnO2, though the diameter of the urchins does approximately double to about 5 µm predominately due to increased length of the nanoneedles (Figures 2b & 2d). This observation is also true for the detached MnO2 particles (Figures S1c & S1d). However, from 3 h to 12 h, the total mass of MnO2 deposits actually decreases due to the loosening of structure and increase of porosity, as confirmed by later BET analysis. The nanoneedle morphology suggests a deposition reaction that is under diffusion control, which typically results in dendritic structures37. A possible growth mechanism of the α-MnO2 nanoparticles is illustrated in Figure 3. The H2SO4 is added to functionalize the surface of carbon fiber paper, providing the active nucleation sites for [MnO6] octahedra formation and further growth of MnO2 crystalline38,39. A series of chemical reaction during hydrothermal process is described in Eq. (1). 4KMnO4 + 2H2SO4 =>

4MnO2 + 2H2O + 2K2SO4 + 3O2

(2)

As the reaction proceeds, not only does the length of the nanoneedles increase, but also the porosity within the MnO2 and this leads to the generation of internal stresses and subsequent detachment.

6


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Figure 2. SEM images of the surfaces of the MnO2 deposited on to carbon fibre paper: (a) and (b) Mn-3h and (c) and (d) Mn-12h.

Figure 3. Proposed growth mechanism of the α-MnO2 nanoneedles on carbon fiber paper

Figure 4 shows the N2 adsorption–desorption isotherms (BET curves) for the two MnO2/CFP composites, with pore-size distributions displayed as insertions. For both materials, the shapes of the hysteresis loop between the adsorption and desorption branches are typical type-IV isotherms, 7



indicating the existence of mesoporosity, as the hysteresis loop is caused by the capillary condensation in mesopores40. It was determined that the specific surface area increased from 37.6 m2 g-1 to 42.3 m2 g-1 as the reaction time was extended from 3 h to 12 h, while the range of diameters of the pore increased slightly from 2-20 nm to 10-50 nm. However, the vast majority of pores remained below 20 nm, well within the accepted limit for mesoporosity41, with the mean diameter changing from 6.5 nm to 14 nm. The BET analysis of the CFP that had been ground into a powder revealed a surface area of only 0.37 m2 g-1, which is less than 1% of the MnO2/CFP composites; whereas activated carbon had a very large surface area of 633 m2 g-1, with a mean pore size of 2.8 nm (microporous). However, for electro-adsorption it is desirable that the electrode material maintains a controllable pore distribution in the range of 10 nm and 50 nm to improve ion adsorption and form a stable electrical double layer42-45. Therefore, despite having smaller specific surface areas the prepared α-MnO2/CFPs have a more promising pore morphology for ion adsorption performance than activated carbon46-49.

Figure 4. N2 adsorption–desorption isothermal curves for the MnO2/CFP composites. (a) Mn-3h and (b) Mn-12h. Symbols: black squares are adsorption; and red circles are desorption. The insets are the respective pore-size distributions. 8


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The X-Ray Diffraction (XRD) patterns of the two composites shown in Figure 5 are almost identical and all the lines can be index to α-MnO2 in tetragonal structure (JCPDS-ICDD card no. 00-44-0141), confirming that extending the reaction time does not change the basic structure of the deposits. The two peaks located at 26.5° and 54.6° in both samples can be attributed to the contribution from the CFP substrate, which can also be seen in its own XRD pattern (Figure S2a). The powder XRD patterns of the particles confirm that this is the same tetragonal α-MnO2 are shown in (Figures S2b & 2c). In addition, the absence of peaks due to other chemical phases indicates that the as-prepared sample is of high phase purity.

Figure 5. XRD patterns for the MnO2/CFP composites. (a) Mn-3h/CFP; (b) Mn-12h/CFP. The solid squares indicate the positions of the peaks for α-MnO2 according to ICDD No. 000-044-0141 and the solid spheres indicate the position of peaks contributed by CFP.

Figure 6 shows Raman spectra of the CFP, Mn-3h/CFP and Mn-12h/CFP composites. The CFP spectrum is dominated by the “G” and “D” band at 1579 cm-1 and 1357 cm-1 that are characteristics of carbon based materials.50 However, these peaks are absent from the spectra of the MnO2/CFP composite materials, which display the four characteristic bands for α-MnO2, confirming complete 9



coverage of the carbon fibers by a thick MnO2 layer.51,52 The major peak at 642 cm-1 refers to the symmetric stretching vibration perpendicular to the direction of the [MnO6] octahedral double chains, and peak at 574 cm-1 corresponds to the stretching vibration mode in the basal plane of [MnO6] sheets. The weaker peaks at 178 cm-1 and 360 cm-1 are attributed to the external mode derived from the translational motion of [MnO6] units and bending mode of Mn-O, respectively. Raman spectra of powders formed during the synthesis process were very similar to those of the MnO2/CFP composite (Figure S3).

642

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) CFP (b) Mn-3h/CFP (c) Mn-12h/CFP

574 178

360

(b) 1579

(c) 1357

(a) 200

400

600

800

1000

1200

1400

1600

1800

Raman Shift (cm-1)

Figure 6. Raman spectra of a) CFP and as-prepared b) Mn-3h/CFP and c) Mn-12h/CFP composites

XPS investigations revealed no significant differences between the two MnO2/CFP composites, with all the peaks being similar to the finds of Zhang et al.53 who investigate the use of MnO2/carbon papers composites for use in lithium-air batteries. In the Mn 2p spectra (Figures 7a & 7b)), two symmetrical peaks of binding energies at 654.5 eV and 642.6 eV with a separation of energy of 11.9 eV are observed, which correspond to Mn 2p1/2 and 2p3/2. There are two peaks in the Mn 3s spectra located at 89 eV and 84.2 eV in the Mn-3h/CFP and 89.2 eV and 84.4 eV in the Mn-12h/CFP 10


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(Figures 7c & 7d)), with the separation of 4.8 eV being a good indication that the manganese is in the Mn(IV).53 In the O 1s spectra (Figures 7e & 7f)), the major peak at 530.2 eV can be attributed to the Mn-O bonds in the manganese dioxide, while the shoulder at 531.8 eV can be assigned to Mn-O-H bonds, suggesting that the MnO2 is not fully dehydrated. As for the C 1s spectra (Figures &g & 7h)), the strongest peak at 284.6 eV corresponds to the sp2 C=C bond of CFP substrate and the shoulder at 285.8 eV signifies the presence of sp3 C-C bonds, while the peaks at 286.4 eV and 288.7 eV could suggest that existence of the some oxygen-containing functional groups.54,55 The XPS features of the MnO2 powders are consistent with their composite counterparts, although there is an additional peak in the O 1s spectrum of the Mn-12h powder 533.2 eV that may suggest some oxygen-carbon bonds are present, likely adsorbed carbon from the external atmosphere (Figure S4).56

11



Figure 7. X-ray photoelectron spectrum (XPS) of (a) Mn 2p of Mn-3h/CFP, (b) Mn 2p of Mn-12h/CFP, (c) Mn 3s of Mn-3h/CFP, (d) Mn 3s of Mn-12h/CFP, (e) O 1s of Mn-3h/CFP, (f) O 1s of Mn-12h/CFP, (g) C 1s of Mn-3h/CFP and (h) 12


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C 1s of Mn-12h/CFP.

2.2. Electrochemical Performance in A Three-electrode System Cyclic voltammetry (CV) was used to determine the capacitances of the α-MnO2/CFP electrodes, with the areal capacitance (F cm-2) as well as gravimetric capacitance (F g-1) being calculated from57:

= (

() (3)

)

= (

)

( ) (4)

where A is the geometric surface area of the working electrode, m is the mass of the active material, υ is the sweep rate, I is the current recorded and (Va−Vc) is the potential window of the scan. The performance of the α-MnO2/CFP composites was compared to an activated carbon (AC) electrode of the same geometric area, with the anodic currents collected during the positive scans being used to calculate the capacitances.

At a scan rate of 50 mV s-1 areal capacitances of 0.21 F cm-2 (13.6 F g-1) and 0.2 F cm-2 (30 F g-1) were recorded for the Mn-3h and Mn-12h samples, respectively, much higher than the 0.09 F cm-2 (5.4 F g-1) recorded for the bare AC electrode (Figure 8a). The gravimetric capacitances, in respect to mass of active material are given in brackets. The improvement capacitance can be explained by larger mesoporous surface area, where the pores are well distributed and interconnected, thus exhibiting better ion storage capacity. In addition, as a metal oxide electrode, MnO2 can also engage in Faradic charge transfer to act as a pseudo-capacitor via the reactions58-62:

(MnO2)surface+ H+ + e− ↔ (MnOO−H+)surface

(5) 13



(MnO2)surface/bulk + M+ + e− ↔ (MnOO−M+)surface/bulk

(6)

where M+ refers to Na+, K+ or other alkali cations. It has been reported by Toupin et al.58 that the intercalation of H+ only occurs at the surface of the material, while alkali cations can access both surface and bulk. Benhaddad et al.63 reported that in neutral solutions Reactions (5) and (6) give rise to the presence of a redox couple close to 0.3 V vs SCE. However, although broad redox peaks close to 0.3 V vs SCE can be seen in the CV collected at 1 mV s-1, (Figure S5a) their small size suggests that charge capacity is dominated by adsorption at the double layer capacitor rather than Faradaic processes. This is positive for the proposed application of heavy metal ion removal, where the Faradaic processes in Reactions (5) and (6) are likely to be parasitic.

Figure 8b shows the dependence of the areal & gravimetric capacitances of the Mn-12h sample, as determined from cyclic voltammetry, as a function of scan rate (Figure S5b). It can be seen that the areal capacitance decreases considerably as the scan rate increases, from 0.84 F cm-2 (126.7 F g-1) at 1 mV s-1 to 0.009 F cm-2 (1.35 F g-1) at 500 mV s-1. This dramatic change can be explained by the ions being only able to reach the near-surface pores instead of an interior access at high scan rates64-66.

14


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Figure 8. (a) Cyclic Voltammetry of α-MnO2/CFP 3 h (black line) and 12 h (blue line) composites and AC (red line) electrodes in 1 M NaCl at scan rate of 50mV s-1 and (b) Dependence of the areal and gravimetric capacitances of Mn-12h/CFP electrodes on scan rate.

2.3. Electro-sorption Performance Figure 9 shows the Ni2+ ion concentration, as determined via ICP-OES, through one compete charging and discharging cycle in the CDI/electro-adsorption apparatus (see Experimental section for details). For comparison purposes, Figure 9 shows data for the two α-MnO2/CFP composites as wells as for activated carbon and acid-treated (for increased wettability) plain carbon fiber paper electrodes. It can be seen that the α-MnO2/CFP composites have much higher nickel ion removal efficiencies than the control electrodes, with Table 1 showing the extent of nickel adsorbed on to the various materials at the end of the 60-minute charging period. The Mn-12h composite adsorbs/plates out more than twice as much nickel as activated carbon and the observation that the CFP on its own removes virtually no nickel implies that adsorption on the composites takes place at the MnO2. In terms of the specific removal capacitance, the performance of α-MnO2/CFP is even more impressive; at 16.4 mg Ni2+ per gram of active material, it is more than six times higher than activated carbon at 2.5 mg Ni2+ per gram of AC, with the original CFP substrate at only 0.034 Ni2+ mg per gram of CFP.

15



In addition, preliminary experiments on the adsorption of Ni2+ with the coexistence of 200 ppm Na+ ions, in the form of NaCl, have also been conducted with the MnO2/CFP composites. It was found that under such conditions both the Mn-3h/CFP and Mn-12h/CFP are only able to reduce nickel form 100 ppm to 20 ppm, with about 20% of the Na+ ions also being removed (Figure S6). Nevertheless, for the Mn-12h/CFP this still represented a very respectable 12 mg Ni2+ per gram of active material.

Table 1 shows the removal efficiency (η) of the nickel ions calculated from:

η=

C0 - Cf C0

× 100%

(6)

where C0 and Cf are the initial and final nickel ion concentrations, respectively, along with the nickel-ion adsorption capacity (M) calculated by:

M = (C0 – Cf) × V / m

(7)

where V is the volume of the solution passed over the electrodes; m is the mass of active material on the electrode. It can be seen that the performance of the MnO2/CFP composites is approximately twice as good as the AC electrode.

16


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Table 1. Nickel ion removal efficiencies and amount of ion removal in one hour from 100 ppm Ni2+ at a pumping rate of 50 ml min-1 for various electrode materials.

Concentration of Nickel Ion (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

80

60

40

20

CFP AC Mn-3h/CFP Mn-12h/CFP

Discharge starts from 60 mins

0

20

40

60

80

100

120

Time (min) Figure 9. Nickel ion adsorption/desorption behaviors of all test electrode materials during the first cycle. Concentrations were determined via ICP-OES. Symbols: red squares are Mn-3h/CFP; red circles are Mn-12h/CFP; inverted black triangles are AC; and black triangles are uncoated CPF.

Figure 10 shows the Ni2+ ion concentration through three charge/discharge cycles using Mn-12h composite electrodes. The first cycle has an adsorption efficiency of 88.9%, with full regeneration of the initial 100 ppm Ni2+ ion concentration being achieved when 0 V was maintained across the two 17



electrodes for 120 minutes, with the subsequent adsorption efficiencies being 93.8% and 95.5%. Hence, at the end of the third cycle the Ni2+ concentration was only 4.5 ppm, well below the 10 ppm limit for requirement for discharge into public sewers in Singapore. However, in subsequent cycles the release of nickel ion was no longer 100%, which implies that either some of the Ni was irreversibly deposited or some precipitation was occurring, although none was visible to the naked-eye.

Unfortunately, although the MnO2/CFP electrodes were successful in the removal of Ni2+ ions, the ICP-OES detected manganese ions in the treated solution. Presumably, this was due to some reduction of the MnO2 particles to soluble Mn2+ when the composite was acting as a cathode for nickel deposition, with the mass of the electrodes decreasing by 0.01 g (

Carbon Paper Composite

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