Article pubs.acs.org/IECR
Enhancement of Electrochemical Oxidation of Ammonia and Ammonium Carbonate over Pt Black Catalysts through Interaction with Manganese Dioxide Nanoparticles Dongsu Song,† Ki Rak Lee,‡ Seung Bin Park,† and Jong-In Han*,‡ †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea ‡ Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea S Supporting Information *
ABSTRACT: In this study, the potential of ammonia (NH3) and ammonium carbonate ((NH4)2CO3) was investigated as alternative fuels of the fuel cell platform. Considering that the anodic oxidation activity of the ammonia compounds is the key for the overall cell performance, this study focused on its improvement by way of including a catalytic additive, which aided in decreasing the use of a precious metal catalyst. Manganese dioxide (MnO2) was chosen for that purpose. Four different types, specifically alpha (α), beta (β), gamma (γ), and delta (δ), were prepared and tested via an electrochemical analysis. A cyclic voltammetry (CV) test was performed with physically mixed catalytic compositions as nanocomposites, such as 20:80, 50:50, 80:20, 90:10, and 95:5. Among these, only 95 wt % Pt and 5 wt % γ-MnO2 exhibited an intended activity increase. Although the replaced amount of Pt was not very substantial, the increase in power density at least doubled in a single cell test. This study supported the notion that ammonia and its derivative can indeed serve as feasible fuel alternatives when a right catalyst is used, though further and systematic investigation is warranted for the realization.
1. INTRODUCTION Hydrogen is counted as an ideal fuel and energy carrier, because of its lightweight, ample abundance, and clean product.1 Storage, however, is quite a challenging issue to be not readily solvable.2 One potential hydrogen carrier is ammonia compounds such as ammonia molecule (NH3) and ammonium carbonate ((NH4)2CO3); even the carbonate salt contains 8.3 wt % hydrogen that is higher than the DOE 2015 target (5.5 wt %) as a board hydrogen storage system.3,4 It also has an added advantage of easy transport,5 though the theoretical energy density is comparatively low. Recently, ammonia and ammonium derivatives have been attempted to be obtained from renewable energy sources, such as biomass, food waste, and animal feces.6 Besides, ammonia is an absorbent for carbon dioxide (CO2) capture, and this ammonia-based technology is in fact near commercialization.7,8 Ammonia, upon being consumed in CO2 capture, ends up being ammonium carbonate or ammonium bicarbonate as follows:
Considering the reduced state, the ammonia derivatives can even directly serve as fuels for the fuel cell, termed ammonia fuel cell.10 Ammonia, however, is generally regarded as a poison for proton exchange membrane fuel cells (PEMFCs); 1 ppm ammonia may cause a significant performance drop in hydrogen PEMFC.11 Accordingly, other configurations without such a detrimental effect, such as alkaline membrane fuel cell, must be opted for the purpose. The electrochemical oxidation of ammonia requires catalysts, noble metals like especially platinum (Pt), but its cost is prohibitively high and yet its activity far from satisfactory. To this end, it is a common practice to incorporate co-catalysts like metal oxides. Manganese dioxide (MnO2), among many, showed the highest ammonia adsorption property, so there was a case of being used as an ammonia sensor.12 MnO2 has various crystalline phases, such as alpha (α), beta (β), gamma (γ), delta (δ), and epsilon (ε). Different structures have distinct activities and thus are used accordingly.13,14 The aim of this study was to examine if any of these MnO2 phases could act as a co-catalyst and enhance the electrochemical ammonia oxidation by way of improving ammonia adsorption property. We found that only gamma MnO2 exhibited such a trait. Cyclic voltammetry (CV), single cell test, and X-ray photoelectron spectroscopy (XPS) were employed to figure out an optimum catalytic composition.
aNH3 + bCO2 + cH 2O → (NH4)2 CO3 ; NH4HCO3 ; NH4COONH 2
(1)
Ammonium carbonate exists as a solid at room temperature, meaning that ammonium density in its most common state is high, which is advantageous in its application as a hydrogen carrier. In an aqueous solution, (NH4)2CO3 is ionized into NH4+ and CO32− ions9 (NH4)2 CO3 → 2NH4 + + CO32 − © 2014 American Chemical Society
Received: Revised: Accepted: Published:
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2. EXPERIMENTS 2.1. Synthesis of MnO2 Nanoparticles. Various crystalline manganese dioxides were prepared as follows: i) alpha (α)-MnO2 was prepared by redox-reaction between MnSO4 and KMnO4 as described by Devaraj and Munichandraich. Ten mmol of KMnO4 (Aldrich) and 15 mmol of MnSO4·H2O (Aldrich) were dissolved in 200 mL of distilled water and continuously stirred for 6 h at room temperature. Dark-brownish precipitates were formed and then washed with distilled water and ethanol to remove excess ions.15 ii) beta (β) MnO2 was synthesized by heat-treatment of gamma MnO2 at 350 °C for 4 h. The detailed preparation method of the gamma phase was as below.16 iii) gamma (γ) MnO2: A 200 mL solution of 16 mmol of MnSO4·H2O and (NH4)2S2O8 (Aldrich) was stirred in an oil bath at 90 °C for 12 h.15 Black precipitates were filtered, washed, and dried in vacuum. iv) delta (δ) MnO2: 6.8 mmol of KMnO4 and 40 mmol of urea (Aldrich) were mixed in 15 mL of distilled water and treated by ultrasonication for 10 min to form a homogeneous solution and then transferred into a Teflon-lined bottle (40 mL), sealed, and maintained at 90 °C for 24 h. The precipitation was filtered, washed, and dried at 50 °C for 1 day.17 2.2. Characterization of MnO2 Nanoparticles. X-ray diffraction (XRD) analysis was performed by using a RIGAKU D/MAX-2500 with Cu Kα radiation in a continuous scan mode over a 2θ range between 20 and 80° at a scan rate of 2°/min. Transmission electron microscopy (TEM) study was performed to investigate the size and shape of the gamma MnO2 and to observe the morphology of the gamma MnO2 using a JEOL 2000FX. X-ray photoelectron spectrometry (XPS) analysis was carried out by Thermo VG Scientific Sigma Probe to investigate the effect of MnO2 addition into Pt. 2.3. Electrochemical Characterization of Pt-MnO2 Composite Catalysts over Ammonia and Ammonium Carbonate Oxidation. A thin film method was fabricated to prepare a working electrode of a three electrode system for the electrochemical experiments. Specific amounts of MnO2 nanoparticles and commercial Pt black (Aldrich) were put into vials with various Pt/MnO2 weight ratios of 100:0, 95:5, 90:10, 80:20, 50:50, 20:80, and, as a control, 0:100. Each mixture was homogeneously dispersed in distilled water through sonication, and then 10 μL of the dispersion was dripped on a glassy carbon working electrode (0.4 cm). Catalyst loading was 0.0159 mg/cm2 and then drying it in air. Ten μL of 5 wt % ionomer solution (Tokuyama) was dripped on the catalyst layer to provide mechanical strength in the catalyst layer. For the electrochemical analysis, a beaker-type three electrode cell was employed, and Pt wire and Ag/AgCl electrode (BAS Co., Ltd., 012167 RE-1B) were used as the counter and reference electrodes, respectively. 2.4. Fabrication and Characterization of Membrane Electrode Assembly (MEA). Two type of anodes were prepared by (1) pure Pt black and (2) 5 wt % gamma MnO2 + 95 wt % Pt black mixed catalysts, respectively. Pt black catalyst (Aldrich) was used for the cathodic electrodes. Catalytic inks were prepared by mixing each catalyst powder, anion exchange ionomer solution, distilled water, and isopropyl solution. Membrane electrode assemblies (MEAs) were fabricated via a transfer method. At first, the inks got through sonication for acquiring homogeneous mixing of each ink and then were
pasted on carbon-cloth (SCCG-5N, CNL Energy) until a total catalyst loading of 4 mg/cm2 was obtained. Twenty-five wt % ammonia and 25 wt % ammonium carbonate solution were supplied into the anode compartment at flow rates of 20 cc/ min. Humidified oxygen was fed into cathode chamber at a flow rate of 50 cc/min. A single cell with a MEA having 9 cm2 of active area was operated at 80 °C. The single cell test was carried out by an electrochemical analyzer (CH instruments 700a) with a scan rate of 15 mV/s. All data from a single cell test were collected at least 1 h operation.
3. RESULT AND DISCUSSION Various phases of prepared manganese dioxide and Pt black were analyzed by XRD, and the results are presented in Figure 1a and 1b, respectively. All the MnO2 phases that were
Figure 1. X-ray diffraction spectra of various types of MnO2 (a) and Pt black (b).
prepared in this study appeared to accord well with previous reports (Figure 1a). Pt showed a crystal-structure size of 25 Ȧ 3 at 2θ = 67.6° (Figure 1b). TEM image of γ-MnO2 revealed that the synthesized γ-MnO2, which exhibited the activity higher than Pt black alone, has a spherical and also rod-like shapes (Figure S1). Electrochemical activities of the catalysts over ammonia oxidation were analyzed via a cyclic voltammetry (CV) test using ammonia solution (0.2 and 0.4 M) and ammonium carbonate as electrolytes over Pt black only (Figure 2). In addition, the proton adsorption/desorption area of the Pt black was evaluated from the CV result obtained in a 1 M H2SO4 solution in order to estimate an electrochemically active surface area (EAS) (92.3 m2 g−1) (Figure S1). Ammonium ion (NH4+) 14674
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all this, the substantial improvement of electrochemical oxidation of the ammonia compounds could be achieved by adding a co-catalyst that has high affinity toward ammonia and/ or low adsorption energy of Nads and carbonate to the main catalyst. To this end, MnO2 was added as a co-catalyst because of the following reasons: 1) MnO2, though tested at 503 K, was reported to have the highest catalytic activity among tested catalysts over gaseous ammonia oxidation28 (MnO2 > CuO > CaO2 > NiO > Bi2O3 > Fe2O3 > V2O5 > TiO2 > CdO > PbO > ZnO > SnO2 > ZrO2 > MoO3 > WO3), and 2) it has the capacity of ammonia adsorption and hence was used as an ammonia sensor.13,29 Catalytic composition and mass of composites of Pt black and MnO2 used in this study are summarized in Table 2.
Figure 2. Cyclic voltammogram of Pt black in 1 M KOH + 0.4 M NH4OH (red line), 1 M KOH + 0.2 M NH4OH (black line), and 1 M KOH + 0.2 M (NH4)2CO3 (blue line) solutions at scan rate 15 mV s−1 from −0.9 V to 0.7 V.
Table 2. Summary of the Catalytic Composition and Mass of Pt Black
showed electrochemical oxidation activity at −0.35 to −0.3 V, and this oxidation might happen as follows (eqs 3−6)18−22 NH3 + OH− → NH 2 (ads) + H 2O + e−
(3)
NH 2 (ads) + OH− → NH (ads) + H 2O + e−
(4)
NH (ads) + OH− → N (ads) + H 2O + e−
(5)
2N (ads) → 2M + N2
(6)
composition (mass ratio) Pt:MnO2 Pt:MnO2 Pt:MnO2 Pt:MnO2 Pt:MnO2
Table 1. Summary of Electrochemical Analysis Results with Pt Black Catalyst
1 M KOH + 0.2 M NH4OH 1 M KOH + 0.4 M NH4OH 1 M KOH + 0.2 M (NH4)2CO3
I (mA cm−2)
mass activity (A gpt−1)
specific activity (mA m−2)
5.13
32.3
350
5.57
35.0
379
3.50
22.0
238
20:80 50:50 80:20 90:10 95:5
3.18 7.95 12.7 14.3 15.1
× × × × ×
10−3 10−3 10−3 10−3 10−3
Figures 3 and 4 showed the oxidation activities of ammonia and ammonium carbonate with various types of MnO2 and composition with respect to Pt black catalyst, respectively. An especially high current density of 5.38 mA/cm2 for ammonia and 3.86 mA/cm2 for ammonium carbonate were observed in the only 5 wt % gamma MnO2 mixed Pt black catalyst at −0.33 V and −0.32 V, respectively. The onset potential was shifted to a lowered voltage with the co-catalyst incorporated. The increase MnO2 content resulted in the reduction of current density regardless of MnO2 phase type. However, catalytic composition consisting of Pt:MnO2 = 95:5 exhibited higher oxidation activity over both NH3 and (NH4)2CO3 than Pt black only. Five wt % γ-MnO2 caused to enhance the mass activity for NH3 (16.4%) and (NH4)2CO3 (10.0%) and so did 10 and 20 wt % to a less degree. Specific activities of the catalyst ratio of Pt:γ-MnO2 = 95:5 were evaluated by dividing mass activities by an electrochemically active surface area (EAS), which are summarized in Table 3. Figure 5 showed the catalytic activity over ammonia and ammonium carbonate oxidation with Pt black and the γ-MnO2 ratio of 95:5 based on specific Pt black weight, respectively. The mass-based activities of Pt black and the γ-MnO2 ratio of 95:5 are summarized in Table 4. The further increase in the mass ratio of γ-MnO2 resulted in a rather reduced activity, which was attributable to conductivity change of catalysts, named the percolation effect; γ-MnO2 could act as an insulator due to its semiconductive nature and thus disrupt the electron flow in the catalyst cluster.30 Based on the optimal 5 wt % γ-MnO2 composite catalyst, a membrane electrode assembly (MEA) was manufactured, and a single cell was operated (Figure 6). The oxidation activity was evaluated with pure Pt black. When NH4OH was an ammonia source and electrolyte, the maximum power density of the MnO2-containing composite based fuel cell was found to be 0.41 mW/cm2 at 0.40 V, which was approximately 5 times higher than that with the pure Pt (0.10 V). Similarly, when (NH4)2CO3 was used, the maximum power density was 0.078
where the current densities of ammonia oxidation were 3.50 mA cm−2 (1 M KOH + 0.2 M (NH4)2CO3, 5.13 mA cm−2 (1 M KOH + 0.2 M NH4OH), and 5.57 mA cm−2 (1 M KOH + 0.4 M NH4OH). Mass-based oxidation activities were 220 A g−1 (1 M KOH + 0.2 M (NH4)2CO3), 323 A g−1 (1 M KOH + 0.2 M NH4OH), and 350 A g−1 (1 M KOH + 0.4 M NH4OH). Pt is generally recognized as the most active catalyst for the electrochemical oxidation of ammonia, just like for many chemicals.23−25 The catalytic activity, however, was found to be substantially suppressed when it came to the oxidation of ammonium carbonate: the Pt black showed the oxidation activity of only about 37% on ammonium carbonate compared with that of ammonia (Table 1). This degraded result can be
electrolyte type
= = = = =
Pt (mg cm−2)
expounded by nitrogen absorption. On the surface of catalyst, ammonia is oxidized and converted into diatom nitrogen molecule (and water), some of which stay and cover EAS and as a result render the catalytic activity dramatically reduced. Ammonium ion (NH4+) is likely to follow the same mechanism. Considering one molecule of ammonium carbonate has two ammonium ions, its ammonia oxidation activity appeared to be rather low. It could be explained by the existence of carbonate ion (CO32−) and its adsorption on the Pt surface, especially the (111) plane;26,27 carbonate ion might act like Nads species as in the case of ammonia oxidation. In light of 14675
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Figure 3. Cyclic voltammetry results of various catalytic compositions with 1 M KOH + 0.2 M NH4OH.
Figure 4. Cyclic voltammetry results of various catalytic compositions with 1 M KOH + 0.2 M (NH4)2CO3.
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Table 3. Summary of Electrochemical Analysis Results with Pt Black and Gamma MnO2 Catalyst Composite (95:5) EAS (m2 g−1)
I (mA cm−2)
mass activity (A gpt−1)
Electrolyte − 1 M KOH + 0.2 M NH4OH 98.9 5.38 35.6 Electrolyte − 1 M KOH + 0.2 M (NH4)2CO3 98.9 3.83 25.4
specific activity (mA m−2) 360 257
Figure 6. I−V test results of the AEM fuel cells using catalytic composition of (a) NH3 and (b) (NH4)2CO3 as a fuel, respectively; (1) Pt black as an anode catalyst (solid circle) and (2) ‘Pt:γ-MnO2 = 95:5’ as an anode catalyst (open circle), respectively.
Interaction between Pt and the γ-MnO2 was investigated using X-ray photoelectron spectroscopy (XPS), and the peak shift of Pt is shown in Figure 7. The binding energy of Pt 4f7/2
Figure 5. Cyclic voltammetry results of various catalytic compositions of Pt black and gamma MnO2.
Table 4. Summary of Pt Specific Current Density with Various Pt:γ-MnO2 Ratios Pt:γ-MnO2 ratio
NH4OH
(NH4)2CO3
Pt black 20:80 50:50 80:20 90:10 95:5
5.14 0.152 1.79 4.12 4.35 5.66
3.46 0.150 2.11 3.04 3.06 4.00
Figure 7. Pt 4f7/2 XPS results of the MEAs which consist of (1) pure Pt and (2) 5 wt % γ-MnO2 mixed Pt catalysts as their anode electrode.
was found to move to a higher level with 5 wt % γ-MnO2, which is consistent with the reported Pt−Mn catalysts.31 It should be noted that γ-MnO2 was dissolved over time and even more so at acidic conditions. Because it might be problematic in the long term operation of AEM fuel cells, this issue must be dealt with appropriately.
mW/cm2 at 0.401 V, and this value is about 80% higher than that with the pure Pt (0.15 V). It was evident that the addition of 5 wt % γ-MnO2 dramatically enhanced the cell performance. This result strongly supports that manganese dioxide indeed has the ability to adsorb an ammonium source (both ammonia molecule and ammonium ion) on its surface and synergistically aid the catalytic activity of the main catalyst Pt, indicating that the Pt- γ-MnO2 nanocomposite is a promising anode catalyst for the ammonia-based fuel cell.
4. CONCLUSION The ammonia oxidation activity of the commercial Pt black catalyst was improved by simply mixing with cheap γ-MnO2 nanoparticles. In the 5 wt % γ-MnO2 mixed catalyst, the mass 14677
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(13) Patil, L. A.; Sonawane, L. S.; Patil, D. G. Room Temperature Ammonia Gas Sensing Using MnO2-Modified ZnO Thick Film Resistors. J. Mod. Phys. 2011, 2, 1215. (14) Yang, Y.; Xiao, L.; Zhao, Y.; Wang, F. Hydrothermal Synthesis and Electrochemical Characterization of α-MnO2 Nanorods as Cathode Material for Lithium Batteries. Int. J. Electrochem. Sci. 2008, 3, 67. (15) Tao, X.; Du, J.; Sun, Y.; Zhou, S.; Xia, Y.; Huang, H.; Gan, Y.; Zhang, W.; Li, X. Exploring the Energy Storage Mechanism of High Performance MnO2 Electrochemical Capacitor Electrodes: An In Situ Atomic Force Microscopy Study in Aqueous Electrolyte. Adv. Funct. Mater. 2013, 23, 4745. (16) Wei, Q.; Wang, X.; Yang, X.; Shu, H.; Ju, B.; Hu, B.; Song, Y. The effects of crystal structure of the precursor MnO2 on electrochemical properties of spinel LiMn2O4. J. Solid State Electrochem. 2012, 16, 3651. (17) Zhu, G.; Li, H.; Deng, L.; Liu, Z.-H. Low-temperature synthesis of δ-MnO2 with large surface area and its capacitance. Mater. Lett. 2010, 64, 1763. (18) Bunce, H. J.; Bejan, D. Mechanism of electrochemical oxidation of ammonia. Electrochim. Acta 2011, 56, 8085. (19) Lomocso, T. L.; Baranova, E. A. Electrochemical oxidation of ammonia on carbon-supported bi-metallic PtM (M = Ir, Pd, SnOx) nanoparticles. Electrochim. Acta 2011, 56, 8551. (20) Gerischer, H.; Mauerer, A. Untersuchungen Zur anodischen Oxidation von Ammoniak an Platin-Elektroden. J. Electroanal. Chem. 1970, 25, 421. (21) de Vooys, A. C. A.; Koper, M. T. M.; van Santen, R. A.; van Veen, J. A. R. The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes. J. Electroanal. Chem. 2001, 506, 127. (22) Lee, K. R.; Song, D.; Park, S. B.; Han, J.-I. A direct ammonium carbonate fuel cell with an anion exchange membrane. RSC Adv. 2014, 4, 5638. (23) Gland, J. L.; Korchak, V. N. Ammonia oxidation on a stepped platinum single-crystal. J. Catal. 1978, 53, 9. (24) Vidal-Iglesias, F. J.; Solla-Gullón, J.; Rodríguez, P.; Herrero, E.; Montiel, V.; Feliu, J. M.; Aldaz, A. Shape-dependent electrocatalysis: ammonia oxidation on platinum nanoparticles with preferential (1 0 0) surfaces. Electrochem. Commun. 2004, 6, 1080. (25) Imbihl, R.; Scheibe, A.; Zeng, Y. F.; Günther, S.; Kraehnert, R.; Kondratenko, V. A.; Baerns, M.; Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure. Phys. Chem. Chem. Phys. 2007, 9, 3522. (26) Markovits, A.; Garcia-Hernandez, M.; Ricart, J. M.; Illas, F. Theoretical Study of Bonding of Carbon Trioxide and Carbonate on Pt(111): Relevance to the Interpretation of “in Situ” Vibrational Spectroscopy. J. Phys. Chem. B 1999, 103, 509. (27) Berna, A.; Rodes, A.; Feliu, J. M.; Illas, F.; Gil, A.; Clotet, A.; Ricart, J. M. Structural and Spectroelectrochemical Study of Carbonate and Bicarbonate Adsorbed on Pt(111) and Pd/Pt(111) Electrodes. J. Phys. Chem. B 2004, 108, 17928. (28) Chenko, N. I., II Catalytic oxidation of ammonia. Russ. Chem. Rev. 1976, 45, 2168. (29) Prasher, R. Effect of Aggregation Kinetics on the Thermal Conductivity of Nanoscale Colloidal Solutions (Nanofluid). Nano Lett. 2006, 6, 1528. (30) Zhang, W.; Zeng, C.; Kong, M.; Pan, Y.; Yang, Z. Waterevaporation-induced self-assembly of α-MnO2 hierarchical hollow nanospheres and their applications in ammonia gas sensing. Sens. Actuators, B 2012, 162, 292. (31) Ammam, M.; Prest, L. E.; Pauric, A. D.; Easton, E. B. Synthesis, Characterization and Catalytic Activity of Binary PtMn/C Alloy Catalysts towards Ethanol Oxidation. J. Electrochem. Soc. 2012, 159, B195.
and specific activities of NH4OH increased by 2.86 and 10.2%, respectively, compared with those of the pure Pt black catalyst. Also, the addition of manganese dioxide enhanced mass and specific activities of ammonium carbonate oxidation by 10.2 and 15.4%. The improved ammonia oxidation activity was also confirmed in each single cell test. It was speculated that the electronic state of Pt was modified due to the introduction of the γ-MnO2 particles. This γ-MnO2 composite catalyst provides a convenient, easy, and economical way to fabricate enhanced performance MEAs, especially suited for new fuel cells powered by ammonia compounds. This study supports that with systematic and unceasing investigation, the promising ammonia-based fuel cell technology can indeed be realized.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1 and S2 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +82-42-350-3629. Fax: +82-42-350-3610. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Advanced Biomass R&D Center (ABC) of Global Frontier Project (ABC-2012053875) and the National Research Foundation of Korea (NRF2012M1A2A2026587) funded by the Korea government Ministry of Education, Science and Technology (MEST).
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REFERENCES
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