Electrophoretic deposition of cobalt ferrite and ...

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Nanoengineering Department, Dr. Lloyd Brown from Thermochemical Solutions LLC.,. Dr. Dave Genders and Dr. Peter Symons from Electrosynthesis Inc., and ...
ECS Transactions, 58 (42) 1-9 (2014) 10.1149/05842.0001ecst ©The Electrochemical Society

Electrophoretic deposition of cobalt ferrite and platinum cobalt nanoparticles as electrocatalysts R. Tanakit, W. Luc, and J.B. Talbot Department of NanoEngineering University of California, San Diego La Jolla, California 92093, USA

Cobalt ferrite and platinum cobalt nanoparticles were deposited using electrophoretic deposition (EPD) from two different bath chemistries. The first bath contained 2 g/L particles suspended in 90 vol.% water and 10 vol.% isopropanol with 0.4 g/L hexadecyltrimethlyammonium bromide. The second bath contained 2 g/L particles suspended in 100 % ethanol. The deposits were tested for electrocatalytic properties for the oxidation of ammonium sulfite to ammonium sulfate. Linear sweep voltammetry showed that both EPD deposits of cobalt ferrite and platinum cobalt nanoparticles had electrocatalytic activity. Introduction Nanoparticle electrocatalysts are being studied in support of the development of a solar sulfur ammonia (SA) thermochemical cycle for splitting water to produce hydrogen [1]. The hydrogen production step in the SA cycle consists of the electrolytic oxidation of ammonium sulfite to ammonium sulfate as follows for a basic medium [2]: anode reaction: SO32- + 2OH-  SO42- + H2O + 2ecathode reaction: 2H2O + 2e-  H2 + 2OHoverall: SO32- + H2O  SO42- + H2

Eo = -0.936 V Eo = -0.828 V Eocell = 0.108 V

[1] [2] [3]

The anodic reaction (1) is kinetically slow, so improvements in the electrocatalysts are being sought. Nanoparticle catalysts not only potentially reduced the amount of catalyst used due to increased surface area, it has been shown that there can be large increases in reactivity due to quantum confinement effects [3]. Electrophoretic deposition (EPD) of catalyst powders have been used by other researchers for a catalytic wall reactor [4]. Electrocatalytic particles that have been deposited by EPD include Ni/La2O3/γ-Al2O3 and Co–Me/ZnO in isopropanol onto a stainless steel substrate [5]. Films of cobalt ferrite (CoFe2O4) and platinum cobalt (Pt3Co) have been identified as potential compounds to catalyze the reaction of ammonium sulfite to ammonium sulfate. EPD processes to deposit nanoparticles of these catalysts were developed. The deposits were then tested for electrochemical activity using linear sweep voltammetry.

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ECS Transactions, 58 (42) 1-9 (2014)

Experimental Nanoparticle synthesis. Cobalt ferrite nanoparticles were synthesized using a coprecipitation method as follows [6]:

Co2+ + 2Fe3+ + 8OH- → CoFe2O4 + 4H2O

[4]

A 0.68 M NaOH solution was heated to 80 ± 1 °C under constant stirring. Then 0.085 M Co(CH3COO)2·4H2O and 0.17 M Fe(NO3)3·9H2O were mixed and poured into the heated sodium hydroxide solution. The temperature of the solution was raised to 100 ± 2 °C and stirred for two hours which allowed for the precipitation of black particles. The solution was cooled to room temperature and washed twice with 200 ml of distilled water. A neodymium magnet was used to separate the magnetic particles from the distilled water. Particles were placed drop-wise from solution onto a copper substrate and dried in air for analysis. The particles were characterized using images and energy-dispersive X-ray spectroscopy (EDX) using an FEI XL30 FE SEM scanning electron microscope (SEM). Electrophoretic deposition methods. EPD was conducted in two bath chemistries at room temperature. First, 2 g/L of nanoparticles were mixed in a solution of 90 vol% water, 10 vol% isopropanol, with 0.4 g/L of hexadecyltrimethylammonium bromide (CTAB). The second bath contained 2 g/L of nanoparticles in 100% ethanol. The pH was adjusted of 5 using nitric acid. Nanoparticles of 30 wt% platinum cobalt on carbon were purchased from Sigma Aldrich. The pH of the baths was measured using an Orion model SA 720 pH meter. Zeta potential of the nanoparticles was measured using a Brookhaven Instruments Corporation ZetaPlus machine. As the zeta potential was found to be positive in the EPD baths, electrophoretic deposition took place on the cathode.

The substrate and a 4 cm x 8 cm aluminum sheet were positioned vertically and parallel 1.0 to 2.2 cm apart in a beaker containing the suspension and connected to a E3612A Agilend DC power supply to apply a constant voltage as shown in Figure 1. The substrate was bound by two Teflon plates that exposed the deposition area, that faced the anode. The substrate area was 4.91 cm2 for EPD of cobalt ferrite particles and 3.75 cm2 for platinum cobalt particles. Typically, voltage and deposition duration were varied in order to yield a thin, uniform deposit. For EPD with cobalt ferrite in the CTAB bath, a constant current was applied. EPD was done either in a quiescent bath or with sonication by placing the beaker of suspension in a Branson 1200 Sonicator. Aluminum foil and graphite paper substrates were weighed before and after the EPD process using a Sartorius 1712 MP8 Silver balance to determine the deposit weight. Particle adhesion tests were performed by soaking deposited samples in water for 24 hours and re-weighing the samples. The deposit uniformity was observed by SEM imaging. Electrochemical testing. Platinum cobalt and cobalt ferrite nanoparticles were deposited by EPD onto graphite paper substrates. The deposit densities varied from 0.3 mg/cm2 to 0.8 mg/cm2 depending upon the EPD bath chemistry and material deposited. The deposits were tested for electrocatalytic activity in a standard three-electrode electrochemical cell filled with 100 ml of 2 M (NH4)2SO3 solution at room temperature. The 4.0 cm2 EPD deposit, 12.8 cm2 graphite paper counter electrode and a calomel reference electrode (SCE) were used. Linear sweep voltammetry was conducted from 0.0

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ECS Transactions, 58 (42) 1-9 (2014)

V to 1.0 V vs SCE using a Princeton Applied Research VeraSTAT 3 potentiostat at a scan rate of 0.5 V/s.

Figure 1: Electrophoretic deposition experimental set up.

Results and Discussion Nanoparticle synthesis. The composition of the synthesized cobalt ferrite particles determined by EDX was 9.56 at% Co, 18.10 at% Fe, and 72.34 at% O, which corresponds to an atomic ratio of Co:Fe:O of 1:1.9:7.6. The excess oxygen from the EDX analysis is most likely from the copper substrate and adsorption of oxygen onto the sample. Therefore, the synthesized particles were most likely CoFe2O4. The particle size was measured using an SEM image to be 20 ± 5 nm. The purchased platinum cobalt particles (Sigma Aldrich) were analyzed by EDX and the composition was 73.69 at% Pt, 26.31 at% Co indicating Pt3Co. SEM images showed that the Pt3Co-loaded carbon particle size was 50 ± 15 nm.

Electrophoretic deposition. The zeta potential of the cobalt ferrite nanoparticles in the CTAB bath chemistry at a pH of 6.2 was 20 ± 2 mV. Figure 2 compares the zeta potential of cobalt ferrite in 90 vol% water and 10 vol% isopropanol solution with and without CTAB as a function of pH as changed with the addition of nitric acid and sodium hydroxide. The zeta potential was only positive at low pH without CTAB, but remained positive with addition of CTAB over a wide range of pH values.

EPD of cobalt ferrite nanoparticles onto an aluminum substrate was done using a constant current of 30 mA from a CTAB bath. Figure 3 shows a linear increase in deposit density with time up to 160 s. A micrograph of the top of a deposit of cobalt ferrite nanoparticles on aluminum from the CTAB bath made at 10 mA for 2 min is shown in Figure 4a. The image shows a narrow particle distribution of the nanosized particles with multiple layers visible. EPD was also done under sonification of the bath. Figure 4b shows an SEM image of cobalt ferrite nanoparticles using the CTAB bath with sonication during EPD, which shows smaller, scattered agglomerates. Figure 5a shows a deposit of cobalt ferrite nanoparticles on graphite paper from a CTAB bath made at 30 V for 1 min. The

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ECS Transactions, 58 (42) 1-9 (2014)

micrograph shows large agglomerates distributed on the graphite fibers. Figure 5b shows cobalt ferrite nanoparticles on graphite paper deposited from a CTAB bath under sonication at 30 V for 1 min. The deposited particle agglomerates are smaller than agglomerates from EPD without sonication. In the 100% ethanol bath, cobalt ferrite particles stayed suspended at a pH of 5.0, and thus, all depositions were conducted at this pH. Figure 6a shows a micrograph of a deposit of cobalt ferrite particles on the aluminum substrate deposited for 1 min with an applied voltage of 35 V. In comparison to the deposit from the CTAB bath, the deposit from the ethanol bath seemed more evenly distributed and thinner (micrographs not shown). Figure 7a and 7b shows deposits of cobalt ferrite nanoparticles from an ethanol bath on graphite paper both without and with sonication of the bath, respectively. The particle agglomerates deposited with sonication are smaller than agglomerates deposited without sonication of the bath. The zeta potential of the platinum cobalt nanoparticles in the CTAB bath chemistry was 60 ± 5 mV at a pH of 7.9. The EPD of platinum cobalt nanoparticles shows a linear increase in deposit weight with the duration of EPD at 30 V as shown in Figure 8. Figure 9a shows an SEM micrograph of platinum cobalt nanoparticles deposited onto aluminum for 25 s at an applied voltage of 20 V. The deposit exhibits small agglomerates distributed across the surface of the substrate. The deposit appears to be thin, but is not uniform. Particle sizes of 35 - 65 nm can be observed. Figure 9b shows platinum cobalt nanoparticle deposits from the CTAB bath with sonication. The deposit performed with sonication is thinner and agglomerates are larger, but more scattered.

Figure 2: Zeta potential vs pH for cobalt ferrite particles in a 90 vol% water, 10 vol% IPA solution and in a CTAB solution.

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ECS Transactions, 58 (42) 1-9 (2014)

Figure 3: Deposit density vs. time for cobalt ferrite nanoparticles deposited on an aluminum substrate at 30 mA.

(a)

(b)

Figure 4: SEM micrographs of cobalt ferrite nanoparticles deposited on an aluminum substrate from a CTAB bath for (a) 2 min with a 10 mA current and (b) for 15 s with an applied voltage of 27 V in a sonication bath.

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ECS Transactions, 58 (42) 1-9 (2014)

(a)

(b)

Figure 5: SEM micrographs of cobalt ferrite nanoparticles deposited on a graphite paper substrate from a CTAB bath for (a) 1 min with an applied voltage of 30 V and (b) 1 min with an applied voltage of 30 V in a sonication bath.

(a)

(b)

Figure 6: SEM micrograph of cobalt ferrite nanoparticles deposited on an aluminum substrate using a 100% ethanol bath for (a) 1 min with an applied voltage of 35 V and (b) in a sonication bath for 2 min with an applied voltage of 33 V.

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(a)

(b)

Figure 7: SEM micrograph of cobalt ferrite nanoparticles deposited on a graphite paper substrate using a 100% ethanol bath for (a) 1 min with an applied voltage of 35 V and (b) in a sonication bath for 1 min with an applied voltage of 20 V.

Figure 8: Deposit density vs. time for platinum cobalt nanoparticles deposited on an aluminum substrate in CTAB bath chemistry.

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ECS Transactions, 58 (42) 1-9 (2014)

(a)

(b)

Figure 9: SEM micrograph of platinum cobalt nanoparticles deposited on an aluminum substrate from a CTAB bath (a) after 25 s with an applied voltage of 20 V and (b) in a sonication bath for 1 min with an applied voltage of 21 V.

Adhesion testing: Deposits on aluminum and graphite paper substrates made by EPD from both baths were tested for adhesion by soaking in water for 24 hours. Deposits weights did not notably change, which indicates reasonable adhesion of the deposits.

Electrocatalytic testing. Linear sweep voltammetry was used to compare the electrocatalytic activity of cobalt ferrite and platinum cobalt nanoparticles deposited on graphite paper using EPD. Cobalt ferrite was deposited at 35 V for 60 s from a 100% ethanol bath. Platinum cobalt was deposited with an applied voltage of 13V for 15 s from a CTAB bath. As shown in Figure 8, both cobalt ferrite and platinum cobalt nanoparticle deposits show enhanced electrochemical activity compared to graphite paper, that is a higher current density for all voltages scanned. As the particle size and loading of the cobalt ferrite and platinum cobalt nanoparticles are different, it is difficult to directly compare their electrocatalytic activity, which will be done in future work.

Conclusions Electrophoretic deposition of both cobalt ferrite and platinum cobalt nanoparticles was achieved from two different bath chemistries. Cobalt ferrite deposits from a 100% ethanol bath at a pH of 5 gave a 3-5 layer deposit that was evenly distributed across the substrate. The platinum cobalt particles on carbon were large and deposited in small agglomerates from both baths. Sonication of the bath during EPD resulted in smaller agglomerates that were more evenly distributed in comparison with deposits made without sonication. Linear sweep voltammetry showed that EPD deposits of both cobalt ferrite and platinum cobalt nanoparticles on graphite paper were electrocatalytic for the ammonia sulfite to ammonia sulfate anodic reaction.

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ECS Transactions, 58 (42) 1-9 (2014)

Figure 10: Electrocatalytic activity of EPD deposits showing current density vs. voltage.

Acknowledgments This work was funded by the Fuel Cell Technologies Office of the U.S. Department of Energy under grant number DE-FG36-07GO17002 through a subcontract with SAIC. The authors thank Dr. Richard Herz from the UC San Diego Nanoengineering Department, Dr. Lloyd Brown from Thermochemical Solutions LLC., Dr. Dave Genders and Dr. Peter Symons from Electrosynthesis Inc., and Roger Davenport and Robin Taylor from SAIC.

References

1. J. Littlefield, M. Wang, L.C. Brown, R.K. Herz and J.B. Talbot, Energy Procedia, 29, 616 (2012). 2. W.M. Haynes, Ed., CRC Handbook of Chemistry and Physics, 94th Edition, Taylor and Francis Group, LLC, p. 5-80 (2012). 3. M. Valden, X. Lai, and D.W. Goodman, Science, 281, 1647-1650 (1998). 4. R. Nedyalkova, C. Torras, J. Salvado, and D. Montane, Fuel Processing Technology, 91, 1040-1048 (2010). 5. R. Nedyalkova, A. Casanovas, J. Llorca, and D. Montane, International Journal of Hydrogen Energy, 34, 6 (2009). 6. Z. Zi, Y. Sun, X. Zhu, Z. Yang, J. Dai, and W. Song, Journal of Magnetism and Magnetic Materials, 321, 1251-1255 (2009).

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