Novel RuCoSe as non-platinum catalysts for oxygen reduction reaction in microbial fuel cells

Journal of Power Sources 362 (2017) 140e146

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Novel RuCoSe as non-platinum catalysts for oxygen reduction reaction in microbial fuel cells Shmuel Rozenfeld a, Michal Schechter a, Hanan Teller b, Rivka Cahan a, Alex Schechter b, * a b

Department of Chemical Engineering, Ariel University, Ariel 40700, Israel Department of Chemical Sciences, Ariel University, Ariel 40700, Israel

h i g h l i g h t s
Novel RuCoSe catalysts for microbial fuel cells were synthesized and characterized.
Comparable power to Pt in MFC and better tolerance to organic contaminations was shown.
Electrochemical study of kinetic current point to specific optimized RuxCoySe ratio.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2017 Received in revised form 2 July 2017 Accepted 4 July 2017

Microbial electrochemical cells (MECs) are explored for the conversion of acetate directly to electrical energy. This device utilizes a Geobacter sulfurreducens anode and a novel RuCoSe air cathode. RuCoSe synthesized in selected compositions by a borohydride reduction method produces amorphous structures of powdered agglomerates. Oxygen reduction reaction (ORR) was measured in a phosphate buffer solution pH 7 using a rotating disc electrode (RDE), from which the kinetic current (ik) was measured as a function of potential and composition. The results show that ik of RuxCoySe catalysts increases in the range of XRu ¼ 0.25 > x > 0.7 and y < 0.15 for all tested potentials. A poisoning study of RuCoSe and Pt catalysts in a high concentration acetate solution shows improved tolerance of RuCoSe to this fuel at acetate concentration 500 mM. MEC discharge plots under physiological conditions show that ~ RuCo2Se (sample S3) has a peak power density of 750 mW cm2 which is comparable with Pt 900 mW cm2. © 2017 Elsevier B.V. All rights reserved.

Keywords: Microbial electrochemical cell Catalysts Oxygen reduction reaction Acetate Geobacter sulfurreducens

1. Introduction Microbial fuel cells (MFCs) have been explored as an interesting device for converting waste water directly to electrical energy [1,2]. These devices utilize microbial biofilm at the anode to oxidize organic molecules such as acetate [3], phenols [4,5], toluene [6] and domestic or industrial waste water [7e9]. The most suitable electron acceptor in MFC cathodes is oxygen, which is readily available, non-toxic and offers high half-cell standard potential, while also being used in many other sustainable alternative energy technologies [10]. However, the sluggish oxygen reduction reaction (ORR) to water results in large energy loss in the form of over-potential at the cathode. This limitation is one of the most difficult barriers in MFC technology development [11].

* Corresponding author. E-mail address: [email protected] (A. Schechter). http://dx.doi.org/10.1016/j.jpowsour.2017.07.022 0378-7753/© 2017 Elsevier B.V. All rights reserved.

In the last decade, several cathode materials and cathode chamber designs have been proposed to enhance the ORR activity and power production in MFCs. The earliest ones used soluble electron acceptors such as ferricyanide [12], permanganate [13], dichromate [2] and persulfate [14]. A more advanced approach applies oxygen reduction catalysts to the electrodes using platinum (Pt), non-Pt bio-cathodes [15] and enzymatic activated cathodes [16]. In spite of the well-known high electrochemical activity of platinized cathodes, it is very unlikely to become practical in MFCs due to the high cost relative to the low overall power density produced by these devices. Several non-platinum catalysts have been proposed over the years as alternative ORR materials in MFC, which can be divided into metal-containing and carbon-based catalysts. Transition metal porphyrins and phthalocyanine, pyrolyzed organo-metallic complexes (e.g. CoTMPP and Iron(II) Phthalocyanine) [17] have been identified as the most promising carbon-

S. Rozenfeld et al. / Journal of Power Sources 362 (2017) 140e146

based catalysts. However, carbonized macrocyclic catalysts have a low density of ORR active sites and, therefore, require much higher loading (X 50) to reach similar currents produced by platinized cathodes. In addition, the cost of a carbonized precursor such as CoTMPP is nearly equal to that of Pt [18]. Other carbon-based catalysts have been extensively investigated. Among the most interesting materials is nitrogen-doped carbon nanotubes offering enhanced power output that approaches the level of Pt in MFC [18]. A somewhat different approach utilizing nitric acid treated graphite to enhance the ORR was proposed by Erable et al. [19]. All these carbon-based catalysts have very high surface area and contain some trace of iron or cobalt, which is said to facilitate the cathodic reaction. It is well documented that carbon steel can easily corrode under anaerobic conditions [20], and, therefore, may not be suitable for long term operation in MFC. Catalysts based on metal alloys such as Pt-Ni [21], Pt-Pd, and PtFe [22] were studied. A wide range of metal oxides were prepared and tested in MFCs, including several manganese dioxides MnO2 [23], spinel MnCoO4 and MnCo2O4 [24], Co3O4 [25] V2O5 [26], NiO2 [27] and PbO2 [28]. Some of these catalysts were presented as having the potential to replace Pt under the same experimental setup, despite their toxicity (e.g. PbO2) and limited stability (MnO2). Nevertheless, the vast majority of these publications present very little kinetic analysis of ORR parameters under a bacterial growing medium. Ruthenium metal is known to have moderate ORR catalytic activity and extremely high stability in acid and alkaline solutions [29]. Ruthenium-based transition metal chalcogenides ORR exhibit lower ORR overpotentials and relatively high current density. Ternary and binary Ru chalcogenides, such as Mo6-xRuxLy [30,31] and RuxLy where L ¼ Se [32], S [33], and Te [34] have been reported. Few of these catalysts, especially those which are seleniumbased, demonstrated both high electroactivity towards oxygen reduction as well as good tolerance to methanol contamination, which is a crucial issue in direct methanol fuel cells (DMFC). In his respect, RuxSey exhibits the best performance. In this work, we studied a new class of catalysts for ORR based on ruthenium-cobalt-selenide (RuCoSe) as a stable and cost effective alternative to Pt catalysts in MFCs. While cobalt-based catalysts have been reported in ORR and oxygen evolution reaction (OER) [35], RuCoSe catalysts have not been reported before in MFC or any other ORR context. We have systematically prepared and studied materials with several Ru to Co atomic compositions in a buffer solution and further compared their activity with Pt under microbial fuel operating conditions. 2. Materials and methods 2.1. Catalyst and electrode preparation RuCoSe catalysts with different Ru:Co:Se atomic ratios were prepared by dissolving CoCl2 (Acros Organics), RuCl3 (STREM) and H2SeO3 (STREM) in deionized water in proper stoichiometric ratios. A solution of 0.1 M NaBH4 (STREM) was gradually added to the stirred solution. The attained suspended solid particle products were separated by a centrifugation, washed with water and ethanol three times and dried. The Ru2Se catalyst was prepared by a microwave assisted polyol method. Hydrous RuCl3 salt was dissolved in ethylene glycol along with H2SeO3 to attain a Ru to Se atomic ratio of 2 to 1. The prepared solution was irradiated under Ar atmosphere in a microwave oven equipped with a condenser for 5 min. The attained powder was washed by water and ethanol and dried. A catalyst ink solution comprised of 60% catalyst, 20% carbon black (Vulcan XC72R, CABOT(, 10% Nafion (IonPower) and 10%

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Teflon (DuPont) was mechanically spread on 4 cm2 commercial Teflon treated carbon cloth (ELAT-LT-1400W, E-TEK). After drying, the final loading of the catalysts was 2.5 mg cm2.

2.2. Catalyst characterization Thermal analysis measurements were conducted using a thermal analyzer (Scinco Co. STA S-1500), combining simultaneous thermal gravimetric and heat flux measurement, under N2 atmosphere at a heating rate of 10 /min. SEM/EDS measurements were conducted using the JEOL JSM 7000F system. XRD measurements were conducted using the Bruker AXS D8 Advance Powder X-ray Diffractometer (using CuKa l ¼ 1.5418 Å radiation). RRDE measurements were performed using the PINE instruments MSRX rotator using a 5 mm diameter glassy carbon disc electrode equipped with a concentric Pt ring. Slow scan LSV (2 mV s1) measurements were conducted using the CHI 760C electrochemical analyzer. The catalyst is applied to the surface of the electrode from a suspension containing the catalyst (45%wt), carbon black (45%wt) and 10% Nafion from a commercial 5%wt solution mixed in a water/isopropanol solution. A drop of the slurry is sputtered on a 5 mm glassy carbon disc and dried at room temperature for 15min. A typical nano-catalyst loading is 50 mg/ cm2. A commercial Ag/AgCl electrode (Metrohm) and Pt wire (99.99, stream) are used as reference and counter electrodes, respectively. The electrodes were cycled for tens of CV cycles at 100 mV s1 in 0.5 M H2SO4 solution under a pure N2 atmosphere in order to activate the catalyst and reach a stable behavior.

2.3. Microbial fuel cell assembly and operation 2.3.1. Bacterial strain and growth conditions A pure culture of Geobacter sulfurreducens (DSMZ12127) was grown on Geobacter medium (826. GEOBACTER MEDIUM, DSMZ) (GM) in a sealed medium bottle at 30  C with agitation of 100 RPM. After 10 days, the bottom of the bottle was covered with red aggregates of bacterial cells. A fraction enriched with the aggregates was inoculated to the MFC anode chamber.

2.3.2. MFC set-up The MFC was comprised of a dual-glass chamber separated by a proton-selective membrane (Nafion 115; Ionpower, USA) (Fig. 1). The volume of each chamber was 250 ml. The anode chamber contained 150 ml GM and 50 ml PB pH 6.8 (50 mM final concentration), carbon cloth bacterial anode and Ag/AgCl reference electrode. The cathode chamber contained 200 mL of 50 mM PB pH 6.8 solution, and catalyst-coated carbon cloth (0.5 mg catalyst cm2). All parts were autoclaved prior to each experiment, except for the reference electrode, which was rinsed with 70% ethanol followed by distilled water. The anode and the cathode were connected through an external 1000 U resistor (Resistance Decade Box 727270, Tenma, USA). The MFC was placed in a thermostatic bath at 30  C and the anode chamber was agitated slowly using a magnetic stir bar. The cathode chamber was aerated through a 0.45-mmpore-size filter (Whatman, USA) to maintain an oxygenated environment while preventing contamination. The MFC steady state current-voltage polarization curve was measured using a decade resistance box from 4 MU (OCP/OCV) to 0 U. Stable voltage and current points were recorded for the anode, cathode and the complete cell at each resistance step after 10 min. Every experiment was performed at least in triplicate.

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Fig. 1. MFC dual-chamber scheme, left: anode chamber, right: cathode chamber.

3. Results and discussion 3.1. Synthesized and material characterization Synthesis of RuCoSe catalysts was carried out by a classical wet reduction process applying excess of sodium borohydride in an aqueous solution of RuCl3 and CoCl2 and H2SO4. Using this method produced colloidal particles, which were analyzed by Electron Dispersive Spectroscopy (EDS) to confirm their elemental composition. Table 1 provides a list of RuCoSe catalysts and the atomic ratio of each compound. Non-stoichiometric RuxCoySez catalysts were prepared with 0.24  X ¼ 0.66; 0  Y ¼ 0.49; 0.22  Z ¼ 0.35. These atomic ratios were selected to attain a metal to chalcogenide ratio higher than 2:1 in accordance with Ru2Se previously reported as an active catalyst in acid solution [36]. Compounds with selected Ru to Co atomic ratios were examined with the aim of finding the most active composition for ORR in MFC conditions. Fig. 2 which presents a typical SEM image of the attained catalysts shows agglomerated particles. These agglomerates consist of 40e60 nm semi spherical shaped particles. XRD analysis of these compounds depicted no specific diffraction peaks except for the microwave synthesized Ru2Se sample (S5), which displays peaks at 2Ɵ values of 37.7 (100), 42.2 (002), 43.4 (101), 57.9 (102), 68.1 (110) and 78.0deg; (103), indicative of Ru2Se (not shown). Materials attained by low temperature aqueous reduction have acquired an amorphous structure. Temperature annealing of these products results in phase separation and Table 1 Ru, Co and Se atomic percentage in the synthesized catalysts, calculated from EDX measurements. Sample

S1 S2 S3 S4 S5

Atomic % in catalyst Ru

Co

Se

43.2 49.5 28.3 24.0 66.7

35.3 15.4 48.8 46.2 -

21.5 35.1 22.9 29.8 33.3

Fig. 2. Typical SEM image of nano-catalyst Ru0.5Co0.15Se0.25 (sample S2).

formation of mostly RuSe and CoSe phases. Thermal analysis of RuCoSe samples was conducted using TGA/ DTA methods to study phase transition and stability of the catalysts in the temperature range of RT -500  C. Gradual decrease of mass by 13% is observed in the temperature range of 35e350  C in Fig. 3a. The TGA and DTA patterns do not vary significantly across different formulations containing selenium. In all samples (S1-S5), the thermograms show endothermic peaks around 200  C and 320  C indicative of SeO3 and SeO4 species present on the particle surface. Decomposition of these species results in mass loss seen in the TGA measurement. It was previously suggested by Tributsch et al. that surface selenous species and Ru-Se complexes present at different oxidation states lower the kinetic barriers for oxygen reduction, and result in the favored 4e reduction directly to water. This reaction requirement for two adjacent vacant Ru sites is disturbed by high Se surface concentration [37].

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Fig. 3. (a) TGA and (b) DTA curves of Ru0.24Co0.46Se0.30 -catalyst (sample S4).

3.2. Electrochemical characterization Samples S1-5 were evaluated with respect to oxygen reduction activity. Slow scan linear sweep voltammetries (LSV) were measured using a rotating disk electrode (RDE) coated by 50 mg/ cm2 of catalysts deposited on the glassy carbon disc electrode in a phosphate buffer solution (Fig. 4). All materials synthesized and evaluated in this study show some electroactivity towards ORR. However, the onset overpotentials of ORR on these catalysts seem to be relatively high in comparison to those of commercial Pt measured for comparison. The onset potential of Pt catalyst at pH 7 was 0.2 V followed by samples S2, S5 at 0.1 V and S1, S3, S4 at 0.17 V (vs. Ag/AgCl). Moreover, diffusion limiting currents, which serve as an indication of fast reaction, were observed only on Pt electrodes. Hence, in general, it can be seen that the reaction becomes more sluggish as the Ru relative concentration decreases. In order to evaluate the reaction's kinetic currents (ik) governing the ORR separately from the diffusion effect, ik as a function of potential was calculated from the KouteckyLevich equation:

1 1 1 1 1 ¼ þ ¼ þ i id ik ik Bu1=2 where i ¼ the measured current density, id ¼ diffusion limiting

Fig. 4. Current vs. Potential curves obtained from RDE measurements of oxygen reduction reaction on synthesized catalysts at pH ¼ 7 (Geobacter bacterium medium containing 0.1 M phosphate buffer, scan rate ¼ 2 mV s1, 600 rpm).

current, ik ¼ kinetic current, u ¼ rotation rate, and B ¼ 0.62nFC0(D0)3/2 n1/6 where n ¼ overall number of electrons transferred in ORR, F ¼ the Faraday constant, C0 ¼ bulk concentration of O2, D0 ¼ diffusion coefficient of O2, and n ¼ kinematic viscosity of the electrolyte. The intercept of a linear regression fitting applied to 1/i vs 1/u1/2 plot at u / 0 is equal to 1/ik. The collection of the kinetic currents at each selected potential provides the potential profile of pure activation controlled kinetic current of each catalyst. A three dimensional plot of the kinetic currents as a function of ruthenium or cobalt fraction in the relevant potentials range is depicted in Fig. 5. The kinetic currents of RuCoSe catalyst increases with Ru content range 0.25
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Fig. 5. Kinetic current densities for O2 reduction on Ru-Co-Se catalysts with different Ru and Co molar fractions, calculated from RDE/LSV measurements. 0.1 M phosphate buffer, scan rate ¼ 2 mV s1.

Fig. 6. Current vs. Potential curves obtained from RDE measurements of oxygen reduction reaction on Ru2Se (sample S5) and Ru-Co-Se (sample S4) catalysts at pH ¼ 7 in air and O2 saturated solutions (anode: Geobacter bacterium medium containing 0.1 M phosphate buffer, scan rate ¼ 2 mV s1, 600 rpm).

elucidate the structure and of the active site, which is beyond the scope of this work. Microbial anodes are expected to use a variety of hydrocarbon substrates. Yet acetate is the most widely reported as a model carbon source in MFC. Acetate ions are accessible to the cathode catalysts and are prone to react at high potentials of the oxygen electrode. An important aspect of the catalysts prepared in this work, with respect to operation in actual MFCs, is their stability in acetate solutions. Oxygen reduction on RuCoSe (sample S4) and Pt for comparison is measured in solutions containing sodium acetate in increasing concentrations using the RDE technique (Fig. 7). Both materials show gradual current decrease at high concentrations of acetate. However, the studied catalyst in Fig. 7b exhibits current decrease of 29% at an acetate ion concentration of 200 mM and

potential of 0.6 vs. Further increase in the acetate concentration to 500 mM has no effect on the voltammetric behavior of ORR on RuCoSe (S4) selected catalyst. Acetate influence on Pt seems to be causing a gradual decrease of the catalytic cathodic O2 reaction (Fig. 7a) that reaches 30% and 49% at 500 mM and 1 M respectively, at the same potential. We attribute this to a previously reported high tolerance of ORR on RuSe based catalysts in a high concentration of methanol [38]. The supernatant solution in MFCs contains a wide range of organic substances including amino acids, proteins and fatty acids, which can block the catalytic sites of Pt due to its well-known C-H bond activation property. Selected synthesized RuCoSe materials were measured as cathode materials in laboratory scale MFCs utilizing the same anode (Fig. 1). Commercial Pt was also measured for comparison. MFC anode performance is governed by the microbial environment development in the anodic chamber. Hence, it is somewhat difficult to compare the performance of different fuel cell assemblies. In order to test the cathode materials studied in this research, 3 parallel MFCs were set up in which the anode side was kept with no change whereas the different catalyst cathodes were replaced and tested. The comparison of different electrodes in all studied MFCs showed the same trends. A typical comparison is presented in Fig. 8. Fig. 8a shows the I-V cathode discharge curves of Pt and S3S5 and a typical anode curve. These curves emphasize the strong dependence of the I-V curves on the cathodic oxygen reduction process rather than on the anodic reaction with a low resistance (small slope). Fig. 8b and c shows the I-V cathode discharge curves of Pt and S3-S5 and the corresponding power curves. In contrast to the trend presented in Figs. 4 and 5, which displayed low ORR currents with decreasing Ru content in the buffer solution, in the MFC configuration the currents increase with the Ru content. Moreover, currents measured on the Pt cathode are only 15% higher in the case of the sample with a corresponding power of 0.75 W cm2 - only 20% lower than the peak power of S3 (Fig. 8c). We attribute this behavior to improved tolerance of this cathode in practical solutions containing constituents, in particular acetate, which hamper the ORR activity on Pt. Similarly, ORR on ruthenium

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Fig. 7. The effect of sodium acetate concentration on the oxygen reduction on (a) Pt and (b) Ru-Co-Se (sample S4) catalysts in 0.1 M phosphate buffer Geobacter medium.

Fig. 8. I-V steady state discharge of cathode half cell (a), full MFC (b) and power density vs. current (c) curves of MFCs containing Pt and Ru-Co-Se catalysts measured in Geobacter medium.

rich catalysts seem to be negatively influenced by the presence of these constituents, which has not been observed in pure PBS solutions. These results emphasize the need for stable catalysts in MFC practical conditions, where RuCoSe with optimized Ru to Co atomic ratio can be considered as a cost effective alternative to Pt. 4. Conclusions RuCoSe catalysts with selected atomic compositions were synthesized using simple sodium borohydride reduction and microwave assisted reduction processes. All the attained materials showed ORR activity in pH 7 buffer solution, yet electrocatalysts

with higher Ru content exhibit lower currents and higher overpotentials at various potentials of interest. Improved stability was seen in RuCoSe catalysts with respect to acetate substrate/fuel contamination. This property supports higher currents and power density in RuCoSe with low Ru content in MFC, due to higher tolerance of these catalysts to contaminations of the microbial cell solution. The sample S3 peak power result represents only 20% lower peak power values as compared to commercial Pt black powder. This relatively high activity of these catalysts in microbial cell solutions makes RuCoSe catalysts stable and cost effective cathode catalysts for MFCs.

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Acknowledgments The Authors would like to thank the Israel Ministry of Environmental Protection for the kind support of this work (grant number: 122-4-2).

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