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Effect of RuO2–CoS2 anode nanostructured on performance of H2S electrolytic splitting system
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Effect of RuO2eCoS2 anode nanostructured on performance of H2S electrolytic splitting system Jonathan Mbah a,*, Sesha Srinivasan b, Burton Krakow c, John Wolan c, Yogi Goswami c, Elias Stefanakos c, Narayana Appathurai d a
Department of Chemical Engineering, Tuskegee University, Luther H. Foster Hall, Suite 513, Tuskegee, AL 36088, USA Department of Physics, Tuskegee University, Tuskegee, AL 36088, USA c Clean Energy Research Center, University of South Florida, Tampa, FL 33620, USA d CSRF/Synchrotron Radiation Center, Stoughton, WI 53589, USA b
article info
abstract
Article history:
An innovative, nanostructured composite, anode electrocatalyst, material has
Received 9 June 2010
developed for the electrolytic splitting of (100%) H2S feed content gas operating at 135 kPa
been
Received in revised form
and 150 C. A new class of anode electrocatalyst with general composition, RuO2eCoS2 has
30 July 2010
shown great stability and desired properties at typical operating conditions. This config-
Accepted 5 August 2010
uration showed stable electrochemical operation over the period of 24 h and also exhibited a maximum current density of (0.019 A/cm2). The kinetic behaviors of various anode-based
Keywords:
electrocatalysts demonstrated that, exchange current density, which is a direct measure of
Electrocatalyst
the electrochemical reaction, increased with RuO2eCoS2-based anodes. Moreover, high
Nanostructured
levels of feed utilization were possible using these materials. Electrochemical performance,
Electrolytic cell
current density, and sulfur tolerance were enhanced compared to the other tested anode
H2S splitting
configurations. The structural, microstructural and surface behavior of RuO2eCoS2 anode
RuO2eCoS2
electrocatalyst was investigated in detail. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Anode
1.
Introduction
The ability to electrochemically split H2S in a fuel cell was carried out in the late 1980s [1]. Subsequently, this led to the study of other electrocatalysts for H2S splitting in electrolytic cell. There are very few investigations on H2S splitting using a solid membrane. Most of the previous studies on H2S splitting were based on aqueous media. Eltron Research, Inc. investigated the application of mixed (oxide and proton) solid electrolytes using the Claus process in an electrolytic system operating at temperature above 700 C [2]. A significant decrease in the cell voltage with an increase in the H2S feed
made them to suggest that the anode electrocatalyst sites were being deactivated by elemental sulfur. Active anode materials (Li2S/CoS1.035 and WS2) were found to enhance H2S electrolytic cell performance tremendously [3]. But de-lamination of the electrolytic cells was a major concern at these operating temperatures because of the increased sulfur viscosity. In a bid to circumvent deactivation of catalyst by sulfur, a solid oxide H2SeO2 fuel cell, utilizing Nafion membrane, operating at 120e145 C with anode catalysts Pd/ C, Pt/C, Pd/Pt/C and MoS2, admixed with 35% Teflonized carbon was found to show sulfur tolerance. However, a high operating pressure range (235e510 kPa) was required to keep
* Corresponding author. Tel.: þ1 334 727 8972; fax: þ1 334 724 4188. E-mail addresses: [email protected], [email protected] (J. Mbah). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.023
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the membrane hydrated [4]. A pyrochlore-based anode material, Gd2Ti1.4Mo0.6O7, showed remarkable tolerance to sulfur-containing fuels used in fuel cells application [5]. The anode/electrolyte interfacial resistance was only 0.2 U cm2 at 950 C in a fuel gas mixture of 10% H2S and 90% H2, demonstrating a peak power density of 342 mW/cm2. The fuel cell operated under these conditions continuously for 6 days without any observable degradation, suggesting that Gd2Ti1.4Mo0.6O7 anode exhibits not only excellent stability but also good catalytic activity toward the splitting of H2S containing gas. But a major draw back in this application is the high temperature of operation. In order to be a suitable candidate for the electrochemical splitting of H2S, an anode material must possess good electrical conductivity and sulfur tolerance at operating temperatures, in addition to good catalytic activity. Pt anode catalysts have potential applications but are susceptible to sulfur poisoning and most metals and their oxides are severely corroded by H2S at elevated temperatures [6,7]. Cobalt and iron fischer-tropsch catalysts showed that average molecular weight of hydrocarbon increased to a maximum when colbalt supported catalyst was exposed continuously to H2S while loss of activity was observed with unpromoted iron as a result of sulfur poisoning [8]. Transition metals of bimetallic sulfides have shown good promise in splitting H2S [9]. As reported by the authors the increased active surface area of the catalyst was responsible for the high activity obtained indicating that bimetallic transition metals have great potential in splitting H2S gas to elemental sulfur and hydrogen. In another study, the high activity exhibited by composite metals was attributed to the formation of heteronuclear metalemetal bond which consequently affects the reactivity of the bonded metals towards sulfur and ultimately minimizes sulfur poisoning [10]. A US Patent [11] reported the use of disordered multicomponent material which included at least a transition element with another element that served as a modifier and consists of sulfur and oxygen to split H2S gas. A high number of catalytic active site was available some of which were given up to counter the effect of sulfur poisoning of the active sites. They reported improvement in the kinetic data and system performance which primarily resulted from using these disordered materials. The system operated uninterrupted over an extended period of time. None of these studies incorporated the usage of hydrogen proton selective membrane to split H2S. However, the bottom line as proven from all these previous studies is that high density of catalytic active sites and catalytic activity is desirable for H2S splitting. This present study was designed to improve the rate of electrode reactions of H2S splitting by developing high active catalytic anode materials using the technique of mechanical attrition to synthesize nanostructured composite with high catalytic efficiency [12]. To enhance the active sites of the electrocatalyst, we designed our composite material such that one of its constituents served as a modifier while the other was used as a host material. Our intention is to synthesize a disordered material with high catalytic surface [13] that could be used as anode electrocatalyst in conjunction with a hydrogen proton selective membrane to split H2S to elemental sulfur and hydrogen in an electrochemical cell.
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It was found in our previous publication [14] that the current density, which reflects the rate at which electrochemical reaction takes place in the cell, was significantly reduced in the presence of sulfur. This made us to conclude that the electrocatalyst was rendered inactive as soon as sulfur was produced due to the blockage of the catalytic active sites. Thus, sulfur poisoning drastically minimized the catalytic efficiency of the material. Furthermore, at the cell anode, sulfur poisoning tends to increase the polarization resistance which physically affects the anode structure bringing about de-lamination of the cell electrode. As a result, an insulation layer was formed which inhibited the transport of hydrogen protons and electrons through the electrolyte and the electrode respectively causing the cell performance to drop. Herein, we have demonstrated that a laboratory synthesized nanostructured composite material, RuO2eCoS2, with high catalytic active surface, was used to improve the efficiency of H2S electrolytic splitting system at 150 C and 135 kPa with no deleterious effect in the electrode performance after an extended period of operation. In this study, RuO2 and CoS2 were used as the modifier and the host materials respectively.
2.
Experimental details
2.1.
Materials
The sources of starting materials and the purity of the catalysts materials used were: ruthenium (IV) oxide, RuO2, (Sigma Aldrich, 99.9%), platinum black, Pt black, (Sigma Aldrich, 99.9%), p-dichlorobenzene (Sigma Aldrich 99.8%), cobalt disulfide, CoS2, (Sigma Aldrich, 99%), ethanol (Sigma Aldrich, 98%), acetone (Sigma Aldrich, 95.5%). These materials were used as received. H2S (Specialty Gases of America, 99.5%), auto-ignition temperature, 260 C, density, 1.363 g/L was used.
2.2.
Catalyst preparation
A mixture of p-dichlorobenzene, CoS2, and RuO2 was used to prepare the anode electrocatalyst. The ratio of the respective components was 0.5:3:3 by mass. The anode catalyst materials, with the ratios specified, are mixed in porcelain mortar and milled for 5 h in a high-energy planetary ball mill. The mechanical attrition process was initiated by loading the specified ratio quantities into a ball mill referred to as Fritsch pulversette planetary mono mill, P6 in an inert atmosphere. 10 g of powders and 15 stainless steel balls (1.5 cm in diameter, 14 g in mass) were sealed under inert, into the stainless steel vials (80 cm3 volume). The rotation speed of the milling vials (u, anticlockwise rotation) which are fixed onto a rotating disk and the rotation speed of disk (U, clockwise rotation) can be set independently. Milling condition was associated by 3 essential parameters (U (rpm)/u (rpm)/Dt (h)) where Dt was the duration of the milling process. The milling parameters such as ball to powder masses ratio and milling speed were optimized to 1/20, [15] and 300 rpm, respectively. To get a uniform mixture, ethanol was added to the combination to make a suspension that was mixed thoroughly. Then ethanol was allowed to evaporate slowly to leave the mixture as powders.
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The powdery mixture were heat-treated in N2 atmosphere at 150 C for 2 h and allowed to cool under N2 to room temperature in an autosorb-1C apparatus (Quantachrome Inc.) in the out gassing station. The resulting composite material was then used to prepare the anode catalysts. The fine crystal powder was stored in a dessicator prior to characterization. The specific surface area determinations of the composite material were carried out with Autosorb-1 software using a BET Multipoint approach. Prior to this, the p-dichlorobenzene was removed under gentle heating (12 h) at 70 C in an inert atmosphere, followed by a 2 C/min ramp to 174 C, the boiling point of p-dichlorobenzene, leaving behind open porosity in the electrocatalyst layers.
2.3. XRD structural characterization of RuO2eCoS2 nanocomposite The powder X-ray diffraction (XRD) of the synthesized RuO2eCoS2 nanoclusters was carried out by a Philips X’pert diffractometer with Cu Ka radiation of known wavelength, ˚ . The incident and the diffraction slit width l ¼ 1.54060 A employed for the measurements are 1 and 2 respectively and mask size is 0.10 cm. Phase identification and particle size calculations have been carried out by PANanalytical X’pert Highscore software version 1.OF [16].
2.4.
Microstructural characterization (SEM)
The morphology of the synthesized material was obtained by Hitachi S800 scanning electron microscopy (SEM). A fixed working distance of 10 mm and a voltage of 25 kV were used. EDAX genesis software was used to analyze the SEM images.
2.5.
Electrochemical method
Our electrochemical cell includes a test-bed with porous electrodes in which a solid proton conducting membrane separates the electrodes. A simplified schematic of the laboratory-fabricated electrochemical cell is shown in Fig. 1. The process consists of passing H2S gas through the anode chamber to contact a catalytic anode, where electrochemical reaction takes place to produce elemental sulfur, protons and electrons. The protons pass through cesium hydrogen sulfate (CsHSO4)-based membrane from the anode chamber to the cathode chamber, where they react with electrons from the
Fig. 1 e Schematic of electrolyzer cell module.
catalytic cathode to produce hydrogen gas. The CsHSO4-based membrane separating both electrodes is impermeable to fuel and product flows, but allows the transport of ionic species between both sides of the cell. Both the anode and cathode chambers are filled with carbon felt which conducts electron. During the process, both the anode and the cathode are maintained at 135 kPa and 150 C, a temperature at which sulfur has a low viscosity and the solid electrolyte is in its hyperprotonic phase [17]. LabView software from National Instrument was used for in-situ electrochemical measurements. Such measurements include current, cell pressures and total cell resistances. Liquid sulfur is collected from the anode compartment and hydrogen is removed from the cathode compartment. Samples were withdrawn from both anode and cathode compartments periodically for analysis.
3.
Results and discussion
3.1.
Surface area characterization
The catalytic active surface area of the RuO2eCoS2 nanocomposite was obtained by using a multipoint BET method which requires a minimum of three points in the appropriate relative pressure range as shown in Fig. 2. At high relative pressures, the rate of adsorption equals the rate of desorption which is very essential for material catalytic performance. The specific surface area calculated from the total surface area for the composite was (10.96 m2/g). This was sufficient to sustain the reaction without interruption by providing sufficient compensation for sulfur poisoning as well as providing enough area for the reaction to proceed. Table 1 shows a list of different anode configurations and their corresponding BET surface areas. Our intention and the focus of the catalyst preparation were to use the mechanical attrition to minimize the particle sizes of the composite material in order to increase the exposed surfaces which are crucial in enhancing the electrode kinetics, and also to reduce the resistance of the material. Since resistance impedes the flow of electrons, the reduced particle size offers lower resistance when compared to the individual resistances from pure ball milled RuO2 and CoS2. The active surfaces of the
Fig. 2 e Multipoint BET surface area plot for anode metal sulfide (RuO2eCoS2) nanocomposite.
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Table 1 e Various anode configurations and their BET catalytic active surface areas.
individual tested ball milled RuO2 and CoS2 are small compared to the prepared composite. The sulfur blockage observed with the pure RuO2 and CoS2 may be attributed to the insufficient sites available to accommodate the effect of sulfur poisoning which subsequently lead to overall poor cell performance. As mentioned previously, this occurrence decreases the rate of chemical reaction which adversely affects the overall performance of the system.
3.2.
Resistivity of anode catalysts
The electrical resistivity of anode catalyst samples with the structure Au/anode catalyst/Au are listed in Table 2. The resistivity of composite catalyst pellet RuO2eCoS2 at 150 C measured by LabView software acquired from National Instruments is less compared to the values obtained for pure ball milled CoS2 and RuO2 electrocatalysts. As explained in section [3.1], reduced particle sizes offer low resistances to flow of electrons when compared to larger particles. Since the mobility of free electrons will depend on carrier concentration, smaller particle sizes yield more molar carrier concentration than larger sizes. In the light of this observation, the present anode catalyst with the RuO2eCoS2 configuration was utilized due to superior electrical conductivity for use as anode catalysts under operating conditions. The contributions from these resistances are small compared to the total cell resistances. This suggests that the large difference in current densities observed may not be significantly due to the resistances of these materials, but primarily due to the availability of catalytic active sites provided by the composite anode materials. However, it was necessary to reduce the internal ohmic resistance of the cell slightly using RuO2eCoS2 anode composite. Note, the total resistances experienced by the electrolytic cell results from contributions due to, activation overvoltage loss (hact), ohmic loss (hohmic) and from concentration loss (hcon). Maximum current density as shown in Table 3 at 0.9 V measured for the RuO2eCoS2 composite was 0.019 A/m2, which was higher than that for CoS2 (0.009 A/cm2) using the same electrolyte thickness. To this end, we have achieved minimizing the
Table 2 e Cell area specific ohmic resistance (RA). This shows different anode electrocatalysts at operating temperature of 150 C.
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Table 3 e Current density and conversion efficiency for different electrocatalyst configurations.
contributions from both hact and hcon by using the anode composite RuO2eCoS2.
3.3.
Stability of H2S electrolytic cell anode materials
Electrocatalysis offers a convenient route to the synthesis of H2S. Requisites of the anode material are a low overpotential for product evolution and high stability. We have tested three groups of anode electrocatalysts for their suitability as endurance materials for H2S splitting using a laboratory fabricated electrolytic cell apparatus as described in section [2.5]. The results of the electrochemical application are discussed below.
3.3.1.
X-ray diffraction characterization
XRD was employed for structural characterization in terms of phase identification, structure determination, and crystal orientation. Fig. 3 (a, b, and c) illustrates the XRD pattern of synthesized composite, as prepared, after 12 h exposure to H2S, and after 24 h exposure to H2S feed content gas, respectively. The diffractogram shows no phase or compositional changes after exposure to H2S gas at the cell operating temperature of 150 C. The structural analysis of RuO2eCoS2 was carried out by varying diffraction angle 2q from 25 to 75 . The average crystal size of the RuO2eCoS2 powder was determined by using the Scherrer formula. The average crystal size of 20e60 nm was obtained and consists of welldefined diffraction peaks. XRD analysis shows that the structure is preserved for the three samples with different sample history. As a result, the stability of RuO2eCoS2 when exposed to H2S (for prolonged periods of time) made it an attractive anode candidate for H2S splitting.
Fig. 3 e XRD pattern of RuO2eCoS2 composite: (a) as prepared; (b) after 12 h exposure to 100% H2S feed content at 150 C; (c) after 24 h exposure to 10 H2S feed content at 150 C.
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3.3.2.
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SEM analysis
The composite RuO2eCoS2 has a relatively high surface and expanded area metallic nanoparticles facing away from the electrolyte. Fig. 4(aed) shows SEM micrographs of a porous electrode (composite nanoparticles on carbon mesh support for strength). These electrodes have tortuous pathways within them to expose orders of magnitude larger surface areas to reacting H2S and to also allow the escape of the liquid sulfur product. The increased numbers of catalytically active sites as pointed out earlier enables the materials to be more resistant to poisoning. This is because these materials exhibit a certain number of catalytically active sites that can be sacrificed to the effects of poisonous sulfur production shown in Fig. 4 (b) while a large number of unpoisoned sites are preserved. In Fig. 4 (c and d) which consists of non composite materials, the catalytic efficiency of the material is substantially less than that possible if a greater number of catalytically active sites were available. This is due to concentration polarization caused by sulfur blockage of the diffusion layers. In all the experimental runs involving RuO2eCoS2 composite, there was only a trace amount of sulfur deposition on the anode surface.
3.3.3.
Electrochemical performance of anode electrocatalysts
Fig. 5 compares the performance of four cells using, RuO2eCoS2 (Fig. 5a), ball milled CoS2 (Fig. 5b), ball milled RuO2 (Fig. 5c) and no ball milled RuO2 (Fig. 5d), as anode materials. There were no signs of de-lamination of the anode in all cases investigated. The performance of the RuO2eCoS2
nanocomposite anode was far superior to any of the anode materials. All the configurations maintained their performances for over 12 h. If no noticeable decline in output after this time period was observed, the cell was deemed adequate for performance testing. Both systems exhibited similar electrochemical behavior and deviations were probably caused by external factors (current collection, lead wires, etc.). Since the main goal of this study was to identify and develop catalytic active materials for H2S splitting in electrolytic cell, a large number of anode materials and experimental cells used in this study required the use of low cost materials with no system optimization configurations. For commercial applicability of this work, full system optimization will need to be carried out. Fig. 6, depicts the effects of various anode configurations on current densities, j, as a function of external applied voltages. Curve (a) represents the performance of the cell based on RuO2eCoS2 anode. Curves (b-d) represent the performances of cells based on ball milled CoS2, ball milled RuO2 and no ball milled RuO2 anodes respectively. At the low region where there are uniform potential gradients, current densities are proportional to potential gradients and are uniform; hence Ohm’s law is appropriate. At high voltages, concentration gradient comes into play. By utilizing this nanostructured anode composite, we were able to maintain a high current density at operating potential. This is because the catalytic surface area provided by the composite highly minimized the activation barrier and also reduced most of the effects associated with concentration polarization; more gases were able
Fig. 4 e Fig. 3 SEM images of surfaces of electrode. (a) RuO2eCoS2 electrocatalyst nanocomposite before electrolysis; (b) RuO2eCoS2 electrocatalyst nanocomposite after electrolysis; (c) ball milled RuO2 electrocatalyst after electrolysis; (d) ball milled CoS2 electrocatalyst after electrolysis.
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20
Transfer of electrons from the chemisorbed hydrogen atoms to the electrode, releasing Hþ into the electrolyte:
a
18
2 M.H/ M þ e þ Hþ
Current density, m A/cm
2
16 14 12
(4)
Mass transport of the Hþ ions away from the electrode:
10
2 Hþ /Hþ
b
8 6 4
c d
2 0 0
2
4
6
8
10
12
14
Time, h
Fig. 5 e Testing for four cells with different anodes (Vcell [ 0.9 V, T [ 150 C, Fuel [ 0.25 cm3/min) H2S was the fuel. (a) RuO2eCoS2; (b) ball milled CoS2; (c) ball milled RuO2; (d) no ball mill RuO2.
to diffuse to the reaction interface and the reaction equilibrium is shifted toward product.
3.3.4.
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Charge transfer reactions
The anode electrode reactions consist of basic steps as shown below. Mass transport of H2S to the electrode: H2 SðbulkÞ /H2 Sðnear electrodeÞ
(1)
Absorption of H2S onto the electrode surface: H2 Sðnear electrodeÞ þ M/M.H2 S
(2)
Separation of the H2S molecules into 2 individually bound (chemisorbed) hydrogen atoms and the release of sulfur on the electrode surface: ðM.H2 þ M/2MHÞ þ 1=8S8
(3)
The overall reaction is limited by electron transfer step between chemisorbed hydrogen and the metal electrode (Eq. (4)). In this equation, M.H represents a hydrogen atom chemisorbed on the metal surface and (M þ e-) represents a liberated metal surface site and a free electron in the metal. A decrease in activation barrier represents the catalytic influence of the surface of the electrode. A catalytic electrode is one which significantly lowers the activation barrier for the reaction. Because it appears as an exponent in Butler-Volmer kinetics, even small decreases in the activation barrier can cause large effects. Using a highly catalytic electrode therefore provides a way to dramatically increase the exchange current density ( j0) as evidenced in the improved value of current density ( j ) obtained in Fig. 6a by utilizing RuO2eCoS2 composite material when compared to other tested configurations.
3.3.5. Tafel slope and exchange current densities for anode configurations Improving kinetic performance stems from increasing j0. To increase the value of j0, we have increased the number of possible reaction sites (i.e., increase the reaction interface roughness) by incorporating a novel composite metal sulfide, RuO2eCoS2 electrocatalyst, in the electrode. The exchange current density was determined experimentally by extrapolating plots of _n j vs. h to h ¼ 0, where h is the activation overvoltage. This is a direct measure of the reaction rate constant at the electrolytic cell electrode. Fig. 7 shows plots for the three anode configurations. In all three cases of anode configurations n ¼ 2 representing the number of electrons transferred in the electrochemical reaction. The anode composite consisting of RuO2eCoS2 shows great potential in reducing the activation barrier as a result of the increased catalytic active sites available for the reaction to proceed without sulfur poisoning. This is reflected by the high exchange current density (2.65 102 A/ cm2) realized compared to the values for the ball milled CoS2 and RuO2 configurations (1.61 102 A/cm2) and (8.4 103 A/ cm2) respectively. More also, the value of the transfer coefficient (a) in Fig. 7 for the composite material is (0.14) which is higher than that obtained for ball milled CoS2 and RuO2 (0.08) and (0.07) respectively, suggesting that net current density is higher with the composite material, hence signifying a rapid electrochemical reaction compared to other tested anode materials.
3.4.
Fig. 6 e Current density for different anode configurations with 100% H2S feed gas content. (a) RuO2eCoS2; (b) ball milled CoS2; (c) ball milled RuO2; (d) no ball milled RuO2.
(5)
Fuel utilization
Table 3 summarizes the measured data in tabular form at specific operating voltage, comparing typical conditions to tested parameters. The conversion efficiency depends on many factors which include the flow rate of H2S to the reaction sites, the availability of active sites, and the thickness of
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Fig. 7 e Tafel plots for anode configurations at operating temperature 150 C and 100% H2S feed content.
the electrolytes. As shown in Table 3, for the same electrolyte thickness and active area, H2S consumption was more than doubled for RuO2eCoS2 when compared to both ball milled RuO2 and CoS2. This suggests that the difference could be attributed to faster electrode kinetics observed with RuO2eCoS2 as explained in sections [3.1 and 3.3.2]. The application of the composite RuO2eCoS2 increased the current density by over an order of magnitude and the efficiency of the system doubled and tripled respectively when compared to both ball milled RuO2 and CoS2. The increased electrode kinetics suggests large active sites available for the reaction to progress, some of which were sacrificed to the effect of sulfur poisoning. This was also supported by the SEM image (see Fig. 4) after 12 h of electrolysis. The results of our measurements show that electrocatalyst configuration prepared from high surface composite nanometals (RuO2eCoS2) demonstrated the optimum performance, and also exhibited sulfur tolerance after extended hours of operation.
4.
Conclusion
We have successfully synthesized a novel nanostructured anode electrocatalyst for the electrolytic splitting of (100%) H2S feed content gas operating at 135 kPa and 150 C. This class of material with general composition, RuO2eCoS2 is stable and has desired properties at tested conditions. It was able to deliver a maximum current density of 0.019 A/cm2 at 0.9 V while maintaining its integrity. The difference in electrochemical performance of this cell compared to other tested materials is directly attributable to the anode material developed in this study. The kinetic behavior of the three anode-based electrocatalysts as captured in the Tafel plots showed that the novel composite material favors the kinetic of the anode reaction.
This implies that by using this material, high H2S feedstock utilization is possible. The overall analyses show that there is an improvement in electrochemical performance, current density, and sulfur tolerance when compared to the other tested anode configurations.
Acknowledgements Authors acknowledge the US Department of Energy (DOE Award #: DE-FG26-05NT42540) for funding. USF Nanomaterial and Nanomanufacturing Research Center for analytical studies is highly acknowledged. We wish to thank Alfred P. Sloan Foundation for financial support.
references
[1] Pujare NU, Semkow KW, Sammells AF. A direct H2S/air solid oxide fuel cell. J Electrochem Soc 1987;134:2639. [2] A.F. Sammels. Electrochemical H2S conversion. US Patent 4920015; 1990. [3] Yates C, Winnick J. Anode materials for a hydrogen sulfide solid oxide fuel cell. J Electrochem Soc 1999;146:2841. [4] Chuang KT, Donini JC, Sanger AR, Slavov SV. A protonconducting solid state H2S-O2 fuel cell: 2. production of liquid sulfur at 120e1458 C. Int J Hydrogen Energy 2000;25: 887e94. [5] S. Zha, Z. Cheng, M. Liu. Gd2Ti2-XMoXO7-based anode materials for H2S-air solid oxide fuel cells. 2005;1(7):293e302. [6] Kirk TJ, Winnick J. Use of metal sulfides as anode catalysts in H2S SOFC. J Electrochem Soc 1993;140:3494. [7] Urfels L, Ge´lin P, Primet M, Tena E. Complete oxidation of methane at low temperature over Pt catalysts supported on high surface area SnO2. Top Catal 2004;30-31(1):427e32. [8] Bartholomew CH, Bowman RM. Sulfur poisoning of cobalt and iron Fischer-Tropsch catalysts. Appl Catal 1985;15(1): 59e67.
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[9] Chivers T, Hyne JB, Lau C. The thermal decomposition of hydrogen sulfide over transition metal sulfides. Int J Hydrogen Energy 1980;5:499. [10] Rodriguez JA. The chemical properties of bimetallic surfaces: importance of ensemble and electronic effects in the adsorption of sulfur and SO2. Prog Surf Sci 2006;81:141e89. [11] V. Srinivasan, P. Naum, S. Krishna. Hydrogen sulfide decomposition cell and catalytic materials therefor. US Patent 4544461; 1985. [12] Kuhn WE, Friedman IL, Summers W, Szegvari A. Powder metallurgy (ASM metals handbook 7). Metals Park, OH: American Society of Metals; 1985. p. 56.
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[13] Jurado JF, Ortiz E, Vargas RA. Measurement science and technology. Meas Sci Technol 1997;8:1151e5. [14] Mbah J, Krakow B, Stefanakos E, Wolan J. Electrolytic splitting of H2S using CsHSO4 membrane. J.Electrochem Soc 2008;155(11):E166e70. [15] Shingu PH. In: Mechanical alloying. Mater. Sci. Forum; 1991. p.88e90. [16] PANalytical X’pert Highscore (PW 3209) Software. Version 1. 0F; 2004. [17] Uda T, Boysen DA, Haile SM. Thermodynamic, thermomechanical, and electrochemical evaluation of CsHSO4. Solid State Ionics 2005;176:127e33.
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journal homepage: www.elsevier.com/locate/he
Effect of RuO2eCoS2 anode nanostructured on performance of H2S electrolytic splitting system Jonathan Mbah a,*, Sesha Srinivasan b, Burton Krakow c, John Wolan c, Yogi Goswami c, Elias Stefanakos c, Narayana Appathurai d a
Department of Chemical Engineering, Tuskegee University, Luther H. Foster Hall, Suite 513, Tuskegee, AL 36088, USA Department of Physics, Tuskegee University, Tuskegee, AL 36088, USA c Clean Energy Research Center, University of South Florida, Tampa, FL 33620, USA d CSRF/Synchrotron Radiation Center, Stoughton, WI 53589, USA b
article info
abstract
Article history:
An innovative, nanostructured composite, anode electrocatalyst, material has
Received 9 June 2010
developed for the electrolytic splitting of (100%) H2S feed content gas operating at 135 kPa
been
Received in revised form
and 150 C. A new class of anode electrocatalyst with general composition, RuO2eCoS2 has
30 July 2010
shown great stability and desired properties at typical operating conditions. This config-
Accepted 5 August 2010
uration showed stable electrochemical operation over the period of 24 h and also exhibited a maximum current density of (0.019 A/cm2). The kinetic behaviors of various anode-based
Keywords:
electrocatalysts demonstrated that, exchange current density, which is a direct measure of
Electrocatalyst
the electrochemical reaction, increased with RuO2eCoS2-based anodes. Moreover, high
Nanostructured
levels of feed utilization were possible using these materials. Electrochemical performance,
Electrolytic cell
current density, and sulfur tolerance were enhanced compared to the other tested anode
H2S splitting
configurations. The structural, microstructural and surface behavior of RuO2eCoS2 anode
RuO2eCoS2
electrocatalyst was investigated in detail. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Anode
1.
Introduction
The ability to electrochemically split H2S in a fuel cell was carried out in the late 1980s [1]. Subsequently, this led to the study of other electrocatalysts for H2S splitting in electrolytic cell. There are very few investigations on H2S splitting using a solid membrane. Most of the previous studies on H2S splitting were based on aqueous media. Eltron Research, Inc. investigated the application of mixed (oxide and proton) solid electrolytes using the Claus process in an electrolytic system operating at temperature above 700 C [2]. A significant decrease in the cell voltage with an increase in the H2S feed
made them to suggest that the anode electrocatalyst sites were being deactivated by elemental sulfur. Active anode materials (Li2S/CoS1.035 and WS2) were found to enhance H2S electrolytic cell performance tremendously [3]. But de-lamination of the electrolytic cells was a major concern at these operating temperatures because of the increased sulfur viscosity. In a bid to circumvent deactivation of catalyst by sulfur, a solid oxide H2SeO2 fuel cell, utilizing Nafion membrane, operating at 120e145 C with anode catalysts Pd/ C, Pt/C, Pd/Pt/C and MoS2, admixed with 35% Teflonized carbon was found to show sulfur tolerance. However, a high operating pressure range (235e510 kPa) was required to keep
* Corresponding author. Tel.: þ1 334 727 8972; fax: þ1 334 724 4188. E-mail addresses: [email protected], [email protected] (J. Mbah). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.023
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the membrane hydrated [4]. A pyrochlore-based anode material, Gd2Ti1.4Mo0.6O7, showed remarkable tolerance to sulfur-containing fuels used in fuel cells application [5]. The anode/electrolyte interfacial resistance was only 0.2 U cm2 at 950 C in a fuel gas mixture of 10% H2S and 90% H2, demonstrating a peak power density of 342 mW/cm2. The fuel cell operated under these conditions continuously for 6 days without any observable degradation, suggesting that Gd2Ti1.4Mo0.6O7 anode exhibits not only excellent stability but also good catalytic activity toward the splitting of H2S containing gas. But a major draw back in this application is the high temperature of operation. In order to be a suitable candidate for the electrochemical splitting of H2S, an anode material must possess good electrical conductivity and sulfur tolerance at operating temperatures, in addition to good catalytic activity. Pt anode catalysts have potential applications but are susceptible to sulfur poisoning and most metals and their oxides are severely corroded by H2S at elevated temperatures [6,7]. Cobalt and iron fischer-tropsch catalysts showed that average molecular weight of hydrocarbon increased to a maximum when colbalt supported catalyst was exposed continuously to H2S while loss of activity was observed with unpromoted iron as a result of sulfur poisoning [8]. Transition metals of bimetallic sulfides have shown good promise in splitting H2S [9]. As reported by the authors the increased active surface area of the catalyst was responsible for the high activity obtained indicating that bimetallic transition metals have great potential in splitting H2S gas to elemental sulfur and hydrogen. In another study, the high activity exhibited by composite metals was attributed to the formation of heteronuclear metalemetal bond which consequently affects the reactivity of the bonded metals towards sulfur and ultimately minimizes sulfur poisoning [10]. A US Patent [11] reported the use of disordered multicomponent material which included at least a transition element with another element that served as a modifier and consists of sulfur and oxygen to split H2S gas. A high number of catalytic active site was available some of which were given up to counter the effect of sulfur poisoning of the active sites. They reported improvement in the kinetic data and system performance which primarily resulted from using these disordered materials. The system operated uninterrupted over an extended period of time. None of these studies incorporated the usage of hydrogen proton selective membrane to split H2S. However, the bottom line as proven from all these previous studies is that high density of catalytic active sites and catalytic activity is desirable for H2S splitting. This present study was designed to improve the rate of electrode reactions of H2S splitting by developing high active catalytic anode materials using the technique of mechanical attrition to synthesize nanostructured composite with high catalytic efficiency [12]. To enhance the active sites of the electrocatalyst, we designed our composite material such that one of its constituents served as a modifier while the other was used as a host material. Our intention is to synthesize a disordered material with high catalytic surface [13] that could be used as anode electrocatalyst in conjunction with a hydrogen proton selective membrane to split H2S to elemental sulfur and hydrogen in an electrochemical cell.
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It was found in our previous publication [14] that the current density, which reflects the rate at which electrochemical reaction takes place in the cell, was significantly reduced in the presence of sulfur. This made us to conclude that the electrocatalyst was rendered inactive as soon as sulfur was produced due to the blockage of the catalytic active sites. Thus, sulfur poisoning drastically minimized the catalytic efficiency of the material. Furthermore, at the cell anode, sulfur poisoning tends to increase the polarization resistance which physically affects the anode structure bringing about de-lamination of the cell electrode. As a result, an insulation layer was formed which inhibited the transport of hydrogen protons and electrons through the electrolyte and the electrode respectively causing the cell performance to drop. Herein, we have demonstrated that a laboratory synthesized nanostructured composite material, RuO2eCoS2, with high catalytic active surface, was used to improve the efficiency of H2S electrolytic splitting system at 150 C and 135 kPa with no deleterious effect in the electrode performance after an extended period of operation. In this study, RuO2 and CoS2 were used as the modifier and the host materials respectively.
2.
Experimental details
2.1.
Materials
The sources of starting materials and the purity of the catalysts materials used were: ruthenium (IV) oxide, RuO2, (Sigma Aldrich, 99.9%), platinum black, Pt black, (Sigma Aldrich, 99.9%), p-dichlorobenzene (Sigma Aldrich 99.8%), cobalt disulfide, CoS2, (Sigma Aldrich, 99%), ethanol (Sigma Aldrich, 98%), acetone (Sigma Aldrich, 95.5%). These materials were used as received. H2S (Specialty Gases of America, 99.5%), auto-ignition temperature, 260 C, density, 1.363 g/L was used.
2.2.
Catalyst preparation
A mixture of p-dichlorobenzene, CoS2, and RuO2 was used to prepare the anode electrocatalyst. The ratio of the respective components was 0.5:3:3 by mass. The anode catalyst materials, with the ratios specified, are mixed in porcelain mortar and milled for 5 h in a high-energy planetary ball mill. The mechanical attrition process was initiated by loading the specified ratio quantities into a ball mill referred to as Fritsch pulversette planetary mono mill, P6 in an inert atmosphere. 10 g of powders and 15 stainless steel balls (1.5 cm in diameter, 14 g in mass) were sealed under inert, into the stainless steel vials (80 cm3 volume). The rotation speed of the milling vials (u, anticlockwise rotation) which are fixed onto a rotating disk and the rotation speed of disk (U, clockwise rotation) can be set independently. Milling condition was associated by 3 essential parameters (U (rpm)/u (rpm)/Dt (h)) where Dt was the duration of the milling process. The milling parameters such as ball to powder masses ratio and milling speed were optimized to 1/20, [15] and 300 rpm, respectively. To get a uniform mixture, ethanol was added to the combination to make a suspension that was mixed thoroughly. Then ethanol was allowed to evaporate slowly to leave the mixture as powders.
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The powdery mixture were heat-treated in N2 atmosphere at 150 C for 2 h and allowed to cool under N2 to room temperature in an autosorb-1C apparatus (Quantachrome Inc.) in the out gassing station. The resulting composite material was then used to prepare the anode catalysts. The fine crystal powder was stored in a dessicator prior to characterization. The specific surface area determinations of the composite material were carried out with Autosorb-1 software using a BET Multipoint approach. Prior to this, the p-dichlorobenzene was removed under gentle heating (12 h) at 70 C in an inert atmosphere, followed by a 2 C/min ramp to 174 C, the boiling point of p-dichlorobenzene, leaving behind open porosity in the electrocatalyst layers.
2.3. XRD structural characterization of RuO2eCoS2 nanocomposite The powder X-ray diffraction (XRD) of the synthesized RuO2eCoS2 nanoclusters was carried out by a Philips X’pert diffractometer with Cu Ka radiation of known wavelength, ˚ . The incident and the diffraction slit width l ¼ 1.54060 A employed for the measurements are 1 and 2 respectively and mask size is 0.10 cm. Phase identification and particle size calculations have been carried out by PANanalytical X’pert Highscore software version 1.OF [16].
2.4.
Microstructural characterization (SEM)
The morphology of the synthesized material was obtained by Hitachi S800 scanning electron microscopy (SEM). A fixed working distance of 10 mm and a voltage of 25 kV were used. EDAX genesis software was used to analyze the SEM images.
2.5.
Electrochemical method
Our electrochemical cell includes a test-bed with porous electrodes in which a solid proton conducting membrane separates the electrodes. A simplified schematic of the laboratory-fabricated electrochemical cell is shown in Fig. 1. The process consists of passing H2S gas through the anode chamber to contact a catalytic anode, where electrochemical reaction takes place to produce elemental sulfur, protons and electrons. The protons pass through cesium hydrogen sulfate (CsHSO4)-based membrane from the anode chamber to the cathode chamber, where they react with electrons from the
Fig. 1 e Schematic of electrolyzer cell module.
catalytic cathode to produce hydrogen gas. The CsHSO4-based membrane separating both electrodes is impermeable to fuel and product flows, but allows the transport of ionic species between both sides of the cell. Both the anode and cathode chambers are filled with carbon felt which conducts electron. During the process, both the anode and the cathode are maintained at 135 kPa and 150 C, a temperature at which sulfur has a low viscosity and the solid electrolyte is in its hyperprotonic phase [17]. LabView software from National Instrument was used for in-situ electrochemical measurements. Such measurements include current, cell pressures and total cell resistances. Liquid sulfur is collected from the anode compartment and hydrogen is removed from the cathode compartment. Samples were withdrawn from both anode and cathode compartments periodically for analysis.
3.
Results and discussion
3.1.
Surface area characterization
The catalytic active surface area of the RuO2eCoS2 nanocomposite was obtained by using a multipoint BET method which requires a minimum of three points in the appropriate relative pressure range as shown in Fig. 2. At high relative pressures, the rate of adsorption equals the rate of desorption which is very essential for material catalytic performance. The specific surface area calculated from the total surface area for the composite was (10.96 m2/g). This was sufficient to sustain the reaction without interruption by providing sufficient compensation for sulfur poisoning as well as providing enough area for the reaction to proceed. Table 1 shows a list of different anode configurations and their corresponding BET surface areas. Our intention and the focus of the catalyst preparation were to use the mechanical attrition to minimize the particle sizes of the composite material in order to increase the exposed surfaces which are crucial in enhancing the electrode kinetics, and also to reduce the resistance of the material. Since resistance impedes the flow of electrons, the reduced particle size offers lower resistance when compared to the individual resistances from pure ball milled RuO2 and CoS2. The active surfaces of the
Fig. 2 e Multipoint BET surface area plot for anode metal sulfide (RuO2eCoS2) nanocomposite.
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Table 1 e Various anode configurations and their BET catalytic active surface areas.
individual tested ball milled RuO2 and CoS2 are small compared to the prepared composite. The sulfur blockage observed with the pure RuO2 and CoS2 may be attributed to the insufficient sites available to accommodate the effect of sulfur poisoning which subsequently lead to overall poor cell performance. As mentioned previously, this occurrence decreases the rate of chemical reaction which adversely affects the overall performance of the system.
3.2.
Resistivity of anode catalysts
The electrical resistivity of anode catalyst samples with the structure Au/anode catalyst/Au are listed in Table 2. The resistivity of composite catalyst pellet RuO2eCoS2 at 150 C measured by LabView software acquired from National Instruments is less compared to the values obtained for pure ball milled CoS2 and RuO2 electrocatalysts. As explained in section [3.1], reduced particle sizes offer low resistances to flow of electrons when compared to larger particles. Since the mobility of free electrons will depend on carrier concentration, smaller particle sizes yield more molar carrier concentration than larger sizes. In the light of this observation, the present anode catalyst with the RuO2eCoS2 configuration was utilized due to superior electrical conductivity for use as anode catalysts under operating conditions. The contributions from these resistances are small compared to the total cell resistances. This suggests that the large difference in current densities observed may not be significantly due to the resistances of these materials, but primarily due to the availability of catalytic active sites provided by the composite anode materials. However, it was necessary to reduce the internal ohmic resistance of the cell slightly using RuO2eCoS2 anode composite. Note, the total resistances experienced by the electrolytic cell results from contributions due to, activation overvoltage loss (hact), ohmic loss (hohmic) and from concentration loss (hcon). Maximum current density as shown in Table 3 at 0.9 V measured for the RuO2eCoS2 composite was 0.019 A/m2, which was higher than that for CoS2 (0.009 A/cm2) using the same electrolyte thickness. To this end, we have achieved minimizing the
Table 2 e Cell area specific ohmic resistance (RA). This shows different anode electrocatalysts at operating temperature of 150 C.
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Table 3 e Current density and conversion efficiency for different electrocatalyst configurations.
contributions from both hact and hcon by using the anode composite RuO2eCoS2.
3.3.
Stability of H2S electrolytic cell anode materials
Electrocatalysis offers a convenient route to the synthesis of H2S. Requisites of the anode material are a low overpotential for product evolution and high stability. We have tested three groups of anode electrocatalysts for their suitability as endurance materials for H2S splitting using a laboratory fabricated electrolytic cell apparatus as described in section [2.5]. The results of the electrochemical application are discussed below.
3.3.1.
X-ray diffraction characterization
XRD was employed for structural characterization in terms of phase identification, structure determination, and crystal orientation. Fig. 3 (a, b, and c) illustrates the XRD pattern of synthesized composite, as prepared, after 12 h exposure to H2S, and after 24 h exposure to H2S feed content gas, respectively. The diffractogram shows no phase or compositional changes after exposure to H2S gas at the cell operating temperature of 150 C. The structural analysis of RuO2eCoS2 was carried out by varying diffraction angle 2q from 25 to 75 . The average crystal size of the RuO2eCoS2 powder was determined by using the Scherrer formula. The average crystal size of 20e60 nm was obtained and consists of welldefined diffraction peaks. XRD analysis shows that the structure is preserved for the three samples with different sample history. As a result, the stability of RuO2eCoS2 when exposed to H2S (for prolonged periods of time) made it an attractive anode candidate for H2S splitting.
Fig. 3 e XRD pattern of RuO2eCoS2 composite: (a) as prepared; (b) after 12 h exposure to 100% H2S feed content at 150 C; (c) after 24 h exposure to 10 H2S feed content at 150 C.
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3.3.2.
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SEM analysis
The composite RuO2eCoS2 has a relatively high surface and expanded area metallic nanoparticles facing away from the electrolyte. Fig. 4(aed) shows SEM micrographs of a porous electrode (composite nanoparticles on carbon mesh support for strength). These electrodes have tortuous pathways within them to expose orders of magnitude larger surface areas to reacting H2S and to also allow the escape of the liquid sulfur product. The increased numbers of catalytically active sites as pointed out earlier enables the materials to be more resistant to poisoning. This is because these materials exhibit a certain number of catalytically active sites that can be sacrificed to the effects of poisonous sulfur production shown in Fig. 4 (b) while a large number of unpoisoned sites are preserved. In Fig. 4 (c and d) which consists of non composite materials, the catalytic efficiency of the material is substantially less than that possible if a greater number of catalytically active sites were available. This is due to concentration polarization caused by sulfur blockage of the diffusion layers. In all the experimental runs involving RuO2eCoS2 composite, there was only a trace amount of sulfur deposition on the anode surface.
3.3.3.
Electrochemical performance of anode electrocatalysts
Fig. 5 compares the performance of four cells using, RuO2eCoS2 (Fig. 5a), ball milled CoS2 (Fig. 5b), ball milled RuO2 (Fig. 5c) and no ball milled RuO2 (Fig. 5d), as anode materials. There were no signs of de-lamination of the anode in all cases investigated. The performance of the RuO2eCoS2
nanocomposite anode was far superior to any of the anode materials. All the configurations maintained their performances for over 12 h. If no noticeable decline in output after this time period was observed, the cell was deemed adequate for performance testing. Both systems exhibited similar electrochemical behavior and deviations were probably caused by external factors (current collection, lead wires, etc.). Since the main goal of this study was to identify and develop catalytic active materials for H2S splitting in electrolytic cell, a large number of anode materials and experimental cells used in this study required the use of low cost materials with no system optimization configurations. For commercial applicability of this work, full system optimization will need to be carried out. Fig. 6, depicts the effects of various anode configurations on current densities, j, as a function of external applied voltages. Curve (a) represents the performance of the cell based on RuO2eCoS2 anode. Curves (b-d) represent the performances of cells based on ball milled CoS2, ball milled RuO2 and no ball milled RuO2 anodes respectively. At the low region where there are uniform potential gradients, current densities are proportional to potential gradients and are uniform; hence Ohm’s law is appropriate. At high voltages, concentration gradient comes into play. By utilizing this nanostructured anode composite, we were able to maintain a high current density at operating potential. This is because the catalytic surface area provided by the composite highly minimized the activation barrier and also reduced most of the effects associated with concentration polarization; more gases were able
Fig. 4 e Fig. 3 SEM images of surfaces of electrode. (a) RuO2eCoS2 electrocatalyst nanocomposite before electrolysis; (b) RuO2eCoS2 electrocatalyst nanocomposite after electrolysis; (c) ball milled RuO2 electrocatalyst after electrolysis; (d) ball milled CoS2 electrocatalyst after electrolysis.
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20
Transfer of electrons from the chemisorbed hydrogen atoms to the electrode, releasing Hþ into the electrolyte:
a
18
2 M.H/ M þ e þ Hþ
Current density, m A/cm
2
16 14 12
(4)
Mass transport of the Hþ ions away from the electrode:
10
2 Hþ /Hþ
b
8 6 4
c d
2 0 0
2
4
6
8
10
12
14
Time, h
Fig. 5 e Testing for four cells with different anodes (Vcell [ 0.9 V, T [ 150 C, Fuel [ 0.25 cm3/min) H2S was the fuel. (a) RuO2eCoS2; (b) ball milled CoS2; (c) ball milled RuO2; (d) no ball mill RuO2.
to diffuse to the reaction interface and the reaction equilibrium is shifted toward product.
3.3.4.
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Charge transfer reactions
The anode electrode reactions consist of basic steps as shown below. Mass transport of H2S to the electrode: H2 SðbulkÞ /H2 Sðnear electrodeÞ
(1)
Absorption of H2S onto the electrode surface: H2 Sðnear electrodeÞ þ M/M.H2 S
(2)
Separation of the H2S molecules into 2 individually bound (chemisorbed) hydrogen atoms and the release of sulfur on the electrode surface: ðM.H2 þ M/2MHÞ þ 1=8S8
(3)
The overall reaction is limited by electron transfer step between chemisorbed hydrogen and the metal electrode (Eq. (4)). In this equation, M.H represents a hydrogen atom chemisorbed on the metal surface and (M þ e-) represents a liberated metal surface site and a free electron in the metal. A decrease in activation barrier represents the catalytic influence of the surface of the electrode. A catalytic electrode is one which significantly lowers the activation barrier for the reaction. Because it appears as an exponent in Butler-Volmer kinetics, even small decreases in the activation barrier can cause large effects. Using a highly catalytic electrode therefore provides a way to dramatically increase the exchange current density ( j0) as evidenced in the improved value of current density ( j ) obtained in Fig. 6a by utilizing RuO2eCoS2 composite material when compared to other tested configurations.
3.3.5. Tafel slope and exchange current densities for anode configurations Improving kinetic performance stems from increasing j0. To increase the value of j0, we have increased the number of possible reaction sites (i.e., increase the reaction interface roughness) by incorporating a novel composite metal sulfide, RuO2eCoS2 electrocatalyst, in the electrode. The exchange current density was determined experimentally by extrapolating plots of _n j vs. h to h ¼ 0, where h is the activation overvoltage. This is a direct measure of the reaction rate constant at the electrolytic cell electrode. Fig. 7 shows plots for the three anode configurations. In all three cases of anode configurations n ¼ 2 representing the number of electrons transferred in the electrochemical reaction. The anode composite consisting of RuO2eCoS2 shows great potential in reducing the activation barrier as a result of the increased catalytic active sites available for the reaction to proceed without sulfur poisoning. This is reflected by the high exchange current density (2.65 102 A/ cm2) realized compared to the values for the ball milled CoS2 and RuO2 configurations (1.61 102 A/cm2) and (8.4 103 A/ cm2) respectively. More also, the value of the transfer coefficient (a) in Fig. 7 for the composite material is (0.14) which is higher than that obtained for ball milled CoS2 and RuO2 (0.08) and (0.07) respectively, suggesting that net current density is higher with the composite material, hence signifying a rapid electrochemical reaction compared to other tested anode materials.
3.4.
Fig. 6 e Current density for different anode configurations with 100% H2S feed gas content. (a) RuO2eCoS2; (b) ball milled CoS2; (c) ball milled RuO2; (d) no ball milled RuO2.
(5)
Fuel utilization
Table 3 summarizes the measured data in tabular form at specific operating voltage, comparing typical conditions to tested parameters. The conversion efficiency depends on many factors which include the flow rate of H2S to the reaction sites, the availability of active sites, and the thickness of
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Fig. 7 e Tafel plots for anode configurations at operating temperature 150 C and 100% H2S feed content.
the electrolytes. As shown in Table 3, for the same electrolyte thickness and active area, H2S consumption was more than doubled for RuO2eCoS2 when compared to both ball milled RuO2 and CoS2. This suggests that the difference could be attributed to faster electrode kinetics observed with RuO2eCoS2 as explained in sections [3.1 and 3.3.2]. The application of the composite RuO2eCoS2 increased the current density by over an order of magnitude and the efficiency of the system doubled and tripled respectively when compared to both ball milled RuO2 and CoS2. The increased electrode kinetics suggests large active sites available for the reaction to progress, some of which were sacrificed to the effect of sulfur poisoning. This was also supported by the SEM image (see Fig. 4) after 12 h of electrolysis. The results of our measurements show that electrocatalyst configuration prepared from high surface composite nanometals (RuO2eCoS2) demonstrated the optimum performance, and also exhibited sulfur tolerance after extended hours of operation.
4.
Conclusion
We have successfully synthesized a novel nanostructured anode electrocatalyst for the electrolytic splitting of (100%) H2S feed content gas operating at 135 kPa and 150 C. This class of material with general composition, RuO2eCoS2 is stable and has desired properties at tested conditions. It was able to deliver a maximum current density of 0.019 A/cm2 at 0.9 V while maintaining its integrity. The difference in electrochemical performance of this cell compared to other tested materials is directly attributable to the anode material developed in this study. The kinetic behavior of the three anode-based electrocatalysts as captured in the Tafel plots showed that the novel composite material favors the kinetic of the anode reaction.
This implies that by using this material, high H2S feedstock utilization is possible. The overall analyses show that there is an improvement in electrochemical performance, current density, and sulfur tolerance when compared to the other tested anode configurations.
Acknowledgements Authors acknowledge the US Department of Energy (DOE Award #: DE-FG26-05NT42540) for funding. USF Nanomaterial and Nanomanufacturing Research Center for analytical studies is highly acknowledged. We wish to thank Alfred P. Sloan Foundation for financial support.
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