Anodic Reaction Mechanism Research Articles

(Invited) Understanding Anode and Cathode Reaction Mechanisms for Proton Conducting Intermediate Temperature Solid Oxide Fuel Cells

The anode and cathode reaction processes for conventional solid oxide fuel cells (SOFC) have been widely studied. In comparison, there are many unknowns about the basic electrochemical behaviors and mechanisms for the anode and cathode reactions of proton conducting SOFC, especially at intermediate temperature (IT) range of ~400-700oC when the cells approach ideal proton conducting behaviors. At intermediate temperature, due to the change of the dominating ionic conducting species from oxide ion to proton, the fundamental reaction pathways would change for both the anode and cathode reactions. Accompanied with such changes, many interesting electrochemical behaviors arise for proton conducting IT-SOFC. For example, it is well known that the Ni-based cermet anode for conventional oxide ion SOFC are susceptible to poisoning by sub-ppm level hydrogen sulfide (H2S) and not very sensitive to carbon monoxide (CO). In comparison, literature seems to suggest that the Ni-proton conducting oxide cermet as the anode for proton conducting IT-SOFC shows much better resistance against low ppm-level H2S but is poisoned by percentage level CO. This study aims to understand the fundamental reaction processes for both the anode (hydrogen electrode) and cathode (air electrode) of proton conducting IT-SOFCs. Electrochemical tests are carried out based on the concept of “controlled poisoning”, which is to selectively slow down a specific part of the electrode reaction using a specially chosen “poison” in order to help understand the fundamentals about the electrode reactions for proton-conducting IT-SOFCs. In particular, for the anode, H2S is used as a “Ni poison” and CO2 as a proton conducting oxide poison, while for the cathode, either H2O or CO2 is used. The measured electrochemical behaviors for different anode- or electrolyte-supported full cells and symmetrical cells including those with patterned (metal) electrodes are compared with those of conventional oxide ion based SOFCs. The results for those experiments aimed at understanding the similarity and differences between proton conducting SOFC and conventional oxide ion conducting SOFC will be presented, and their implications on the anode and cathode reaction mechanisms and the roles (including electrocatalytic roles) of proton conducting oxides in those electrode reactions for proton conducting IT-SOFC will be discussed.

Anodic Reaction Mechanisms in Direct Carbon Fuel Cells Using Carbon Particles Dispersed in Molten Carbonate

Highly efficient technologies for the conversion of solid fuels, such as coal and biomass, to electricity are required for clean energy production and sustainable energy supply. Direct carbon fuel cells (DCFCs), which convert the chemical energy in solid carbon directly into electricity without gasification, have attracted much attention because of their theoretical ability to achieve 100 % efficiency. However, solid carbon particles have disadvantages in terms of fluidity when compared to gaseous fuels. Thus, carbon particles are dispersed into molten carbonate to form a carbon/carbonate slurry in which the carbon particles can move. Then, the anode is inserted into the carbon/carbonate slurry for discharge in the DCFC [1-5]. Carbon dispersed into the molten carbonate is easily transported to the anode, leading to an improvement of the formation of triple phase boundary (carbon, electrolyte, and anode). During discharge, carbon is supposed to be consumed through the complete electrochemical oxidation (C+2CO3 2-→3CO2+4e- (R1)) which releases four electrons per carbon. However, several studies have suggested a possible formation of CO from partial electrochemical oxidation (C+1/2CO3 2-→3/2CO+e- (R2)) which releases one electron per carbon, or even from the Boudouard reaction (C+CO2→2CO (R3)) where CO2 produced during discharge reacts with carbon particles [3,6]. Although the partial oxidation (R2) and the Boudouard reaction (R3) reduce the DCFC efficiency, little effort has been made to study the reaction mechanisms in the DCFC. The aim of this study is to investigate the anodic reaction mechanisms in the DCFC using carbon/carbonate slurry experimentally and theoretically. Figure shows setup of the DCFC using carbon/carbonate slurry stirred by gas bubbling. The working electrode (WE), counter electrode (CE), and reference electrode (RE) were made from gold sheets. Cell discharge was controlled with a potentio/galvano-stat. The reactor was heated using an electric furnace to 1073 K. Activated carbon particle with an average diameter of 44.1 μm was used as fuel. Ternary molten carbonate (Li2CO3/Na2CO3/K2CO3 (= 16.9/25/58.1 mol%)) was used as electrolyte. The carbon content in the carbonate was set to 1.0 wt% or 2.0 wt%. The carbon/carbonate slurry was contained in the porous alumina tube having an average pore diameter of 120 nm. This allowed the separation between the carbon/carbonate slurry and the molten carbonate electrolyte. In the setup, the carbon/carbonate slurry was able to be stirred by gas bubbling. CO and CO2 concentrations in off-gases were measured by FID with methanizer, and CO and CO2 production rates were determined at the open circuit mode or constant discharge mode at 20 mA/cm2. Moreover, to study the impact of Boudouard reaction (R3) on CO formation, CO2 bubbling with a flow rate of 1.0 ml/min, which was approximately 5 times of theoretical CO2 produced during discharge at 20 mA/cm2, was used in the carbon/carbonate slurry at the open circuit condition, and CO concentration in off gases was measured. As a result, an amount of CO was formed when the carbon/carbonate slurry was not stirred by Ar gas bubbling during the constant discharge at both the carbon contents. At the open circuit condition, CO was not formed by CO2 bubbling (1.0 ml/min) simulated CO2 produced during discharge. This indicated that the impact of Boudouard reaction (R3) was insignificant in the DCFC. Moreover, measured CO and CO2 production rates were well balanced on the calculation based on two electrochemical oxidations (R1 and R2). And, the partial electrochemical oxidation accounted for approximately 80 % in the carbon consumption at the carbon content of 2.0 wt%, whereas that accounted for approximately 40 % at the carbon content of 1.0 wt%. It was shown that the ratio of partial electrochemical oxidation depended on the carbon concentration near the anode. On the other hand, when the carbon/carbonate slurry was stirred by Ar bubbling with a flow rate of 50 ml/min, only CO2 was formed and the production rate of CO2 corresponded to the theoretical rate based on R1 at both the carbon contents. This indicated the carbon particles were consumed only by the complete electrochemical oxidation (R1). It was shown that stirring the carbon/carbonate slurry was effective to enhance the complete electrochemical oxidation. Reference [1] N. J. Cherepy et al., J. Electrochem. Soc., 152 (2005) A80-A87. [2] X. Li et al., Ind. Eng. Chem. Res., 47 (2008) 9670-9677. [3] H. Watanabe et al., J. Power Sources, 273 (2015) 340-350. [4] H. Watanabe et al., Energy and Fuels, 30 (2016) 1835-1840 [5] H. Watanabe et al., J. Chem. Eng. Jpn., 49 (2016) 237-242 [6] T.M. Gür, J. Electrochem. Soc., 157 (2010) B751-B759 Figure 1

Anodic Reaction Mechanisms in Direct Carbon Fuel Cells Using Carbon Particles Dispersed in Molten Carbonate

This study aims to investigate the anodic reaction mechanisms in direct carbon fuel cell using a carbon/carbonate slurry stirred by Ar bubbling. CO2 and CO production rates were measured during the constant discharge. Gas products were calculated based on complete (C+2CO3 2-→3CO2+4e-) and partial electrochemical oxidation (C+1/2CO3 2-→3/2CO+e-). The impact of Boudouard reaction (C+CO2→2CO) on CO production was also discussed. As a result, an amount of CO was formed without Ar bubbling, whereas CO was not formed with Ar bubbling. The impact of CO2 bubbling on CO formation was insignificant, and calculations of gas products almost agreed with experiments of those. These indicated that CO was formed through not Boudouard reaction but the partial electrochemical oxidation. Moreover, partial electrochemical oxidation helped to explain the trends of open circuit voltages. It was shown that the anodic reactions consisted of complete and partial electrochemical oxidations.

High-Performance Direct Dimethyl Ether Fuel Cell Operating with an Advanced Activation Process

The direct methanol fuel cell (DMFC) has been of interest due to the absence of a difficult to split carbon-carbon bond in methanol, high energy content of the liquid fuel and facile handling. Other established high energy-density liquid fuels, such as ethanol, are not yet amenable to direct electrochemical oxidation due to the lack of electrocatalysts for an efficient carbon-carbon bond scission. A related fuel to methanol, with strong direct feed fuel cell potential is dimethyl ether (DME). DME possesses many of the same physical advantages that methanol does.1-3 It is similar to methanol in terms of theoretical open cell voltage (1.18 vs. 1.21 V at 25°C) and does not require a carbon-carbon bond scission. DME is also a clean and, unlike methanol, nontoxic fuel that undergoes facile biodegradation and can be easily synthesized from recycled carbon dioxide and renewable energy.4 Due to its lower dipole moment, DME is less subject to membrane crossover than methanol; it also has a higher energy content than methanol (8.2 vs. 6.1 kWh/kg).5,6Complete electro-oxidation of DME consumes 3 equivalents of water and yields 12 equivalents of electrons: CH3 OCH3+3H2O→2CO2+12H++12e- (1) While the direct DME fuel cell (DDMEFC) performance has by now been demonstrated to match that of the state-of-the-art DMFC,7 DDMEFC still requires development to become fully viable. In addition to the needed improvements in the anode catalyst activity, the catalyst deactivation and its durability continue to pose a challenge. Several aspects of optimization have been explored of which break-in has been one area of attention.8 In this work, membrane electrode assembly (MEA) preparation and initial break-in were carried out according to standard procedure.1 Prior to operation in the DDMEFC mode, the fuel cell was operated on humidified hydrogen/air to condition the membrane. When the current at 0.7 V plateaued the break-in was stopped, fuel cell polarization recorded on H2/air, and the anode feed switched to preheated, humidified DME under a pressure of approximately 1.75 bar gauge (2.56 bar absolute).1 Performance at this point reproduced previously observed performance on DME under similar conditions. Several DDMEFC polarization plots were recorded, leading a gradual performance loss. In an attempt to recover lost performance, the cell was switched back to break-in conditions on H2/air and after break in, DDMEFC polarization plots were recorded again. With each additional iteration of this procedure, the fuel cell performance increased, up to ~0.3 A cm-2 at 0.5 V after a thirteenth iteration (Figure 1). As a result of this conditioning, the DDMEFC power density achieved reached 0.18 W cm-2 – a 50% increase over the highest power density value reported in the literature with a commercial PtRu catalyst (0.12 W cm-2).7 In this presentation, we will provide details of the procedure used and discuss the causes of such a significant performance improvement. We will also comment on the DDMEFC performance durability in the context of the conditioning approach used in this study. Acknowledgment Financial support from the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy through Fuel Cell Technologies Office is gratefully acknowledged. REFERENCES 1 Li, Q., Wu, G., Johnston, C. M., Zelenay, P. "Direct Dimethyl Ether Fuel Cell with Much Improved Performance." Electrocatalysis 5, 310-317 (2014). 2 Jensen, J. O., Vassiliev, A., Olsen, M., Li, Q., Pan, C., Cleemann, L. N., Steenberg, T., Hjuler, H. A., Bjerrum, N. "Direct dimethyl ether fueling of a high temperature polymer fuel cell." Journal of Power Sources 211, 173-176 (2012). 3 Li, Q., Wen, X., Wu, G., Chung, H. T., Zelenay, P. "High-Activity PtRuPd/C Catalyst for Direct Dimethyl Ether Fuel Cell." Angewandte Chemie International Edition(In Press). 4 Olah, G. A., Goeppert, A., Prakash, G. S. "Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons." The Journal of Organic Chemistry 74, 487-498 (2008). 5 Müller, J. T., Urban, P. M., Hölderich, W. F., Colbow, K. M., Zhang, J., Wilkinson, D. "Electro‐oxidation of dimethyl ether in a polymer‐electrolyte‐membrane fuel cell." Journal of the Electrochemical Society 147, 4058-4060 (2000). 6 Mizutani, I., Liu, Y., Mitsushima, S., Ota, K.-i., Kamiya, N. "Anode reaction mechanism and crossover in direct dimethyl ether fuel cell." Journal of Power Sources 156, 183-189 (2006). 7 Chung, H. T., Dumont, J. H., Martinez, U., Zelenay, P. Catalyst Development for Dimethyl Ether Electrooxidation. in Meeting abstracts. 929-929p (The Electrochemical Society). 8 Qi, Z., Kaufman, A. "Quick and effective activation of proton-exchange membrane fuel cells." Journal of Power Sources 114, 21-31 (2003). Figure 1

Determination and explanation of the pH-related critical fluoride concentration of pure titanium in acidic solutions using electrochemical methods

The critical fluoride concentrations for titanium were determined experimentally in acidic solutions at pH=1.0∼5.0 using open circuit potential measurements combined with potentiodynamic polarization tests. The results indicated that the dependence of the critical fluoride concentration on pH (∂pF/∂pH) could be classified into two pH regions: ∂pF/∂pH=−0.17 at region I (pH=1.0∼2.5) and ∂pF/∂pH=−0.78 at region II (pH=3.0∼5.0), respectively. Electrochemical parameters for titanium in fluoride-containing acidic solutions were obtained by analyzing the polarization curves quantitatively. It was found that hydrogen evolution reaction controlled by charge transfer, and oxygen reduction reaction controlled by mass transfer dominated the cathodic reactions at Region I and Region II, respectively. The ∂pF/∂pH values were calculated using the obtained parameters and were in line with the experimental results. Based on the calculations, the different ∂pF/∂pH values between Region I and Region II were mainly determined by the anodic parameter, ∂lgIm/∂pH, rather than the cathodic parameters. Accordingly, anodic reaction mechanisms were proposed considering the existence forms of the fluoride. They could be fully utilized to qualitatively explain the phenomenon of pH-related critical fluoride concentration.

Electrooxidation of cyanide ion on a platinized Ti electrode

Decontamination of highly concentrated cyanide solutions by the method of electrochemical oxidation is presented in this work. By using a Ti electrode covered with 600 nm average thickness layer of Pt, the effective electrooxidation of 0.1 M KCN solution is possible when 25 mA cm−2 current density is applied. Current efficiencies of ~60 % are achieved, and almost full removal of the cyanide ions (from 0.1 to 0.00016 M) are reached when 69 kC L−1 of charge is passed. The kinetics of electrochemical reaction of cyanide oxidation was investigated by voltammetry and fast Fourier transform electrochemical impedance spectroscopy (EIS). The kinetic parameters of cyanide ion electrooxidation reaction are determined and the mechanism of anodic reaction is suggested by considering the analysis of EIS data.

Corrosion inhibition of mild steel in sulphuric acid using a bicyclic thiadiazolidine

The inhibitory effect of 3a,6a-diphenyltetrahydro-1H-imidazo [4,5-c] [1, 2, 5] thiadiazole-5(3H)-thione 2,2-dioxide (TTU) on the corrosion behaviour of mild steel in 0.5 M H2SO4, at (30 ± 0.5) °C was studied by gravimetric, potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscopy measurements. The effect of inhibitor concentration on the corrosion rate, surface coverage and inhibition efficiency is investigated. Results show that TTU exerts a strong inhibiting effect on mild steel corrosion and acts as a cathodic-type inhibitor. TTU does not affect the mechanism of the cathodic reaction while the anodic reaction mechanism changes upon addition of the inhibitor. Possible mechanistic pathways for the inhibition process are proposed. The inhibition efficiency of TTU may be due to either the adsorption of inhibitor molecules building a protective film or the formation of an insoluble complex of the inhibitor with metal cations. TTU adsorption obeys the Langmuir model.

Performance of Direct Carbon Fuel Cells Operated on Coal and Effect of Operation Mode

Previous experimental results have shown that a system with molten carbonate and solid oxide electrolyte is feasible for Direct Carbon Fuel Cell (DCFC). A study is presented to investigate cell performance with a range of solid carbon (i.e. coals, biochars, graphite) and operation mode in this hybrid electrolyte system. The results show that less crystalline coal with high fixed carbon, low sulfur, medium volatile material and moisture is best suited to this system. Using high rank of fuel such as anthracite coal, good cell performance can be obtained only by elevating temperature and with adequate pretreatment to remove impurities. Discussion of cell operation indicates that cell degradation and operation failure were due to coal agglomeration, ash buildup, and limited fuel supply in potentiostatic mode. Instead, galvanostatic operation gave stable cell performance over 60 hours. This result allows better understanding of anode reaction mechanism on the hybrid electrolyte system. Thus, long-term operation is promised when suitable solid fuel and optimized operation parameters are applied.

H2 Oxidation on Doped Yttrium Chromites Anode of Solid Oxide Fuel Cell

Doped yttrium chromites as potential anodes for SOFC are studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Ni doping. Electrochemical impedance spectra results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes and improves the charge transfer process probably by increasing the amount of adsorbed H on the electrode surface. A model of H2 oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

H2 oxidation on doped yttrium chromites/yttrium stabilized zirconia anode of solid oxide fuel cell

Co and Ni doped yttrium chromites as potential anodes for solid oxide fuel cell (SOFC) are studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3–1/2 for Ni doped yttrium chromites. A model of H2 oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.

Anodic Dissolution of Spheroidal Graphite Cast Iron with Different Pearlite Areas in Sulfuric Acid Solutions

The rate equation of anodic dissolution reaction of spheroidal graphite cast iron in sulfuric acid solutions at 298 K has been studied. The cast irons have different areas of pearlite. The anodic Tafel slope of 0.043 V decade−1and the reaction order with respect to the hydroxyl ion activity of 1 are obtained by the linear potential sweep technique. The anodic current density does not depend on the area of pearlite. There is no difference in the anodic dissolution reaction mechanisms between pure iron and spheroidal graphite cast iron. The anodic current density of the cast iron is higher than that of the pure iron.

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen (1-7). The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion(2). Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation(8-12). It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition(8, 10, 13, 14) ("coking") and sulfur poisoning(11, 15) and the manner in which surface modifications stave off this degradation(16). The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated(17-22). In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

Indole Alkaloids of Alstonia angustifolia var. latifolia as Green Inhibitor for Mild Steel Corrosion in 1 M HCl Media

The inhibition effect of mild steel (MS) corrosion in 1 M HCl was studied by the addition of indole alkaloids (crude) isolated from Alstonia angustifolia var. latifolia (A. latifolia) leaves at 303 K. Potentiodynamic polarization, impedance spectroscopy, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) analyses were used for this study. Results show that the isolated alkaloid extract of A. latifolia is a good inhibitor and exhibited maximum inhibition efficiency (above 80%) at concentrations between 3 and 5 mg/L. Polarization measurements indicated that the inhibitor does not alter the mechanism of either anodic or cathodic reactions and acted as mixed-type inhibitor. The inhibition efficiencies of both electrochemical techniques are found to be in good agreement and adsorption of inhibitor follows Langmuir isotherm. Adsorption of inhibitor over metal surface was well supported by the SEM studies, while FTIR studies evidenced the presence of indole alkaloids as green inhibitor that reduces the rate of corrosion.

Mathematical modelling of direct borohydride fuel cells

A detailed mathematical model for the direct borohydride/O2 fuel cell is developed. The activation polarizations, mass-transport limitations and resistances to charge transport in each component of the cell are explicitly incorporated. The anode kinetic mechanism is based on direct borohydride oxidation, borohydride hydrolysis and the full Tafel–Volmer–Heyrovsky mechanism for hydrogen evolution and oxidation. The mixed potential at the anode is calculated using the mixed-potential theory. The model results are compared to experimental data across a range of operating conditions and component properties, including the reactant concentrations, the anolyte flow rate, the ionomer volume fractions and the membrane/ionomer conductivity. A good qualitative fit to the experimental data is demonstrated. In order to gain insight into the anode reaction mechanism, the performance on both Pt and Ni anodes is simulated and compared to experimental observations. A detailed analysis of the results provides an explanation for the different performance on these catalysts.

Anodization process of Sn in oxalic acid at low applied voltages

This study investigated the anodization of tin in oxalic acid electrolyte at low applied voltages of U ≤ 5 V. Under such conditions, the anodization product consists of micron-sized SnC 2O 4 crystalline particles and the morphology of the product changes during the anodization process in three distinctive stages. In the initial stage I, Sn dissolves into the electrolyte at high current density. This process is associated with the precipitation of highly oriented SnC 2O 4 crystals on the Sn surface. Stage II is a passivation process associated with the formation of a SnC 2O 4 surface layer with unique pyramidal microstructure via in situ conversion. Stage III is the localized creation of smaller SnC 2O 4 polyhedron crystals. The anodic reaction mechanisms for each stage are discussed in detail and a mathematical model is established to quantify the critical condition that induces the anodic passivation.

Double layer structure in solid oxide fuel cell anode/electrolyte interfaces: A Monte Carlo study

The double layer structure of solid oxide fuel cell anode/electrolyte interfaces is simulated by Markov Chain Monte Carlo methods. A case study is carried out on lanthanum strontium vanadate (LSV)/yttria-stabilized zirconia (YSZ). The density of oxygen vacancies directly adjacent to the LSV/YSZ interface is one order of magnitude higher than the bulk value of YSZ. The spatial variation of oxygen vacancies in the double layer region exhibits exponential decay behaviour. The double layer undergoes pronounced relaxations when the interfaces are under anodic biases ranging from 0 to 150 mV. The results indicate that 70–80% of the oxygen vacancies are immobilized in the Helmholtz–Perrin layer. The rationale presented in this work has wide applications on elucidating anodic reaction mechanisms and potential distributions across anode/electrolyte interfaces.

Research on Polarization Curve and Anode Discharge Mechanism in Electrolysis Cells for Preparation of Al-Li Alloys

At 953K, anodic over voltage by electrolyzing LiCl and Li2CO3 were measured respectively by continuous pulse oscillograph method in a mixture fused salt of LiCl-LiF in a laboratory electrolyte cell and the change of polarization curve obtained was studied as well. According to kinetics of electrode process theory, we calculated and analyzed anode reaction mechanism and reaction rate-determining step. The results indicate that when over voltage entered into the Tafel zone with current density in range of 0.1 to 0.3A·cm-2, electrode reaction of LiCl is controlled by electrochemical polarization but electrode reaction of Li2CO3 is controlled by chemical polarization. With current density increase further beyond the Tafel zone (i>0.4A·cm-2), electrode reaction is changing gradually from dual control influenced by electrochemistry or chemistry united with concentration polarization to single concentration polarization. While electrolyzing Li2CO3, it is complexing oxygen ions discharging on carbon anode slowly that determine the whole electrode reaction rate.

Effect of contact type between anode and carbonaceous fuels on direct carbon fuel cell reaction characteristics

The contact between the anode and the carbonaceous fuel has a strong effect on the direct carbon fuel cell (DCFC) reaction characteristics. These effects are experimentally investigated by measuring the electrochemical behavior of a detached anode, an anode in physical contact with the fuel and an anode with carbon deposited on the surface in a DCFC. The results show that for the detached type DCFC, the reaction characteristics are closely related to the anode gas. In an Ar atmosphere, the main anode reactions are the electrochemical reaction to produce O2 and the carbon gasification with the formed O2. In a CO2 atmosphere, the main anode reactions are the carbon gasification with CO2 and the electrochemical oxidization of the formed CO. For the physical contact type DCFC, the anode reaction mechanisms are the same as for the detached type DCFC with no electrochemical oxidization of carbon at the physical contact interface between the carbonaceous fuel and the anode. Thus, the increased contact does not result in better performance. The carbon-deposited type DCFC has better performance with a significant activation polarization due to the electrochemical oxidization of the deposited carbon.

Mechanism of electro-generative-leaching of chalcopyrite-MnO 2 in presence of Acidithiobacillus ferrooxidans

To clarify the role and mechanism of Acidithiobacillus ferrooxidans ( A. ferrooxidans) in bio-electro-generative-leaching (BEGL), an experiment was made on the electro-generative leaching of chalcopyrite -MnO 2 in the presence of the bacteria which grew respectively in Fe(II) and S 0 media. A dual cell system with chalcopyrite anode and MnO 2 cathode was used to study the relationship between time and both of electric quantity and dissolved rate of the two minerals in BEGL. The results show that the dissolved rates for Cu 2+ and Fe 2+ under the action of the bacteria cultivated by S 0 medium are almost 2 times faster than those by Fe(II). And the leaching ratio for Mn 2+ and the electric output increase by near 3 times. The oxidation residue of chalcopyrite was characterized by SEM and XRD, whose patterns are similar to those of raw ore in BEGL. The mechanism of anodic reaction for CuFeS 2-MnO 2 leaching in the presence of A. ferrooxidans cultivated by S 0 medium is proposed as a successive reaction of two independent sub-processes. The first stage is the dissolution of chalcopyrite to produce Cu 2+, Fe 2+ and sulfur, and the second stage is bio-oxidation of sulfur, which is the control step of the process. However, dissolution of MnO 2 lasts until the reaction of chalcopyrite stops or the ores exhaust in two types of leaching.