Anode Structures

Figure C shows an electron photomicrograph of a broken planar SOFC. The thick portion on the left is the porous anode structure. This is an anode-supported cell, meaning that in addition to collecting current and supporting the anode reaction, the anode layer stiffens the whole cell. The layer on the right is the cathode, and the interface between the two is the thin electrolyte. One of the challenges of this design is to ensure that the rates of expansion of the cathode and the anode match. If the anode expands faster than the cathode, the planar cell tends to curl like a potato chip when the temperature changes.

Catalytic promotion of carbon deposition from carbon-containing fuels. Carbon deposited in the anode will generally cause irreversible damage (disintegration) of the anode structure. 

R. G. Allen, C. Lim, L. X. Yang, K. Scott, and S. Roy. Novel anode structure for the direct methanol fuel cell. Journal of Power Sources 143 (2005) 142-149. 

However, two penalties, both associated with the energy density, arise from the disordered anode structure (1) a smaller Coulombic capacity than the theoretical value for LiCe and (2) a sloping potential profile during both charging and discharging. 

Because the performance of an SOFC depends strongly on the anode structure, it is useful to consider how the anode works on a microscopic scale. The electrochemical reaction can only occur at the three-phase boundary (TPB), which is defined as the line at which the electrolyte, the electron-... 

The previous discussion has focused on the properties of perovskite materials rather than on their performance as anodes. The number of actual fuel-cell studies is more limited, but this literature has been reviewed recently by Irvine. Various perovskites have been investigated as potential SOFC anode materials however, these early efforts were hampered by low electrochemical activity toward methane oxidation,poor anode structure,or insufficient electrode conductivity. Most recently, Tao and Irvine demonstrated that an anode based on (Lao.75Sro.25)o.9Cro.5Mno.503 can provide reasonable power densities at 1173 K in 3% humidified CH4. Barnett and co-workers also reported stable power generation with methane and propane fuels on an anode based on LaCr03 however, they reported that the addition of Ni, in levels too small to affect the conductivity, was crucial in providing activity for the electrochemical oxidation reactions. 

S. Ye, Fuel cell anode structure for voltage reversal tolerance, US Patent 7608358, assigned to BDF IP Holdings Ltd. (Vancouber, BC, CA), October 27, 2009. 

Each electrolytic application demands a unique approach to anode structure design and fabrication. Factors such as current distribution, gas release, ability to maintain structural tolerances, electrical resistance, and the practicality of recoaiing must be taken into account. The most commercially accepted design for diaphragm chlorine cells is that of the expandable anode (Fig. I). 

In mercury chlorine cells, it has been found that cells operate at lower voltages when filled with anode structures comprised or triangular rods or of vertical blades. 

Environmental and Safety f actors. The primary environmental concern for the coaling plant is actually the residual material on the anode structures being returned for recoating. Therefore the anode user must enact effective cleaning procedures prior to shipment. Overall, the DSA [Electrode Corp.) has made chlorine manufacture cleaner, more consistent, simpler, and therefore safer. 

Fig. 3.2 Schematic representation of the porous anode structure. Black circles represent electronic conductive sites, while grey circles represent ionic conductive sites.

Figure 3.2 represents a schematic of the anode structure, highlighting the electronic and ionic conductive sites, while Figure 3.3 represents the schematic representation used for modeling purposes. 

The fuel processing operation of an SOFC critically depends on the anode structure and composition, since the electrochemical reaction can only take place at the three-phase boundary. If there is a breakdown in connectivity in any one of the three phases, the reaction cannot take place. Besides, if ions from the electrolyte cannot reach the reaction site, if the gas-phase fuel molecules cannot reach the reaction site, or if electrons cannot be removed from the reaction site, this site cannot contribute to the performance of the cell [5],... 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

At p. 307 of Williams (2002), MIT enters the direct hydrocarbon field presenting alternative anode structures, asserted to be an improvement on copper-based anodes, as immediately above. See Figure A.5 on methane direct oxidation in Appendix A. 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

Knights S etal, 2001a, Fuel Cell Anode Structure for Voltage Reversal Tolerance. WO 01/15247. 

One of the first attempts at modeling SOFCs with KMC simulations was reported by Modak and Lusk [32]. In their study, their model was restricted to capture the behavior of the electrolyte, YSZ, as a function of the open-circuit voltage, and comparisons were made with analytical predictions (Guoy-Chapman model). The paper focused on the oxygen concentration distribution within the electrolyte at the TPB, the voltage profile across the electrolyte, and the electric field within the electrolyte. Furthermore, the influences of the temperature and relative permittivity of the electrolyte on these features were captured. In order to accelerate the convergence of the simulations and to facilitate comparison with analytic models, a one-dimensional (1-D) model was implemented, and the cathode and anode structures and reactions were completely neglected. 

Alternatively, an additional layer constructed by using fine nickel powder, L1A102, and NiO is positioned between the anode and the electrolyte and filled with molten carbonate electrolyte. The purpose of this additional layer is to prevent gas crossover from one electrode to the other if cracks develop in the electrolyte structure. This bubble barrier layer serves as a reinforcement of the electrolyte matrix. This bubble pressure barrier (BPB) can be fabricated as an integral part of the anode structure. Typically, the pores of this barrier layer are smaller than the anode pores and provide ionic transport through the cell. ... 

The anode is stabilized by using mixed powders of Ni-Cr, Ni-Al, or refractory oxides and sintered at 900-1100°C under a reducing atmosphere to provide a creep resistant anode structure. The function of the additives is to reduce the loss of porosity during sintering and develop creep resistant materials. The creep is referred to as the shrinkage in thickness and change in shape. The sintering resistance is increased by the additives, which are usually metals or oxides of metals. 

The timing pulse from the multichannel plate is processed in the usual way in the time-measurement block. The pulses from the anode structure of the detector, A1 through A4, are converted by four ADCs on the TCSPC board. The position of the photon is calculated in a digital arithmetic unit. The unit has to deliver one X Y data pair within 100 ns or less to keep the dead time within the common standard of advanced TCSPC. The solution to the problem is pipelining ... 

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