Electrode micro structured

Ribeiro, J. and De Andrade, A. R. (2004), Characterization of Ru02-Ta205 coated titanium electrode micro structure, morphology, and electrochemical investigation. J. Electrochem. Soc., 151(10) D106-D112. 

However, due to the transport limitation the concentration of the chemical species at the TPB is different from that in the fuel and oxidizer channels. The concentration overpotential strongly depends on electrode micro-structure. A high tortuosity and low porosity can lead to high concentration overpotentials. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Figure 3 and 4 show SEM images for the surface of anode catalyst and the cross section of ESC (NiO-YSZ-CeOa I YSZ I (LaSr)Mn03), respectively. The micro structure of the catalyst electrode was characterized by SEM (Hitachi Co., S-4200). The morphology of particles over Ni0-YSZ-Ce02 was uniformly distributed. 

The micro structured platelets, hold in a non-conducting housing, were realized by etching of metal foils and laser cutting techniques [69]. Owing to the small Nemst diffusion layer thickness, fast mass transfer between the electrodes is achievable. The electrode surface area normalized by cell volume amounts to 40 000 m m". This value clearly exceeds the specific surface areas of conventional mono- and bipolar cells of 10-100 m m. ... 

Ni-YSZ cermets deposited by RF sputtering (230 nm) were found to have micro-structural features consisting of columnar grains 13 to 75 nm long and 9 to 22 nm wide, and showed good adhesion to the YSZ layer on which they were deposited [128], In a three-layer Ni-YSZ-Ni film deposited on NiO by RF sputtering in another study, the YSZ layer exhibited a columnar structure with some pinholes [129], Microstructural and electrochemical features of Pt electrodes patterned by lithography on YSZ have also been studied [130,131]. 

Li+ intercalation material (V. M. Cepak and C. R. Martin, unpublished). These results, which will be the subject of a future paper, show that other synthetic methodologies, in addition to CVD, can be used to make micro-structured battery electrodes like those described here. In addition, the underlying microtubular current collector does not have to be Au. Microtubules composed of graphite [35] or other metals [1,3] (e.g., Ni) could be used. Finally, for the advantages noted above to be realized in practical cells, large-scale template-fabrication methods would have to be developed. 

An electrochemical reaction, the reduction of benzoquinone, is exemplarily described. An electrochemical micro structured reactor is divided into a cathode and anode chamber by a Nafion hollow-fiber tube. The anode chamber is equipped with a platinum electrode and the cathode chamber contains the analyte and carbon or zinc electrodes. Current density and flow rate are controlled to maximize current efficiency as determined by analysis of the formed hydroquinone by an electrochemical detector. Hydroquinone is extracted subsequently in a micro extractor from the resulting product stream [84],... 

Fig. 14.13 Micro structure of S11O2 electrodes with nanometer coating and S11O2 electrodes with normal coating...

LIGA = lithographic-galvanic this term refers to the manufacturing process applied for making micro structured honeycomb electrodes. 

J. Xie, D. L. Wood III, K. L. More, P. Atanassov, and R. L. Borup, Micro-structural Changes of Membrane Electrode Assemblies During PEFC Durability Testing at High Humidity Conditions, /. Electrochem. Soc., 12, AlOl 1 (2005). 

SOFC anodes is typically a complex inter-networks of ionically and electronically conducting phases, and gas-filled porosity. Control of the composition and micro-structure is critical for the activity of electrodes [6]. Percolating networks of three-phase boundaries formed by the electronic phase, ionic phase, and the gas-phase are important for high electrochemical performance of the cell. A three-dimensional reconstruction of a typical state of the art Ni/YSZ anode and its three-phase boundaries reproduced from [7] is shown in Fig. 1.2. There are numerous techniques by which the anodes can be fabricated [8,9]. In all cases the NiO-YSZ active layer as fired is a dense material, and most of the porosity results during the reduction process [7]. Zhu et. al [10] reported that a continuous porosity of more than 30% is required to facilitate the transport of reactants and products to and away from the three-phase boundary (TPB). 

The operation of the cell is associated with various irreversibilities and leads to various potential losses. In the case of electrodes the total resistance comprises of the internal resistance, contact resistance, activation polarization resistance, and concentration polarization resistance. Internal resistance refers to the resistance for electron transport, which is usually determined by the electronic conductivity and the thickness of the electrode structure. Contact resistance refers to the poor contact between the electrode and the electrolyte structure. All resistive losses are functions of local current density. However, one can minimize the overpotential losses by appropriate choice of electrode material and controlling the micro-structural properties during manufacturing process. 

The total resistance on the anode/cathode side comprises of internal resistance towards the transport of electrons (also ions in the case of mixed ionic electronic conductors) and contact resistance, i.e., the resistance caused by the poor adherence between the anode and the electrolyte. The magnitude of aU these resistances depends on the particular material of construction and the micro-structure of the porous electrode. 

The quest to predict the effect of other cell components on cell performance and the need to interpret the influence of micro structural properties lead to the development of other simple educative models. An electrolyte model to investigate the migration of 0 ions and diffusion of free electrons is developed by Chan et al. [85]. Interconnects one of the most important cell component is probably the least numerically investigated cell component. Tanner et al. reported interconnect model to investigate symmetry effects [86]. Model incorporating micro-structure of electrodes has been reported by Xia et al. [87]. 

As illustrated in Equation (17.20) for parallel electrodes and a uniform current distribution, the ohmic drop decreases with decrease in the inter-electrode gap and with increase in the electrolyte conductivity. In microstructured reactors, the small interelectrode gap together with the conductivity increase due to the coupling of the electrode processes leads to a substantial reduction in the ohmic perudty [7, 8j. Hence micro-structured designs permit one to minimize the cell voltage [Equation (17.19)], the specific energy consumption of the electrochemical cell [Equation (17.18)] and the heat generation terms [Equation (17.21)]. 

No other material shows as much versatility as an electrode as does electrically conducting, CVD diamond. The material can be used in electroanalysis to provide low detection limits for analytes with superb precision and stability for high current density electrolysis (1-10 A/cm ) in aggressive solution environments without any morphological or micro-structural degradation and as an optically transparent electrode (OTE) for spectroelectrochemical measurements in the ultraviolet-visible (UV-Vis) and infrared (IR) regions of the electromagnetic spectrum. 

Cheng, X., Chen, L., Peng, C., Chen, Z., Zhang, Y., Fan, Q. 2004. Catalyst micro-structure examination of PEMFC membrane electrode assemblies vs. time. Journal of the Electrochemical Society, 151, A48-A52. 

Kim S and Mench M M (2007), Physical degradation of membrane electrode assemblies undeigoing freeze/thaw cycling Micro-structure effects . Journal of Power Sources, 174,206-220. 

FIGURE 9.6 MEA with layer delamination after freeze-thaw cycles. (Reprinted from /. Power Sources, 174, Kim, S. and Mench, M. M., Physical degradation of membrane electrode assemblies undergoing freeze/thaw cycling Micro-structure effects, 206 220, Copyright (2007), with permission from Elsevier.)... 

Strutwolf J, O Sullivan CK (2007) Micro structures by selective desorption of self-assembled monolayer from polycrystalline gold electrodes. Electroanalysis 19 1467-1475... 

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