A triple phase boundary (TPB) is a geometrical class of phase boundary and the location of contact between three different phases. A simple example of a TPB is a coastline where land, air and sea meet to create an energetic location driven by solar, wind and wave energy capable of supporting a high level of biodiversity. This concept is particularly important in the description of electrodes in fuel cells and batteries. For example for fuel cells, the three phases are an ion conductor (electrolyte), an electronconductor, and a virtual "porosity" phase for transporting gaseous or liquid fuelmolecules. The electrochemical reactions that fuel cells use to produce electricity occur in the presence of these three phases. Triple phase boundaries are thus the electrochemically active sites within electrodes.
The oxygen reduction reaction that occurs at a solid oxide fuel cell's (SOFC) cathode, can be written as follows:
O 2(gas) + 4e−(electrode) → 2O2− (electrolyte)
Different mechanisms bring these reactants to a TPB to carry out this reaction.[1] The kinetics of this reaction is one of the limiting factors in cell performance, so increasing the TPB density will increase the reaction rate, and thus increase cell performance.[2] Analogously, TPB density will also influence the kinetics of the oxidation reaction that occurs between oxygen ions and fuel on the anode side of the cell. Transport to and from each TPB will also affect kinetics, so optimization of the pathways to get reactants and products to the active area is also an important consideration. Researchers working with fuel cells are increasingly using 3D imaging techniques like FIB-SEM and X-raynanotomogrpahy to measure TPB density as a way of characterizing cell activity.[3][4] Recently, processing techniques such as infiltration have been shown to substantially increase TPB density, leading to higher efficiency and, potentially, more commercially viable SOFCs.[5]
In systems consisting of only three phases, triple phase boundaries are geometrically closed loop linear features that do not intersect other TPBs and do not as such form a network. The simplest TPB shape is easily visualised using two arbitrarily sized intersecting spheres of different phase suspended in free space (see figure 3[6]) which creates a circular TPB at the intersection of the spheres. However, in electrodes TPB loops typically have highly complex and stochastic shapes in three dimensions (3D). TPBs thus have the units of length. For electrodes normalising the TPB length to TPB density provides an important microstructure parameter for the description of electrode and thus cell performance that is independent of electrode dimensions. TPB density is normally a volumetric density and is measured in units of inverse square length, typically μm−2 (i.e. μm/μm3) due to the scale of typical electrode microstructural features.
Active TPBedit
Triple phase boundaries are only electrochemically active if each and every "phase" is connected to reaction species sources and destinations to complete the electrochemical reaction. Active TPBs are often referred to as percolated TPBs. For example in an SOFC Ni-YSZ anode cermet the TPB must:
Have access to hydrogen from the anode gas inlet and be able to exhaust steam to the anode gas outlet via the pore phase network
Have access to oxygen ions transported from the electrolyte YSZ electrolyte phase network
Be able to conduct electrons from the TPB through the electron conducting nickel network to the anode current collector
In addition to increasing the TPB density it is obviously advantageous to increase the ratio of active to total TPB density to increase electrode/cell performance electrode.
^Fehribach, Joseph D.; O'Hayre, Ryan (January 2009). "Triple Phase Boundaries in Solid-Oxide Cathodes". SIAM Journal on Applied Mathematics. 70 (2): 510–530. doi:10.1137/080722667. ISSN 0036-1399. S2CID 17872247.
^O’Hayre, Ryan; Prinz, Fritz B. (2004). "The Air/Platinum/Nafion Triple-Phase Boundary: Characteristics, Scaling, and Implications for Fuel Cells". Journal of the Electrochemical Society. 151 (5): A756. Bibcode:2004JElS..151A.756O. doi:10.1149/1.1701868.
^Vivet, N.; Chupin, S.; Estrade, E.; Richard, A.; Bonnamy, S.; Rochais, D.; Bruneton, E. (December 2011). "Effect of Ni content in SOFC Ni-YSZ cermets: A three-dimensional study by FIB-SEM tomography". Journal of Power Sources. 196 (23): 9989–9997. Bibcode:2011JPS...196.9989V. doi:10.1016/j.jpowsour.2011.07.010.
^Song, Bowen; Ruiz-Trejo, Enrique; Bertei, Antonio; Brandon, Nigel P. (January 2018). "Quantification of the degradation of Ni-YSZ anodes upon redox cycling". Journal of Power Sources. 374: 61–68. Bibcode:2018JPS...374...61S. doi:10.1016/j.jpowsour.2017.11.024. hdl:10044/1/53328.
^Song, B.; Ruiz-Trejo, E.; Brandon, N.P. (August 2018). "Enhanced mechanical stability of Ni-YSZ scaffold demonstrated by nanoindentation and Electrochemical Impedance Spectroscopy". Journal of Power Sources. 395: 205–211. Bibcode:2018JPS...395..205S. doi:10.1016/j.jpowsour.2018.05.075. hdl:10044/1/60309.
^Jørgensen, P.S.; Hansen, K.V.; Larsen, R.; Bowen, J.R. (2010-12-15). "High accuracy interface characterization of three phase material systems in three dimensions". Journal of Power Sources. 195 (24): 8168–8176. Bibcode:2010JPS...195.8168J. doi:10.1016/j.jpowsour.2010.06.083.
March 04, 2024
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A triple phase boundary TPB is a geometrical class of phase boundary and the location of contact between three different phases A simple example of a TPB is a coastline where land air and sea meet to create an energetic location driven by solar wind and wave energy capable of supporting a high level of biodiversity This concept is particularly important in the description of electrodes in fuel cells and batteries For example for fuel cells the three phases are an ion conductor electrolyte an electron conductor and a virtual porosity phase for transporting gaseous or liquid fuel molecules The electrochemical reactions that fuel cells use to produce electricity occur in the presence of these three phases Triple phase boundaries are thus the electrochemically active sites within electrodes The oxygen reduction reaction that occurs at a solid oxide fuel cell s SOFC cathode can be written as follows O2 gas 4e electrode 2O2 electrolyte Different mechanisms bring these reactants to a TPB to carry out this reaction 1 The kinetics of this reaction is one of the limiting factors in cell performance so increasing the TPB density will increase the reaction rate and thus increase cell performance 2 Analogously TPB density will also influence the kinetics of the oxidation reaction that occurs between oxygen ions and fuel on the anode side of the cell Transport to and from each TPB will also affect kinetics so optimization of the pathways to get reactants and products to the active area is also an important consideration Researchers working with fuel cells are increasingly using 3D imaging techniques like FIB SEM and X ray nanotomogrpahy to measure TPB density as a way of characterizing cell activity 3 4 Recently processing techniques such as infiltration have been shown to substantially increase TPB density leading to higher efficiency and potentially more commercially viable SOFCs 5 Contents 1 Units 2 Active TPB 3 See also 4 ReferencesUnits editIn systems consisting of only three phases triple phase boundaries are geometrically closed loop linear features that do not intersect other TPBs and do not as such form a network The simplest TPB shape is easily visualised using two arbitrarily sized intersecting spheres of different phase suspended in free space see figure 3 6 which creates a circular TPB at the intersection of the spheres However in electrodes TPB loops typically have highly complex and stochastic shapes in three dimensions 3D TPBs thus have the units of length For electrodes normalising the TPB length to TPB density provides an important microstructure parameter for the description of electrode and thus cell performance that is independent of electrode dimensions TPB density is normally a volumetric density and is measured in units of inverse square length typically mm 2 i e mm mm3 due to the scale of typical electrode microstructural features Active TPB editTriple phase boundaries are only electrochemically active if each and every phase is connected to reaction species sources and destinations to complete the electrochemical reaction Active TPBs are often referred to as percolated TPBs For example in an SOFC Ni YSZ anode cermet the TPB must Have access to hydrogen from the anode gas inlet and be able to exhaust steam to the anode gas outlet via the pore phase network Have access to oxygen ions transported from the electrolyte YSZ electrolyte phase network Be able to conduct electrons from the TPB through the electron conducting nickel network to the anode current collectorIn addition to increasing the TPB density it is obviously advantageous to increase the ratio of active to total TPB density to increase electrode cell performance electrode See also editGlossary of fuel cell termsReferences edit Fehribach Joseph D O Hayre Ryan January 2009 Triple Phase Boundaries in Solid Oxide Cathodes SIAM Journal on Applied Mathematics 70 2 510 530 doi 10 1137 080722667 ISSN 0036 1399 S2CID 17872247 O Hayre Ryan Prinz Fritz B 2004 The Air Platinum Nafion Triple Phase Boundary Characteristics Scaling and Implications for Fuel Cells Journal of the Electrochemical Society 151 5 A756 Bibcode 2004JElS 151A 756O doi 10 1149 1 1701868 Vivet N Chupin S Estrade E Richard A Bonnamy S Rochais D Bruneton E December 2011 Effect of Ni content in SOFC Ni YSZ cermets A three dimensional study by FIB SEM tomography Journal of Power Sources 196 23 9989 9997 Bibcode 2011JPS 196 9989V doi 10 1016 j jpowsour 2011 07 010 Song Bowen Ruiz Trejo Enrique Bertei Antonio Brandon Nigel P January 2018 Quantification of the degradation of Ni YSZ anodes upon redox cycling Journal of Power Sources 374 61 68 Bibcode 2018JPS 374 61S doi 10 1016 j jpowsour 2017 11 024 hdl 10044 1 53328 Song B Ruiz Trejo E Brandon N P August 2018 Enhanced mechanical stability of Ni YSZ scaffold demonstrated by nanoindentation and Electrochemical Impedance Spectroscopy Journal of Power Sources 395 205 211 Bibcode 2018JPS 395 205S doi 10 1016 j jpowsour 2018 05 075 hdl 10044 1 60309 Jorgensen P S Hansen K V Larsen R Bowen J R 2010 12 15 High accuracy interface characterization of three phase material systems in three dimensions Journal of Power Sources 195 24 8168 8176 Bibcode 2010JPS 195 8168J doi 10 1016 j jpowsour 2010 06 083 Retrieved from https en wikipedia org w index php title Triple phase boundary amp oldid 1193568946, wikipedia, wiki, book, books, library,
