Triple phase boundary

Last updated

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.

Contents

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-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]

Units

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 TPB

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:

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.

See also

Related Research Articles

Fuel cell Device that converts the chemical energy from a fuel into electricity

A fuel cell is an electrochemical cell that converts the chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from metals and their ions or oxides that are commonly already present in the battery, except in flow batteries. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.

Lithium-ion battery Rechargeable battery type

A lithium-ion battery or Li-ion battery is a type of rechargeable battery. Lithium-ion batteries are commonly used for portable electronics and electric vehicles and are growing in popularity for military and aerospace applications. A prototype Li-ion battery was developed by Akira Yoshino in 1985, based on earlier research by John Goodenough, Stanley Whittingham, Rachid Yazami and Koichi Mizushima during the 1970s–1980s, and then a commercial Li-ion battery was developed by a Sony and Asahi Kasei team led by Yoshio Nishi in 1991.

A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.

Proton-exchange membrane fuel cell

Proton-exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.

Solid oxide fuel cell fuel cell that has a ceramic electrolyte

A solid oxide fuel cell is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.

Flow battery A type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids

A flow battery, or redox flow battery, is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 volts.

In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential implies the cell requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density is achieved.

Genoa Joint Laboratories (GJL) is a scientific research activity founded in 2002, combining expertise in electroceramics and electrochemistry of three facilities: National Research Council - Institute for Energetics and Interphases (CNR-IENI), Department of Chemical and Process Engineering with University of Genova (DICHeP), and the Department of Chemistry and Industrial Chemistry with University of Genova (DCCI), all located in Genoa, Italy.

Yttria-stabilized zirconia A ceramic with room temperature stable cubic crystal structure

Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (ZrO2) and "yttria" (Y2O3), hence the name.

A nanowire battery uses nanowires to increase the surface area of one or both of its electrodes. Some designs, variations of the lithium-ion battery have been announced, although none are commercially available. All of the concepts replace the traditional graphite anode and could improve battery performance.

Lithium-ion capacitor hybrid supercapacitor

A lithium-ion capacitor (LIC) is a hybrid type of capacitor classified as a type of supercapacitor. Activated carbon is typically used as the cathode. The anode of the LIC consists of carbon material which is pre-doped with lithium ions. This pre-doping process lowers the potential of the anode and allows a relatively high output voltage compared with other supercapacitors.

Lithium–sulfur battery type of rechargeable battery, notable for its high specific energy

The lithium–sulfur battery is a type of rechargeable battery, notable for its high specific energy. The low atomic weight of lithium and moderate atomic weight of sulfur means that Li–S batteries are relatively light. They were used on the longest and highest-altitude unmanned solar-powered aeroplane flight by Zephyr 6 in August 2008.

Solid oxide electrolyzer cell Type of fuel cell

A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas and oxygen. The production of pure hydrogen is compelling because it is a clean fuel that can be stored easily, thus making it a potential alternative to batteries, which have a low storage capacity and create high amounts of waste materials. Electrolysis is currently the most promising method of hydrogen production from water due to high efficiency of conversion and relatively low required energy input when compared to thermochemical and photocatalytic methods.

Nanoarchitectures for lithium-ion batteries are attempts to employ nanotechnology to improve the design of lithium-ion batteries. Research in lithium-ion batteries focuses on improving energy density, power density, safety, durability and cost.

The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.

A metal–air electrochemical cell is an electrochemical cell that uses an anode made from pure metal and an external cathode of ambient air, typically with an aqueous or aprotic electrolyte. During discharging of a metal–air electrochemical cell, a reduction reaction occurs in the ambient air cathode while the metal anode is oxidized. The specific capacity and energy density of metal–air electrochemical cells is higher than that of lithium-ion batteries, making them a prime candidate for use in electric vehicles. However, complications associated with the metal anodes, catalysts, and electrolytes have hindered development and implementation of metal–air batteries.

Alkaline anion exchange membrane fuel cell

An alkaline anion exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion exchange membrane to separate the anode and cathode compartments.

Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas on research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.

Lithium–silicon battery is a name used for a subclass of lithium-ion battery technology that employs a silicon-based anode and lithium ions as the charge carriers. Silicon has a much larger specific capacity than graphite. Silicon's large volume change when lithium is inserted is one of the main obstacles along with high reactivity in the charged state to commercializing this type of anode. Commercial battery anodes may have small amounts of silicon, boosting their performance slightly. The amounts are closely held trade secrets, limited as of 2018 to at most 10% of the anode.

Mixed conductor Mixed ion-electron conductor

Mixed conductors, also known as mixed ion-electron conductors(MIEC), are a single-phase material that has significant conduction ionically and electronically. Due to the mixed conduction, a formally neutral species can transport in a solid and therefore mass storage and redistribution are enabled. Mixed conductors are well known in conjugation with high-temperature superconductivity and are able to capacitate rapid solid-state reactions.

References

  1. 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.
  2. 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. doi:10.1149/1.1701868.
  3. 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.
  4. 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 .
  5. 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 .
  6. 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.