Three-phase boundaries anodes

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 hydrogen oxidation within a fuel cell occurs partly at the anode and the cathode. Different models were supposed for the detailed reaction mechanisms of the hydrogen at Ni-YSZ (yttria stabilised zirconia) cermet anodes. The major differences of the models were found with regard to the location where the chemical and electrochemical reactions occur at the TPB (three-phase boundary of the gaseous phase, the electrode and the electrolyte). However, it is assumed that the hydrogen is adsorbed at the anode, ionised and the electrons are used within an external electrical circuit to convert the electrical potential between the anode and the cathode into work. Oxygen is adsorbed at the cathode and ionised by the electrons of the load. The electrolyte leads the oxide ion from the cathode to the anode. The hydrogen ions (protons) and the oxide ion form a molecule of water. The anodic reaction is... 

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],... 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Ceria has also been used as the ceramic part in nickel - or ruthenium - cermet anodes for hydrogen oxidation." Beneficial effects have been reported and interpreted as being probably due to the broadening of the three-phase boundary zone width. 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

Fig. 14 Anodic reactions at the three-phase boundary of a SOFC V°° = oxygen vacancy in solid conducting oxide.

The three-phase boundary, prescribing exclusive locations at which reactions proceed, is well defined in regular composite media, as encountered more genuinely in SOFC anodes [120,128], In CCLs of PEFCs it cannot be defined precisely, as indicated in Fig. 11(b). Here, catalyst particles acquire individual activities depending on their immediate environment. 

It is very important for a high-performance electrode to have both a highly active electrocatalytic reaction zone and a sufficient gas-supply network in its microstructure. The conventional anode material used so far is Ni-YSZ cermet prepared from pm-sized NiO and YSZ particles as shown in Fig. 3A. Because all reactants (fuel gas, electrons, and oxide ions) must meet together at the reaction sites, the so-called "three phase boundary (TPB) zone is the effective... 

Porous ceramics, with high electronic conduction and chemical stability in the fuel gas are required for anode and cathode. The electrode should be porous and have homogeneous microstructures because the electrode reaction occurs on three phase boundary (TPB), which consists of electrolyte, electrode and gas. The reaction site increases with TPB length. Using a new technique of spray dry process, a new... 

The fundamental studies of the catalytic reactions mechanisms that occur near the three-phase boundary in the anode of a solid oxide fuel cell still deserving attention and investigation. Many in situ sp>ectroscopies such as Raman and infrared spectroscopy are routinely used in catalysis research to characterize surface intermediates and reaction mechanisms. It is very difficult to apply in situ spectroscopy techniques to an operating SOFC anode (Atkinson et al., 2004). Recently research groups (Liu et al., 2002 Guo et al., 2010) presented their methodologies on the possible ways to apply the infrared emission spectroscopy to characterize working SOFC anodes. 

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). 

Typical anodes are cermets composed of metals and ceramics. The metallic phase is required for the transport of electrons which are released at the three phase boundary (TPB), while the ceramic phase facilitates the transport of oxygen ions. The ceramic is often made of... 

---