Model-guided design of a high performance and durability Ni nanofiber/ceria matrix solid oxide fuel cell electrode

Abstract

Mixed ionic electronic conductors (MIECs) have attracted increasing attention as anode materials for solid oxide fuel cells (SOFCs) and they hold great promise for lowering the operation temperature of SOFCs. However, there has been a lack of understanding of the performance-limiting factors and guidelines for rational design of composite metal-MIEC electrodes. Using a newly-developed approach based on 3D-tomography and electrochemical impedance spectroscopy, here for the first time we quantify the contribution of the dual-phase boundary (DPB) relative to the three-phase boundary (TPB) reaction pathway on real MIEC electrodes. A new design strategy is developed for Ni/gadolinium doped ceria (CGO) electrodes (a typical MIEC electrode) based on the quantitative analyses and a novel Ni/CGO fiber-matrix structure is proposed and fabricated by combining electrospinning and tape-casting methods using commercial powders. With only 11.5 vol% nickel, the designer Ni/CGO fiber-matrix electrode shows 32% and 67% lower polarization resistance than a nano-Ni impregnated CGO scaffold electrode and conventional cermet electrode respectively. The results in this paper demonstrate quantitatively using real electrode structures that enhancing DPB and hydrogen kinetics are more efficient strategies to enhance electrode performance than simply increasing TPB.

Optimizing Solid Oxide Fuel Cell Performance to Re-evaluate Its Role in the Mobility Sector

“…The pronounced negative SI when decreasing ε fe,dl by 10% at 1023.15 K indicates a mass transport limitation in the Ni-CGO FMS layer, further complicating the structural optimization in such a way that a dedicated trade-off between maximizing A gas/io V , porosity, and ion conduction has to be made, with a growing influence of gas transport at higher temperatures and fuel consumptions. This is in agreement with the considerations of the DPB design strategy reported by Ouyang et al 23 Thus, additional system simulations considering "MSC_FMS_infil" RUs with a further advanced ("Adv") microstructure were performed. For the case "Adv_A gas/io V ", the specific CGO/pore phase interfacial area in the anode was increased by 50%, for "Adv_t elyt " the dense CGO|YSZ|CGO electrolyte thickness was decreased from 6 to 3 μm, and for "Adv_t fe,dl ", the Crofer 22 H support layer thickness was reduced from 200 to 150 μm.…”

“…To systematically identify the performance bottlenecks and uncover possible avenues to further improve the system, a sensitivity analysis of electrochemical and microstructural parameters was conducted for the "MSC_FMS_infil" configuration by performing isothermal RU simulations at 0.8 V and 973.15−1023.15 K, as shown in Figure 8b. The dimensionless sensitivity index (SI) is calculated with respect to P el,stack V according to a ± 10% change in each parameter P, From the sensitivity plot, it can be deduced that, aside from the interconnect rib-air electrode contact resistance R contact,ae→ic , which causes the highest variation of P el,stack V and, accordingly, contributes the most to the observed cell resistance, A gas/io V , the electrolyte thickness t elyt , and the Crofer 22 H support layer thickness t fe,dl all lead to SI > 0.085 at 973.15 K. Thus, when R contact,ae→ic is assumed to be fixed, the model predicts that significant performance improvements are possible by either refining the Ni-CGO electrospun fiber-network to increase the CGO-pore phase interfacial area, e.g., by optimizing processing conditions 23 or by further reducing the layer thicknesses t elyt and t fe,dl . Moreover, it can be extracted that ion conduction in the anode CGO matrix, i.e., σ io,fe eff , causes a high SI of P el,stack V of 0.082−0.093 in the respective temperature interval, which underlines the primary importance of a well-percolating backbone structure across the relatively thin functional layer.…”

Integrated Multiscale Modeling of Solid Oxide Electrodes, Cells, Stacks, and Systems

“…The results showed up to an 85 % increase in power density over commercial stacks was possible in a 130-kW system by optimizing the cell architecture, especially, the fuel electrode surface area, the metal support thickness, and the electrolyte thickness. This significant increase was achieved whilst maintaining over 50 % system efficiency and average stack temperatures below 973.15 K. Moreover, care was taken that the final optimized cell design was manufacturable by basing it on promising architectures already reported in the literature [18,19]. This optimized system easily surpassed industrial targets prescribed for SOFC-based range extenders and can also potentially serve as a prime mover for large-scale transport applications with further optimization of the system dynamics.…”

Non-equilibrium thermodynamics of mixed ionic-electronic conductive electrodes and their interfaces: a Ni/CGO study

“…However, charge transfer at the 3PB is still possible and must be considered in the model even though the availability of appropriate 3PB sites is substantially less than that of 2PB sites. 38 At the 3PB, the shi in free energy under bias is expressed as:…”