Graphene‐based Oxygen Reduction Electrodes for Low Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are attracting attention as next-generation energy conversion devices due to their high efficiency, eco-friendliness, and scalability. However, the high operation temperature of conventional SOFCs (> 800°C) has posed several practical issues such as thermal degradation and limited material choice. To overcome this limitation, low-temperature SOFC (LT-SOFC, operation temperature <600°C) is actively being researched these days. Ni-YSZ (nickel-yttria stabilized zirconia) is commonly used as anode material in SOFCs. Ni-YSZ anodes are reasonably conductive at high operating temperatures and stable under harsh reducing conditions. However, at lower operating temperatures, the low conductivity of Ni-YSZ becomes an issue for use as an anode. Using doped ceria instead of YSZ is considered an alternative because the doped ceria has higher conductivity compared to YSZ due to the co-existence of multivalent states, i.e., Ce3+ and Ce4+, which can be further improved by doping. Moreover, if one adopts a carefully designed anode structure, e.g., gradient structure, the electrochemical performance of Ni-doped ceria anode can be further improved. Employing nanoscale components, e.g., thin films, are also one of the strategies to improve the electrode kinetics at the low operating temperature of LT-SOFCs. Thin film electrodes with short electronic and ionic paths as well as high surface area contribute to low ohmic and activation resistances. While high uniformity, reproducibility and throughout are practical concerns in thin film fabrication, reactive sputtering, which can produce uniform ceramic thin films from metal target in reactive gas environment in high reproducibility ad deposition rate, is a suitable candidate for thin film components for SOFCs. In this study, we fabricate a Ni-Samaria doped ceria(SDC) gradient anode by reactive co-sputtering. We prepare and characterize the cells with homogeneous and gradient-structured Ni-SDC thin film anodes. The gradient Ni-SDC thin film anode is composed of three layers, whose compositions are graded in a way that the Ni content increases from bottom (near electrolyte-anode interface) to top (anode top-surface). Possibly due to the combination of facilitated ionic conduction near electrolyte-anode interface and higher catalytic activity at anode surface, we observe the improved anode kinetics with lower activation resistance in the cell with gradient-structured Ni-SDC anode compared to the one with homogeneous anode.

Density functional theory was used to design a non-noble metal catalyst for low temperature oxygen reduction. A core–shell catalyst of Ag|AgxCu5-xZn8 was found to have an electronic structure predicted to be near optimal for weak oxygen bonding, and the activation energy for dissociation was low. This catalyst was made via evaporation and sputtering and tested as the anode and cathode of low temperature solid oxide fuel cells. The new catalyst showed charge transfer resistance 16 times higher than platinum on the cathode side, and five times lower on the anode side at 400 °C. The resistance to common catalyst poisons CO and H2S is enhanced compared to Pt. Further, the catalyst was tested as the cathode of a proton exchange membrane fuel cell by testing the half cell reaction in a rotating disk electrode. The exchange current density was within a factor of 3 lower than Pt, while the charge transfer coefficient was a factor of 3 higher than Pt. The Ag|AgxCu5-xZn8 catalyst is a promising start to develop non...

: Using DFT and experimental methods, the following topics have been deeply explored. (1) Interaction between oxygen and molten carbonate; (2) Fabrication and test of SOFCs with MC modified LSM cathodes; (3) Low-temperature SOFCs using MC modified LSCF/GDC cathode; (4) Spectroscopic and DFT evidence of pyrocarbonate. The results point out that molten carbonate modified cathodes can significantly reduce the operating temperature of SOFCs by enhancing the ORR process at low temperatures of 500-650 deg C. The mechanism of ORR with the presence of MC was proposed through DFT modeling and the theoretical results agree well with the experiments.

Mixed metallic oxides are getting increasing attention as novel electrode materials for energy conversion devices. However, low mixed ionic-electronic conductivity and high operating temperature hamper the practical applications of these devices. This study reports an effective strategy to improve the conductivity and performance of the fuel cell at low temperature by partially incorporating graphene in the Li0.1Cu0.2Zn0.7-oxide (LCZ) composite. The proposed cathode material is synthesized via the cost effective conventional solid-state route. Graphene incorporated LCZ shows excellent performance, which is attributed to the favorable charge transport paths offering low area-specific resistance. An X-ray diffractometer (XRD) and scanning electron microscope (SEM) are employed for microstructural and surface morphological analyses, respectively. Electrical conductivities of all the materials are determined by the DC four probe method, and interestingly, LCZ-1.5% graphene exhibits an excellent conductivity of 3.5 S/cm in air atmosphere at a temperature of 450 °C with a minimum value of 0.057 Ωcm2 area-specific resistance (ASR) that demonstrates significantly good performance. Moreover, the three-layer fuel cell device is fabricated using sodium carbonated Sm0.2Ce0.8O (NSDC) as an electrolyte, which can operate at low temperatures exhibiting open circuit voltage 0.95 V and shows a peak power density, i.e., 267.5 mW/cm2 with hydrogen as the fuel.

The praseodymium nickelate oxide Pr2NiO4+δ, a mixed conducting oxide, was evaluated as cathode for Low Temperature Solid Oxide Fuel Cell at T = 873 K, using zirconia-type electrolytes. The electrochemical performance of the porous cathode has been improved by optimisation of its microstructure combined with the use of a doped ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr2NiO4+δ powders with a median particle size of 0.4 µm. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions and tested. The best results were obtained when using a dense Ce0.8Gd0.2O1.9 (20CGO) barrier layer. Moreover, we developed a new route for depositing ceria thin films: the supercritical fluid deposition (SFD) technique. Interestingly, we could evidence that the deposit of a dense interlayer of a few tens of nanometer contributes to the lowering of the polarization resistance, but also that this result can be achieved with undoped ceria, although this material is never used for SOFC because of its low ionic conductivity level.

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The sluggish kinetics of oxygen reduction reaction (ORR) at low temperatures and the fast degradation of the cathode are the main obstacles to the commercialization of solid oxide fuel cells (SOFCs). However, it is still very challenging to achieve both high catalytic activity and favorable stability for single‐phase materials. Herein, a highly active and durable nanocomposite cathode (Ba0.5Sr0.5)0.75Pr0.25Co0.575Fe0.3W0.125O3‐δ (BSPCFW) for low‐temperature SOFC (≤650 °C) is presented, which self‐assembles into two cubic perovskites: the simple perovskite Pr0.38Ba0.25Sr0.37Co0.62Fe0.38O3‐δ (PBSCF‐c) and the B‐site cations ordered double perovskite Ba1.30Sr0.70Co1.0Fe0.25W0.75O6‐δ (BSCFW‐c). The former PBSCF‐c serves as the highly conductive and active catalyst for ORR, while the latter BSCFW‐c with a large lattice parameter introduces a key beneficial lattice tensile strain into the PBSCF‐c phase through a coherent interface, which significantly promotes the ORR activity at low temperatures with the area specific resistance of 0.034 Ω cm2 at 650 °C, and the long‐term stability of 2 years storage and 1280 h operation in symmetrical cell. The introduction of the beneficial lattice tensile strain in self‐assembled composite catalysts is an effective way to synergistically enhance the electrochemical activity and durability of the electrode materials for electrochemical devices.

Development of trimetallic Ni–Cu–Zn anode for low temperature solid oxide fuel cells with composite electrolyte

Solid Oxide Fuel Cells (SOFCs) are energy conversion devices known for their significantly higher power density compared to other fuel cell types. However, their high operating temperatures pose challenges related to thermal stability. To address this, research is focusing on Low-Temperature SOFCs (LT-SOFCs), which function at lower temperatures and exhibit enhanced electrochemical performance. While various electrode materials are utilized in SOFCs, platinum (Pt) stands out for its excellent electronic conductivity and catalytic activity. Unfortunately, at the operating temperatures of SOFCs, Pt tends to agglomerate, leading to a rapid reduction in the triple phase boundary (TPB) and a subsequent decline in electrochemical reactions. In this study, LT-SOFCs were fabricated with a Praseodymium Oxide (PrOx) capping layer applied to a porous Pt cathode using sputtering, with various thicknesses achieved by adjusting the deposition time. The electrochemical performance of the LT-SOFCs was measured at 500oC. Additionally, the degradation behavior of the LT-SOFCs was assessed by applying a constant voltage of 0.5 V for 48 hours. Scanning Electron Microscopy (SEM) analysis was also conducted on the PrOx capping layer thin films under the same operating conditions.

A gadolinia-doped ceria (GDC) capping layer was fabricated on a metal Pt cathode via a sputtering technique to enhance the thermal stability and oxygen-reduction reaction (ORR) kinetics. By varying the thickness of the GDC thin film on the Pt cathode, we fabricated thermally durable low-temperature solid oxide fuel cells (LT-SOFCs) on porous anodized aluminum oxide (AAO) substrates while securing very high performance. The GDC protective layers subsequently mitigated the agglomeration of the Pt morphology by hindering the coarsening of metal grains, and enhanced the cathodic kinetics as a result of the increased TPB density at the Pt-electrolyte interface. Enhanced performance of fuel cells and thermo-stable behavior with the GDC capping layer were demonstrated by employing transmission electron microscopy (TEM) and electrochemical analysis. An enhanced peak power density of 258 mW/cm2 was obtained with a degradation of 56.4% of the Pt cathode over 25 hours of operation at 450°C.

Proc. 6 th International Symposium on Solid Oxide Fuel Cells (SOFC-VI) ed. S. C. Singhal et al., Honolulu, Hawaii, Oct. 17-22, 1999 LBNL-43185 May, 1999 PROMISES AND PROBLEMS WITH METALLIC INTERCONNECTS FOR REDUCED TEMPERATURE SOLID OXIDE FUEL CELLS Peggy Y. Hou, Keqin Huang † and Wate T. Bakker* Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley, CA 94720 Texas Materials Institute, ETC 9.102 The University of Texas at Austin Austin, TX 78713 *Electric Power Research Institute 3412 Hillview Avenue Palo Alto, CA 94304 ABSTRACT The oxidation kinetics and electrical properties of oxide scales thermally grown on the surface of a commercial iron-based alloy have been investigated up to 1000 o C. The effect of surface coating to improve performance was studied, and it was found that a double coating of Y and Ni oxide was most beneficial. While the Y oxide reduced the oxidation rate, the Ni oxide increased the Cr 2 O 3 scale conductivity. Different electrodes were applied to the alloy or to pre-oxidized samples. Some of the electrodes, such as LaSrCoO 3 (LSCo), reacted strongly with the alloy upon oxidation by increasing the oxidation rate and forming a mixed Fe/Co/Cr interlayer that impeded conduction. Application of these electrodes to a pre-oxidized surface eliminated these problems. Preliminary results in reducing atmosphere using Ce 0.8 +Sm 0.2 O 1.9 +Ni and Pt electrodes are also reported. The possibility of using Cr 2 O 3 forming alloys as interconnects for low temperature solid oxide fuel cell is discussed in light of these results. INTRODUCTION The trend in planar solid oxide fuel cell (SOFC) development is towards lower temperature operation. This has several advantages such as the elimination of interdiffusion between electrodes and electrolytes, ease of internal reforming and the use of relatively low cost metallic interconnects and balance of plant components. Several methods to operate at low temperatures have been identified and demonstrated at a single cell level. The most prominent are the use of thin film, conventional doped zirconia electrolytes (1,2) and the use of a recently discovered new electrolyte, doped lanthanum

Effects of In substitution in Y1−xInxBaCo3ZnO7+δ (0 ≤ x ≤ 0.5) cathodes for intermediate temperature solid oxide fuel cells

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd1.5Ba1.5CoFeMnO9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (RP) of NBCFM is 1.1273 Ω cm2~0.1587 Ω cm2 in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10−6 K−1 at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce0.8Sm0.2O2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10−6 K−1 to 15.25 × 10−6 K−1. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10−4 to 10−3 cm s−1 over the range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm2 at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm2). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.

The porous and mixed ionic electronic cathode conductor plays a vital role in the efficient working of low‐temperature solid oxide fuel cells. Perovskite composite cathode material La0.3Sr0.7Fe0.4Ti0.6O3‐δ (LSFT) is synthesized using the sol–gel method. Graphene oxide (GO) is integrated in 1 and 1.5 wt.% into the perovskite LSFT cathode to use as a cathode for low‐temperature solid oxide fuel cell (LT‐SOFC). Crystal structure analysis is studied by XRD and the crystalline size of samples is determined in the range of 103–73 nm. SEM analysis is used to characterize homogeneous and porous morphology. The FTIR spectral analysis is examined for proposed nanocomposite cathode materials. The electrical conductivities are carried out by four probe method in air atmosphere in the temperature range 300–600 °C and enhanced conductivity is obtained 7 Scm−1 for LSFT‐1.5 wt.% GO sample. For the same sample, a minimum value of area‐specific resistance (ASR) of 0.02 Ωcm2 is obtained. The performance of three‐layer fuel cells is tested by supplying H2 fueled at the anode and oxygen at the cathode terminal of the cell and peak power density is determined to be 362 mWcm−2 for GO incorporated fuel cell. The prepared cathode can be proposed as a potential candidate for low‐temperature solid oxide fuel cells.

Fast oxygen reduction reaction (ORR) at the cathode is a key requirement for the realization of low-temperature solid oxide fuel cells (SOFCs). While the design of three-dimensional (3D) structures has emerged as a new and promising approach to improving the electrochemical performance of SOFC cathodes, achieving versatile structures and structural stability is still challenging. In this study, we demonstrate a novel architectural design for a superior cathode with fast ORR activity. By employing a completely new fabrication process comprising a 3D printing technique and pulsed laser deposition (PLD), we design 3D La0.8Sr0.2CoO3-δ (LSC) micro-nano structures with the desired shape. 3D-printed yttria-stabilized ZrO2 (YSZ) microstructures significantly increase the ratio of surface area to volume while maintaining suitable ionic conductivity comparable to that of single-crystalline YSZ substrates. Scanning electron microscopy and energy dispersive X-ray microanalysis reveal the formation of crack- or void-free YSZ microstructures and the uniform deposition of LSC films by PLD on the YSZ microstructures. The 3D LSC micro-nano structures show significantly enhanced oxygen surface exchange coefficients (kchem) extracted from electrical conductivity relaxation (ECR) measurements by up to 3 orders of magnitude relative to the bulk LSC. Furthermore, electrochemical impedance spectroscopy measurements verify the kchem values from ECR and no directional difference in the measured ORR activity depending on the shape of 3D microstructures. The dramatic enhancement of the ORR activity of LSC is attributed to the increased film surface areas resulting from the 3D YSZ microstructures.