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1. Theoretical understanding of stability of the oxygen electrode in a proton-conductor based solid oxide electrolysis cell

   Yudong Wang, Barbara Marchetti (May 2023, International journal of hydrogen energy)

   The oxygen electrode in a proton-conductor based solid oxide cells is often a triple-conducting material that enables the transport and exchange of electrons (e-), oxygen ions (O2-), and protons (H+), thus expanding active areas to enhance the oxygen electrode activity. In this work, a theoretical model was developed to understand stability of tri-conducting oxygen electrode by studying chemical potentials of neutral species (i.e., μ_(O_2)^ , μ_(H_2)^ , and μ_(H_2 O)^ ) as functions of transport properties, operating parameters, and cell geometry. Our theoretical understanding shows that: (1) In a conventional oxygen-ion based solid oxide cell, a high μ_(O_2)^ (thus high oxygen partial pressure) exists in the oxygen electrode during the electrolysis mode, which may lead to the formation of cracks at the electrode/electrolyte interface. While in a proton-conductor based solid oxide cell, the μ_(O_2)^ is reduced significantly, suppressing the crack formation, and resulting in improved performance stability. (2) In a typical proton-conductor based solid oxide electrolyzer, the dependence of μ_(O_2)^ on the Faradaic efficiency is negligible. Hence, approaches to block the electronic current can improve the electrolysis efficiency while achieving stability. (3) The difference of the μ_(O_2)^ (thus p_(O_2)^ ) between the oxygen electrode and gas phase can be reduced by using higher ionic conducting components and improving electrode kinetics, which lead to further improvement of electrode stability. 

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2. Tuning electrochemical and transport processes to achieve extreme performance and efficiency in solid oxide cells

   https://doi.org/10.1039/d0ta04555a  

   Park, Beom-Kyeong; Scipioni, Roberto; Zhang, Qian; Cox, Dalton; Voorhees, Peter W.; Barnett, Scott A. (June 2020, Journal of Materials Chemistry A)

   Solid oxide cells (SOCs) have important applications as fuel cells and electrolyzers. The application for storage of renewable electricity is also becoming increasingly relevant; however, it is difficult to meet stringent area-specific resistance (ASR) and long-term stability targets needed to achieve required efficiency and cost. Here we show a new SOC that utilizes a very thin Gd-doped ceria (GDC)/yttria-stabilized zirconia (YSZ) bi-layer electrolyte, Ni–YSZ cell support with enhanced porosity, and electrode surface modification using PrO x and GDC nanocatalysts to achieve unprecedented low ASR values < 0.1 Ω cm 2 , fuel cell power density ∼3 W cm −2 , and electrolysis current density ∼4 A cm −2 at 800 °C. Besides this exceptionally high performance, fuel cell and electrolysis life tests suggest very promising stability in fuel cell and steam electrolysis modes. Electrochemical impedance spectroscopy analysis done using a novel impedance subtraction method shows how rate-limiting electrode processes are impacted by the new SOC materials and design. 

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3. A robust solid oxide electrolyzer for highly efficient electrochemical reforming of methane and steam

   https://doi.org/10.1039/c9ta00467j  

   Liu, Tong; Liu, Hao; Zhang, Xiaoyu; Lei, Libin; Zhang, Yanxiang; Yuan, Zhihao; Chen, Fanglin; Wang, Yao (June 2019, Journal of Materials Chemistry A)

   In this work, a robust solid oxide electrolysis cell with Sr 2 Fe 1.5 Mo 0.5 O 6−δ –Ce 0.8 Sm 0.2 O 1.9 (SFM–SDC) based electrodes has been utilized to verify the conceptual process of partial oxidation of methane (POM) assisted steam electrolysis, which can produce syngas and hydrogen simultaneously. When the cathode is fed with 74%H 2 –26%H 2 O and operated at 850 °C, the open circuit voltage (OCV), the minimum energy barrier required to overcome the oxygen partial gradient, is remarkably reduced from 0.940 to −0.012 V after changing the feed gas in the anode chamber from air to methane, indicating that the electricity consumption of the steam electrolysis process could be significantly reduced and compensated by the use of low grade thermal energy from external heat sources. It is found that after ruthenium (Ru) impregnation, the electrolysis current density of the electrolyzer is effectively enhanced from −0.54 to −1.06 A cm −2 at 0.6 V and 850 °C, while the electrode polarization resistance under OCV conditions and 850 °C is significantly decreased from 0.516 to 0.367 Ω cm 2 . Long-term durability testing demonstrates that no obvious degradation but a slight improvement is observed for the electrolyzer, which is possibly due to the activation of the SFM–SDC electrode during operation. These results indicate that the robust Ru infiltrated solid oxide electrolyzer is a very promising candidate for POM assisted steam electrolysis applications. Our result will provide insight to improve the electrode catalysts used in POM assisted steam electrolysis. 

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4. Evolving contact mechanics and microstructure formation dynamics of the lithium metal-Li7La3Zr2O12 interface

   https://doi.org/10.1038/s41467-021-26632-x  

   Chang, Wesley; May, Richard; Wang, Michael; Thorsteinsson, Gunnar; Sakamoto, Jeff; Marbella, Lauren; Steingart, Daniel (December 2021, Nature Communications)

   Abstract The dynamic behavior of the interface between the lithium metal electrode and a solid-state electrolyte plays a critical role in all-solid-state battery performance. The evolution of this interface throughout cycling involves multiscale mechanical and chemical heterogeneity at the micro- and nano-scale. These features are dependent on operating conditions such as current density and stack pressure. Here we report the coupling of operando acoustic transmission measurements with nuclear magnetic resonance spectroscopy and magnetic resonance imaging to correlate changes in interfacial mechanics (such as contact loss and crack formation) with the growth of lithium microstructures during cell cycling. Together, the techniques reveal the chemo-mechanical behavior that governs lithium metal and Li 7 La 3 Zr 2 O 12 interfacial dynamics at various stack pressure regimes and with voltage polarization. 

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5. Robust redox-reversible perovskite type steam electrolyser electrode decorated with in situ exsolved metallic nanoparticles

   https://doi.org/10.1039/c9ta06309a  

   Liu, Tong; Zhao, Yiqian; Zhang, Xiaoyu; Zhang, Hong; Jiang, Guang; Zhao, Wen; Guo, Jiayi; Chen, Fanglin; Yan, Mufu; Zhang, Yanxiang; et al (January 2020, Journal of Materials Chemistry A)

   Redox stabilities of the hydrogen electrode with in situ exsolved Fe–Ni nanoparticles from Sr 2 Fe 1.4 Ni 0.1 Mo 0.5 O 6−δ (SFMNi) perovskite are studied by analyzing the evolution of the phase composition and morphology during the redox cycles. It is found that certain amount of the exsolved nanoparticles have been oxidized to the transition metal oxide (Ni,Fe)O instead of reincorporating into the parent perovskite lattice upon re-oxidizing at 800 °C in air. However, the (Ni,Fe)O secondary phases show no adverse effect on the subsequent reduction treatment. The redox reversibility mechanism is explained by the regular-solution model. The electrodes are almost fully recovered in the reducing atmosphere, and the symmetrical cells measured under 9.7% H 2 –3% H 2 O–87.3% N 2 conditions show a stable specific area polarization resistance of around 1.93 Ω cm 2 at 800 °C during 13 redox cycles. Single cells using the Ni–Fe nanoparticles structured electrode exhibit a stable electrode polarization resistance of about 0.598 Ω cm 2 at 800 °C under open circuit voltage conditions and a steady electrolysis current density of about −653 mA cm −2 at 1.5 V during the steam electrolysis process over 5 redox cycles. These results indicate that the SFMNi material is a very promising electrode candidate for steam electrolysis application with robust redox reversibility. 

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