Future cost and performance of water electrolysis: An expert elicitation study

Correction for ‘Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage’ by Turgut M. Gur, Energy Environ. Sci., 2018, DOI: 10.1039/c8ee01419a.

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Correction: Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage

This article provides a perspective on solid oxide fuel cells operating at low temperature, defined here to be the range from ∼400 °C to 650 °C. These low-temperature solid oxide fuel cells (LT-SOFCs) have seen considerable research and development and are widely viewed as the “next generation” technology, following the 650–850 °C SOFCs that are currently undergoing commercialization. LT-SOFCs have potential advantages for conventional SOFC applications such as stationary power generation, and may be viable for new portable and transportation power applications, along with electrolytic fuel production and energy storage. The characteristics of electrolyte and electrode materials are reviewed, with a focus on materials that have demonstrated good properties and cell performance at low temperature. Only oxygen-ion-conducting electrolytes are considered here. Anode materials are discussed, primarily the various Ni–cermet anode compositions that yield good low-temperature performance. Mixed ionically and electronically conducting cathode materials are described in detail, reflecting the extensive research activity that has aimed at providing useful oxygen reduction kinetics at low operating temperature. Cell design, materials compatibility, processing methods, and resulting microstructures are discussed, along with their role in determining cell performance. Results from state of the art LT-SOFCs are presented, and future prospects are discussed.

A perspective on low-temperature solid oxide fuel cells

High-temperature solid oxide electrolysis cells (SOECs) are advanced electrochemical energy storage and conversion devices with high conversion/energy efficiencies. They offer attractive high-temperature co-electrolysis routes that reduce extra CO2 emissions, enable large-scale energy storage/conversion and facilitate the integration of renewable energies into the electric grid. Exciting new research has focused on CO2 electrochemical activation/conversion through a co-electrolysis process based on the assumption that difficult CO double bonds can be activated effectively through this electrochemical method. Based on existing investigations, this paper puts forth a comprehensive overview of recent and past developments in co-electrolysis with SOECs for CO2 conversion and utilization. Here, we discuss in detail the approaches of CO2 conversion, the developmental history, the basic principles, the economic feasibility of CO2/H2O co-electrolysis, and the diverse range of fuel electrodes as well as oxygen electrode materials. SOEC performance measurements, characterization and simulations are classified and presented in this paper. SOEC cell and stack designs, fabrications and scale-ups are also summarized and described. In particular, insights into CO2 electrochemical conversions, solid oxide cell material behaviors and degradation mechanisms are highlighted to obtain a better understanding of the high temperature electrolysis process in SOECs. Proposed research directions are also outlined to provide guidelines for future research.

A review of high temperature co-electrolysis of H2O and CO2 to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology

Reversible fuel cells based on both proton exchange membrane fuel cell and solid oxide fuel cell technologies have been proposed to address energy storage and conversion challenges and to provide versatile pathways for renewable fuels production. Both technologies suffer challenges associated with cost, durability, low round-trip efficiency and the need to separate H2O from the product fuel. Here, we present a reversible protonic ceramic electrochemical cell based on an yttrium and ytterbium co-doped barium cerate–zirconate electrolyte and a triple-conducting oxide air/steam (reversible) electrode that addresses many of these issues. Our reversible protonic ceramic electrochemical cell achieves a high Faradaic efficiency (90–98%) and can operate endothermically with a >97% overall electric-to-hydrogen energy conversion efficiency (based on the lower heating value of H2) at a current density of −1,000 mA cm−2. Even higher efficiencies are obtained for H2O electrolysis with co-fed CO2 to produce CO and CH4. We demonstrate a repeatable round-trip (electricity-to-hydrogen-to-electricity) efficiency of >75% and stable operation, with a degradation rate of <30 mV over 1,000 h. Reversible electrochemical cells can operate in both fuel cell and electrolysis modes to interconvert between chemical and electrical energy. Here, Duan et al. design a reversible protonic ceramic electrochemical cell that operates stably at 500–600 °C, with high Faradaic and round-trip efficiencies, by minimizing electronic leakage.

Highly efficient reversible protonic ceramic electrochemical cells for power generation and fuel production

Correction for ‘Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4’ by S. H. Jensen et al., Energy Environ. Sci., 2015, 8, 2471–2479.

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Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4

Modeling and experimental performance of an intermediate temperature reversible solid oxide cell for high-efficiency, distributed-scale electrical energy storage

Anode-supported solid oxide cells (SOCs) with thin bi-layer Y0.16Zr0.92O2−δ (YSZ)/Gd0.1Ce0.9O1.95 (GDC) electrolytes were prepared by a reduced-temperature (1250 °C) co-firing process enabled by the addition of a Fe2O3 sintering aid. The Fe2O3 amounts in the layers affected the formation of voids at the GDC/YSZ interface; the case with 1 mol% Fe2O3 in the YSZ layer and 2 mol% Fe2O3 in the GDC layer yielded minimal interfacial voids, presumably because of optimized shrinkage matching between the electrolyte layers during co-firing. The best cells yield fuel cell power density at 0.7 V in air and humidified hydrogen of 1.74 W cm−2 (800 °C) and 1.0 W cm−2 (700 °C). Under electrolysis conditions, i.e., air and 50 vol% H2O–50 vol% H2, the best cell area specific resistance is 0.12 Ω cm2 at 800 °C and 0.27 Ω cm2 at 700 °C. This excellent cell performance was explained by a number of factors related to the reduced firing temperature: (1) low electrolyte resistance due to minimization of YSZ/GDC interdiffusion; (2) minimal zirconate phase formation between the YSZ and the La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) cathode because of the dense GDC barrier layer; (3) high three phase boundary density in the Ni–YSZ anode functional layer; and (4) good pore connectivity in the Ni–YSZ support. Preliminary life testing under fuel cell and electrolysis operation shows promising cell stability.

Solid oxide cells with zirconia/ceria Bi-Layer electrolytes fabricated by reduced temperature firing

Solid oxide fuel cells (SOFCs) with thin (La0.9Sr0.1)0.98Ga0.8Mg0.2O3-δ (LSGM) electrolytes are primary candidates for achieving high (> 1 W cm-2) power density at intermediate (< 650 °C) temperatures. Although high power density LSGM-electrolyte SOFCs have been reported, it is still necessary to develop a fabrication process suitable for large-scale manufacturing and to minimize the amount of LSGM used. Here we show that SOFCs made with a novel processing method and a Sr0.8La0.2TiO3-α (SLT) oxide support can achieve high power density at intermediate temperature. The SLT support is advantageous, especially compared to LSGM supports, because of its low materials cost, electronic conductivity, and good mechanical strength. The novel process is to first co-fire the ceramic layers – porous SLT support, porous LSGM layer, and dense LSGM layer – followed by infiltration of nano-scale Ni into the porous layers. Low polarization resistance of 0.188 Ωcm2 was achieved at 650 °C for a cell with an optimized anode functional layer (AFL) and an (La,Sr)(Fe,Co)O3 cathode. Maximum power density reached 1.12 W cm−2 at 650 °C, limited primarily by cathode polarization and ohmic resistances, so there is considerable potential to further improve the power density.

Zhan Gao

Papers

A perspective on low-temperature solid oxide fuel cells

Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4

Modeling and experimental performance of an intermediate temperature reversible solid oxide cell for high-efficiency, distributed-scale electrical energy storage

Solid oxide cells with zirconia/ceria Bi-Layer electrolytes fabricated by reduced temperature firing

A High Power Density Intermediate‐Temperature Solid Oxide Fuel Cell with Thin (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O3‐δ Electrolyte and Nano‐Scale Anode