Review on zirconate-cerate-based electrolytes for proton-conducting solid oxide fuel cell

The performance of low-to-intermediate temperature (400–800?°C) solid oxide fuel cells (SOFCs) depends on the properties of electrolyte used. SOFC performance can be enhanced by replacing electrolyte materials from conventional oxide ion (O^2-) conductors with proton (H⁺) conductors because H⁺ conductors have higher ionic conductivity and theoretical electrical efficiency than O^2- conductors within the target temperature range. Electrolytes based on cerate and/or zirconate have been proposed as potential H⁺ conductors. Cerate-based electrolytes have the highest H⁺ conductivity, but they are chemically and thermally unstable during redox cycles, whereas zirconate-based electrolytes exhibit the opposite properties. Thus, tailoring the properties of cerate and/or zirconate electrolytes by doping with rare-earth metals has become a main concern for many researchers to further improve the ionic conductivity and stability of electrolytes. This article provides an overview on the properties of four types of cerate and/or zirconate electrolytes including cerate-based, zirconate-based, single-doped cerate–zirconate and hybrid-doped cerate–zirconate. The properties of the proton electrolytes such as ionic conductivity, chemical stability and sinterability are also systematically discussed. This review further provides a summary of the performance of SOFCs operated with cerate and/or zirconate proton conductors and the actual potential of these materials as alternative electrolytes for proton-conducting SOFC application.     

Efficient hydrogen production for industry and electricity storage via high-temperature electrolysis

The paper describes the development status of Sunfire's reversible solid oxide cell (RSOC) technology. Here, Sunfire is a pioneer in the field of high-temperature electrolysers (HTE) for renewable hydrogen production which can be operated as a fuel cell for power generation in a reverse mode. The maturity of the technology is improved stepwise so that first applications in the field of hydrogen production for industry and electricity storage can be tackled. Three application examples where larger scale prototype has been installed will be discussed: 1) A power-to-power electricity storage based on hydrogen, 2) a RSOC unit that is installed in an iron and steel works, and 3) a pressurized SOEC prototype which will be integrated with a methanation unit. Results show the potentials of the technology in connection with fluctuating renewable energy sources.     

Simulation of the transient behavior of tubular solid oxide electrolyzer cells under fast load variations

Solid oxide electrolyzer cells pose a promising technology for the production of hydrogen gained from renewables, such as wind and PV. Due to the fluctuating nature of these sources, the transient behavior of SOECs under various load cases plays a crucial role in terms of their long-time stability, degradation behavior, conversion efficiency and application. This study presents a dynamic, 2D-FEM model of a single tubular SOEC. The transient operational behavior of the cell under fast load variations and different flow configurations is assessed based on the conducted simulations.     

Theoretical considerations on the modelling of transport in a three-phase electrode and application to a proton conducting solid oxide electrolysis cell

This paper presents a numerical model for fuel cells and electrolysis cells that use cermets as electrodes. The mass and charge continuity equations were demonstrated inside the electrodes and in the divergence term, surface ratios were used instead of more usual volumetric ratios. The Butler–Volmer equation for electrokinetics was used with concentration coefficients in order to predict correctly the concentration effects on the value of the transfer current density in the cermets. In addition, it was considered that the reaction takes place inside all of cermet's volume inste1 ad of a thin layer near the electrolyte. The model was tested for the proton-conducting SOEC technology on a generic cell design and the calculations were performed with COMSOL Multiphysics 4.1™. A parametric analysis was carried out on a proton-conducting SOEC in galvanostatic mode in order to evaluate the influence of parameters on oxygen production across the electrode in the anodic compartment. This analysis showed that the structural parameters of the cermets such as grain radius or volumetric ratios of the conductors play a major role in the distribution of reaction kinetics.     

Exergoeconomic analysis of hydrogen production using a standalone high-temperature electrolyzer

The conventional hydrogen production methods, primarily steam methane reforming and coal gasification, produce massive amounts of greenhouse gas emissions which significantly cause impacts on the environment. An alternative hydrogen production method is high-temperature electrolysis using Solid Oxide Electrolyzer that combines both high conversion efficiency and saleable high purity hydrogen production. The produced hydrogen can feed the various industrial processes at different scales in addition to offering an environmentally friendly storage option. The scope of this paper is to examine the economic feasibility of this technology through the utilization of the exergoeconomic concept, which traces the flow of exergy through the system and price both waste and products. Therefore, a standalone solid oxide electrolyzer of a 1MWe is considered for hydrogen production using renewably generated electricity. Having the detailed exergy analysis conducted in earlier studies, the focus of this article is on the costing of each exergy stream to determine the exergy cost and the potential changes outcomes as a result of the system operating or design parameters optimization. It is found that the cost of hydrogen production through the modular high-temperature electrolyzer varies between $3-$9/kg with an average of about $5.7/kg, respectively.     

Investigation of the effect of ion transition type on performance in solid oxide fuel cells fueled hydrogen and coal gas

Renewable energy will be a panacea for environmental difficulties due to the extensive usage of carbon-rich fuels as a main source of energy. As a result, hydrogen-fueled solid oxide fuel cell is a revolutionary clean technology that has a great contribution in solving the current energy and environmental-related challenges. Thus, a 3D model of hydrogen and coal gases fueled solid oxide fuel cell (H₂–SOFC) using different electrolytes has been developed and simulated using COMSOL commercial software to explore the performance of electrolyte supported SOFC. The performance of the developed model has been studied and characterized using different differential equations. Accordingly, it has been found that the performance of hydrogen-fueled oxide ion conducting electrolytes (SOFC–O) is lower than that of protonic conducting one (SOFC–H) at 800 °C. Furthermore, a numerical simulation has been conducted to investigate the result of temperature changes on SOFC performance at 400 °C, 600 °C, and 800 °C for proton-conducting SOFC and 800 °C and 1000 °C for oxygen-conducting SOFC. It has been demonstrated that SOFC–O shows a better performance at high temperatures compared with SOFC–H while SOFC–H can be an agreeable selection at medium temperatures. Therefore, this study reveals that the temperature augments the performance of both electrolytes, yet at higher working temperatures SOFC–H becomes more advantageous than SOFC–O to use hydrogen and coal gas as a primary fuel. Besides, the effect of channel height was also analyzed numerically and the finding disclosed that decreasing the channel height emerges in a curtly current path. Thus, it can be reasoned out that the performance of SOFC decreases when the channel height is increased.     

Mechanical behavior of Ce0.9Gd0.1O1.95-La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen electrode with a coral microstructure for solid oxide fuel cell and solid oxide electrolyzer cell

The objective of this work is to investigate the mechanical behavior of CGO-LSCF composite developed by electrostatic spray deposition as an oxygen electrode for Solid Oxide Fuel Cell and Solid Oxide Electrolysis Cell. The coating is characterized by a highly porous morphology designated coral microstructure. Its mechanical behavior was studied by scratch and ultramicroindentation tests and a model of material degradation under progressive compressive loading has been proposed. The coral's damage mechanism involves three regimes: at very low loads stresses are concentrated at the tips of individual corals that fracture and fill the spaces between corals (regime I); as load increases, generalized fracture of the corals occurs and the material starts compacting into an increasingly dense layer (regime II); finally, at the highest loads, the material behaves like an almost fully dense (regime III). As load increases during testing porosity decreases from about 60 to about 5 vol% in the compacted material. The transitions between regimes are associated to increases in the contact stress and the same damage mechanism is found during scratching and indentation. Hardness increases from about 2–100 MPa, while the Young's modulus varies in the range 1–18 GPa, as the porosity decreases. Calculations of the real contact pressure during loading allowed estimating a yield stress of 83 MPa that can be considered as a low limit for the materials fracture strength.     

Operational challenges for low and high temperature electrolyzers exploiting curtailed wind energy for hydrogen production

Understanding the system performance of different electrolyzers could aid potential investors achieve maximum return on their investment. To realize this, system response characteristics to 4 different summarized data sets of curtailed renewable energy is obtained from the Irish network and was investigated using models of both a Low Temperature Electrolyzer (LTE) and a High Temperature Electrolyzer (HTE). The results indicate that statistical parameters intrinsic to the method of data extraction along with the thermal response time of the electrolyzers influence the hydrogen output. A maximum hydrogen production of 5.97 kTonne/year is generated by a 0.5 MW HTE when the electrical current is sent as a yearly average. Additionally, the high thermal response time in a HTE causes a maximum change in the overall flowrate of 65.7% between the 4 scenarios, when compared to 7.7% in the LTE. This evaluation of electrolyzer performance will aid investors in determining scenario specific application of P2G for maximizing hydrogen production.     

High-temperature Co-electrolysis of CO2/H2O and direct methanation over Co-impregnated SOEC. Bimetallic synergy between Co and Ni

To study the synergy between the transition metals for enhancing the electrochemical and chemical activity, a series of SOECs were modified with a small amount of Co ions, namely 1.8, 3.6, and 5.4 wt% in the reduced state. The addition of βCD into the precursor solution allowed for extremely fine dispersion of Co species across the Ni-YSZ cermet structure. The sample containing 3.6 wt% Co reached an outstanding over 2.5-times-higher concentration of CH₄ in the outlet stream. At the same time, the Co greatly enhanced the electrochemical efficiency of water and CO₂ co-electrolysis. Full characterization involving STXM imaging allowed for better understanding of the synergy between the Ni and Co host metal and made it possible to find the causes of the increased activity. It revealed the complexity of the substructures formed within the electrode. A novel discovery was described regarding the NiCo₂O₄ spinel structure subjected to the O₂-TPO measurements. Despite the applied oxidizing atmosphere, the catalyst evolved oxygen at elevated temperatures in a reversible manner. The performance tests indicated the roles of both rWGS and direct electrolysis of CO₂ in the electroreduction process. The addition of Co did not influence the prolonged degradation of the cell.     

Preparation of BaZr0.1Ce0.7Y0.2O3−δ thin membrane based on a novel method-drop coating

The thin membrane of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BYCZ) was fabricated on the porous NiO-BYCZ anode substrate through a novel method-drop coating. The intact cell was assembled with the cathode of La_0.7Sr_0.3FeO_3−δ/BYCZ and tested from 600 to 700 ^°C with humidified hydrogen (∼3% H₂O) as the fuel and the static air as the oxidant. We investigated the influence of different contents of BYCZ in the slurry and different pre-fired temperatures to the open-circuit potential and performance of the prepared cells. The results showed that the cell with 5% (wt) BYCZ in the slurry and pre-fired temperature of 500 °C had the highest open-circuit potential which indicated the densest electrolyte and the highest power density. Double-layer-drop-coating with the anode functional layer was also adopted and improved the cell performance to 377 mW cm⁻² at 700 °C.