Bobing Hu

A nanostructured ceramic fuel electrode for efficient CO2/H2O electrolysis without safe gas

There is increasing interest in converting CO2/H2O to syngas via solid oxide electrolysis cells (SOECs) driven by renewable and nuclear energies. The electrolysis reaction is usually conducted through Ni–YSZ (yttria stabilized zirconia) cermets, state-of-the-art fuel electrodes for SOECs. However, one obvious problem for practical applications is the usage of CO/H2 safe gas, which must be supplied to maintain the electrode performance. This work reports a safe gas free ceramic electrode for efficient CO2/H2O electrolysis. The electrode has a heterogeneously porous structure with Sr2Fe1.5Mo0.5O6−δ (SFM) electrocatalyst nanoparticles deposited onto the inner surface of the YSZ scaffold fabricated by a modified phase-inversion tape-casting method. The nanostructured SFM–YSZ electrodes have demonstrated excellent performance for CO2–H2O electrolysis. For example, the electrode polarization resistance is 0.25 Ω cm2 under open circuit conditions while the current density is 1.1 A cm−2 at 1.5 V for dry CO2 electrolysis at 800 °C. The performance is comparable with those reported for the Ni–YSZ fuel electrodes, where safe gas must be supplied. In addition, the performance is up to one order of magnitude better than those reported for other ceramic electrodes such as La0.75Sr0.25Cr0.5Mn0.5O3−δ and La0.2Sr0.8TiO3+δ. Furthermore, the electrode exhibits good stability in the short-term test at 1.3 V for CO2-20 vol% H2O co-electrolysis, which produces a syngas with a H2/CO ratio close to 2. The reduced interfacial polarization resistance, high current density, and good stability show that the nanostructured SFM–YSZ fuel electrode is highly effective for CO2/H2O electrolysis without using the safe gas, which is critical for practical applications.

Electrical conductivity relaxation of Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O1.9 dual-phase composites

The oxygen incorporation kinetics of Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O1.9 (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of SDC phase volume fraction using electrical conductivity relaxation. It is shown that the oxygen re-equilibration kinetics in the range of oxygen partial pressure (pO2) from 0.01 to 1 atm is limited by the surface exchange rate. The effective surface exchange coefficient of the composites is found to increase profoundly upon increasing the phase volume fraction of the oxide electrolyte phase SDC. The results are interpreted to reflect the synergistic oxygen incorporation at the SFM–SDC–gas triple phase boundaries (TPBs), which occurs in addition to the direct incorporation via the surface of the perovskite mixed conductor SFM. Already at a SDC phase volume fraction of 0.105, the uptake of oxygen via the synergistic TPB route (referred to as route III), following a step change in the surrounding pO2, comprises more than 75% of the overall uptake of oxygen by the composite. It is further concluded that under the conditions of the experiments the two-phase SFM–SDC boundaries allow for a facile exchange of oxygen ions between both involved phases.

Measuring oxygen surface exchange kinetics on mixed-conducting composites by electrical conductivity relaxation

The oxygen release kinetics of mixed-conducting Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O2−δ (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of the SDC phase volume fraction using electrical conductivity relaxation (ECR) under reducing atmospheres, extending our previous work on the oxygen incorporation kinetics of these composites under oxidizing conditions. Gas mixtures of H2/H2O and CO/CO2 were used to control step changes in the oxygen partial pressure (pO2) in the range 10−24 to 10−20 atm. At the conditions of the experiments, oxygen re-equilibration is entirely controlled by the surface exchange kinetics. A model is developed which allows deconvolution of the effective time constant of the relaxation process in terms of the intrinsic contributions of the components to oxygen surface exchange and synergetic contributions caused by heterogeneous interfaces. The oxygen surface exchange kinetics under H2/H2O atmosphere is found to be a weighted average of the intrinsic contributions of SFM and SDC phases. No evidence is found for an enhanced exchange rate at the SFM–SDC–gas triple phase boundaries (TPB). Synergetic contributions arise under CO/CO2 atmosphere, enhancing the rate of oxygen surface exchange up to a factor of 2.4. The obtained results are discussed in terms of the surface microstructure of the composites from image analysis. Overall, the results of this and our previous study confirm that the triple phase boundaries in SFM–SDC composites significantly accelerate the oxygen incorporation kinetics under oxidizing conditions, but only modestly, or even negligibly, influence the oxygen release kinetics under reducing conditions.

Influence of ionic conductivity of the nano-particulate coating phase on oxygen surface exchange of La0.58Sr0.4Co0.2Fe0.8O3−δ

The oxygen surface exchange kinetics of mixed-conducting perovskite La0.58Sr0.4Co0.2Fe0.8O3 d (LSCF) ceramics coated with a porous nano-particulate layer of either gadolinea (Gd2O3), ceria (CeO2) or 20 mol% Gd-doped ceria (GCO) was determined by electrical conductivity relaxation (ECR). The measurements were performed in the temperature range 700–900 C, following pO2-step changes between 0.2 and 0.4 atm. The apparent value of the surface exchange coefficient, kchem, is found to vary with the loading amount and ionic conductivity of the coated phase whilst, as expected, the chemical diffusion coefficient Dchem remains invariant with the applied coating. Partial coverage of the LSCF surface with non-ionic conductive Gd2O3 or CeO2 lowers the value of kchem relative to that observed for bare LSCF, which is attributed to surface blocking effects. In contrast, partial coverage of LSCF with GCO electrolyte particles enhances the apparent value of kchem up to a factor of 6 compared to bare LSCF. The data of pulse isotope exchange (PIE) measurements show that the surface exchange reaction on bare LSCF is predominantly limited by dissociative adsorption of O2. Different mechanisms for the improved oxygen surface exchange kinetics after partially covering the LSCF surface with GCO are discussed.

A novel fuel electrode enabling direct CO2 electrolysis with excellent and stable cell performance

Solid oxide electrolysis cells (SOECs) can directly convert CO2 to CO and O2 that are important building blocks for chemical production and other applications. However, the use of SOECs for direct CO2 electrolysis has been hampered mainly due to the absence of a stable, highly catalytically active and cost effective cathode (fuel electrode) material. Here we report a ceramic SOEC cathode material of perovskite-structured Sr1.9Fe1.5Mo0.4Ni0.1O6−δ for direct CO2 electrolysis. By annealing at 800 °C in H2, homogeneously dispersed nano-sized NiFe alloy nanoparticles are exsolved from the Sr1.9Fe1.5Mo0.4Ni0.1O6−δ perovskite lattice. The exsolved NiFe nanoparticles significantly enhance the chemical adsorption and surface reaction kinetics of CO2 with the cathode. SOECs with the novel cathode have demonstrated a peak current density of 2.16 A cm−2 under an applied voltage of 1.5 V at 800 °C and have demonstrated stable direct CO2 electrolysis performance during 500 h of operation under current density above 1 A cm−2 at 800 °C.