High-temperature electrocatalysis and key materials in solid oxide electrolysis cells

Solid oxide electrolysis cells(SOECs)can convert electricity to chemicals with high efficiency at ~600-900℃,and have attracted widespread attention in renewable energy conversion and storage.SOECs operate in the inverse mode of solid oxide fuel cells(SOFCs)and therefore inherit most of the advantages of SOFC materials and energy conversion processes.However,the external bias that drives the electrochemical process will strongly change the chemical environments in both in the cathode and anode,therefore necessitating careful reconsideration of key materials and electrocatalysis processes.More importantly,SOECs provide a unique advantage of electrothermal catalysis,especially in converting stable low-carbon alkanes such as methane to ethylene with high selectivity.Here,we review the state-of-the-art of SOEC research progress in electrothermal catalysis and key materials and provide a future perspective.

Enhanced Oxidative Coupling of Methane with Oxygen Permeation Membrane

Oxidative coupling of methane to ethylene is of high importance to the future of light olefin industry.However,the carbon atom efficiency is normally below 50%in gas phase reaction which is limited to the overoxidation of methane to carbon dioxide with oxidants.Here we present an alternative approach of electrochemical oxidation of methane in an oxygen permeation membrane reactor and show the highest conversion of methane and C2 selectivity of 28%and 40.2%at 1150℃,respectively.We prepare the 100-μm-thick perovskite(La0.8 Sr0.2)1-x Cr0.5 Fe0.5 O3-δ(LSCrF)dense membrane(La0.8 Sr0.2)1-xCr0.5 Fe0.5O3-δ(LSCrF–Fe)(x=0,0.02,0.05 and 0.10)scaffolds while the excess of Fe would be exsolved on porous skeleton to create metal-oxide interfaces toward methane oxidation.The metal-oxide interfaces not only facilitate the activation of C–H bond in methane but also enhance the coking resistance.

Porous Iron Single Crystals at 2 cm Scale Delivering Enhanced Electrocatalysis Performance

Porous single crystals would significantly enhance their catalysis functionalities owing to the combination of structural coherence and porous microstructures. Porous single crystals have wormhole microstructures and then we define them as wormcrystals. The twisted surfaces in porous microstructures would produce surface lattice distortions that give rise to high-energy active surfaces. Here we grow porous iron single crystals at an unprecedented 2 cm scale with a lattice reconstruction strategy and create high-energy surfaces through the control of lattice distortions within a thickness region of 1~2 nm. The porous iron crystal therefore boosts electrochemical reduction of nitrobenzene to aminobenzene with ~100% conversion and > 95% selectivity. The exceptionally high current densities with porous iron crystals represent the first level electrocatalysis performance. The current work would open a new pathway not only to the creation of high energy surfaces but also to the growth of porous single crystals at large scales in wealth of other materials.

Porous Iron-and Cobalt-based Single Crystals with Enhanced Electrocatalysis Performance

Porous single crystals have the characteristics of long-range ordering structure and large specific surface area,which will significantly enhance their electrochemical performance.Here,we report a method different from the conventional porous single crystal growth method.This method is to directly convert single crystal precursors Co_(3)O_(4) and Fe_(3)O_(4) into Co_(2)N and Fe_(2)N,and then further reduces them to porous single crystals Co and Fe particles under H2/Ar atmosphere.The removal of O^(2–)in the lattice channel at the pressure of 25~300 torr and the temperature of 300~600℃ will promote nitridation of the single-crystalline Co–O and Fe–O frames,and further remove N^(3–)in H2/Ar atmosphere and recrystallize as Co and Fe.These porous single crystals exhibit enhanced electrochemical properties due to their structural coherence and highly active surface.We demonstrated that the aminobenzene yield was up to 91%and the selectivity reached 92%in the electrochemical reduction of nitrobenzene.