Study on Water Splitting of the 214-Type Perovskite Oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga)

We present a study on the electrocatalysis of 214-type perovskite oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga) with semiconducting-like behavior synthesized using the sol-gel method. Among these five catalysts, PrSrCoO4 exhibits the optimal electrochemical performance in both the oxygen evolution reaction and the hydrogen evolution reaction, mainly due to its larger electrical conductivity, mass activity, and turnover frequency. Importantly, the weak dependency of LSV curves in a KOH solution with different pH values, revealing the adsorbate evolving mechanism in PrSrCoO4, and the density functional theory (DFT) calculations indicate that PrSrCoO4 has a smaller Gibbs free energy and a higher density of states near the Fermi level, which accelerates the electrochemical water splitting. The mutual substitution of different rare-earth elements will change the unit-cell parameters, regulate the electronic states of catalytic active site Co ions, and further affect their catalytic performance. Furthermore, the magnetic results indicate strong spin-orbit coupling in the electroactive sites of Co ions in SmSrCoO4 and EuSrCoO4, whereas the magnetic moments of Co ions in the other three catalysts mainly arise from the spin itself. Our experimental results expand the electrochemical applications of 214-type perovskite oxides and provide a good platform for a deeper understanding of their catalytic mechanisms.

Multiple-phase evolution and electrical transport of Sr4-xYxCo4O12-δ (x = 0-1.0): an ordered phase transition process

The transition metal oxide (TMO) SrCoO_3-δ family with rich structural diversity has been widely studied in the phase transition and energy application fields. We report the multiple-phase structure evolution, phase transitions during sintering, and electrical transport of A-site doped Sr_4-xY_xCo₄O_12-δ (x = 0-1.0) ceramics. Sr₆Co₅O₁₅ (x = 0) adopts a hexagonal structure (H), Sr_4-xY_xCo₄O_12-δ (x = 0.2-0.4) ceramics adopts a cubic perovskite (CP) structure, and Sr_4-xY_xCo₄O_10.5+δ' (x = 0.8-1.0) ceramics adopts an ordered-tetragonal (OT) structure; moreover, their phase transitions during the sintering processing of samples are systematically investigated. Combining the thermal analysis and X-ray diffraction results, the exothermic peak and weight gain of Sr₃YCo₄O_10.5 (x = 1.0, T) at 1042 °C are considered to correspond to an ordered phase transition (T → OT) occurring. Finally, a systematic phase schema of the Sr_4-xY_xCo₄O_12-δ (x = 0-1.0) state dependence on the Y content and sintering temperature is obtained. The high-energy Y-O bond stabilizes the high-temperature CP structure (x = 0.2-0.4) and induces a structural evolution from the CP to OT structure (x = 0.8-1.0). In addition, all Sr_4-xY_xCo₄O_12-δ (x = 0-1.0) ceramics show semiconductive electrical transport behavior. Sr₆Co₅O₁₅ (H) with a one-dimensional chain structure has the highest resistivity, while Sr_3.8Y_0.2Co₄O_12-δ (CP) with a three-dimensional corner-sharing structure exhibits the lowest resistivity, and Sr_4-xY_xCo₄O_12-δ (x = 0.2-1.0) ceramics show an increasing tendency in resistivity due to the hole carrier Co⁴⁺ converting to Co³⁺. We studied multiple-phase evolution and ordered phase transition in Sr_4-xY_xCo₄O_12-δ (x = 0-1.0) ceramics through Y-O bonding.

Facile Synthesis and Origin of Enhanced Electrochemical Oxygen Evolution Reaction Performance of 2H-Hexagonal Ba2CoMnO6-δ as a New Member in Double Perovskite Oxides

Perovskite oxides have been considered promising oxygen evolution reaction (OER) electrocatalysts due to their high intrinsic activity. Yet, their poor long-term electrochemical and structural stability is still controversial. In this work, we apply an A-site management strategy to tune the activity and stability of a new hexagonal double perovskite oxide. We synthesized the previously inaccessible 2H-Ba₂CoMnO_6-δ (BCM) perovskite oxide via the universal sol-gel method followed by a novel air-quench method. The new 2H-BCM perovskite oxide exhibits outstanding OER activity with an overpotential of 288 mV at 10 mA cm^-2 and excellent long-term stability without segregation or structural change. To understand the origin of outstanding OER performance of BCM, we substitute divalent Ba with trivalent La at the A-site and investigate crystal and electronic structure change. Fermi level and valence band analysis presents a decline in the work function with the Ba amount, suggesting a structure-oxygen vacancy-work function-activity relationship for Ba _x La_2-x CoMnO_6-δ (x = 0, 0.5, 1, 1.5, 2) electrocatalysts. Our work suggests a novel production strategy to explore the single-phase new structures and develop enhanced OER catalysts.