Surprisingly High Activity for Oxygen Reduction Reaction of Selected Oxides Lacking Long Oxygen-Ion Diffusion Paths at Intermediate Temperatures: A Case Study of Cobalt-Free BaFeO_3‑δ

The widespread application of solid oxide fuel cell technology requires the development of innovative electrodes with high activity for oxygen reduction reaction (ORR) at intermediate temperatures. Here, we demonstrate that a cobalt-free parent oxide BaFeO_3‑δ (BF), which lacks long-range oxygen-ion diffusion paths, has surprisingly high electrocatalytic activity for ORR. Both in situ high-temperature X-ray diffraction analysis on room-temperature powder and transmission electron microscopy on quenched powder are applied to investigate the crystal structure of BF. Despite the lack of long oxygen-ion diffusion paths, the easy redox of iron cations as demonstrated by thermal gravimetric analysis (TGA) and oxygen temperature-programmed desorption and the high oxygen vacancy concentration as supported by iodometric titration and TGA benefit the reduction of oxygen to oxygen ions. Moreover, the electrical conductivity relaxation technique in conjunction with a transient thermogravimetric study reveals very high surface exchange kinetics of BF oxide. At 700 °C, the area specific resistance of BF cathode, as expressed by a symmetrical cell configuration, is only ∼0.021 Ω cm², and the derived single fuel cell achieves high power output with a peak power density of 870 mW cm^–2. It suggests that an undoped BF parent oxide can be used as a high-efficiency catalyst for ORR

Highly Active and Stable Cobalt-Free Hafnium-doped SrFe_0.9Hf_0.1O_3−δ Perovskite Cathode for Solid Oxide Fuel Cells

Sluggish oxygen reduction reaction (ORR) kinetics and chemical instability of cathode materials hinder the practical application of solid oxide fuel cells (SOFCs). Here we report a Co-free Hf-doped SrFe_0.9Hf_0.1O_3−δ (SFHf) perovskite oxide as a potential cathode focusing on enhancing the ORR activity and chemical stability. We find that SFHf exhibits a high ORR activity, stable cubic crystal structure, and improved chemical stability toward CO₂ poisoning compared to undoped SrFeO_3−δ. The SFHf cathode has a polarization area-specific resistance as low as 0.193 Ω cm² at 600 °C in a SFHf|Sm_0.2Ce_0.8O_1.9 (SDC)|SFHf symmetrical cell and has a maximum power density as high as 1.94 W cm^–2 at 700 °C in an anode-supported fuel cell (Ni+(ZrO₂)_0.92(Y₂O₃)_0.08 (YSZ)|YSZ|SDC|SFHf). The ORR activity maintains stable for a period of 120 h in air and in CO₂-containing atmosphere. The attractive ORR activity is attributed to the moderate concentration of oxygen vacancy and electrical conductivity, as well as the fast oxygen kinetics at the operation temperature. The improved chemical stability is related to the doping of the redox-inactive Hf cation in the Fe site of SrFeO_3−δ by decreasing oxygen vacancy concentration and increasing metal–oxygen bond energy. This work proposes an effective strategy in the design of highly active and stable cathodes for SOFCs

Highly Active and Stable Cobalt-Free Hafnium-doped SrFe_0.9Hf_0.1O_3−δ Perovskite Cathode for Solid Oxide Fuel Cells

Sluggish oxygen reduction reaction (ORR) kinetics and chemical instability of cathode materials hinder the practical application of solid oxide fuel cells (SOFCs). Here we report a Co-free Hf-doped SrFe_0.9Hf_0.1O_3−δ (SFHf) perovskite oxide as a potential cathode focusing on enhancing the ORR activity and chemical stability. We find that SFHf exhibits a high ORR activity, stable cubic crystal structure, and improved chemical stability toward CO₂ poisoning compared to undoped SrFeO_3−δ. The SFHf cathode has a polarization area-specific resistance as low as 0.193 Ω cm² at 600 °C in a SFHf|Sm_0.2Ce_0.8O_1.9 (SDC)|SFHf symmetrical cell and has a maximum power density as high as 1.94 W cm^–2 at 700 °C in an anode-supported fuel cell (Ni+(ZrO₂)_0.92(Y₂O₃)_0.08 (YSZ)|YSZ|SDC|SFHf). The ORR activity maintains stable for a period of 120 h in air and in CO₂-containing atmosphere. The attractive ORR activity is attributed to the moderate concentration of oxygen vacancy and electrical conductivity, as well as the fast oxygen kinetics at the operation temperature. The improved chemical stability is related to the doping of the redox-inactive Hf cation in the Fe site of SrFeO_3−δ by decreasing oxygen vacancy concentration and increasing metal–oxygen bond energy. This work proposes an effective strategy in the design of highly active and stable cathodes for SOFCs

Water Splitting with an Enhanced Bifunctional Double Perovskite

The rational design of highly active and durable electrocatalysts for overall water splitting is a formidable challenge. In this work, a double perovskite oxide, i.e., NdBaMn₂O_5.5, is proposed as a bifunctional electrode material for water electrolysis. Layered NdBaMn₂O_5.5 demonstrates significant improvement in catalyzing oxygen and hydrogen evolution reactions (OER and HER, respectively), in contrast to other related materials, including disordered Nd_0.5Ba_0.5MnO_3−δ as well as NdBaMn₂O_5.5−δ and NdBaMn₂O_5.5+δ (δ < 0.5). Importantly, NdBaMn₂O_5.5 has an OER intrinsic activity (∼24 times) and a mass activity (∼2.5 times) much higher than those of the benchmark RuO₂ at 1.7 V versus the reversible hydrogen electrode. In addition, NdBaMn₂O_5.5 achieves a better overall water splitting activity at large potentials (>1.75 V) and catalytic durability in comparison to those of Pt/C–RuO₂, making it a promising candidate electrode material for water electrolyzers. The substantially enhanced performance is attributed to the approximately half-filled e_g orbit occupancy, optimized O p-band center location, and distorted structure. Interestingly, for the investigated perovskite oxides, OER and HER activity seem to be correlated; i.e., the material achieving a higher OER activity is also more active in catalyzing HER