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

A Green Route to a Na₂FePO₄F‑Based Cathode for Sodium Ion Batteries of High Rate and Long Cycling Life

Sodium ion batteries (SIBs) are considered one of the most promising alternatives for large-scale energy storage due largely to the abundance and low cost of sodium. However, the lack of high-performance cathode materials at low cost represents a major obstacle toward broad commercialization of SIB technology. In this work, we report a green route strategy that allows cost-effective fabrication of carbon-coated Na₂FePO₄F cathode for SIBs. By using vitamin C as a green organic carbon source and environmentally friendly water-based polyacrylic latex as the binder, we have demonstrated that the Na₂FePO₄F phase in the as-derived Na₂FePO₄F/C electrode shows a high reversible capacity of 117 mAh g^–1 at a cycling rate of 0.1 C. More attractively, excellent rate capability is achieved while retaining outstanding cycling stability (∼85% capacity retention after 1000 charge–discharge cycles at a rate of 4 C). Further, in operando X-ray diffraction has been used to probe the evolution of phase structures during the charge–discharge process, confirming the structural robustness of the Na₂FePO₄F/C cathode (even when charged to 4.5 V). Accordingly, the poor initial Coulombic efficiency of some anode materials may be compensated by extracting more sodium ions from Na₂FePO₄F/C cathode at higher potentials (up to 4.5 V)

Na_0.86Co_0.95Fe_0.05O₂ Layered Oxide As Highly Efficient Water Oxidation Electrocatalyst in Alkaline Media

Electrochemical energy storage and conversion technologies, such as water-splitting devices, rechargeable metal-air batteries, and regenerative fuel cells, are promising alternatives to traditional nonrenewable energy systems. Given the sluggish oxygen evolution reaction (OER) in the above renewable-energy technologies, the development of efficient OER electrocatalysts with high performance is of great importance. Here, we demonstrate a layer-structured oxide Na_0.86Co_0.95Fe_0.05O₂ (NCF0.05) as a novel electrocatalyst for efficient water oxidation in alkaline media. NCF0.05 shows enhanced performance, including lower overpotential, lower Tafel slope and better stability than the parent Na_0.86CoO₂ (NC). Especially, the OER performance of NCF0.05 is comparable to the state-of-the-art IrO₂ catalyst. This enhanced catalytic activity of NCF0.05 may be ascribed to the unusual synergistic interplay between Fe and Co. A possible dual-metal-site mechanism was also proposed for OER on NCF0.05

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

Enhanced Sulfur Tolerance of Nickel-Based Anodes for Oxygen-Ion Conducting Solid Oxide Fuel Cells by Incorporating a Secondary Water Storing Phase

In this work, a Ni+BaZr_0.4Ce_0.4Y_0.2O_3‑δ (Ni+BZCY) anode with high water storage capability is used to increase the sulfur tolerance of nickel electrocatalysts for solid oxide fuel cells (SOFCs) with an oxygen-ion conducting Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. Attractive power outputs are still obtained for the cell with a Ni+BZCY anode that operates on hydrogen fuels containing 100–1000 ppm of H₂S, while for a similar cell with a Ni+SDC anode, it displays a much reduced performance by introducing only 100 ppm of H₂S into hydrogen. Operating on a hydrogen fuel containing 100 ppm of H₂S at 600 °C and a fixed current density of 200 mA cm^–2, a stable power output of 148 mW cm^–2 is well maintained for a cell with a Ni+BZCY anode within a test period of 700 min, while it was decreased from an initial value of 137 mW cm^–2 to only 81 mW cm^–2 for a similar cell with a Ni+SDC anode after a test period of only 150 min. After the stability test, a loss of the Ni percolating network and reaction between nickel and sulfur appeared over the Ni+SDC anode, but it is not observed for the Ni+BZCY anode. This result highly promises the use of water-storing BZCY as an anode component to improve sulfur tolerance for SOFCs with an oxygen-ion conducting SDC electrolyte

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

Systematic Study of Oxygen Evolution Activity and Stability on La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ Perovskite Electrocatalysts in Alkaline Media

Perovskite oxide is an attractive low-cost alternative catalyst for oxygen evolution reaction (OER) relative to the precious metal oxide-based electrocatalysts (IrO₂ and RuO₂). In this work, a series of Sr-doped La-based perovskite oxide catalysts with compositions of La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ (<i>x</i> = 0, 0.2, 0.5, 0.8, and 1) are synthesized and characterized. The OER-specific activities in alkaline solution increase in the order of LaFeO_3−δ (LF), La_0.8Sr_0.2FeO_3−δ (LSF-0.2), La_0.5Sr_0.5FeO_3−δ (LSF-0.5), SrFeO_3−δ (SF), and La_0.2Sr_0.8FeO_3−δ (LSF-0.8). We establish a direct correlation between the enhancement in the specific activity and the amount of surface oxygen vacancies as well as the surface Fe oxidation states. The improved specific activity for LSF-0.8 is clearly linked to the optimum amount of surface oxygen vacancies and surface Fe oxidation states. We also find that the OER performance stability is a function of the crystal structure and the deviation in the surface La and/or Sr composition(s) from their bulk stoichiometric compositions. The cubic structure and lower deviation, as is the case for LSF-0.8, led to a higher OER performance stability. These surface performance relations provide a promising guideline for constructing efficient water oxidation