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Water splitting is a key step in a number of sustainable energy technologies including hydrogen production, fuel cells, and rechargeable metal-air batteries. The oxygen evolution reaction (OER) is a component reaction of the electrochemical splitting of water into hydrogen and oxygen.
2H2O + Energy ↔ 2H2 + O2
The OER takes place at the anode of an electrolyzer, while the hydrogen evolution reaction takes place at the cathode. The energy required for the reaction is supplied by an electronic current.
Currently, a large overpotential is required to accelerate the OER. For this reason, water splitting technologies for hydrogen production are not very competitive as the increased energy required results in more expensive hydrogen compared with production from natural gas. Therefore, research is presently focused on the search for cost-effective, efficient and stable catalysts for the OER that can reduce the required overpotential. RuO2 and IrO2 have demonstrated high catalytic activity in the OER, however high costs have limited their widespread use. New research published recently in Nature Communications highlights the potential of doped double perovskite nanofibers as the next generation of OER catalysts.
Perovskite oxides (ABO3, where A is a rare earth or alkaline earth metal and B is a transition metal) have been proposed as cost-effective OER catalysts. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has demonstrated activity an order of magnitude higher than IrO2. However, the low stability of BSCF during OER cycles originating from surface amorphization means that further development of perovskite oxide catalysts for the OER is required. Attempts to improve the activity and stability of perovskite oxides catalysts for the OER have included doping, nanostructuring, and surface modifications.
Double perovskites (AA′B2O5+δ) have enhanced stability compared with conventional perovskites and PrBaCo2O5+δ (PBC) has been found to have a comparable intrinsic (surface-area-normalized) activity to BSCF. Unfortunately, the preparation of double perovskites requires high calcination temperatures, resulting in low surface areas, therefore limiting their mass-normalized catalytic activities and reducing the potential applications of double perovskite OER catalysts.
A team of researchers from the Georgia Institute of Technology have identified ultrafine PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) nanofibers as highly efficient and stable catalysts for the OER. The team doped PBC with Sr and Fe to enhance the intrinsic activity, and synthesized nanofibers to increase the surface area of the catalyst, therefore maximizing the activity per catalyst mass.
The PBSCF catalysts were tested in the OER. It was found that doping unstructured PBSCF catalysts with Sr and Fe increased the intrinsic catalyst activity by ~4.7 times, yielding a much higher intrinsic activity than commercial IrO2 catalysts. Synthesizing ultrafine nanofibers ~20 nm in diameter increased the catalyst surface area from 1.52 to 18.81 m2 g-1. Nanostructuring not only increased the mass-normalized activity of PSCF by ~72 times but also increased the intrinsic activity by ~1.6 times compared with unstructured PBSCF powder, resulting in an outstanding catalyst for the OER. The ultrafine PBSCF nanofibers delivered a current density of 10 mA cm-2 at an overpotential of 0.358 V and demonstrated a mass-normalized activity ~2.5 times higher than that of a commercial IrO2 catalyst. Furthermore, stability tests revealed that while commercial IrO2 catalysts experienced a slight loss of catalytic activity over 12 hours of the OER, the activity of the ultrafine PBSCF nanofibers remained stable.
The ultrafine PBSCF nanofibers combine high intrinsic activity with high surface area and high stability; therefore, they represent the next generation of electrocatalysts for the OER. These results may have important technological implications and increase the viability of sustainable technologies based on water splitting.
Zhao B., Zhang L., Zhen D., Yoo S., Ding Y, Chen D., Chen Y., Zhang Q., Doyle B., Xiong X., Liu M., Nature Communications, 2017, 8, 14586.