Co3+ -Rich Na1.95 CoP2 O7 Phosphates as Efficient Bifunctional Catalysts for Oxygen Evolution and Reduction Reactions in Alkaline Solution

Potassium Cobalt Pyrophosphate as a Nonprecious Bifunctional Electrocatalyst for Zinc-Air Batteries

Economic and sustainable (ecological) energy storage forms a major pillar of the global energy sector. Bifunctional electrocatalysts, based on oxygen electrolysis, play a key role in the development of rechargeable metal-air batteries. Pursuing precious metal-free economic catalysts, here, we report K₂CoP₂O₇ pyrophosphate as a robust cathode for secondary zinc-air batteries with efficient oxygen evolution and oxygen reduction (OER||ORR) activity. Prepared by autocombustion, nanoscale K₂CoP₂O₇ exhibited excellent oxygen reduction and evolution reactions among all phosphate-based electrocatalysts. In particular, the OER activity surpassed that of commercial RuO₂ with low overpotential (0.27 V). First-principles calculations revealed that the bifunctional activity is rooted in the Co active site with the CoO₅ local coordination in the most stable (110) surface. This nanostructured (tetragonal) pyrophosphate can be harnessed as an economic bifunctional catalyst for zinc-air batteries.

Silver decorated cobalt carbonate to enable high bifunctional activity for oxygen electrocatalysis and rechargeable Zn-air batteries

Rechargeable zinc-air batteries (ZABs) is primarily driven by the couple of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Currently,it is still challenging to develop cost-effective, highly efficient, and robust bifunctional catalysts for ZABs. Herein, a novel silver decorated cobalt carbonate (Ag@CoCO₃) hybrid catalyst is proposed as the potential bifunctional catalyst to drive OER and ORR for ZABs. Engineering Ag nanoparticles onto the surface of CoCO₃ microsphere not only facilitates the charge transfer, but also modulates the electronic structure, which are beneficial to intrinsic bifunctional activity. As a result, this Ag@CoCO₃ catalyst yields a substantially enhanced bifunctionality compared to the pristine CoCO₃ catalyst. Moreover, the homemade Ag@CoCO₃ based ZABs provides a high peak power density of 146 mW cm^-2, superior to 107 mW cm^-2 for CoCO₃ based ZABs and 111 mW cm^-2 for commercial Pt/C-IrO₂ based ZABs.

Selenium vacancy triggered atomic disordering of Co0.85Se nanoparticles towards a highly-active electrocatalyst for water oxidation

Selenium vacancy engineering has been realized in Co0.85Se nanoparticles via an anoxic melting strategy, where the vacancy content can be continuously controlled to modulate atomic disordering. The resulting Co0.85Se-30 catalyst requires a super low overpotential of 243 mV to achieve 10 mA cm-2 for the OER with a Tafel slope of 45.5 mV dec-1 and 70 h stability. In-depth electrochemical analysis finds that the outstanding properties are chiefly attributed to the dynamic Co-centers, giving the highest intrinsic activity (jCo = 6.49 A g-1 at η = 270 mV) and lowest apparent activation energy (42.43 kJ mol-1).

Coordination environment evolution of Co(ii) during dehydration and re-crystallization processes of KCoPO4·H2O towards enhanced electrocatalytic oxygen evolution reaction

Development of efficient and stable electrodes for electrocatalytic oxygen evolution reaction (OER) is essential for energy storage and conversion applications, such as hydrogen generation from water splitting, rechargeable metal–air batteries and renewable fuel cells. Alkali metal cobalt phosphates show great potential as OER electrocatalysts. Herein, an original electrode design strategy is reported to realize an efficient OER electrocatalyst through engineering the coordination geometry of Co(ii) in KCoPO₄·H₂O by a facile dehydration process. Experimental result indicated that the dehydration treatment is accompanied by a structural transformation from orthorhombic KCoPO₄·H₂O to hexagonal KCoPO₄, involving a concomitant coordination geometry evolution of Co(ii) from octahedral to tetrahedral configuration. More significantly, the local structural evolution leads to an advantageous electronic effect, i.e. increased Co–O covalency, resulting in an enhanced intrinsic OER activity. To be specific, the as-produced KCoPO₄ can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Thus, this present research provides a new way of developing alkali metal transition-metal phosphates for efficient and stable electrocatalytic oxygen evolution reaction.

Coordination environment evolution of potassium cobalt phosphate towards enhanced electrocatalytic oxygen evolution reaction.