Prussian Blue Analogues and Their Derivatives for Water Splitting Reactions

Electricity produced from renewable energy sources such as solar, wind, hydro, and marine power usually suffers from intermittency, thus requiring additional storage capacity. Conversion of the electricity to chemical fuels for future use is an effective solution. Hydrogen (H₂) is an ideal energy carrier due to its high gravimetric energy density, which can be produced from the electrolysis of water (H₂O → H₂ + O₂). However, the efficiency of electrochemical water splitting is impeded by the kinetic energy barriers for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Efficient electrocatalysts are required to lower the overpotentials for HER and OER, thus improving the energy conversion efficiency and reducing the production cost of H₂. Design of optimal electrocatalysts should take the operating condition and the type of electrolyzer (proton exchange membrane electrolysis, alkaline electrolysis, anion exchange membrane electrolysis, and high-temperature solid oxide water electrolysis) into account. In alkaline conditions, non-noble metal-based electrocatalysts are more likely to provide competitive activity over noble metal-based materials (Pt, Ir, Ru, and Pd) on both cathode and anode. Currently, active and robust non-noble metal-based electrocatalysts for HER and OER are urgent. The purpose of this Ph.D. project is to develop efficient and non-noble metal-based electrocatalysts for water splitting in alkaline solutions.

In this Ph.D. thesis, Prussian blue analogues (PBAs), enjoying controllable metal compositions, abundant CN groups, and easy preparation, were explored as the precursors to synthesize active transitional metal-based electrocatalysts for HER and OER. Two efficient electrocatalysts were synthesized by morphology engineering and composition design in this project. Effects of the morphology, composition, and properties upon the optimized electrocatalysts were systematically investigated and explained, as summarized below:

1. Bimetallic NiFeP catalyst coated on NiP rods on Ni foam were successfully synthesized (NiFeP@NiP@NF). The self-supported and interfacial-connected structure favors mass transfer and reduces electrical resistance for electrocatalysis. After optimization of the thickness of NiFe PBA and the phosphidation temperature, the sample exhibited excellent OER performance with a low overpotential of 227 mV at 10 mA cm^-2 and no obvious degradation for 120 h. It can also be applied as an HER electrocatalyst, delivering 10 mA cm^-2 at an overpotential of 105 mV. Therefore, the prepared NiFeP@NiP@NF was tentatively used in an electrolyzer as both cathode and anode, showing a voltage bias of 1.57 V for 10 mA cm^-2 and good stability. Advantages of the structure and composition of NiFeP@NiP@NF for electrocatalytic performance were carefully discussed and explained.

2. CoFe PBAs with different cation species (NH₄⁺, K⁺) in the interstitial spaces can influence the composition, morphology, and crystalline phases of their derivatives after heat-treatment in Ar atmosphere, leading to different performance for OER in 1.0 M KOH. The derivative of CoFe PBA (filled with NH₄⁺) performed best, with overpotentials of 270 and 305 mV at 1 and 10 mA cm^-2, respectively, and remarkable stability. The sample worked well with a Pt/C cathode in a single-cell alkaline electrolyzer, delivering 100 mA cm^-2 at 80 ℃ at a cell voltage of 1.66 V in 1.0 M KOH solution for 100 h with negligible degradation. The mechanisms of material transformation and the corresponding effects on the OER catalytic performance were thoroughly explored.