Constructing an Adaptive Heterojunction as a Highly Active Catalyst for the Oxygen Evolution Reaction

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Coupling Interface Constructions of FeNi3-MoO2 Heterostructures for Efficient Urea Oxidation and Hydrogen Evolution Reaction

Urea electrolysis has prospects for urea-containing wastewater purification and hydrogen (H₂) production, but the shortage of cost-effective catalysts restricts its development. In this work, the tomentum-like FeNi₃-MoO₂ heterojunction nanosheets array self-supported on nickel foam (NF) as bifunctional catalyst is prepared by facile hydrothermal and annealing method. Only 1.29 V and -50.8 mV is required to obtain ±10 mA cm^-2 for urea oxidation and hydrogen evolution reaction (UOR and HER), respectively, showing great bifunctional catalytic activity. For overall urea electrolysis, it only needs 1.37 V to reach 10 mA cm^-2 and can last at 100 mA cm^-2 for 70 h without obvious activity attenuation, showing outstanding durability. Coupling interface constructions of FeNi₃-MoO₂ heterostructures, novel morphology with a mesoporous and self-supporting structure could be the reason for this good performance. This work thus proposes a promising catalyst for boosting UOR and HER to realize efficient overall urea electrolysis.

Active Phase on SrCo1-x Fe x O3-δ (0 ≤ x ≤ 0.5) Perovskite for Water Oxidation: Reconstructed Surface versus Remaining Bulk

Perovskite oxides based on earth-abundant transition metals have been extensively explored as promising oxygen evolution reaction (OER) catalysts in alkaline media. The (electro)chemically induced transformation of their initially crystalline surface into an amorphous state has been reported for a few highly active perovskite catalysts. However, little knowledge is available to distinguish the contribution of the amorphized surface from that of the remaining bulk toward the OER. In this work, we utilize the promoting effects of two types of Fe modification, i.e., bulk Fe dopant and Fe ions absorbed from the electrolyte, on the OER activity of SrCoO_3-δ model perovskite to identify the active phase. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the surface amorphization of SrCoO_3-δ as well as SrCo_0.8Fe_0.2O_3-δ after potential cycling in Fe-free KOH solution. By further cycling in Fe-spiked electrolyte, Fe was incorporated into the amorphized surface of SrCoO_3-δ (SrCoO_3-δ + Fe³⁺), yielding approximately sixfold increase in activity. Despite the difference in remaining perovskites, SrCoO_3-δ + Fe³⁺ and SrCo_0.8Fe_0.2O_3-δ exhibited remarkably similar activity. These results reflect that the in situ developed surface species are directly responsible for the measured OER activity, whereas the remaining bulk phases have little impact.

Mechanism of the Loss of Capacity of LiNiO2 Electrodes for Use in Aqueous Li-Ion Batteries: Unveiling a Fundamental Cause of Deterioration in an Aqueous Electrolyte through In Situ Raman Observation

This study investigated the fundamental mechanisms of the loss of capacity of LiNiO₂ (LNO) electrodes for Li⁺ insertion/deinsertion with a special focus on the origin of this deterioration in an aqueous system. In situ Raman spectra revealed that the intercalation of H⁺ ions formed a NiOOH_x film at the surface of LNO during the initial electrochemical cycles; this NiOOH_x film was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy analysis. The formation of an electrochemically inactive spinel-like phase (Ni₃O₄) at the subsurface was triggered by the absence of Li in the NiOOH_x film at the surface. These structural changes of LNO, accelerated by the intercalation of H⁺ ions, were considered to be the fundamental cause of the greater loss of capacity in the aqueous system.