A highly efficient FeP/CeO2-NF hybrid electrode for the oxygen evolution reaction

Effect of Cooperative Redox Property and Oxygen Vacancies on Bifunctional OER and HER Activities of Solvothermally Synthesized CeO2/CuO Composites

Herein, we report the synthesis of the CeO₂/CuO composite as a bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrocatalyst in a basic medium. The electrocatalyst with an optimum 1:1 CeO₂/CuO shows low OER and HER overpotentials of 410 and 245 mV, respectively. The Tafel slopes of 60.2 and 108.4 mV/dec are measured for OER and HER, respectively. More importantly, the 1:1 CeO₂/CuO composite electrocatalyst requires only a 1.61 V cell voltage to split water to achieve 10 mA/cm² in a two-electrode cell. The role of oxygen vacancies and the cooperative redox activity at the interface of the CeO₂ and CuO phases is explained in the light of Raman and XPS studies, which play the determining factor for the enhanced bifunctional activity of the 1:1 CeO₂/CuO composite. This work provides guidance for the optimization and design of a low-cost alternative electrocatalyst to replace the expensive noble-metal-based electrocatalyst for overall water splitting.

Enhanced activity promoted by amorphous metal oxyhydroxides on CeO2 for alkaline oxygen evolution reaction

Herein, we demonstrate a direct growth of amorphous metal oxyhydroxide (AMO) attached on CeO₂ by a galvanic replacement mechanism as advanced oxygen evolution reaction (OER) catalyst. In this unique structure, the CeO₂ substrate not only offers high specific surface area for the formation of AMO, but also provides high conductivity, guaranteeing the promoted electron transfer for the catalytic reaction. In addition, the AMO on the surface of the CeO₂ exposes abundant active sites for the OER. Benefiting from the above advantages, the as-prepared AMO@CeO₂ supported on nickel foam (AMO@CeO₂/NF) exhibits excellent OER performance with low overpotential of 261 mV at 10 mA cm^-2, high turnover frequency of 0.07 s^-1 at 20 mA cm^-2 and superior stability in 1.0 M KOH.

Interface Engineering of Heterogeneous CeO2-CoO Nanofibers with Rich Oxygen Vacancies for Enhanced Electrocatalytic Oxygen Evolution Performance

The development of highly efficient and cheap electrocatalysts for the oxygen evolution reaction (OER) is highly desirable in typical water-splitting electrolyzers to achieve renewable energy production, yet it still remains a huge challenge. Herein, we have presented a simple procedure to construct a new nanofibrous hybrid structure with the interface connecting the surface of CeO₂ and CoO as a high-performance electrocatalyst toward the OER through an electrospinning-calcination-reduction process. The resultant CeO₂-CoO nanofibers exhibit excellent electrocatalytic properties with a small overpotential of 296 mV at 10 mA cm^-2 for the OER, which is superior to many previously reported nonprecious metal-based and commercial RuO₂ catalysts. Furthermore, the prepared CeO₂-CoO nanofibers display remarkable long-term stability, which can be maintained for 130 h with nearly no attenuation of OER activity in an alkaline electrolyte. A combined experimental and theoretical investigation reveals that the excellent OER properties of CeO₂-CoO nanofibers are due to the unique interfacial architecture between CeO₂ and CoO, where abundant oxygen vacancies can be generated due to the incomplete matching of atomic positions of two parts, leading to the formation of many low-coordinated Co sites with high OER catalytic activity. This research provides a practical and promising opportunity for the application of heterostructured nonprecious metal oxide catalysts for high-efficiency electrochemical water oxidation.

Designing of Highly Efficient Oxygen Evolution Reaction Electrocatalysts Utilizing A Correlation Factor: Theory and Experiment

The theoretical prediction of the catalytic activity is very beneficial for the design of highly efficient catalysts. At present, most theoretical descriptors focus on estimating the catalytic activity and understanding the enhancement mechanism of catalysts, while it is also quite important to find a factor to correlate the descriptors with preparation methods. In this work, a correlation factor, the d electron density of transition metal ions, was developed to correlate the d band center values of transition metal ions with the preparation methods of amorphization and Al introduction. According to the results of theoretical simulations, the correlation factor not only exhibited favorable linear relationships with the theoretical overpotentials of (CoFeAl_x)₃O₄ and (CoFeAl_x)₃O₄ + (CoFeAl_x)OOH systems but also correlated with two preparation methods by altering the volume of systems. Based on theoretical guidance, the electrocatalytic activities of the prepared (CoFeAl_x)₃O₄ specimens were gradually improved by the preparation methods of amorphization and Al introduction, and the Am-CoFeAl-2-10h specimen exhibited a low kinetic barrier of 268 mV, fast charge transfer rate, and stable electrocatalytic activity. This strategy could be applied to design highly efficient catalysts by adjusting the correlation factor of the active site with suitable preparation methods.

Recent Advances in Multimetal and Doped Transition-Metal Phosphides for the Hydrogen Evolution Reaction at Different pH values

Hydrogen is a fuel with a potentially zero-carbon footprint viewed as a viable alternative to fossil fuels. It can be produced in a large scale via electrochemical water splitting using electricity derived from renewable sources, but this would require highly active, inexpensive, and stable hydrogen evolution reaction (HER) catalysts to replace the Pt benchmark. Transition-metal phosphides (TMPs) are potential Pt replacements owing to their generally high activity as well as versatility as HER catalysts for different pH media. This review summarizes the recent progress in the development of TMP HER electrocatalysts, focusing on the strategies that have been recently explored to tune the activity in acidic, neutral, and basic media. These strategies are the doping of TMPs with metal and nonmetal elements, fabrication of multimetallic phosphide phases, and construction of multicomponent heterostructures comprising TMPs and another component such as a different TMP or a metal oxide/hydroxide. The synthetic methods utilized to design the catalysts are also presented. Finally, the challenges still remaining and future research directions are discussed.