Tailoring π-symmetry electrons in cobalt–iron phosphide for highly efficient oxygen evolution

Sparkling Synergy: Enhancing Hydrogen Evolution with a Mesoporous CoP/FeP Interface

The reaction kinetics is predominantly determined by the surface and interface engineering of electrocatalysts. Herein, we demonstrate the growth of cobalt monophosphide and iron monophosphide (CoP/FeP) with an effective solid interface. The surface of CoP/FeP is mesoporous, which is obtained by phosphidizing mesoporous CoFe2O4. The CoP/FeP electrode exhibits substantially superior hydrogen evolution reaction (HER) performance compared to CoP and FeP. The overpotentials (η) required to generate 10 mA cm-2 are determined to be around 98 mVRHE (CoP/FeP), 220 mVRHE (FeP), and 265 mVRHE (CoP) in an acidic electrolyte. The exchange current density and Tafel slopes suggest that CoP/FeP has better redox properties and kinetic abilities compared to FeP and CoP. Furthermore, the CoP/FeP electrode exhibits reduced electrochemical impedance and superior surface charge transport characteristics in comparison to both the CoP and FeP electrodes. In addition to having a greater number of catalytically active sites, the turnover frequency of CoP/FeP is approximately 2 and 5 times higher than that of FeP and CoP, respectively. The CoP/FeP electrode maintains a consistent current density of around 25 mA cm-2 for a continuous period of 24 h during the HER, attesting to the excellent durability of the CoP/FeP electrode. In addition, a relationship between differential hydrogen adsorption energy (ΔEH), the corresponding Gibbs free energy change (ΔGH), and the hydrogen coverage on distinct surfaces, namely, CoP, FeP, and CoP/FeP, is established. The calculation findings show that the CoP/FeP surface, which is predominantly exposed with CoP, exhibits the highest catalytic potential for the HER. The estimation of the specific HER activity of the electrodes, normalized to the electrochemically active surface area, corroborates the calculation findings.

Defect engineering induces Mo-regulated Co9Se8/FeNiSe heterostructures with selenium vacancy for enhanced electrocatalytic overall water splitting in alkaline

The pursuit of cost-effective catalysts for electrocatalytic overall water splitting continues to present a significant challenge in the field. A molybdenum (Mo)-regulated Co9Se8/FeNiSe self-supporting electrode material with rich vacancy defects has been prepared by hydrothermal reaction. Doping of Mo atoms not only can form rich selenium vacancy defects to enrich the inherent activity of the catalyst, but also expose more active sites. The intrinsic electronic architecture of the interface catalysis is regulated and optimized through the introduction of heteroatom Mo, resulting in the exceptional catalytic activities of the Mo-Co9Se8/FeNiSe heterostructure. Additionally, the Faraday efficiency of hydrogen (H2) and oxygen (O2) production approaches 100 %. The voltage required for the water-splitting system is only 1.58 V (10 mA cm-2), and 100 h stability test at 100 mA cm-2 demonstrates no decay. This work presents a new perspective for the reasonable design and synthesis of non-precious metal selenide-based bifunctional electrocatalysts.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Tailoring abundant active-oxygen sites of Prussian blue analogues-derived adsorbents for highly efficient Yb(III) capture

The removal of rare earth elements in mineral processing wastewater is highly desirable but still challenging. In this study, three bimetallic Prussian blue analogues (PBA) and six corresponding oxides are prepared by co-precipitation and calcination methods, and then utilized to adsorb aqueous Yb(III) solution. The results of XRD, SEM, BET, and XPS indicate the successful synthesis of all the adsorbents. Among them, three PBA-oxide samples (PBO-800) exhibit the superior adsorption capacities (˃250 mg/g). The adsorption processes of Yb(III) are in accordance with the pseudo-second-order kinetic model and Langmuir model, simultaneously showing the spontaneous and endothermic thermodynamics. Moreover, PBO-800 can be reused after alkaline solution regeneration with less than 10% degradation after five consecutive adsorption-desorption cycles. More importantly, PBO-800 exhibits the impressive separation selectivity of Yb(III) and most light rare earth ions (e.g., 5.51 of Yb/La, 4.03 of Yb/Pr), as well as the selectivity of Yb(III) and alkali metal ions (e.g., 300.5 of Yb/Na, 256.2 of Yb/Ca). According to the characterization analysis and DFT calculation, the adsorption mechanism of Yb(III) by PBO-800 is mainly attributed to the strong interaction between the abundant active-oxygen sites and Yb(III), and the significant electrostatic attraction.

Transformation of CoFe2O4 spinel structure into active and robust CoFe alloy/N-doped carbon electrocatalyst for oxygen evolution reaction

Electrochemical water splitting is an environmentally benign technology employed for H₂ production; however, it is critically hampered by the sluggish kinetics of the oxygen evolution reaction (OER) at the positive electrode. In this work, nitrogen-doped carbon-coated CoFe electrocatalysts were synthesized via a three-step route comprising (1) hydrothermal reaction, (2) in-situ polymerization of dopamine and (3) carbonization. The effect of carbonized polydopamine on the overall physicochemical properties and electrochemical activity of CoFe catalysts was systematically studied. By controlling and optimizing the ratio of CoFe₂O₄ and dopamine contents, a transformation of the CoFe₂O₄ structure to CoFe alloy was observed. It was found that CoFe/NC_30% (prepared with 30% dopamine) exhibits an excellent catalytic activity towards OER. A small overpotential of 340 mV was required to generate a current density of 10 mA cm^-2 in a 1.0 M KOH electrolyte. More importantly, the CoFe/NC_30% catalyst reflected exceptional durability for at least 24 h. This research sheds light on the development of affordable, highly efficient, and durable electrocatalysts for OER.

Current trends and perspectives on emerging Fe-derived noble-metal-free oxygen electrocatalysts

This article discusses recent progress in the development of Fe-derived noble metal-free electrocatalysts, including the strategies used for design, synthesis, and assessment of their performance in alkaline conditions.

Electronic engineering of CoSe/FeSe2 hollow nanospheres for efficient water oxidation

First-row non-precious metal-based catalysts are widely studied and recognized as potential substitutes for precious metal-based catalysts in the oxygen evolution reaction (OER) for hydrogen generation but their application remains challenging. In this study, a unique class of Co-Fe selenide hollow nanospheres (CoSe@FeSe2) is well-designed through a facile hydrothermal method. The in situ formed hybrid composites possess numerous interfaces allowing partial electron transfer via O2- bridges to optimize the adsorption feature of the reaction intermediates, *OH, *O, and *OOH, on the catalysts. The collected surface valence band spectra evidence the optimization of the intermediate adsorption and active sites. The as-synthesized CoSe@FeSe2 exhibits excellent OER activity with a low overpotential of 281 mV to drive a current density of 10 mA cm-2 and a low Tafel slope of 34.3 mV dec-1 in an alkaline electrolyte. Additionally, the advanced catalyst also shows super stability with negligible current density decay after 12 h. This work presents a prototype for the fabrication of highly efficient electrocatalysts using an electronic engineering strategy.