Fluoride mediated conversion of FeOOH into NiFeOOH for outstanding oxygen evolution reaction

Efficiently Electrocatalytic Overall Water Splitting over Ni0.85Se-Ni Foam Electrode Modified with Nanoneedle-Like Ni3Fe Alloy

Electrocatalytic splitting of H2O into H2 and O2 is one of the potential ways to develop clean and storable energy. In this study, the self-supported Ni0.85Se-Ni foam (Ni0.85Se-NF) hybrid electrode was modified with Ni3Fe (Ni3Fe/Ni0.85Se-NF) via a facile selenization and electrochemical deposition method. The Ni3Fe/Ni0.85Se-NF hybrid electrode exhibited excellent performance of electrocatalytic overall water splitting in an alkaline solution (1 m KOH, pH = 13.9), which is much more active than the individual Ni0.85Se-NF and Ni3Fe/NF. In particular, the Ni3Fe/Ni0.85Se-NF hybrid electrode generated 10 mA cm-2 cathodic current at an overpotential of 81 mV, and 10 mA cm-2 anodic current at an overpotential of 209 mV in KOH solution (1 m, pH = 13.9), respectively. In addition, only 1.68 V potential was required to generate 12 mA cm-2 current in a two-electrode system for overall water splitting in an alkaline aqueous solution using the Ni3Fe/Ni0.85Se-NF working as the anode and cathode, respectively. The TEM images and Raman spectra results indicated that the microstructure reconstruction happened on the surface of the Ni3Fe/Ni0.85Se-NF electrode after the HER or OER test. Some Ni and Fe species were found to convert to the NixFe1-xOOH during water electrocatalysis.

Solar-Powered AEM Electrolyzer via PGM-Free (Oxy)hydroxide Anode with Solar to Hydrogen Conversion Efficiency of 12.44

Water electrolyzers powered by renewable energy are emerging as clean and sustainable technology for producing hydrogen without carbon emissions. Specifically, anion exchange membrane (AEM) electrolyzers utilizing non-platinum group metal (non-PGM) catalysts have garnered attention as a cost-effective method for hydrogen production, especially when integrated with solar cells. Nonetheless, the progress of such integrated systems is hindered by inadequate water electrolysis efficiency, primarily caused by poor oxygen evolution reaction (OER) electrodes. To address this issue, a NiFeCo─OOH has developed as an OER electrocatalyst and successfully demonstrated its efficacy in an AEM electrolyzer, which is powered by renewable electricity and integrated with a silicon solar cell.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

Revealing the effect of crystallinity and oxygen vacancies of Fe-Co phosphate on oxygen evolution for high-current water splitting

Strategically tuning the composition and structure of transition metal phosphates (TMPs) holds immense promise in the development of efficient oxygen evolution reaction (OER) electrocatalysts. However, the effect of crystalline phase transformation for TMPs on the catalytic OER activity remains relatively uncharted. In this study, we have deftly orchestrated the reaction process of anion-etched precursor to induce the amorphization process of FeCo-POx from crystalline to amorphous states. The as-obtained amorphous FeCo-POx (A-FeCo-POx) exhibited an optimized OER performance with a low overpotential of 270 mV at a current density of 10 mA cm-2, which could be attributed to the flexibility of its amorphous structure and the synergistic effect of oxygen vacancies. Moreover, when incorporated into an overall water splitting (OWS) device configured as A-FeCo-POx(+)||Pt/C(-), it displayed long-term solid stability, sustaining operation for 300 h at a current density of 200 mA cm-2. This work not only provides valuable insights into understanding the transformation from crystalline to amorphous states, but also establishes the groundwork for the practical utilization of amorphous nanomaterials in the field of water splitting.

Operando spectroscopies capturing surface reconstruction and interfacial electronic regulation by FeOOH@Fe2O3@Ni(OH)2 heterostructures for robust oxygen evolution reaction

Developing high-performance and low-cost electrocatalysts for oxygen evolution reaction (OER) and understanding the phase evolution in the catalytic process are vital to improving the overall efficiency of electrochemical water splitting. Herein, a hybrid heterogeneous FeOOH@Fe₂O₃@Ni(OH)₂ electrocatalyst with robust OER intrinsic activity and a low overpotential of 269 mV to obtain a current density of 100 mA cm^-2 and a Tafel slope value of 60.15 mV dec^-1 is effectively prepared. The dynamic surface evolution has been detected by in-situ Raman spectroscopy, which exposes that FeOOH@Fe₂O₃@Ni(OH)₂ is reconstituted as Ni(Fe)OOH demonstrated as catalytically active species under high potential. X-ray photoelectron spectroscopy analysis indicates that partial electrons of Ni in the heterogeneous interface transfer to Fe. Furthermore, partial Fe doping of NiOOH under high potential accompanied by the oxidized Ni³⁺ with optimized d-orbit electronic configuration for nearly unity e_g occupancy results in proper chemisorption bonding strength for oxygen reaction intermediates and is conducive to enhancing OER reaction kinetics. This work provides ideas that multicomponent heterostructure can adjust the electronic structure of iron and nickel to enhance the intrinsic activity of OER, which could help with the design and synthesis of high-performance OER catalysts used in energy storage and conversion.

Fabrication of Trimetallic Fe-Co-Ni Electrocatalysts for Highly Efficient Oxygen Evolution Reaction

The development of inexpensive and well-activated water-splitting catalysts is required to reduce the use of conventional fossil fuels. In this study, a trimetallic Fe-Co-Ni catalyst was fabricated using a simple ion electrodeposition method. The metal deposition was performed using cyclic voltammetry, which was more efficient than constant-voltage deposition and significantly increased the stability of the catalyst. The synthesized material presented the morphology of a nanoflower in which the nanosheets were agglomerated. The Fe-Co-Ni catalyst exhibited excellent oxygen evolution reaction (OER) properties because the charge-transfer rate was improved owing to the synergistic effect of the metals. The OER was performed in a 1 M KOH solution using a three-electrode system, and the overpotential was 302 mV at 100 mA/cm². In addition, the Fe-Co-Ni catalyst exhibited excellent stability in alkaline solution for more than 48 h at 200 mA/cm². The results show that the method for preparing Fe-Co-Ni significantly improves its catalytic activity, and the resulting material could be used as an economical and efficient catalyst in future.

Function of Internal and External Fe in a Ni-Based Precatalyst System Toward Oxygen Evolution Reaction

It is well known that the "iron" impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M(OH)₂/MOOH (M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the "precatalyst" system. Accordingly, in this work, the Ni_3-xFe_xP (x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni_2.5Fe_0.5P and Ni₂FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni₃P) owing to the large amount of active M(OH)₂/MOOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl₂·4H₂O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni₃P; nevertheless, that the OER activity of Ni₂FeP is still much superior than that of Ni₃P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni₂FeP and that internal Fe is also critical to the phase transformation as well as OER performance.