Doping of Cr to Regulate the Valence State of Cu and Co Contributes to Efficient Water Splitting

Chromium-Doped NiBP Micro-Sphere Electrocatalysts for Green Hydrogen Production under Industrial Operational Conditions

Wide spread adaptation of green hydrogen can help to mitigate the serious climate issues, increasing global energy demands and the development of advanced electrocatalysts robust under industrial conditions is one of the key technological challenges. Herein, chromium-doped nickel-boride-phosphide (Cr/NiBP) micro sphere (MS) electrocatalyst is demonstrated via a two-step hydrothermal approach along with post-annealing. The Cr/NiBP MS demonstrates low hydrogen evolution reaction and oxygen evaluation reaction over potentials of 78 and 250 mV at 100 mA cm-2 in 1 m KOH, out performing most of the reported catalysts. The Cr/NiBP ǁ Cr/NiBP exhibits only 1.54 V at 100 mA cm-2 in 1 m KOH and surpasses the benchmark of RuO2 (+) ǁ Pt/C (-) up to 2000 mA cm-2, which sets it as one of the best bifunctional electrocatalysts. Impressively, it maintains stable performance for over 240 h at 1000 mA cm-2 in 6 m KOH at 60°C, demonstrating rapid response, anti-corrosion resistance, and robust structural integrity to meet the industrial operational conditions. Further, Cr/NiBP ǁ Pt/C exhibits a super-low cell-voltage of 2.25 V at 2000 mA cm-2. The small amount of Cr atoms incorporation can significantly enhance active sites and intrinsic properties, accelerating water dissociation and rapid intermediate formation.

Lanthanum-Promoted Electrocatalyst for the Oxygen Evolution Reaction: Unique Catalyst or Oxide Deconstruction?

A conventional performance metric for electrocatalysts that promote the oxygen evolution reaction (OER) is the current density at a given overpotential. However, the assumption that increased current density at lower overpotentials indicates superior catalyst design is precarious for OER catalysts in the working environment, as the crystalline lattice is prone to deconstruction and amorphization, thus greatly increasing the concentration of catalytic active sites. We show this to be the case for La3+ incorporation into Co3O4. Powder X-ray diffraction (PXRD), Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) reveal smaller domain sizes with decreased long-range order and increased amorphization for La-modified Co3O4. This lattice deconstruction is exacerbated under the conditions of OER as indicated by operando spectroscopies. The overpotential for OER decreases with increasing La3+ concentration, with maximum activity achieved at 17% La incorporation. HRTEM images and electron diffraction patterns clearly show the formation of an amorphous overlayer during OER catalysis that is accelerated with La3+ addition. O 1s XPS spectra after OER show the loss of lattice-oxide and an increase in peak intensities associated with hydroxylated or defective O-atom environments, consistent with Co(O)x(OH)y species in an amorphous overlayer. Our results suggest that improved catalytic activity of oxides incorporated with La3+ ions (and likely other metal ions) is due to an increase in the number of terminal octahedral Co(O)x(OH)y edge sites upon Co3O4 lattice deconstruction, rather than enhanced intrinsic catalysis.

Influence of Copper Valence in CuOx/TiO2 Catalysts on the Selectivity of Carbon Dioxide Photocatalytic Reduction Products

The Cu cocatalyst supported on the surface of TiO2 photocatalysts has demonstrated unique activity and selectivity in photocatalytic CO2 reduction. The valence state of copper significantly influences the catalytic process; however, due to the inherent instability of copper's valence states, the precise role of different valence states in CO2 reduction remains inadequately understood. In this study, CuOx/TiO2 catalysts were synthesized using an in situ growth reduction method, and we investigated the impact of various valence copper species on CO2 photocatalytic reduction. Our results indicate that Cu+ and Cu0 serve as primary active sites, with the selectivity for CH4 and CO products during CO2 photoreduction being closely related to their respective ratios on the catalyst surface. The adsorption and activation mechanisms of CO on both Cu+ and Cu0 surfaces are identified as critical factors determining product selectivity in photocatalytic processes. Furthermore, it is confirmed that Cu+ primarily facilitates CH4 production while Cu0 is responsible for generating CO. This study provides valuable insights into developing highly selective photocatalysts.

Cu-P@silica-CNT-based catalyst for effective electrolytic water splitting in an alkaline medium with hydrazine assistance

In this study, we prepared a potential catalyst as an electrode modifier for electrolytic water splitting. In the preparation step, the amine was decorated with copper-phosphorus. It was immobilized over the silica surface, and the surface was engineered using N-(3-(trimethoxysilyl) propyl)ethylenediamine for the synthesis of the catalysts (AS). The morphological and structural aspects of the catalyst (AFS-Cu-P) were determined using FE-SEM/EDAX, FTIR, elemental analysis, BET, TGA, and XPS. The catalyst's efficacy for the oxygen evolution reaction (OER) was assessed in an alkaline medium with and without hydrazine. The hydrazine oxidation reaction enhanced the sluggish OER and facilitated water splitting. Detailed electrochemical measurements confirmed an increase in the kinetics of the process and a reduction in the activation energy needed to complete the process. The Tafel slopes, charge transfer coefficients, exchange-specific current densities, apparent rate constants, and diffusion coefficients are provided along with their respective values. The results showed that the presence of Cu and CNT is crucial in the conversion process.

Surface reconstruction in amorphous CoFe-based hydroxides/crystalline phosphide heterostructure for accelerated saline water electrolysis

Developing electrocatalysts with high activity and robust performance for large-scale seawater electrolysis to produce hydrogen holds immense significance. Herein, a highly active bifunctional electrode composed of amorphous cobalt-iron layered double hydroxides (CoFeLDH) and crystalline nickel phosphide (Ni2P) (denoted as CoFeLDH@Ni2P), is employed to boost hydrogen production through seawater electrolysis. The strong interface coupling effectively modifies the electronic structure at active sites, thereby accelerating the catalytic reaction kinetics. Impressively, in situ Raman and post-stability analyses demonstrate a unique reconstruction behavior on the CoFeLDH@Ni2P electrode. Bimetal co-incorporated NiOOH (CoFe-NiOOH) and Ni(OH)2 species are formed during the oxygen evolution reaction (OER), while CoFeLDH@Ni2P can transform into Ni(OH)2 species during the hydrogen evolution reaction (HER) process. Furthermore, the highly negatively charged surface selectively rejects Cl- ions by formed PO43-, endowing CoFeLDH@Ni2P with excellent tolerance and promising durability in saline electrolytes. Consequently, the CoFeLDH@Ni2P electrode exhibits an overpotential of 106 mV for HER at 10 mA cm-2 and 308 mV for OER to achieve 100 mA cm-2 in 1.0 M KOH solution. Additionally, the CoFeLDH@Ni2P(+,-) electrolyzer requires a low cell voltage of 1.56 V to deliver 10 mA cm-2 in 1.0 M KOH + Seasalt. This work presents an appealing strategy for the rational design of advanced electrocatalysts with amorphous-crystalline interfaces, which reveals the source of the activity of transition-metal phosphating compounds in saline water electrolysis.

Surface defect-engineered Fe doping in layered Co-based complex as highly efficient bifunctional electrocatalysts for overall water splitting

The electrocatalytic splitting of water to produce hydrogen is regarded as an efficient and promising strategy but is limited by its large overpotential; thus, a highly efficient electrocatalyst is urgently needed. Mixed metal doping is an important strategy in defect engineering because the heteroatoms can change the intrinsic structure to form defects by affecting the atomic coordination mode and adjusting the electronic structure, which is often accompanied by morphological changes. Herein, two-dimensional layered bimetallic Co-pydc containing axially coordinated water molecules was selected by producing surface defects through Fe doping in Co centers as bifunctional electrocatalysts for OER and HER. The optimized Co0.59Fe0.41-pydc possesses outstanding OER performance with the lowest overpotential of 262 mV to reach j = 10 mA cm-2, and Co0.75Fe0.25-pydc possesses superior HER performance with the lowest overpotential of 96 mV at j = 10 mA cm-2. Furthermore, the overall water splitting device assembled with Co0.59Fe0.41-pydc@NF//Co0.59Fe0.41-pydc@NF affords a current density of 10 mA cm-2 at only 1.687 V. This work emphasizes the surface defects formed by tuning the electronic structure of metal centres accompanied with morphological changes of bimetallic dopants for efficient overall water splitting.

Fabricating Ru Atom-Doped Novel FeP4/Fe2PO5 Heterogeneous Interface for Overall Water Splitting in Alkaline Environment

Developing bifunctional electrocatalysts with low-content noble metals and high activity and stability is crucial for water splitting. Herein, we reported a novel Ru doped FeP4/Fe2PO5 heterogeneous interface catalyst (Ru@FeP4/Fe2PO5) for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) by heat treatment coupling electrodeposition strategy. Experiments disclosed that Ru@FeP4/Fe2PO5 proclaimed excellent catalytic activity for the OER (249 mV@100 mA cm-2) and HER (49 mV@10 mA cm-2) in a 1 M KOH environment. More importantly, the mass activity and turnover frequency of Ru@FeP4/Fe2PO5 were 117 and 108 times higher than that of commercial RuO2 at an overpotential of 300 mV during the OER, respectively. In addition, the assembled Ru@FeP4/Fe2PO5 || Ru@FeP4/Fe2PO5 system could retain superior durability in a two-electrode system for 134 h at 300 mA cm-2. Further mechanism studies revealed that Ru atoms in Ru@FeP4/Fe2PO5 act in a key role for the excellent activity during water splitting because the electronic structure of Ru sites could be optimized by the interaction between Ru and Fe atoms at the interface to strengthen the adsorption of reaction intermediates. Besides, the introduction of Ru atoms could also enhance the charge transfer, which effectually accelerates the reaction kinetics. The strategy of anchoring Ru atom on novel heterostructure provides a promising path to boost the overall activity of electrocatalysts for water splitting.