NiCo-Based Electrocatalysts for the Alkaline Oxygen Evolution Reaction: A Review

Heterogeneous electrocatalysis usually involves the charge transfer between the surface of electrocatalysts and corresponding reactants. Thus, modulating the surface electron density of electrocatalysts is an effective strategy to boost the electrocatalytic activity of targeted reactions. Herein, the NiSe2/FeSe2 p-p heterojunction is constructed via a simple selenization method for both oxygen evolution reaction (OER) and urea oxidation reaction (UOR). The designed NiSe2/FeSe2 electrocatalyst exhibits a superior activity towards OER, which only requires a low overpotential of 256 mV to reach a current density of 10 mA cm−2, and surpasses other selenides and even the state-of-the-art RuO2. Impressively, when employed as the UOR electrode, the NiSe2/FeSe2 heterojunction needs only 127 mV overpotential at 50 mA cm-2, remarkably superior to other selenides and confirming the less energy consumption for UOR. The comprehensive analysis demonstrates that the well-designed built-in electric field at the heterointerface of NiSe2/FeSe2 p-p heterojunction due to the difference of energy levels can expedite the charge transfer and thus strengthen the conductivity of heterojunction electrocatalyst. Moreover, the self-driven electron transfer across the NiSe2/FeSe2 heterointerface can induce local charge redistribution at the interface region, which is beneficial for the adsorption of OH− and urea owing to the electrostatic interaction. Therefore, the designed NiSe2/FeSe2 p-p heterojunction with regulated electronic structure displays extraordinary electrocatalytic activity for both the OER and UOR. This study demonstrates a novel strategy to manipulate the surface/interface charge states of electrocatalysts for improving the catalytic activity of OER and UOR, and provides new guidelines for exploring other superior electrocatalysts.

Interfacial engineering of the NiSe2/FeSe2 p-p heterojunction for promoting oxygen evolution reaction and electrocatalytic urea oxidation

Designing and fabricating well-defined heterointerface catalysts with high electrocatalytic performance for oxygen evolution reaction (OER) at the industrial grade current density still remains a huge challenge. Here the flower-like nanosheets with rich Fe2O3/NiFe-layered double hydroxides (LDHs) heterointerfaces were fabricated, and they exhibit superior catalytic activity with a very low overpotential of 220 mV for OER at the industrial grade current density of 500 mA cm− 2 and fast reaction kinetics with a small Tafel slope of 32 mV dec−1. Based on the analyses of operando Raman spectra, DFT theoretical calculations and electrochemical characterizations, the superior electrocatalytic performance of catalysts for OER at the industrial grade current density can be attributed to Fe2O3/NiFe-LDHs heterointerfaces that can obviously promote interfacial electron transfer from Ni2+ to Fe3+ and optimize d-orbit electronic configuration with eg occupancy of Ni close to the unity, resulting in moderate adsorption/desorption energies of oxygenated intermediates, and thus facilitating remarkably electrocatalytic performance and superior intrinsic kinetics for OER in alkaline media. 

Interfacial electronic modulation by Fe2O3/NiFe-LDHs heterostructures for efficient oxygen evolution at high current density

Multiple relaxation behaviors are promising for broad frequency band and strong electromagnetic wave (EMW) absorption based on polarization‐controlled electromagnetic (EM) attenuation. However, rational selection of materials and structure manipulation through tunable substitution or phase control are challenging toward optimization of EMW absorption. Herein, bi‐metallic organic frameworks (B‐MOFs) with various morphologies are employed as EMW absorbers. Remarkably, the polar units can be enhanced by introducing Ni‐metal nodes into the Cu‐coordinated MOFs, rendering the B‐MOFs with self‐polarized properties and consecutive multifrequency EMW absorption behaviors. The maximum reflection loss of acetylene black (ACET) filled NiCu‐MOFs can reach –40.54 dB together with a wide bandwidth (<‐10 dB) of 5.87 GHz at a thickness of 2.5 mm. As a counterpart of the Ni/Cu/C derivatives, significantly increased broad band absorption (6.93 GHz) and multifrequency absorbing and polarization characteristics are also maintained in bimetal coexisting carbonized architectures as prepared by calcination of CuNi‐MOFs. This work demonstrates that the performance of effective absorbing frequency band can be enhanced in multi‐metallic organic frameworks‐based architectures, and paves a novel avenue for developing broadband and strong EMW absorbers.

Tailoring Self‐Polarization of Bimetallic Organic Frameworks with Multiple Polar Units Toward High‐Performance Consecutive Multi‐Band Electromagnetic Wave Absorption at Gigahertz

The exploration of an efficient electrocatalyst for the oxygen evolution reaction (OER) is urgently required for sustainable renewable-energy conversion and storage. Due to the increased chemical complexity, multimetallic catalysts provide flexibility to alter their electronic and crystal structure to attain a superior intrinsic catalytic activity via synergistic effects, which is seldom accomplished using single metal catalysts. However, the high chemical complexity increases the difficulty to prepare elemental homogenous catalysts and reveal their synergistic effect during OER process, which further hinder the design of multimetallic catalysts. Here, high entropy concept is utilized to design an NiFeCoMnAl oxide with amorphous structure as OER catalyst. The direct evidence of active Ni sites is provided by the operando Raman measurements and Fe can modify oxygen intermediates binding energy on Ni sites. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) reveal that the incorporation of Mn can construct the electron-rich environment of active Ni center, and the relatively lower oxidation state of Ni facilitates the self-construction of β-NiOOH intermediates, which shows promoted OER activity as confirmed by density functional theory calculations. Doping Co can enhance the conductivity and doping Al leads to the formation of nanoporous structure through dealloying process, thus each component is essential for improving OER performance. The optimized NiFeCoMnAl catalyst exhibits an overpotential of 190 mV at 10 mA cm−2 in 1 M KOH solution, much superior to the ternary and quaternary counterparts. This work sheds light on understanding the origin of high entropy catalysts’ OER activity and thereby enables the rational design of multinary transition metallic catalysts.

Promoted self-construction of β-NiOOH in amorphous high entropy electrocatalysts for the oxygen evolution reaction

Metal-organic frameworks (MOFs) have recently emerged as promising precursors to construct efficient non-noble metal electrocatalyst for oxygen evolution reaction (OER) Herein, a Co-Ni-Fe spinel oxide-carbonitrides hybrids (CoNiFeOx-NC) electrocatalyst with hierarchical structure was synthesized from Fe-MIL-101-NH2 through a unique ion-exchange based strategy The ion exchange of Fe-MIL-101-NH2 with both Ni and Co ions induced a hierarchically structured 2-D ternary metal MOF shell layer encapsulated 3-D octahedral MOF crystals as a core This prevents the collapse of MOF frameworks during the air calcination process and affords highly porous structure and large surface area Additionally, the unique combination of Co-Ni-Fe in spinel oxides derived from calcination of the hierarchically structured core-shell MOF provides a favorable electronic environment for the adsorption of OER intermediates, which was further verified by the XPS characterizations and DFT calculations DFT study revealed the Ni-Co coordinated Oh sites in the MFe2O4 reverse spinel structures as the main active sites, which tuned the binding strength of oxygen species with a catalyst through electron transfer of Fe→Co→Ni, thereby lowered the energy barriers for OER As a result, the rationally designed CoNiFeOx-NC catalyst manifests superior OER performance with a low overpotential of 265 mV at 50 mA cm−2 and a decent Tafel slope of 641 mV dec-1 The ion-exchange based strategy may serve as a versatile platform for rational design and synthesis of multi-metallic MOF derived electrocatalysts

Zixi Kang

Papers

Hierarchical trimetallic Co-Ni-Fe oxides derived from core-shell structured metal-organic frameworks for highly efficient oxygen evolution reaction