Bifunctional Porous NiFe/NiCo₂O₄/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting

Abstract

3515wileyonlinelibrary.com OER and HER catalysts based on the earth-abundant fi rst row transition metals (Fe, Co, and Ni, etc.) has received extensive research interest. [9][10][11][12][13] Although signifi cant progress has been achieved, great challenges remain for nonprecious catalysts to achieve activity and stability that are comparable to conventional precious metals. To this end, one promising approach is to develop multimetallic/carbon catalysts by taking advantage of abundant metal-metal and metal-carbon synergistic interactions to enhance the performance of nonprecious catalysts. [14][15][16] The NiFe (oxy)hydroxide-based catalysts are regarded as one of the best performing nonprecious OER electrocatalysts in alkaline solutions, [17][18][19][20][21] which can be synthesized by coupling with carbon nanotube or graphene to achieve better conductivity and synergistic effects via hydrothermal, [ 19,22 ] or electrodeposition onto gold, glassy carbon, and nickel foam, etc., conductive current collector substrates. [ 19,23,24 ] The high OER activities of NiFe catalysts are generally attributed to a strong synergistic effect upon the incorporation of Fe, even in trace amount, into NiOOH, although the complete mechanisms and structural characteristics are not yet fully understood. [ 19,23 ] NiFe catalysts also have been reported for HER in alkaline media, [ 25 ] although the synergistic effect for HER is not as signifi cant as that for OER, and the reported activity for HER is relatively low compared to state-of-the-art nonprecious HER catalysts. Nevertheless, using the same catalyst as both the anode and cathode in an electrolysis device is very attractive, which could not only signifi cantly improve the integration and simplifi cation of the water splitting system, but also provide the feasibility of industrial application of water splitting technology.Besides having an effi cient catalyst, rational design of catalyst structure is known to be crucial for improving the electrode performance. In pursuit of creating large surface area and high active site density, various attempts have been devoted to developing 3D nanostructured catalyst materials, [26][27][28] such as mesoporous NiFe nanosheets, [ 24 ] NiCo 2 O 4 nanosheets/halloysite nanotubes, [ 26 ] hierarchically structured carbon microfi bre, [ 29 ] and multilayered TiO 2 nanowire arrays. [ 30 ] Moreover, smart design of a catalyst with macroscopic structure onto conductive porous support (e.g., nickel foam) could afford Bifunctional Porous NiFe/NiCo 2 O 4 /Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Effi cient Whole Cell Water SplittingChanglong Xiao , Yibing Li , Xunyu Lu , and Chuan Zhao * A 3D hierarchical porous catalyst architecture based on earth abundant metals Ni, Fe, and Co has been fabricated through a facile hydrothermal and electrodeposition method for effi cient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The electrode is comprised of three levels of porous structures including the bottom su...

Straightforward synthesis of nitrogen-doped carbon nanotubes as highly active bifunctional electrocatalysts for full water splitting

“…In recent years, clear progress has been made in the development of efficient electrocatalysts with earth-abundant materials for HER [13,14] and OER [3,4,15] in alkaline media. Substantial research efforts have been put on developing versatile bifunctional catalysts with satisfying activity toward both HER and OER, enabling operation in the same electrolyte [8,16]. However, developing such active bifunctional electrocatalysts is challenging due to the incompatibility of pH ranges in which catalysts are stable and active.…”

“…However, developing such active bifunctional electrocatalysts is challenging due to the incompatibility of pH ranges in which catalysts are stable and active. In this view, various efficient bifunctional catalysts based on transition-metal based compounds of Ni [6,8,[17][18][19][20], Co [4,16,21] and doped heteroatoms [22][23][24] in alkaline media have been proposed. However, the transition metal oxide materials suffer from intrinsically low conductivity limiting their performance at high current densities.…”

“…Water electrolysis is a promising method for storing intermittent electrical energy from renewable resources, such as sun and wind, in the form of hydrogen fuel [3]. Electrochemical water splitting in alkaline media includes two half reactions: the hydrogen evolution reaction (HER, 4H 2 have shown high activity toward OER in acidic and alkaline media [4,5], and among the non-noble metal OER electrocatalysts, nickel (Ni) based electrocatalysts have shown very promising performance for OER in alkaline media [6][7][8][9][10]. For HER, Pt based materials are known as the most efficient HER catalysts in both acidic and alkaline media [4,11].…”

Acid promoted Ni/NiO monolithic electrode for overall water splitting in alkaline medium

“…NF, a widely used electrode that serves as a promising substrate for the support of many catalysts, possesses unique features such as good electrical conductivity, intrinsic strength, corrosion resistance, high surface area and low cost [26,32]. However, the intrinsic activity of metallic nickel towards HER or OER under alkaline conditions is not so good.…”

“…As a result, the pulverized catalysts tend to peel off from the substrate especially in a working circumstance where lots of hydrogen and oxygen would continuously bubble up, leading to gradually decreased catalytic efficiency as well as limited life span [25]. In comparison, growing the catalysts directly onto the conductive substrate such as Ni foam (NF) [26,27], carbon cloth [28,29], carbon fiber paper [30] and Ti foil [31] can mitigate the problem of falling off and contact to some extent. However, the matter of the contact interface between electrocatalysts and the current collector as well as pulverization issues is still not fully addressed.…”