In situ modification of Cu substrate enables nickel-copper phosphide nanoarrays for enhanced electrocatalytic hydrogen evolution

https://doi.org/10.1016/j.ijhydene.2022.08.261Get rights and content

Highlights

  • A low-temperature oxidation strategy to in-situ modify copper foam (CF).

  • The NiP2/Cu3P heterogeneous interface facilitated the HER ability.

  • The prepared NiCuP@CF required 113.4 mV@10 mA cm−2 toward HER.

  • The NiCuP@CF exhibited outstanding long-term durability for at least 36h.

Abstract

Searching for non-noble and high active electrocatalysts with excellent durability for hydrogen evolution reaction (HER) is significant for hydrogen production but remains a grand challenge. Here, we report a low-temperature oxidation strategy to modify copper foam (CF) and successfully built a heterogeneous NiP2-Cu3P nanoarray on the modified CF (NiCuP@CF), in which the CF functions as both conductive support and the precursor of the active substances. The resulting catalyst achieves low overpotentials of 113.4 and 266.4 mV to reach the current density of 10 and 100 mA cm−2 towards HER in alkaline electrolyte, respectively, benefiting from the 3D heterostructure arrays with abundant active sites and excellent conductivity. The open channels formed spontaneously in the 3D arrays allow rapid H2 bubbles to escape, which enables the catalyst to exhibit remarkable stability for at least 36 h under different current densities. This work presents a facile and economic modification method for Cu substrate, which will provide a good foundation for the subsequent material optimization, not limited to the heterogeneous metal phosphide catalyst toward HER.

Graphical abstract

The heterointerface between NiP2 and Cu3P is constructed on the in-situ modified copper foam, which is well-controlled via tuning temperature. The electron structure of the sample prepared at 276 K is optimized and thus endows it excellent HER performance.

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Introduction

The ever-increasing energy and environmental issues require the research of clean and sustainable energy sources [1,2]. Hydrogen (H2), as a green and renewable energy, has outstanding advantages of convenient storage and transportation, high mass-energy density, and harmless combustion product of H2O [[3], [4], [5], [6], [7], [8], [9]]. Among many hydrogens production methods, electrochemical water splitting is a green and sustainable process with high purity, which can directly decompose water into oxygen and hydrogen. The applied electric energy can be from solar power, hydropower, power grid, or wind power [3,7,8,10,11]. Noble metal-based materials are efficient electrocatalysts for hydrogen production from water splitting [12], but their high cost and low abundance have limited their large-scale application. Therefore, it is essential to search for efficient and non-noble metal-based electrocatalysts to promote HER [[13], [14], [15]].

Generally, the electrocatalysts for hydrogen production from electrolyzed water can be classified into two categories: powder catalyst and monolithic catalyst. The powdered catalyst requires the addition of a binder (Nafion or PTFE) to immobilize the powder on the working electrode [16,17]. This would increase the resistance of catalysts and cover part of active sites, leading to reduced electrocatalyst performance. On the contrary, the monolithic catalyst, of which the active components are directly grown on the conductive substrate (such as conductive carbon cloth [17,18], nickel foam [[19], [20], [21], [22]], and copper foam [23,24]) can avoid the usage of a binder. Particularly, compared with one-dimensional and two-dimensional nanostructures arrays, three-dimensional (3D) self-supporting monolithic catalytic materials have a large specific surface area, more accessible active sites, favorable diffusion, and transportation, which could promote the overall water splitting electrochemical performance [23,[25], [26], [27], [28]]. Of all the 3D self-supporting electrochemical materials, the copper foam has higher conductivity than that of nickel foam and carbon cloth. Copper has the advantages of low cost and rich in reserves, therefore suiting the electrocatalytic material substrate [29]. For the past few years, researchers have devoted themselves to the synthesis of copper foams based electrocatalysts with specific morphology by using electrodeposition or hydrothermal method [[29], [30], [31], [32], [33], [34]]. However, there are few studies on using copper foam as an efficient hydrogen evolution electrode from water splitting.

Recently, transition metal phosphides (TMPs) have played a vital role in hydrogen generation by water splitting [18,[35], [36], [37]]. Previous density functional theory (DFT) calculation has revealed that the P atom can serve as proton-acceptor centers for adsorbing protons, therefore facilitating the electrochemical HER activity to drive water splitting [[38], [39], [40]]. Sun's group have reported a fabrication of Cu3P nanoarrays on copper foam by a low-temperature phosphating reaction [41]. The Cu3P cathode maintained its activity for at least 25 h and exhibited an overpotential of 143 mV to reach 10 mA cm−2 in acid electrolyte. However, oxygen evolution reaction (OER) catalyst has higher activity in an alkaline environment, while HER catalyst is more inclined to acidic conditions. Therefore, it would be more meaningful to study the efficient metal phosphide electrode for hydrogen evolution in an alkaline medium.

In addition, the construction of bimetallic or polymetallic phosphates can lead to the redistribution of valence electrons and provide more active sites than monometallic phosphates, which could reduce the energy barrier in the process of HER. The CoP/CoMoP nanocages electrode reported by Wang's group show high HER activity over a wide pH range [42]. At the current density of 10 mA cm−2, the CoP/CoMoP electrode had a low overpotential of 72 mV, 44 mV, and 151 mV in alkaline, acidic and neutral environments, respectively. Previous reports have indicated that benefited by the similar lattice parameters, Ni (4s2 3d8) and Cu (4s1 3d10) can be easily blended to form NiCu alloy, which will be conducive to the overall electrocatalytic activity [34]. Because the doping of Ni ions can change the d-band center of Cu ions and improve the HER performance of Cu-based catalysts [43].

It has been reported that in-situ oxidation of the Cu substrate using a strong oxidant in an alkaline environment can produce a series of Cu(OH)2 nanowires [44]. Inspired by the previous study, herein, inspired by the previous study, herein, we designed a heterogeneous Ni2P-Cu3P nanoarray built on porous copper foam for hydrogen evolution under alkaline conditions, in which a low-temperature wet-chemical oxidation strategy was used to in situ synthesize a series of vertical Cu(OH)2 nanowires as precursors. Specifically, the copper foams were directly soaked in the solution containing ammonium persulfate and sodium hydroxide (see details in Experiment section) at four temperatures (271 K, 276 K, 285 K, and room temperature, 298 K) and served as the precursor to load Ni. After low-temperature phosphating, the final samples were labeled as NiCuP@CF-271 K, NiCuP@CF-276 K, NiCuP@CF-285 K, and NiCuP@CF-298 K (room temperature) according to the oxidation temperature at the first synthesis steps. Among them, NiCuP@CF-276 K exhibits outstanding HER activity and is superior to other NiCuP samples, which merely requires a low overpotential of 113.4 mV and 266.4 mV to reach the current density of 10 mA cm−2 and 100 mA cm−2, and shows outstanding stability for at least 36 h. Such adjusted Cu substrate enables nickel-copper phosphide nanoarrays for enhanced HER electrocatalytic. And the heterostructure composed of NiP2 and Cu3P phases can adjust the electronic structure and optimize the adsorption capacity on active sites, thereby facilitating the catalyst ability [45].

Section snippets

Chemical reagents

All the reagents are analytically pure. The copper foam (1 × 3 cm2, thickness: 1 mm 550 g m−2) was successively cleaned with 1 M hydrochloric acid, ethanol, and deionized water respectively, and each step was under sonication for at least 30 min to remove the oxide and oil on the surface.

Synthesis of Cu(OH)2 nanowires precursor

Cu(OH)2 NWs were prepared by chemical oxidation at different temperatures. According to the reaction of formula 2.1, the cleaned copper foam was immersed in a 20 mL deionized water solution containing 0.2 mmol

Result and discussion

3D heterogenous phosphide NiP2-Cu3P nanoarrays were successfully fabricated on the modified copper foam substrate. The fabrication process is shown in Fig. 1a. NiCuP@CF-271 K, NiCuP@CF-276 K, NiCuP@CF-285 K, and NiCuP@CF-298 K are labeled according to different oxidation temperatures for the synthesis of Cu(OH)2 nanowires. The crystal phase structure of the synthesized catalyst was analyzed by X-ray diffraction (XRD). As shown in Fig. 1b, the three characteristic strong peaks of NiCuP@CF-276 K

Conclusion

In conclusion, 3D NiP2/Cu3P heterostructure grown on copper foam with excellent HER performance has been achieved via pretreatment of Cu substrate. The shifted binding energy from XPS results confirm this multi-scale heterojunction regulation strategy can facilitate electron transfer and realize electron redistribution. Thus, the optimum NiCuP@CF-276 K exhibits excellent electrochemical hydrogen evolution property with a low overpotential of 113.4 mV and 266.4 mV at the current density of

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the Foundation of Basic and Applied Basic Research of Guangdong Province (2019B1515120087) and the Guangdong Provincial Key Laboratory of Plant Resources Biorefinery (2021B121204011). We would like to thank the Analysis and Test Center of Guangdong University of technology for the test of XPS, HRTEM, and UPS.

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    These authors contributed equally to this work.

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