Zeolitic nickel phosphate nanorods with open-framework structure (VSB-5) for catalytic application in electro-oxidation of urea

Highlights

• Microporous and crystalline nickel phosphate nanorods are prepared hydrothermally.

• Nickel phosphate nanorods with zeolitic VSB-5 structure are grown on 3D nickel foam.

• Lithium dihydrogen phosphate affects the morphology of nickel phosphate nanorods.

• VSB-5 improves current density, onset potential, and efficiency in urea electrolysis.

Abstract

Microporous and crystalline nickel phosphate nanorods with VSB-5 (Versailles-Santa Barbara-5) structure were uniformly grown around the skeleton of nickel foam under hydrothermal condition in the presence of lithium dihydrogen phosphate. The VSB-5 nanorods exhibited open-framework structure, nanoporosity with a pore diameter of about 1 nm, and typical molecular sieves with zeolitic properties, making them a promising catalyst material for urea oxidation reaction (UOR). A catalyst layer of reticulated VSB-5 nanorods grown on the nickel foam could provide abundant micro-, meso-, and macro-scale pores for hosting electrolyte and expediting the movement of electron, electrolyte, and gaseous products. Electrochemically active surface area (EASA) of the VSB-5 electrode with unique pore structure was considerably greater than that of the nickel hydroxide electrode with granular particles and dense interior. Large EASA and high valence state of nickel species accounted for the superior electrocatalytic performance of the VSB-5 electrode towards UOR compared with the nickel hydroxide electrode. The VSB-5 electrode exhibited higher current density (160 mA cm⁻²) at 0.6 V vs. SCE (saturated calomel electrode) and lower onset potential (0.27 V vs. SCE) than the nickel hydroxide electrode (75 mA cm⁻² and 0.28 V) obtained from the cyclic voltammograms.

Introduction

Urea-containing wastewater from fertilizer industry, agricultural nutrient source, and human/animal urine may cause detrimental impact on the environment owing to the ammonia emission to the atmosphere through the hydrolysis of urea. Several approaches have been proposed to remedy the wastewater with different concentrations of urea. Most of them need many sophisticated devices and expensive chemicals. The electro-oxidation of urea has been demonstrated to be one of the most promising strategies since it remedies the urea-containing wastewater and simultaneously yields hydrogen gas through a simple electrochemical reactor [1]. However, the sluggish kinetics of urea oxidation reaction (UOR) may limit the large-scale treatment of urea-containing wastewater. Thus, various catalysts including expensive metals like Pt and Ru and inexpensive Ni have been used to circumvent the poor kinetics of UOR [2]. Among them, nickel material has been shown to be an eligible catalyst in the alkaline medium on account of its low cost and comparable catalytic capability with rare and expensive metals [3], [4], [5], [6], [7], [8].

In nickel-based catalysts, the electrochemical UOR proceeds at the nickel anode-electrolyte interface in an electrolytic cell, which could be boosted by Ni³⁺ species in the nickel-based catalysts through the electrochemical-catalytic (EC’) mechanism expressed as follows using nickel hydroxide as an example [5],

6Ni(OH)₂(s) + 6OH⁻ ↔ 6NiOOH(s) + 6H₂O(l) + 6e⁻

6NiOOH(s) + CO(NH₂)₂(aq) + H₂O(l) → 6Ni(OH)₂(s) + N₂(g) + CO₂(g)

CO(NH₂)₂(aq) + 6OH⁻ → N₂(g) + 5H₂O(l) + CO₂(g) + 6e⁻

Previous studies have from several laboratories indicated that the Ni²⁺ is electrochemically oxidized to active form (Ni³⁺) through Eq. (1). The Ni³⁺ promotes the UOR and then reduces to inactive Ni²⁺ by urea via Eq. (2). The inactive Ni²⁺ will be electrochemically re-oxidized to Ni³⁺ in anodic process for further oxidation of urea. The overall UOR (EC’) at the anode decomposes the urea to non-toxic products of nitrogen and carbon dioxide through Eq. (3), while hydrogen gas evolves at the cathode as follows

6H₂O(l) + 6e⁻ → 3H₂(g) + 6OH⁻

Though nickel-based catalysts have received considerable advantages in catalyzing UOR, their advances still encounter several challenges such as poor intrinsic conductivity and catalytic activity of some nickel oxide/hydroxide materials. Heteroatom-doping with cobalt, platinum, or rhodium gives rise to high catalytic ability owing to the improvement of intrinsic electronic structure of the host nickel catalysts [7], [9], [10]. A similar effect as the heteroatom doping can be achieved by incorporating conductive additives such as carbon materials to the bulk nickel oxide/hydroxide to form the composite catalysts [6], [11], [12], [13], [14].

Urgent demands for electrocatalysts with superior catalytic capability and low material cost have inspired research into a new class of electrocatalysts. The adjustment of structure and chemical characteristics such as pore distribution, heterostructures, defects, electronic structure, and surface functional groups is of great importance for enhancing the catalytic ability of UOR [15], [16], [17], [18]. The size- and shape-controlled catalysts supported on macroporous nickel foam or carbon aerogel have been shown to alleviate the formation of agglomerate catalyst layer and offer abundant pore conduits to allow reactants and products to go through, leading to improved onset oxidation potential (OOP) and oxidation current density of UOR [18], [19], [20], [21], [22], [23]. The pore size distribution of catalysts plays a key role in UOR. Small pores in catalysts provide large surface area to participate the catalytic reaction, while large pores accommodate large amounts of electrolyte and facilitate the passage of reactants and gas products [24]. Among the porous materials, microporous nickel phosphate is a new class of ordered porous materials featuring large surface area, unsaturated metal coordination sites, and unique frameworks for emerging applications such as hydrogen storage and catalysts [25]. The nickel phosphates named VSB-n (Versailles-Santa Barbara-n) are microporous and crystalline materials with open-framework structures, which are stable over a wide temperature and show typical zeolitic properties [26]. In this work, another zeolitic nickel phosphate with large pore channels, VSB-5, is directly grown on the conductive skeleton of macroporous nickel foam under alkaline hydrothermal conditions. The nickel phosphates with molecular sieve configuration (VSB-5) have been shown to be successful in hydrogen storage, catalysis, and supercapacitor [25], [26], [27], [28]. This unique electrode is rarely studied as a catalyst material in electro-oxidation of small organic molecules until now and thus deserves more detailed investigations for enhanced UOR.