In situ Grown Ni phosphate@Ni₁₂P₅ Nanorod Arrays as a Unique Core–Shell Architecture: Competitive Bifunctional Electrocatalysts for Urea Electrolysis at Large Current Densities

Abstract

It is still a big challenge to develop active, stable, and easy-to-make bifunctional non-noble electrocatalysts for upshifting overall urea-assisted water splitting toward practical environmental applications at large current densities with lower cell voltages. In response, here we report a competitive bifunctional electrocatalyst that can catalyze both the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) by fabricating the in situ grown Ni phosphate (shell)-anchored Ni12P5 nanorod (core) arrays on the 3D Ni foam skeleton (named as Ni12P5/Ni-Pi/NF). Benefiting from the unique hierarchical core–shell nanorod architecture with abundant exposed active sites and improved electron and mass transfer efficiency, such elaborate binder-free arrays could act as a robust 3D UOR anode and can achieve 900 mA cm–2 only at potentials of 1.378 V in 1.0 M KOH with 0.5 M urea. Additionally, this electrode also shows remarkable cathodic HER catalytic activities. Moreover, when constructing an alkaline electrolyzer using the bifunctional electrodes, the integrated system is capable of delivering the current density of 500 mA cm–2 stably for over 6 h at a cell voltage as low as 1.662 V, which is 287 mV less than that for pure water splitting. As such, our result may become a significant step in developing an industrial electrolyzer for meaningful massive electrocatalytic hydrogen (H2) production by urea-assisted water splitting.

A three-dimensional nanostructure of NiFe(OH)X nanoparticles/nickel foam as an efficient electrocatalyst for urea oxidation

“…6 Generally speaking, the water electrolysis strategy is one of the most efficient ways to produce hydrogen energy, which is composed of the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) [7][8][9][10] and is sought aer and loved by many researchers. 11,12 However, as the most important half-reaction of the anode of water electrolysis, oxygen evolution reaction (OER) has sluggish kinetics and is complicated by a four-electron transferring process (4OH À / 2H 2 O + O 2 + 4e À ), which seriously hinders the conversion and application of hydrogen energy process. 13 The urea oxidation reaction (UOR), which was also an anode reaction, has a 6-electron transmission process (CO(NH 2 ) 2 + 6OH À / N 2 + 5H 2 O + CO 2 + 6e À ).…”

Co(OH)2 Nanosheets Array Doped by Cu2+ Ions with Optimal Electronic Structure for Urea‐Assisted Electrolytic Hydrogen Generation

“…Since some Ni-based materials also have good HER catalytic behaviors, they were successfully applied in overall urea electrocatalysis for concurrent H 2 production, spanning from oxide, [11] hydroxide, [12] sulfide, [13] selenide, [14] and phosphide. [15,16] Despite of the current development of the catalysts for overall urea electrolysis, the breakthroughs are expected to be successfully addressed for exploring new catalysts with high performance toward real application. Considering the potential for the oxidation of Co 2 + to Co 3 + is lower than that of Ni 2 + to Ni 3 + , the Co-based materials are more likely favorable for UOR electrocatalysis.…”

“…It was found that nickel‐based materials can be electrochemically oxidized to generate Ni 3+ ions as the active centers for a following chemical oxidation of urea, [9,10] hence, great endeavors have been devoted in pursuit of various Ni‐based catalysts for effective UOR electrolysis. Since some Ni‐based materials also have good HER catalytic behaviors, they were successfully applied in overall urea electrocatalysis for concurrent H 2 production, spanning from oxide, [11] hydroxide, [12] sulfide, [13] selenide, [14] and phosphide [15,16] …”

Progress and Perspectives in Photo‐ and Electrochemical‐Oxidation of Biomass for Sustainable Chemicals and Hydrogen Production

“…Though numerous photocatalytic systems have been reported for overall water splitting, solar-to-hydrogen (STH) efficiencies of powder photocatalytic and photoelectrochemical systems using earth abundant materials remain below ≈5%. [315,316] To a large extent this is due to the [285] 2D Ni-MOF nanosheets, [281] Ni 2 P NF/CC, [295] HS-Ni 2 P/ Ni 0.96 S/NF, [293] Ni(OH) 2 -nanomeshs, [296] NiIr-MOF/NF, [297] NiTe-Ni(OH) 2 , [292] P-CoS 2 /Ti, [298] Ni(OH) 2 nanoflakes, [299] RuO 2 , [300] Ni/Co, [301] NiClO-D, [284] NF/NiMo-Ar, [289] CoMn/CoMn 2 O 4 , [291] (Ni-WO 2 )@C/NF, [302] Ni-NiO-Mo 0.84 Ni 0.16 , IrO 2 , [293] Ni 12 P 5 /Ni-Pi/NF, [300] Ni/C, [303] NF-NiMoO-H 2 , [289] Pt/C. [289] Detailed information about experimental conditions are summarized in Table S14 of the Supporting Information.…”