Synthesis of nanostructuredβ-Ni(OH)₂by electrochemical dissolution–precipitation and its application as a water oxidation catalyst

Abstract

A straightforward electrochemical dissolution-precipitation approach has been developed to synthesize nanostructured β-Ni(OH)2 powders (particle size 10-100 nm, specific surface area ∼100 m(2) g(-1)) from Ni metal anodes. The approach differs from existing electrochemical synthesis methods in that it predominantly results in bulk precipitation of nanoparticles, without significant film growth on either of the electrodes. Heat treatment of the as-synthesized β-Ni(OH)2 afforded NiO with mostly preserved nanostructure and very high specific surface area (≤100 m(2) g(-1), depending on calcination temperature). The as-synthesized β-Ni(OH)2 was found to be an excellent catalyst for the oxygen evolution reaction (OER) in the technologically important water electrolysis process, apparently contradicting recent reports that the α polymorph is required for such high activity. With catalyst loadings <0.1 mg cm(-2), OER current densities of 10 mA cm(-2) were sustained at overpotentials as low as 340 mV, with Tafel slopes of only ∼38 mV/decade. The catalyst was highly stable in alkaline media over the course of electrolysis experiments lasting for several hours. This performance surpasses that of many previously reported earth-abundant OER catalysts and is comparable to that obtained with state-of-the-art RuO2 and IrO2 catalysts.

Improved Electrochemical Phase Diagrams from Theory and Experiment: The Ni–Water System and Its Complex Compounds

“…Ni Pourbaix diagrams have been simulated many times over the last fifty years, 1,[30][31][32] where the experimental ∆ f G's are used. However, none of these Ni Pourbaix diagrams are consistent with various electrochemical observations, e.g., NiO and/or Ni(OH) 2 should be stable at pH 5 ∼ 15, [24][25][26]29,[33][34][35][36][37][38][39][40][41][42] while, in those simulated Ni Poubaix diagrams with a moderate aqueous ion concentration ([I] = 10 −6 mol/L), Ni(OH) 2 is only stable at pH 3 of 9 ∼ 13. 1,30,31 These failures have important consequences because solutions with pH > 13 are critically important for the synthesis, characterization, and application of Ni (hydr)oxides.…”

“…[24][25][26]29,[33][34][35]41 Furthermore, solutions with pH < 9 are also used to synthesize Ni(OH) 2 from Ni metal using a recent dissolution-precipitation approach. 42 For these reasons there is a clear technological motivation for possessing accurate Ni Pourbaix diagrams. The apparent inconsistencies suggests inaccuracies in the experimental ∆ f G's and the resultant Ni Pourbaix diagrams derived from them.…”

Vertically Aligned Porous Nickel(II) Hydroxide Nanosheets Supported on Carbon Paper with Long‐Term Oxygen Evolution Performance

“…In this context, considerable research efforts have recently been de-voted to the search for efficient and inexpensive OER catalysts comprising earth-abundant elements, including transitionmetal oxides, [8][9][10] hydroxides, [11][12][13] perovskites, [14,15] phosphates, [16,17] sulfides, [18,19] and recently reported phosphides. [33][34][35][36][37][38][39][40] For example, Stern and Hu compared the OER performance of bulk Ni(OH) 2 films to that of Ni(OH) 2 nanoparticles and found that the Ni(OH) 2 nanoparticles only neededasmall overpotential (h)o f3 00 mV to deliver ac urrent density of 10 mA cm À2 (h 10 ), and thus, they substantially outperformed bulk Ni(OH) 2 films and demonstratedt he benefito fn anostructuring. Previous work on the OER performance of Ni(OH) 2 was primarily focusedo ne lectrodepositeda nd physically depositedt hin films.…”

“…[33] Gao and co-workerss ynthesized a-Ni(OH) 2 hollow spheres consisting of nanosheet assemblies that showed h 10 = 331 mV and aT afel slope as small as 42 mV dec À1 ,v alues superior to those of state-of-the-art RuO 2 catalysts. In the existing reports, water electrolysis based on Ni(OH) 2 nanostructures was either only tested for as hort periodo ftime (a few hours) [35,36] or exhibited ad egradation trend in the OER activity over time. To overcome this limitation, Zhou et al attempted to attach ultrathin b-Ni(OH) 2 nanoplates onto multiwalled carbon nanotubes (MWCNTs), [35] and the resulting composite exhibitede nhanced OER activity.V ery re-cently,W ang and co-workersa lso reported surprisingly high OER activity for crystalline b-Ni(OH) 2 anchored on oxidized MWCNTs,w hich showedah 10 value as low as 270 mV,a ne xtremely small Tafel slope of 32 mV dec À1 ,a nd av ery high turnover frequency (TOF)o f1 6s À1 at h = 350 mV.…”

“…[34] Although nanostructured Ni(OH) 2 is able to provide ag reater surfacea rea and more electrocatalytically active sites to allow for easy access to the electrolyte, the intrinsic electrical conductivity of Ni(OH) 2 is very poor such that fast charge transfer is difficult to achieve. [33,34,37,39] Moreover,m ost Ni(OH) 2 nanostructures reported for use as OER catalysts were produced in the form of powders, [33][34][35][36][37]39] and Ni(OH) 2 nanostructures directly grown on current collectors have been rarely explored. [37] The excellent OER activity was ascribed to as ynergistic effect between Ni(OH) 2 and the MWCNTsa sw ell as the presence of oxygencontaining groups (e.g.,k etone and carboxylate) on the MWCNTst hat facilitated protont ransfer during the OER.…”