Enhancing and stabilizing α-Fe2O3 photoanode towards neutral water oxidation: Introducing a dual-functional NiCoAl layered double hydroxide overlayer

Hematite (alpha-Fe2O3) is a promising candidate as photoanode for photoelectrochemical (PEC) water splitting. However, it still suffers from low efficiency and serious hydrolysis in neutral pH electrolyte. Herein, NiCoAI layered double hydroxides (NiCoAl-LDH) is successfully in situ constructed onto hematite (alpha-Fe2O3) nanoarrays. The resulting NiCoAl-LDH/alpha-Fe2O3 photoanode exhibits an onset potential as low as 0.55 V vs. reversible hydrogen electrode (RHE) and a significantly enhanced photocurrent density of 2.56 mA cm(-2) at 1.23 V vs. RHE for PEC water oxidation in neutral electrolyte, which is ca. 13, 2 and 1.4 times as high as that of alpha-Fe2O3, NiAl-LDH/alpha-Fe2O3 and CoAl-LDH/alpha-Fe2O3. Comprehensive electrochemical and structural analysis reveal the synergistic effect of Ni and Co should be responsible for the enhanced PEC performance. Ni and Co could cause local structure distortion of MO6 (M = Ni or Co) in NiCoAI-LDH, which (1) improve the conductivity of MA1-LDH, thus enhance the separation and the transfer of photogenerated charges and (2) slightly change the chemical environment of Co and Ni and facilitate the formation of low-valent oxidation states, thus lower the energy barrier of Co2+ or Ni2+ to high oxidation states and accelerate the kinetics of water oxidation. The versatile skeleton and interlayered anions as well as the thicker decoration of NiCoAI-LDH enable NiCoAl-LDH/alpha-Fe2O3 to possess excellent stability in neutral electrolyte. This work reveals the partial substitution of Co (or Ni) with Ni (or Co) in MAI-LDH could produce more efficient materials toward high PEC water splitting and will stimulate further development on ternary LDHs as excellent cocatalysts for other electrode in neutral electrolyte. (C) 2018 Elsevier Inc. All rights reserved.