Electrochemical and Photoelectrochemical Properties of Nickel Oxide (NiO) With Nanostructured Morphology for Photoconversion Applications

Abstract

The cost-effective production of chemicals in electrolytic cells and the conversion of the radiation energy into electrical energy in photoelectrochemical cells (PECs) require the use of electrodes with large surface area, which possess either electrocatalytic or photoelectrocatalytic properties. In this context nanostructured semiconductors are electrodic materials of great relevance because of the possibility of varying their photoelectrocatalytic properties in a controlled fashion via doping, dye-sensitization or modification of the conditions of deposition. Among semiconductors for electrolysers and PECs the class of the transition metal oxides (TMOs) with a particular focus on NiO interests for the chemical-physical inertness in ambient conditions and the intrinsic electroactivity in the solid state. The latter aspect implies the existence of capacitive properties in TMO and NiO electrodes which thus act as charge storage systems. After a comparative analysis of the (photo)electrochemical properties of nanostructured TMO electrodes in the configuration of thin film the use of NiO and analogs for the specific applications of water photoelectrolysis and, secondly, photoelectrochemical conversion of carbon dioxide will be discussed.

Keywords: metal oxide nanostructures; nickel oxide nanoparticle; photoelectrochemical cells; photoelectrochemistry; semiconductor nanostructures; solar energy conversion.

Figure 1

Cyclic voltammetry of NiO prepared via conventional sintering of sprayed NiO nanoparticles (average diameter, ø: 50 nm). Electrolyte composition: 0.7M LiClO₄ in anhydrous propylene carbonate; counter electrode: Li; reference electrode: Li⁺/Li; scan rate: 15mV s⁻¹. NiO thickness: 0.3 μm. Adapted from Awais et al. (2013b).

Figure 2

Cyclic voltammetries of a NiO film (l: 4 μm) at the scan rate of 10mV s⁻¹ in two different electrolytes (red curve: 0.2M LiClO₄ in acetonitrile; blue curve: 0.2M LiI and 0.02 M I₂ in acetonitrile). Potential values are referred to the redox couple Ag/AgCl. Top: voltammograms in the full scale of current; bottom: zoom of the two voltammograms in correspondence of the onset of NiO (red profile) and I⁻ (blue profile) oxidations. Reproduced with permission from Bonomo et al. (2016b).

Figure 3

Light-driven production of H₂ from a PEC of electrolysis, which employs P1-sensitized NiO as photocathode and the co-catalyst Co1 [a Co(II) complex] dissolved in aqueous electrolyte. H₂ is photogenerated in solution when P1-NiO cathode is illuminated and kept polarized at −0.4 V vs. Ag/AgCl. Reproduced with permission from Zannotti et al. (2015).

Figure 4

Light-driven water photoelectrolysis with production of H₂ at the NiO photocathode and O₂ at the TiO₂ photoanode. The scheme depicts a PEC in which both oxide electrodes are sensitized with supramolecular assemblies with PS, i.e., the photosensitive moiety of the assembly, and Cat, i.e., the catalytic center of the assembly, which execute a process of et upon light excitation of PS. Reproduced with permission from Halpin et al. (2009).

Figure 5

Left: depiction of the photoelectroactive material consisting of p-type NiO nanoparticles decorated with the dye-sensitizer coumarin 343 (PS) and the biomimetic Fe-Fe catalyst (Cat) directly anchored on NiO through the phosphonate group. Right: temporal profiles of the current density associated to H₂ generation as a function of the light-switching time when Fe-Fe Cat [1] is not included (black profile), and when is anchored (red profile). Reproduced with permission from Antila et al. (2016).

Figure 6

Definition of a photocathode based on nanostructured NiO for the selective photoelectrochemical reduction of CO₂ to CO. The photocathode is sensitized by a di-nuclear complex of Ru and Re having the dual function of absorbing visible light and transferring electrons to CO₂. Reproduced with permission from Takeda and Ishitani (2010).

Figure 7

Scheme of the photoelectrocatalytic action exerted by a dye-sensitized NiO cathode decorated with the enzyme carbon monoxide dehidrogenase I toward the reduction of CO₂ to CO. The light absorbing unit is P1 (in red). Reproduced with permission from Bachmeier et al. (2014).