Origin of Photoelectrochemical Generation of Dihydrogen by a Dye-Sensitized Photocathode without an Intentionally Introduced Catalyst

Dye-sensitized photocathodes have been observed on several occasions to sustain light-driven H-2 generation without intentionally introduced catalysts. Herein, plausible mechanisms addressing this phenomenon are probed by a combination of long-term photoelectrochemical measurements with concurrent gas chromatography, transient absorption spectroscopy, and inductively coupled mass spectrometry using a perylenemonoimide-sexithiophene-triphenylamine (PMI-6T-TPA) sensitized NiO electrode. The experimental evidence obtained discounts the possibility for direct reduction of hydrogen by the dye and demonstrates that the availability of interfaces between dye molecules and any electrically disconnected NiO particles exposed to the electrolyte solution is critical for photoelectrocatalytic H-2 generation. These interfaces are postulated to serve as photoactive sites for the formation of a hydrogen evolution catalyst, e.g., metallic nickel, which can accept photogenerated electrons from the excited dye molecules. The Ni-0 catalyst can form via photoelectroreduction of Ni2+, which has been found to slowly dissolve from the NiO support into the solutions during the photoelectrochemical measurements. Additionally, dependence of the H-2 generation rate on the anion within the electrolyte has been identified, with the highest rates of 35-40 nmol h(-1) cm(-2) achieved with acetate. The origin of this dependence remains unsolved at this stage but is clearly demonstrated to be not associated with the different rates of dissolution of NiO, the presence of other transition metal contaminants, nor electronic impacts of the anion on the NiO valence band. Overall, the results herein demonstrate that the effects of the chemical nature of the electrolyte, metallic nickel deposited from dissolved Ni2+, and availability of the interfaces between disconnected NiO and adsorbed dye should be considered when interpreting the photoelectrocatalytic performance of dye-sensitized photocathodes for dihydrogen evolution.