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

Abstract

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 Ni 2+ , 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 Ni 2+ , 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. × Our colleague, mentor and friend Prof. Leone Spiccia has passed away before this work could be published. Leone was an outstanding researcher and personality, and is sadly missed. ABSTRACTDye-sensitized photocathodes have been observed on several occasions to sustain light-driven H2 generation without intentionally-introduced catalysts. Herein, plausible mechanisms addressing this phenomenon are probed by a combination of long-term photo-electrochemical measurements with concurrent gas chromatography, transient absorption spectroscopy and inductively coupled mass spectrometry using a perelenmonoimid-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 photo-electrocatalytic H2 generation. These interfaces are postulated to serve as photo-a...

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

“…These strategies are also supported by a clearer view on the mechanisms and limitations of the approach, as addressed in this work, and proposals to circumvent them. In particular, the origin of the low faradaic efficiency towards HER observed here and in previous studies engaging NiO-based photocathodes 21 , 22 , 73 is related to the reduction of traces of O 2 trapped in the mesoporous NiO film and to partial reduction of NiO itself. The latter hypothesis, initially proposed by us, 22 was recently confirmed by Simonov and colleagues.…”

“…The latter hypothesis was previously assessed by us 22 and was recently confirmed by Simonov and coll. 73 …”

“…The latter hypothesis, initially proposed by us, 22 was recently confirmed by Simonov and colleagues. 73 If higher photocurrent densities are reached, these competitive processes may become minor phenomena. However, solutions may as well come from smart coverage of the NiO substrate using atomic layer deposition (ALD) of passivating oxides or alternative surface treatments, including non-hydrolytic sol–gel deposition, 115 already exploited to enhance performances of other metal oxide-based photoelectrodes.…”

Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications

“…One should also keep in mind that photocurrent could also result from the formation of Ni(0) species via reduction by PB6. 79 Since the NiO-Sil-PB6 electrodes could produce photocurrent, we then wanted to demonstrate that the PB6generated after hole injection could transfer an electron to an acceptor molecule. To test this, NiO-Sil-PB6 electrodes were examined in the presence of an irreversible electron acceptor, chloropentaaminecobalt(III) chloride.…”

H2-Evolving Dye-Sensitized Photocathode Based on a Ruthenium–Diacetylide/Cobaloxime Supramolecular Assembly

“…[36] This yield is relatively low but is in the range of state-of-the-art NiO-based dye-sensitized H2evolving photocathodes. [30] Competing reactions, such as the NiO reduction, [37,52] or reduction of the oxygen formed in the anodic compartment (which might diffuse during the course of long-term experiments), might be considered. A slow decrease of the photocurrent (from 2.5 to 1 A.cm -2 ) is also observed during the course of the 4.5 hours measurement, which can be attributedat least partlyto some leaching of the dye-catalyst assembly in the electrolyte.…”