N. Kumar, K.M. Alam, M.M. Rana, S. Chitti, R. Pambala, S.S. Aare, K. Shankar
University of Alberta,
Canada
Keywords: donor-acceptor compounds, carrier dynamics characterization, organic photovoltaics, green hydrogen production
Summary:
Liquid carriers of hydrogen gas constitute an effective means to transport H2 and utilize it at the site of demand. This is a particularly pressing issue because of the low energy density of H2 gas, its high combustibility and diffusivity, and the resulting challenges in the transport and storage of hydrogen. Methanol is a nearly ideal hydrogen carrier because of its high gravimetric density, its low cost and its sourcing from both natural (e.g. biomass) and industrial processes. In this report, we share our success in using a small molecule organic semiconductor to directly generate hydrogen from methanol using sunlight as the energy source. Organic semiconductors (OSCs) have achieved stunning performance in mass market applications such as organic light emitting diode (OLED) displays and chemobiological sensors. Organic photovoltaics (OPV) and organic field effect transistors (OFETs) are being commercialized for renewable energy and printed electronics applications respectively. OSCs can be broadly classified into (1)Small molecules and (2) Polymers. Small molecule organic semiconductors are easy to synthesize and purify but require expensive and low throughput vacuum deposition to form thin films in order to achieve the desired optoelectronic properties. Polymeric organic semiconductors are associated with more complex synthesis and purification steps but are typically more compatible with solution processing and achieve excellent optoelectronic performance in solution processed thin films. Another key technical issue is that the narrow electronic bandgaps demanded in photocatalysis and photovoltaics to optimally harvest a broad swathe of the solar spectrum, are significantly easier to achieve in conjugated polymers compared to small molecules. We introduce functionalized benzoselenadiazole (BSe) organic semiconductors with narrow bandgaps and high photochemical stability that are highly dispersible in common organic solvents. BSe-based OSCs benefit from the introduction of highly polarizable selenium atoms into the conjugated heteroaromatic ring. BSe-based compounds were synthesized by simple 1-3 step reactions involving Suzuki or Stille coupling of widely available precursor compounds. They were purified using column chromatography and their molecular structure was validated using 1H NMR spectroscopy. The vibronic absorption peaks and absorption edge at ~600 nm were found to agree with the results of density functional theory quantum chemical simulations. BSe-based compounds exhibited a strong red fluorescence and excited state lifetimes in the range of a few nanoseconds. From pure methanol suspensions, BSe-based OSCs generated H2 at a rate of 24 μmol g–1 h–1 under AM1.5G one sun illumination, which increased 6-fold to 144 μmol g–1 h–1 when BSe was decorated with Pt nanoparticles in conjunction with the addition of triethanolamine (TEOA) hole scavenger to methanol. The decoration by Pt nanoparticles and addition of TEOA reduced carrier recombination losses and improved the photocatalytic product yield. Time-resolved microwave conductivity was used to evaluate the carrier dynamics in the synthesized BSe compounds. These results point toward a way forward to generate large quantities of green hydrogen from methanol through photocatalysis using non-exotic and mass producible small molecule organic semiconductors.