UC Davis

With the global market transitioning from fossil fuels to more carbon neutral methods of energy harvesting and storage such as alternative fuel production and solar cell engineering, this work discusses two methods for improving energy harvesting (via photovoltaic solar energy absorption) and storage (via synthesized catalysts which require relatively low applied voltage to produce fuels including hydrogen gas and liquid formate). The catalysts produced for this study include a novel application of the addition of external ligands and consequently a secondary coordination sphere (SCS) onto an organometallic cluster center to form an electrocatalyst cluster. By incorporating an SCS, preliminary results have shown enhancement of the reaction rate and catalytic activity due to interactions that work to stabilize charged intermediates by improving the efficiency of substrate binding.

Currently, the specific mechanisms responsible for these stabilization effects are not fully understood, but it is most likely related to the increased negative or positive charge character which the clusters take on due to the addition of the SCS ligands. By employing a combination of cyclic voltammetry (CV), constant potential electrolysis (CPE), infrared (IR) spectroscopy, and IR spectroelectrochemistry (SEC), and by using several different cluster compositions with various net charges, we have achieved proper benchmarking of the electrocatalyst system. This type of characterization gives researchers the ability to probe the effects of various SCSs, cluster geometries, and charge environments on the catalytic performance in hydrogen evolution reactions (HERs) and in carbon dioxide reduction reactions (CO2RRs).

A series of six iron carbonyl clusters modified with net positive, net negative, or un-charged ligand SCSs were synthesized and, to deconvolute the thermodynamic and kinetic effects, electrochemically reacted with various organic acid substrates. It was found that in all cases when a phosphine ligand is added to an iron-based organometallic cluster center (Fe4N(CO)12) there is some increase in the reaction rate and turn-over frequency (TOF) with respect to overpotential (η), but in the case where the addition of a ligands creates a net positive charge on the catalyst, this reaction rate improvement is much more significant. While in HER, hydrogen is the desired product, in a CO2RR, a common product is formate and further experimentation is proposed to introduce modified E. coli bacteria to an aqueous CPE cell to further investigate if the formate produced can be further processed by the organic media to produce other useful commodity fuels such as isobutanol or pyruvate bioplastics.

For the improvement of energy harvesting techniques with solar cell devices, recent studies explore a variety of layer materials and deposition methods such as the window layer, back mirror layer, back surface field layer, and perhaps the most important and functional layer, the absorber layer. Sb2Se3 has been proposed as a promising option as the absorber layer used in photovoltaic devices. While many deposition methods have been explored to achieve a conformal and homogeneous film, very little work has been done on Sb2Se3 electrochemical deposition techniques. This work explores a variety of parameters which can be tuned to affect the morphology of the film as well as composition and performance.

The three main parameters explored to deposit Sb2Se3 onto fluorine-doped tin oxide (FTO) substrates include temperature of deposition, pre-deposition surface treatments, and pre- and post-deposition annealing treatments. Many characterization techniques were performed including but not limited to, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), surface photovoltage (SPV) measurements, Ultraviolet-visible (UV-Vis) spectroscopy, and photoelectrochemical (PEC) measurements of these micro/nanoparticle films. These results show that not only was the target film composition, Sb2Se3, achieved, but also it can be improved by a hydrophilization and KMnO4 surface treatments and by a post-deposition annealing treatment during which the deposited films are annealed at 300°C to 400°C for 3 hours in an argon atmosphere in the presence of pristine selenium powder. As each parameter is separately compared to a control sample, the work explores the precise effect of each parameter on the composition, morphology, band gap, and performance of the Sb2Se3 films. Once optimal deposition parameters and treatments were identified, further pre-deposition annealing treatments were performed displaying that one can deposit selenium seeds onto an FTO surface to achieve films with smaller bang baps and even better film thickness, homogeneity, conformality, and overall morphology leading to competitive absorber layers produced via inexpensive electrochemical deposition techniques. A comparison of two different substrate suppliers/FTO characteristics to determine if the deposition or other specifications of the FTO film affect the resulting deposited Sb2Se3 film morphology. Further work is proposed to continue to explore seeding composition, temperatures, and procedural order to further optimize film morphology and performance.