Hector F. Garces

Growth control of compact CH3NH3PbI3 thin films via enhanced solid-state precursor reaction for efficient planar perovskite solar cells

CH3NH3PbI3 (MAPbI(3)) perovskite thin films that are solution-processed using either a one-step or two-step conventional method typically contain a significant number of defects (voids, pinholes) or PbI2 impurities, which have a detrimental effect on the performance of planar perovskite solar cells (PSCs) fabricated using those films. To overcome this issue, we show that enhancement of the solid-state reaction between inorganic-organic precursors is an effective route for the growth of compact, phase-pure MAPbI(3) perovskite thin films with no voids or pinholes. To ensure uniform solid-state conversion (MAI + PbI2 -> MAPbI(3)) across the entire film thickness, a new successive spin coating/annealing (SSCA) process is used, where MAI is repeatedly infiltrated into a nanoporous PbI2 film, followed by thermal annealing. The mechanisms involved in the SSCA process are elucidated by monitoring the evolution of the phases during the reaction. Owing to these desirable characteristics (high-purity, full-coverage, enhanced smoothness and compactness) of the SSCA MAPbI(3) films, planar PSCs based on these perovskite thin films delivered a maximum power conversion efficiency (PCE) close to 15%. Furthermore, PSCs fabricated using partially converted nanoporous PbI2 thin films delivered a surprising PCE approaching 10%, suggesting continuous MAPbI(3) phase formation throughout the entire film at each spin coating/annealing process. The advantages gained from enhancing the solid-state precursor reactions allow better control of the growth of the perovskite making the SSCA process more robust.

Thin-Film Transformation of NH4PbI3 to CH3NH3PbI3 Perovskite: A Methylamine-Induced Conversion–Healing Process

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

Influence of Formic Acid Impurity on Proton Exchange Membrane Fuel Cell Performance

The effect of trace amounts of formic acid (HCOOH) in hydrogen fuel on proton exchange membrane fuel cell (PEMFC) performance is reported. Long-term stability tests (100 h), periodic cyclic voltammetry scans, and electrochemical impedance spectroscopy analyses are used to evaluate and characterize the effects of this impurity on fuel cell performance. The results show that trace amounts of HCOOH cause degradation in fuel cell performance and significantly contaminate the electrodes. Furthermore, full recovery from the contamination could not be achieved by applying pure hydrogen to the anode while operating the fuel cell. However, this degradation may also be caused by the coarsening or dissolution of Pt, in addition to any permanent effects of HCOOH contamination. Mechanisms of contamination of the electrodes and performance degradation of the PEMFC are also postulated.

Lithium-ion battery electrolyte mobility at nano-confined graphene interfaces.

Interfaces are essential in electrochemical processes, providing a critical nanoscopic design feature for composite electrodes used in Li-ion batteries. Understanding the structure, wetting and mobility at nano-confined interfaces is important for improving the efficiency and lifetime of electrochemical devices. Here we use a Surface Forces Apparatus to quantify the initial wetting of nanometre-confined graphene, gold and mica surfaces by Li-ion battery electrolytes. Our results indicate preferential wetting of confined graphene in comparison with gold or mica surfaces because of specific interactions of the electrolyte with the graphene surface. In addition, wetting of a confined pore proceeds via a profoundly different mechanism compared with wetting of a macroscopic surface. We further reveal the existence of molecularly layered structures of the confined electrolyte. Nanoscopic confinement of less than 4–5 nm and the presence of water decrease the mobility of the electrolyte. These results suggest a lower limit for the pore diameter in nanostructured electrodes.

Epoxidation of cyclopentene by a low cost and environmentally friendly bicarbonate/peroxide/manganese system

The system hydrogen peroxide/sodium bicarbonate/manganese sulfate was used for the first time to epoxidize cyclopentene. Effects of parameters such as type and amount of solvent, ratio of hydrogen peroxide and manganese sulfate to cyclopentene, presence of additives, and reaction time and temperature on the selectivity to cyclopentene oxide were evaluated. Gas chromatography was used to quantify residual cyclopentene and produced cyclopentene oxide using the internal standard method. Type and amount of solvent, addition method, and temperature were important factors to increase the selectivity to cyclopentene oxide. Unlike previous reports on epoxidation of different substrates, additives like sodium acetate and salicylic acid did not improve the selectivity to cyclopentene oxide. One time, single-step addition of hydrogen peroxide/sodium bicarbonate to the solution of cyclopentene/solvent/manganese sulfate produced more cyclopentene oxide than stepwise addition. The maximum selectivity obtained was 56%, possibly due to the high reactivity of cyclopentene that causes the formation of oxidation products different to cyclopentene oxide, which were not detected in the analyzed phase.