Sudesh Kumari

Reduced SnO2 Porous Nanowires with a High Density of Grain Boundaries as Catalysts for Efficient Electrochemical CO2-into-HCOOH Conversion

Electrochemical conversion of CO2 into energy-dense liquids, such as formic acid, is desirable as a hydrogen carrier and a chemical feedstock. SnOx is one of the few catalysts that reduce CO2 into formic acid with high selectivity but at high overpotential and low current density. We show that an electrochemically reduced SnO2 porous nanowire catalyst (Sn-pNWs) with a high density of grain boundaries (GBs) exhibits an energy conversion efficiency of CO2 -into-HCOOH higher than analogous catalysts. HCOOH formation begins at lower overpotential (350 mV) and reaches a steady Faradaic efficiency of ca. 80 % at only -0.8 V vs. RHE. A comparison with commercial SnO2 nanoparticles confirms that the improved CO2 reduction performance of Sn-pNWs is due to the density of GBs within the porous structure, which introduce new catalytically active sites. Produced with a scalable plasma synthesis technology, the catalysts have potential for application in the CO2 conversion industry.

Solar hydrogen production from seawater vapor electrolysis

Solar photovoltaic utilities require large land areas to produce power equivalent to a conventional fossil fuel utility, and also must be coupled to cost-effective energy storage to overcome the intermittency of sunlight and provide reliable, continuous energy generation. To target both of these disadvantages, a method was demonstrated to produce hydrogen fuel from solar energy by splitting seawater vapor from ambient humidity at near-surface ocean conditions. Using a proton exchange membrane electrolyzer with seawater-humidified air at 80% relative humidity at the anode and dry N2 at the cathode, sufficient electrolysis current density was maintained to operate near the maximum energy-conversion point when driven by a triple-junction amorphous Si solar cell of equivalent area. Fluctuations in atmospheric water content were demonstrated to have minimal effect on the solar-to-hydrogen efficiency, remaining ∼6% with relative humidity ≥30%. Direct solar-driven H2 production from seawater vapor was maintained at >4.5 mA cm−2 for >90 hours. The calculated solar-to-hydrogen conversion efficiency before and after 50 h of operation changed from 6.0% to 6.3% using an ambient seawater humidity feedstock as compared to a drop from 6.6% to 0.5% using liquid seawater feedstock.

Intense Pulsed Light Sintering of CH3NH3PbI3 Solar Cells

Perovskite solar cells utilizing a two-step deposited CH3NH3PbI3 thin film were rapidly sintered using an intense pulsed light source. For the first time, a heat treatment has shown the capability of sintering methylammonium lead iodide perovskite and creating large crystal sizes approaching 1 μm without sacrificing surface coverage. Solar cells with an average efficiency of 11.5% and a champion device of 12.3% are reported. The methylammonium lead iodide perovskite was subjected to 2000 J of energy in a 2 ms pulse of light generated by a xenon lamp, resulting in temperatures significantly exceeding the degradation temperature of 150 °C. The process opens up new opportunities in the manufacturability of perovskite solar cells by eliminating the rate-limiting annealing step, and makes it possible to envision a continuous roll-to-roll process similar to the printing press used in the newspaper industry.

A low-noble-metal W1−xIrxO3−δ water oxidation electrocatalyst for acidic media via rapid plasma synthesis

Acid-based electrolysis has many advantages, but to achieve simultaneous activity and stability, commercial water oxidation catalysts rely on noble metal oxides that are expensive and too rare for the global scale. Here, earth-abundant tungsten was used as a structural metal to dilute the noble metal iridium content while maintaining high activity and stability in acid. Mixed-metal oxide catalysts were synthesized using rapid plasma oxidation in which the non-equilibrium reaction environment permitted better formation of a homogenous W1−xIrxO3−δ phase. With an Ir metal content as low as 1%, a competitive and durable overpotential for oxygen evolution was achieved. Relative to high Ir content, low Ir compositions consisted of a more highly crystalline, phase-pure iridium polytungstate which was more catalytically active per Ir content. Moreover, the plasma-synthesized material had a sharp electrocatalytic improvement over an equivalent composition synthesized via standard thermal oxidation, demonstrating the value of non-equilibrium synthesis to find new catalysts.

Photoelectrochemical reduction of CO2 to HCOOH on silicon photocathodes with reduced SnO2 porous nanowire catalysts

Electrochemical reduction of CO2 to liquid products offers a route for the energy-dense storage of intermittent renewable electricity while simultaneously helping to mitigate greenhouse gas emissions. In this work, high-quality Si photocathodes decorated with an earth-abundant Sn porous nanowire catalyst utilized the energy from visible light absorption to provide a photovoltage-assisted conversion of CO2 to liquid HCOOH. The Sn porous nanowire catalysts were selected for their high density of grain boundaries which was previously shown to enhance activity for formic acid formation. A faradaic efficiency of ∼60% with a partial current density of 10 mA cm−2 for HCOOH was achieved at −0.4 V vs. RHE under illumination, which reflected a positive potential shift of ∼400 mV compared to the dark electrocatalytic behavior. The photo-assisted electrolysis efficiency for formic acid was calculated to be 11.0%. The results represent a promising photocathode for a narrow bandgap subcell for a tandem photoelectrode system for unbiased light-driven CO2 electroreduction.

Simulations of non-monolithic tandem solar cell configurations for electrolytic fuel generation

The efficient conversion of solar energy to fuels through electrochemical processes requires optimizing the photovoltage and current for an ideal coupling with the electrolysis reaction. A modular architecture for tandem photovoltaics is explored and modeled as a strategy to drive an arbitrary electrolysis reaction from sunlight to produce the maximum fuel product in a day. Non-monolithic tandem solar cells based on Si and organometal halide perovskites are simulated in two-terminal and four-terminal arrangements and coupled with experimental data on water-splitting and CO2 reduction to predict the performance of an integrated solar fuels system. An appropriately designed four-terminal system is modeled to match or exceed the output of a two-terminal system. The four-terminal configuration leads to a 15.8% increase in daily H2 production with a 1.5 eV/1.12 eV system, and a 5.3% increase with a more ideal 1.74 eV/1.12 eV combination. The four-terminal system is also simulated to match the production of formic acid and increase the production of ethylene by 20.4% in a Cu-catalyzed CO2 reduction process compared to a two-terminal tandem arrangement. The effects of series resistance in non-monolithic tandem devices are modeled as well, showing a much greater tolerance to cell width in the four-terminal systems.