Engineering topics

Hahn, Christopher

Publications and source records attributed to Hahn, Christopher.

60 records · Page 4

DOE OSTI2017

Development of a reactor with carbon catalysts for modular-scale, low-cost electrochemical generation of H 2 O 2

The development of small-scale, decentralized reactors for H 2 O 2 production that can couple to renewable energy sources would be of great benefit, particularly for water purification in the developing world. Herein, we describe our efforts to develop electrochemical reactors for H 2 O 2 generation with high Faradaic efficiencies of >90%, requiring cell voltages of only ~1.6 V. The reactor employs a carbon-based catalyst that demonstrates excellent performance for H 2 O 2 production under alkaline conditions, as demonstrated by fundamental studies involving rotating-ring disk electrode methods. Finally, the low-cost, membrane-free reactor design represents a step towards a continuous, modular-scale, de-centralized production of H 2 O 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Carbon Dioxide Electroreduction using a Silver-Zinc Alloy [CO 2 Electroreduction on a Ag-Zn Alloy]

We report on CO 2 electroreduction activity and selectivity of a polycrystalline AgZn foil in aqueous bicarbonate electrolyte. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements show that the alloy foil was slightly enriched in zinc both at the surface and in the bulk, with a surface alloy composition of 61.3±5.4 at % zinc and with Ag 5 Zn 8 as the most prominent bulk phase. AgZn is active for CO 2 reduction; CO is the main product, likely due to the weak CO binding energy of the surface, with methane and methanol emerging as minor products. Compared to pure silver and pure zinc foils, enhancements in activity and selectivity for methane and methanol are observed. A five-fold increase is observed in the combined partial current densities for methane and methanol at –1.43 V vs. the reversible hydrogen electrode (RHE), representing a four- to six-fold increase in faradaic efficiency. Here, such enhancements indicate the existence of a synergistic effect between silver and zinc at the surface of the alloy that contributes to the enhanced formation of further reduced products.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Electrochemical CO 2 reduction on Au surfaces: mechanistic aspects regarding the formation of major and minor products

In the future, industrial CO 2 electroreduction using renewable energy sources could be a sustainable means to convert CO 2 and water into commodity chemicals at room temperature and atmospheric pressure. This study focuses on the electrocatalytic reduction of CO 2 on polycrystalline Au surfaces, which have high activity and selectivity for CO evolution. Here, we explore the catalytic behavior of polycrystalline Au surfaces by coupling potentiostatic CO 2 electrolysis experiments in an aqueous bicarbonate solution with high sensitivity product detection methods. We observed the production of methanol, in addition to detecting the known products of CO 2 electroreduction on Au: CO, H 2 and formate. We suggest a mechanism that explains Au's evolution of methanol. Specifically, the Au surface does not favor C-O scission, and thus is more selective towards methanol than methane. These insights could aid in the design of electrocatalysts that are selective for CO 2 electroreduction to oxygenates over hydrocarbons.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Tandem Core–Shell Si–Ta 3 N 5 Photoanodes for Photoelectrochemical Water Splitting

Here, nanostructured core–shell Si–Ta 3 N 5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta 3 N 5 for photoelectrochemical water splitting. The core–shell devices were fabricated by atomic layer deposition of amorphous Ta 2 O 5 onto nanostructured Si and subsequent nitridation to crystalline Ta 3 N 5 . Nanostructuring with a thin shell of Ta 3 N 5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta 3 N 5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta 3 N 5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiO x and NiO x water oxidation cocatalysts were deposited onto the Si–Ta 3 N 5 to yield active photoanodes that with NiO x retained 50–60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta 3 N 5 photoanodes reported to date.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Molybdenum Disulfide as a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes

Gallium indium phosphide (GaInP2) is a semiconductor with promising optical and electronic properties for solar water splitting, but its surface stability is problematic as it undergoes significant chemical and electrochemical corrosion in aqueous electrolytes. Molybdenum disulfide (MoS2) nanomaterials are promising to both protect GaInP2 and to improve catalysis since MoS2 is resistant to corrosion and also possesses high activity for the hydrogen evolution reaction (HER). In this work, we demonstrate that GaInP2 photocathodes coated with thin MoS2 surface protecting layers exhibit excellent activity and stability for solar hydrogen production, with no loss in performance (photocurrent onset potential, fill factor, and light limited current density) after 60 hours of operation. This represents a five-hundred fold increase in stability compared to bare p-GaInP2 samples tested in identical conditions.

14 SOLAR ENERGY↗

DOE OSTI2015

Engineering Cobalt Phosphide (CoP) Thin Film Catalysts for Enhanced Hydrogen Evolution Activity on Silicon Photocathodes

Transition metal phosphide catalysts have recently emerged as active, earth abundant alternatives to precious metals for the hydrogen evolution reaction in acid. High performance, scalable catalysts are necessary for the successful implementation of photoelectrochemical water splitting devices, which have the potential to generate hydrogen in a sustainable manner. Herein, a general synthetic route is reported to produce transition metal phosphide thin films, which is used to fabricate cobalt phosphide (CoP) catalysts with high average turnover frequency (TOF avg ), 0.48 H 2 s −1 and 1.0 H 2 s −1 at 100 and 120 mV overpotential, respectively. Furthermore, it is shown that CoP thin films can be applied to silicon photoabsorbers to generate one of the most active precious metal‐free crystalline silicon photocathodes to date, achieving −10 mA cm −2 at +0.345 V vs. reversible hydrogen electrode. The synthesis route presented here provides a platform for both fundamental studies of well‐defined electrocatalysts and the fabrication of high‐performance photoelectrodes.

Hellstern, Thomas R.↗