6 research outputs found
Synergistic Electrochemical CO<sub>2</sub> Reduction and Water Oxidation with a Bipolar Membrane
The
electrochemical conversion of CO<sub>2</sub> and water to value-added
products still suffers from low efficiency, high costs, and high sensitivity
to electrolyte, pH, and contaminants. Here, we present a strategy
for this reaction using a silver catalyst for CO<sub>2</sub> reduction
in a neutral catholyte, separated by a bipolar membrane from a nickel
iron hydroxide oxygen evolution catalyst in a basic anolyte. This
combination of electrolytes provides a favorable environment for both
catalysts and shows the effective use of bicarbonate and KOH to obtain
low cell voltages. This architecture brings down the total cell voltage
by more than 1 V compared to that with conventional use of a Pt counter
electrode and monopolar membranes, and at the same time, it reduces
contamination and improves stability at the cathode
General Considerations for Improving Photovoltage in MetalâInsulatorâSemiconductor Photoanodes
Metalâinsulatorâsemiconductor
(MIS) photoelectrodes
offer a simple alternative to the traditional semiconductorâliquid
junction and the conventional pân junction electrode. Highly
efficient MIS photoanodes require interfacial surface passivating
oxides and high workfunction metals to produce a high photovoltage.
Herein, we investigate and analyze the effect of interfacial oxides
and metal workfunctions on the barrier height and the photovoltage
of a c-Si photoanode. We use two metal components in a bimetal contact
configuration and observe the modulation of the effective barrier
height and the resulting photovoltage as a function of the secondary
outer metal. The photovoltage shows a strong linear dependence by
increasing the inner metal workfunction, with the highest photovoltage
achieved by a MIS photoanode using a platinum inner metal. We also
found that coupling a thin aluminium oxide with an interfacial silicon
oxide and controlling the oxide thickness can significantly improve
the photovoltage of an MIS junction photoanode
In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity
Ni-based oxygen evolution catalysts
(OECs) are cost-effective and
very active materials that can be potentially used for efficient solar-to-fuel
conversion process toward sustainable energy generation. We present
a systematic spectroelectrochemical characterization of two Fe-containing
Ni-based OECs, namely nickel borate (NiÂ(Fe)âB<sub>i</sub>)
and nickel oxyhydroxide (NiÂ(Fe)ÂOOH). Our Raman and X-ray absorption
spectroscopy results show that both OECs are chemically similar, and
that the borate anions do not play an apparent role in the catalytic
process at pH 13. Furthermore, we show spectroscopic evidence for
the generation of negatively charged sites in both OECs (NiOO<sup>â</sup>), which can be described as adsorbed âactive
oxygenâ. Our data conclusively links the OER activity of the
Ni-based OECs with the generation of those sites on the surface of
the OECs. The OER activity of both OECs is strongly pH dependent,
which can be attributed to a deprotonation process of the Ni-based
OECs, leading to the formation of the negatively charged surface sites
that act as OER precursors. This work emphasizes the relevance of
the electrolyte effect to obtain catalytically active phases in Ni-based
OECs, in addition to the key role of the Fe impurities. This effect
should be carefully considered in the development of Ni-based compounds
meant to catalyze the OER at moderate pHs. Complementarily, UVâvis
spectroscopy measurements show strong darkening of those catalysts
in the catalytically active state. This coloration effect is directly
related to the oxidation of nickel and can be an important factor
limiting the efficiency of solar-driven devices utilizing Ni-based
OECs
Hot Carrier Generation and Extraction of Plasmonic Alloy Nanoparticles
The conversion of
light to electrical and chemical energy has the
potential to provide meaningful advances to many aspects of daily
life, including the production of energy, water purification, and
optical sensing. Recently, plasmonic nanoparticles (PNPs) have been
increasingly used in artificial photosynthesis (e.g., water splitting)
devices in order to extend the visible light utilization of semiconductors
to light energies below their band gap. These nanoparticles absorb
light and produce hot electrons and holes that can drive artificial
photosynthesis reactions. For n-type semiconductor photoanodes decorated
with PNPs, hot charge carriers are separated by a process called hot
electron injection (HEI), where hot electrons with sufficient energy
are transferred to the conduction band of the semiconductor. An important parameter that affects
the HEI efficiency is the nanoparticle composition, since the hot
electron energy is sensitive to the electronic band structure of the
metal. Alloy PNPs are of particular importance for semiconductor/PNPs
composites, because by changing the alloy composition their absorption
spectra can be tuned to accurately extend the light absorption of
the semiconductor. This work experimentally compares the HEI efficiency
from Ag, Au, and Ag/Au alloy nanoparticles to TiO<sub>2</sub> photoanodes
for the photoproduction of hydrogen. Alloy PNPs not only exhibit tunable
absorption but can also improve the stability and electronic and catalytic
properties of the pure metal PNPs. In this work, we find that the
Ag/Au alloy PNPs extend the stability of Ag in water to larger applied
potentials while, at the same time, increasing the interband threshold
energy of Au. This increasing of the interband energy of Au suppresses
the visible-light-induced interband excitations, favoring intraband
excitations that result in higher hot electron energies and HEI efficiencies
Solar Water Splitting Combining a BiVO<sub>4</sub> Light Absorber with a Ru-Based Molecular Cocatalyst
We
demonstrate here for the first time the photoelectrochemical
properties of a BiVO<sub>4</sub> photoanode in conjunction with a <i>molecular</i> catalyst. When the Ru-based molecular catalyst
(RuCat) is coupled to a BiVO<sub>4</sub> light-absorber the performance
of this photoanode improves particularly in the low-bias region (<1.0
V vs RHE). The RuCat-BiVO<sub>4</sub> photoanode shows a higher photocurrent
than CoP<sub>i</sub>-BiVO<sub>4</sub> under front illumination, and
a 0.1 V more cathodic onset potential. The former can be partly explained
by the low light absorption of the RuCat (<5% light absorption
in the UVâvisâNIR range). For the latter, we propose
that the linkers in the RuCat reduce the surface recombination in
BiVO<sub>4</sub> to a greater extent than CoP<sub>i</sub>. Finally,
we observe that the fill factor of the RuCat-BiVO<sub>4</sub> JV characteristic
improves after the stability test. The results presented herein not
only show the feasibility and potential of the solid state/molecular
heterojunctions but also represent a proof of principle to improve
conventional all-solid-state systems such as CoP<sub>i</sub>-BiVO<sub>4</sub>
Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene
This work utilizes
EIS to elucidate the impact of catalystâionomer
interactions and cathode hydroxide ion transport resistance (RCL,OHâ) on cell
voltage and product selectivity for the electrochemical conversion
of CO to ethylene. When using the same Cu catalyst and a Nafion ionomer,
varying ink dispersion and electrode deposition methods results in
a change of 2 orders of magnitude for RCL,OHâ and ca. a 25% change in electrode
porosity. Decreasing RCL,OHâ results in improved ethylene Faradaic efficiency (FE), up
to âŒ57%, decrease in hydrogen FE, by âŒ36%, and reduction
in cell voltage by up to 1 V at 700 mA/cm2. Through the
optimization of electrode fabrication conditions, we achieve a maximum
of 48% ethylene with >90% FE for non-hydrogen products in a 25
cm2 membrane electrode assembly at 700 mA/cm2 and
RCL,OHâ is translated to other
material requirements, such as anode porosity. We find that the best
performing electrodes use ink dispersion and deposition techniques
that project well into roll-to-roll processes, demonstrating the scalability
of the optimized process