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
Unifying the Conversation: Membrane Separation Performance in Energy, Water, and Industrial Applications
Dense polymer membranes
enable a diverse range of separations
and
clean energy technologies, including gas separation, water treatment,
and renewable fuel production or conversion. The transport of small
molecular and ionic solutes in the majority of these membranes is
described by the same solution-diffusion mechanism, yet a comparison
of membrane separation performance across applications is rare. A
better understanding of how structureâproperty relationships
and driving forces compare among applications would drive innovation
in membrane development by identifying opportunities for cross-disciplinary
knowledge transfer. Here, we aim to inspire such cross-pollination
by evaluating the selectivity and electrochemical driving forces for
29 separations across nine different applications using a common framework
grounded in the physicochemical characteristics of the permeating
and rejected solutes. Our analysis shows that highly selective membranes
usually exhibit high solute rejection, rather than fast solute permeation,
and often exploit contrasts in the size and charge of solutes rather
than a nonelectrostatic chemical property, polarizability. We also
highlight the power of selective driving forces (e.g., the fact that
applied electric potential acts on charged solutes but not on neutral
ones) to enable effective separation processes, even when the membrane
itself has poor selectivity. We conclude by proposing several research
opportunities that are likely to impact multiple areas of membrane
science. The high-level perspective of membrane separation across
fields presented herein aims to promote cross-pollination and innovation
by enabling comparisons of solute transport and driving forces among
membrane separation applications
Unifying the Conversation: Membrane Separation Performance in Energy, Water, and Industrial Applications
Dense polymer membranes
enable a diverse range of separations
and
clean energy technologies, including gas separation, water treatment,
and renewable fuel production or conversion. The transport of small
molecular and ionic solutes in the majority of these membranes is
described by the same solution-diffusion mechanism, yet a comparison
of membrane separation performance across applications is rare. A
better understanding of how structureâproperty relationships
and driving forces compare among applications would drive innovation
in membrane development by identifying opportunities for cross-disciplinary
knowledge transfer. Here, we aim to inspire such cross-pollination
by evaluating the selectivity and electrochemical driving forces for
29 separations across nine different applications using a common framework
grounded in the physicochemical characteristics of the permeating
and rejected solutes. Our analysis shows that highly selective membranes
usually exhibit high solute rejection, rather than fast solute permeation,
and often exploit contrasts in the size and charge of solutes rather
than a nonelectrostatic chemical property, polarizability. We also
highlight the power of selective driving forces (e.g., the fact that
applied electric potential acts on charged solutes but not on neutral
ones) to enable effective separation processes, even when the membrane
itself has poor selectivity. We conclude by proposing several research
opportunities that are likely to impact multiple areas of membrane
science. The high-level perspective of membrane separation across
fields presented herein aims to promote cross-pollination and innovation
by enabling comparisons of solute transport and driving forces among
membrane separation applications
When Flooding Is Not CatastrophicWoven Gas Diffusion Electrodes Enable Stable CO<sub>2</sub> Electrolysis
Electrochemical CO2 reduction has the potential
to use
excess renewable electricity to produce hydrocarbon chemicals and
fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations
of CO2 mass transfer but are sensitive to flooding from
(hydrostatic) pressure differences, which inhibits upscaling. We investigate
the effect of the flooding behavior on the CO2 reduction
performance. Our study includes six commercial gas diffusion layer
materials with different microstructures (carbon cloth and carbon
paper) and thicknesses coated with a Ag catalyst and exposed to differential
pressures corresponding to different flow regimes (gas breakthrough,
flow-by, and liquid breakthrough). We show that physical electrowetting
further limits the flow-by regime at commercially relevant current
densities (â„200 mA cmâ2), which reduces the
Faradaic efficiency for CO (FECO) for most carbon papers.
However, the carbon cloth GDE maintains its high CO2 reduction
performance despite being flooded with the electrolyte due to its
bimodal pore structure. Exposed to pressure differences equivalent
to 100 cm height, the carbon cloth is able to sustain an average FECO of 69% at 200 mA cmâ2 even when the liquid
continuously breaks through. CO2 electrolyzers with carbon
cloth GDEs are therefore promising for scale-up because they enable
high CO2 reduction efficiency while tolerating a broad
range of flow regimes
When Flooding Is Not CatastrophicWoven Gas Diffusion Electrodes Enable Stable CO<sub>2</sub> Electrolysis
Electrochemical CO2 reduction has the potential
to use
excess renewable electricity to produce hydrocarbon chemicals and
fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations
of CO2 mass transfer but are sensitive to flooding from
(hydrostatic) pressure differences, which inhibits upscaling. We investigate
the effect of the flooding behavior on the CO2 reduction
performance. Our study includes six commercial gas diffusion layer
materials with different microstructures (carbon cloth and carbon
paper) and thicknesses coated with a Ag catalyst and exposed to differential
pressures corresponding to different flow regimes (gas breakthrough,
flow-by, and liquid breakthrough). We show that physical electrowetting
further limits the flow-by regime at commercially relevant current
densities (â„200 mA cmâ2), which reduces the
Faradaic efficiency for CO (FECO) for most carbon papers.
However, the carbon cloth GDE maintains its high CO2 reduction
performance despite being flooded with the electrolyte due to its
bimodal pore structure. Exposed to pressure differences equivalent
to 100 cm height, the carbon cloth is able to sustain an average FECO of 69% at 200 mA cmâ2 even when the liquid
continuously breaks through. CO2 electrolyzers with carbon
cloth GDEs are therefore promising for scale-up because they enable
high CO2 reduction efficiency while tolerating a broad
range of flow regimes
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