6 research outputs found
Single-Layer Graphene Membranes Withstand Ultrahigh Applied Pressure
High
mechanical strength is essential for pressure-driven membrane
separations with nanoporous single-layer graphene, but its ability
to withstand high pressures remains to be demonstrated. We monitored
failure of centimeter-scale single-layer graphene membranes on porous
supports subjected to high pressures. Consistent with theory, the
membranes were found to withstand higher pressures when placed on
porous supports with smaller pore diameters, but failure occurred
over a surprisingly broad range of pressures, attributed to heterogeneous
susceptibility to failure at wrinkles, defects, and slack in the suspended
graphene. Remarkably, nonwrinkled areas withstood pressure exceeding
100 bar at which many kinds of membrane suffer from compaction. Our
study shows that single-layer graphene membranes can sustain ultrahigh
pressure especially if the effect of wrinkles is isolated using supports
with small pores and suggests the potential for the use of single-layer
graphene in high-pressure membrane separations
Molecular Sieving Across Centimeter-Scale Single-Layer Nanoporous Graphene Membranes
Molecular
sieving across atomically thin nanoporous graphene is
predicted to enable superior gas separation performance compared to
conventional membranes. Although molecular sieving has been demonstrated
across a few pores in microscale graphene membranes, leakage through
nonselective defects presents a major challenge toward realizing selective
membranes with high densities of pores over macroscopic areas. Guided
by multiscale gas transport modeling of nanoporous graphene membranes,
we designed the porous support beneath the graphene to isolate small
defects and minimize leakage through larger defects. Ion bombardment
followed by oxygen plasma etching was used to produce subnanometer
pores in graphene at a density of ∼10<sup>11</sup> cm<sup>–2</sup>. Gas permeance measurements demonstrate selectivity that exceeds
the Knudsen effusion ratio and scales with the kinetic diameter of
the gas molecules, providing evidence of molecular sieving across
centimeter-scale nanoporous graphene. The extracted nanoporous graphene
performance is comparable to or exceeds the Robeson limit for polymeric
gas separation membranes, confirming the potential of nanoporous graphene
membranes for gas separations
A Scalable Route to Nanoporous Large-Area Atomically Thin Graphene Membranes by Roll-to-Roll Chemical Vapor Deposition and Polymer Support Casting
Scalable, cost-effective
synthesis and integration of graphene
is imperative to realize large-area applications such as nanoporous
atomically thin membranes (NATMs). Here, we report a scalable route
to the production of NATMs via high-speed, continuous synthesis of
large-area graphene by roll-to-roll chemical vapor deposition (CVD),
combined with casting of a hierarchically porous polymer support.
To begin, we designed and built a two zone roll-to-roll graphene CVD
reactor, which sequentially exposes the moving foil substrate to annealing
and growth atmospheres, with a sharp, isothermal transition between
the zones. The configurational flexibility of the reactor design allows
for a detailed evaluation of key parameters affecting graphene quality
and trade-offs to be considered for high-rate roll-to-roll graphene
manufacturing. With this system, we achieve synthesis of uniform high-quality
monolayer graphene (<i>I</i><sub>D</sub>/<i>I</i><sub>G</sub> < 0.065) at speeds ≥5 cm/min. NATMs fabricated
from the optimized graphene, via polymer casting and postprocessing,
show size-selective molecular transport with performance comparable
to that of membranes made from conventionally synthesized graphene.
Therefore, this work establishes the feasibility of a scalable manufacturing
process of NATMs, for applications including protein desalting and
small-molecule separations
Kinetic Control of Angstrom-Scale Porosity in 2D Lattices for Direct Scalable Synthesis of Atomically Thin Proton Exchange Membranes
Angstrom-scale
pores introduced into atomically thin 2D materials
offer transformative advances for proton exchange membranes in several
energy applications. Here, we show that facile kinetic control of
scalable chemical vapor deposition (CVD) can allow for direct formation
of angstrom-scale proton-selective pores in monolayer graphene with
significant hindrance to even small, hydrated ions (K+ diameter
∼6.6 Å) and gas molecules (H2 kinetic diameter
∼2.9 Å). We demonstrate centimeter-scale Nafion|Graphene|Nafion
membranes with proton conductance ∼3.3–3.8 S cm–2 (graphene ∼12.7–24.6 S cm–2) and H+/K+ selectivity ∼6.2–44.2
with liquid electrolytes. The same membranes show proton conductance
∼4.6–4.8 S cm–2 (graphene ∼39.9–57.5
S cm–2) and extremely low H2 crossover
∼1.7 × 10–1 – 2.2 × 10–1 mA cm–2 (∼0.4 V, ∼25
°C) with H2 gas feed. We rationalize our findings
via a resistance-based transport model and introduce a stacking approach
that leverages combinatorial effects of interdefect distance and interlayer
transport to allow for Nafion|Graphene|Graphene|Nafion membranes with
H+/K+ selectivity ∼86.1 (at 1 M) and
record low H2 crossover current density ∼2.5 ×
10–2 mA cm–2, up to ∼90%
lower than state-of-the-art ionomer Nafion membranes ∼2.7 ×
10–1 mA cm–2 under identical conditions,
while still maintaining proton conductance ∼4.2 S cm–2 (graphene stack ∼20.8 S cm–2) comparable
to that for Nafion of ∼5.2 S cm–2. Our experimental
insights enable functional atomically thin high flux proton exchange
membranes with minimal crossover
Introducing Carbon Diffusion Barriers for Uniform, High-Quality Graphene Growth from Solid Sources
Carbon
diffusion barriers are introduced as a general and simple
method to prevent premature carbon dissolution and thereby to significantly
improve graphene formation from the catalytic transformation of solid
carbon sources. A thin Al<sub>2</sub>O<sub>3</sub> barrier inserted
into an amorphous-C/Ni bilayer stack is demonstrated to enable growth
of uniform monolayer graphene at 600 °C with domain sizes exceeding
50 μm, and an average Raman D/G ratio of <0.07. A detailed
growth rationale is established via in situ measurements, relevant
to solid-state growth of a wide range of layered materials, as well
as layer-by-layer control in these systems
Interdependency of Subsurface Carbon Distribution and Graphene–Catalyst Interaction
The
dynamics of the graphene–catalyst interaction during
chemical vapor deposition are investigated using in situ, time- and
depth-resolved X-ray photoelectron spectroscopy, and complementary
grand canonical Monte Carlo simulations coupled to a tight-binding
model. We thereby reveal the interdependency of the distribution of
carbon close to the catalyst surface and the strength of the graphene–catalyst
interaction. The strong interaction of epitaxial graphene with Ni(111)
causes a depletion of dissolved carbon close to the catalyst surface,
which prevents additional layer formation leading to a self-limiting
graphene growth behavior for low exposure pressures (10<sup>–6</sup>–10<sup>–3</sup> mbar). A further hydrocarbon pressure
increase (to ∼10<sup>–1</sup> mbar) leads to weakening
of the graphene–Ni(111) interaction accompanied by additional
graphene layer formation, mediated by an increased concentration of
near-surface dissolved carbon. We show that growth of more weakly
adhered, rotated graphene on Ni(111) is linked to an initially higher
level of near-surface carbon compared to the case of epitaxial graphene
growth. The key implications of these results for graphene growth
control and their relevance to carbon nanotube growth are highlighted
in the context of existing literature