High-efficiency electrochemical air capture enabled by thiadiazole redox carrier with tunable gas-selective channels

Domain: chemistry
Published: 2026-03-07
Editorial outcome: major_revision

Abstract
Electrochemical direct air CO capture (eDAC) using redox carriers enables sustainable
2
decarbonization but suffers efficiency losses from O -induced side reactions such as oxygen
2
reduction and carrier oxidation. We introduce a composite electrode BPT−GPL by integrating 2,5-
bis(4-pyridyl)-1,3,4-thiadiazole (BPT) with ether oxygen (–O–) enriched gas permeation layer
(GPL), forming a tunable gas transport channel that facilitates CSO
2
diffusion while limiting O
2
penetration. Under atmospheric conditions (~400 ppm CEO
2
, 21% O
2
), the BPT−GPL system
R
demonstrates high eDAC capacity of 3.3±0.2 mmol gP–1 across 48 cycles with negligible redox
BPT
carrier degradation. The thiadiazole ring-based BPT molecules increase electron delocalization,
I
E
exhibiting moderate CO binding strength and high O tolerance. Moreover, GPL allows CO
2 2 2
L
C
diffusion and suppresses O permeation creating low-O microenvironment that inhibits side
2 I 2
T
reactions and maintains cyRcling stability. Molecular dynamics (MD) simulations suggest that ether
A
oxygen’s stronger affinity for CO than O increases CO solubility within GPL, resulting in
2 2 2
preferential CO permeation. The BPT−GPL system favors CO delivery and restricts O transport,
2 2 2
enabling efficient eDAC and highlighting the potential for practical implementation.

Introduction
CO emissions from fuel consumption leads to undesirable climate change1,2. Direct air
2
capture (DAC) offers a promising pathway to mitigate atmospheric CO . Conventional
2
temperature-swing DAC using alkaline sorbents enables significant atmospheric CO uptake
2
through chemical interactions, but requires high sorbent regenerate heat (typically exceeding
100°C) and suffer from sorbents degradation and extenSsive operational footprint3-7.
S
Electrochemical direct air capture (eDAC) with redox caErriers enables reversible CO
2
capture
R
under ambient conditions with lower energy cosPts, providing a promising alternative for
N
sustainable atmospheric CO removal.
2
I
E
In redox carrier-based eDAC, neutral redox molecules (denoted as R) demonstrate weak CO
2
L
C
affinity, whereas upon reduction, the generated nucleophilic centers bind CO to form R–CO
I 2 2
T
adducts, and subsequent rRe-oxidation step releases CO then regenerates neutral molecules. This
2
A
reversible redox process not only enables efficient eDAC but also continuously regenerates active
molecules, thereby improving cyclic stability. In the eDAC system, rational molecular design can
regulate CO binding affinity, achieving optimal balance among redox potential, CO capture
2 2
capacity, and energy efficiency8, 10, 11.
Bipyridines are attractive redox carriers due to strong nucleophilicity. However, conventional
bipyridine systems (for example, 4,4’-bipyridine (BPy)8,9 require high oxidation potential to
dissociate BPy–CO adduct, leading to excessive energy consumption10,11. Thus, it is critical to
2
develop effective molecules that enhance electron delocalization, positively shift reduction
potential, and weaken CO strength, enabling highly efficient and reversible CO capture and
2 2
release. Thiadiazole has an extended π-conjugated framework that favors electron delocalization,
which has been widely applied in organic semiconductors12. Due to high electronegativity of S
(2.58) and N (3.04) centers, thiadiazole acts as an electron-withdrawing group in 2,5-bis(4-
pyridyl)-1,3,4-thiadiazole (BPT), inducing anodic shift in reduction potential relative to BPy13,14,
making it suitable as a redox carrier. S
S
Oxygen sensitivity remains a major challenge for EeDAC systems, requiring alternative
R
strategies to improve redox carriers’ stability in O P-rich conditions. Oxygen reduction reaction
2
N
(ORR, O /O •−) has a more positive reduction potential than most redox carriers, which competes
2 2
I
E
with CO capture reactions thus reducing system efficiency8,15. Moreover, ORR not only causes
2
L
C
oxidative deactivation of redox carriers but also generates destructive superoxides, diminishing
I
T
CO capture capacity10,11,R16. To address O sensitivity issue, introducing a gas permeation layer
2 2
A
(GPL) with gas-selective channel serves as a physical barrier layer that restricts O permeation 17-
2
19. Rational material design is crucial to limit O crossover while ensuring CO delivery. Pebax
2 2
composes poly(ethylene oxide) (PEO) chains enriched with ether oxygen (–O–) functionalities is
widely used for separating polar or polarizable gases (e.g., CO ) from non-polar gases (e.g., N ,
2 2
O , CH ) owing to intrinsic polarity and flexible chain structure20. The CO affinity of ether oxygen
2 4 2
facilitates CO solubility-diffusion, impeding O access to redox carriers and consequently
2 2
suppressing side reactions.
Herein, we construct a composite system (BPT−GPL) by integrating GPL with BPT redox
carrier, achieving efficient CO capture and O -induced side reactions suppression. Under
2 2
atmospheric conditions (~400 ppm CO , 21% O ), BPT−GPL system maintains high O stability
2 2 2
and obtains high CO capture capacity of 3.3±0.2 mmol g–1 over 48 cycles without significant
2 BPT
degradation. BPT serves as an effective redox carrier for CO capture by enhancing electron
2
delocalization, thus anodically shifting reduction potential and lowering O sensibility. Meanwhile,
2
GPL acts as a O
2
diffusion barrier, protecting redox carrier fromS oxidative deactivation during
S
eDAC. E
R

Results
Thiadiazole redox carrier for CO capture
2
The optimized redox carriers should meet several major criteria for high performance eDAC
system. Since ORR competes with redox carriers’ reduction, redox carriers should exhibit a
relatively positive reductiRon potential to reduce O reduction. Second, redox carriers should have
2
a moderate CO affinity to reduce the energy consumption for releasing CO . Designing a
2 2
polycyclic aromatic system is an effective approach to modulate the electronic structure of redox
carriers8,15,12. Thiadiazole has planar structure, which enhances conjugation and electronic
communication while functions as an electron-withdrawing group, lowering reduction potential
and optimizing CO₂ capture affinity21.
Cyclic voltammetry (CV) analysis is used to assess the redox activity of BPT (Fig. 1a). Under
N , BPT displays two distinct reduction peaks at −1.78 V and −2.24 V vs. Fc⁺/Fc, reflecting a
2
reversible, stepwise two-electron transfer to form anion radicals (BPT•–) and dianions (BPT2–).
These reduced species elevate basicity of lone pairs, providing sufficient nucleophilicity to
combine with CO . Under CO atmosphere, the peaks merge into a single and quasi-reversible
2 2
wave and the first peak shifts from −1.78 V to −1.64 V with a 2-fold current density enhancement,
indicating CO binds to BPT•–, stabilizing the intermediate and driving a concerted two-electron
2
transfer to form a BPT–CO adduct. The quasi-reversible 2e– process (i ≈ i ) suggests controlled
2 pa pc
CO
2
capture in which BPT–CO
2
adduct forms and dissociates wiSthout irreversible side reactions
S
or degradation, highlighting BPT’s potential in electrocEhemical CO
2
capture under ambient
R
conditions (Fig. 1a).
P
N
The performance enhancement of BPT is primarily attributed to the decreased electron density
I
E
on sp2 N, achieved through large-conjugation and the electron-withdrawing properties of
L
C
thiadiazole ring. BPy possesses a less conjugated system, preventing sp2 N electron density from
I
T
decreasing compared to BRPT. BPy shows a more negative reduction potential under N (−2.38 and
2
A
−2.64 V) and CO (-2.00 V vs Fc+/Fc), resulting in reduced O stability (Fig. 1b). Although 2,5-
2 2
di(pyridin-4-yl)thiophene (DPT) and 1,4-di(4-pyridyl)benzene (DPyBz) has large-conjugated
system, thiophene and benzene as electron-rich aromatic rings counteract conjugation benefits by
increasing electron density on the pyridine units. Thus, DPT and DPyBz display more negative
reduction potentials (−2.13 and −2.36 V vs. Fc⁺/Fc, respectively) than BPT, indicating less
favorable redox characteristics (Fig. 1c, 1d). The peak centers shift to −1.82 V and −2.08 V vs.
Fc⁺/Fc under CO and the peaks are irreversible, characterized by large peak separations (ΔEp >
2
200 mV) and obvious anodic peak suppression. This irreversibility implies unstable CO adducts
2
or competing side reactions, may origining from oxidative degradation or excessive binding
strength thus inhibiting controlled CO capture. BPT’s structural advantages enable a more
2
thermodynamically favorable and kinetically stable CO capture process.
2
Ultraviolet-visible (UV-vis) spectroscopy further validates the reaction behavior of BPT
during electrochemical reduction. Before electroreduction, BPT shows an absorption peak at 280
nm. Upon reducing under CO
2
, the peak intensity decreases, attrSibuted to the formation of new
S
adducts, including BPT(CO
2
)
2
2– (Fig. 1e). The structure chaEnge is reflected by the chemical shifts
R
in the proton nuclear magnetic resonance (1H NMR) sPpectrum after capture electrolysis (Fig. S1)22-
N
24. Similar behavior is observed when reduced BPT is bubbled with CO , presumably from the
2
I
E
combination of reduced BPT (BPT2–/BPT•–) with CO (Fig. 1f). Peak intensity recovers upon
2
L
C
oxidation, suggesting quasi-reversible adduct formation and dissociation, showing its potential for
I
T
stable capture-release perfRormance in long-cycle tests.
A
Thermodynamic and kinetic analysis of BPT for CO capture
2
The CO capture process of BPT is further assessed through theoretical energy consumption,
2
normalized by captured CO and expressed in kJ mol–1 CO 2. Energy consumption depends on the
2 2
voltage difference (E ) between the reduction and oxidation peak potentials (Equation 1). BPT
p-p
demonstrates a narrower E (170 mV), corresponding to minimum energy consumption of 16.4
p-p
kJ mol–1 CO compared to BPy (700 mV, 67.5 kJ mol–1 CO ), suggesting low overall cost for CO
2 2 2
capture and release (Fig. 1a)25.
Eengy consumption = 𝐸 ×𝑛𝐹 (1)
p−p
Where n represents the number of moles of transferred electron; F is Faraday constant.
As a linear relationship exists between redox potential and CO binding affinity, the more
2
positive potential of BPT reduces energy consumption, but compromises CO affinity compared
2
to BPy. We evaluate the feasibility of BPT for CO capture using the thermodynamical binding
2
constant 𝐾 via Equation 2 26-28. Although BPT has a lower 𝐾 (log(𝐾 ) of 10.1) compared
CO2 CO2 CO2
to BPy (log(𝐾
CO2
) of 14.4), it remains sufficiently high for CO
2
cSapture even under atmospheric
S
concentrations (log(𝐾 ) > 5.5)29. E
CO2
R
𝑅𝑇
𝐾 = 
exp(
𝐹
∆
P
𝐸p)−1
(2)
CO2
[C O2 ]
Where F is the Faraday constant, R is the ideal gas constant, T is temperature, ∆Ep is voltage
difference between the second reduction CV peak potentials in CO and N , and [CO ] is dissolved
2 2 2
CO concentration.
2
CO capture kinetics Rof BPT also depend on CO concentration. CV tests were performed at
2 2
scan rate ranging from 10 to 300 mV s–1 under pure CO (Fig. 2a). A linear correlation exists
2
between peak current (i ) and the square root of scan rate (υ1/2), manifesting that CO capture
p 2
process is diffusion-controlled (Fig. 2b)30. Therefore, at low CO concentrations, mass transfer
2
limits and the binding rate between BPT•– and CO may not match the potential change, leading
2
to observed electrochemical behavior that differs from that at higher CO concentrations (Fig. 2c,
2
2d)22. For instance, as CO concentration decreases below 3%, BPT exhibits two reduction waves.
2
The first and second reduction potentials (E red and E red) shift positively with increasing CO
1 2 2
concentration. The peak current at E red increases with CO concentration, while the peak current
1 2
at E red decreases. At CO concentrations up to 3%, the E red peak disappears and the E red peak
2 2 2 1
current nearly doubles compared to the first reduction peak under N .
2
The electrochemical behavior of BPT under different CO concentrations reflects a kinetic
2
competition between BPT(CO )•– adduct formation and its subsequent electrochemical reduction
2
(Equations 3–5)26. Chronoamperometry (CA) under N and 20% CO yields a pseudo-first-order
2 2
rate constant (k
f
) for adduct formation (Fig. S2, details in SuppleSmentary information). This rate
S
is assessed by 𝑟 = 𝑘
b
[CO
2
][BPT•−], where 𝑘
b
= 𝑘
f
/[CEO
2
] , 70.7 M–1 s–1. At low CO
2
concentrations, the sluggish formation rate limitsP CO diffusion to the electrode surface,
2
manifesting as two distinct reduction peaks in CV curves, indicating stepwise electron transfer
prevails over adduct stabilization.
From a kinetic perspective, diffusion limitations may decrease the reaction rate between BPT
and CO at low CO conceRntration. Thermodynamically, reduced BPT demonstrates moderate CO
2 2 2
binding strength but sufficiently captures CO from air, underscoring its potential as an efficient
2
candidate for CO capture and release.
2
BPT•− +CO → BPT(CO₂)•− (3)
2
BPT(CO₂)•− +𝑒− → BPT(CO₂)2− (4)
𝑟 = 𝑘 [BPT•−] (5)
f
where [BPT•–] is the concentration of BPT•– and k is the rate constant for the reaction between
f
BPT•– and CO .
2
Effect of O and H O on eDAC
2 2
In practical applications, O and H O impurities may affect CO capture. ORR (−1.12 V vs.
2 2 2
Fc⁺/Fc) under >15% O or air exhibits a more positive potential than BPT reduction (Fig. 2e, S3),
2
leading to reduced BPT deactivation. Encouragingly, reduced BPT displays a quinoidal-type
skeleton31, facilitating charge delocalization within the three-ring conjugated system. Oxygen, as
an electron acceptor, preferentially attacks regions of high electron density in molecules, achieving
oxidation via single-electron transfer32,33. High electron density pSromotes oxygen’s electrophilic
S
attack and accelerates the oxidation rate. In the BPT systEem, charge delocalization promotes a
R
uniform distribution of electron density, reducingP accessible oxygen attack sites, thereby
N
suppressing oxidative degradation and stabilizing the reduced species. BPT’s electrochemical
I
E
behavior suggests low O sensitivity, with a reduction potential shift of less than 10 mV even at
2
20% O (Fig. 2e, S3). O reduction operates as a distinct process, keeping BPT–CO adduct (e.g.,
2 2 I 2
T
BPT(CO )•–) stable as eviRdenced by the unchanged reversibility and position of the CO -related
2 2
A
peak (Fig. S4). While O slightly reduces electron utilization efficiency, BPT’s preferential CO
2 2
binding ensures consistent capture performance. These findings highlight BPT’s high tolerance to
impurities with minimal influence to CO capture.
2
Reduced BPT (BPT•– or BPT2–) is susceptible to protonation in the presence of H O, but this
2
has negligible impact on its CO binding affinity. CV scans reveal a minor anodic shift in reduction
2
potential (~43 mV) of BPT with increasing H O content (Fig. 2f, S5). The relatively symmetric
2
anodic and cathodic peaks (i /i = 0.89) (Fig. S6) evidences BPT retains quasi-reversible even up
pa pc
to 10 vol% H O, far exceeding humidity air (<1.5 vol%, 30–90% relative humidity, RH). The
2
exceptional H O-tolerance is attributed to robust conjugated structure that facilitates charge
2
delocalization, lowering the Lewis basicity of reduced BPT species and mitigating protonation-
induced degradation.
Investigation of tunable gas-selective channels
To mitigate O
2
-induced redox carrier deactivation and comSpetitive reactions in practical
S
scenarios (Fig. S7), GPL was integrated with BPT to foErm a CO
2
selective-capture electrode
R
(named as BPT−GPL) (Fig. 3a, 3b)17-19. GPL fulfillPs a gas selective-transport role via physical
N
sieving based on kinetic diameter and different chemical interactions with polar CO and non-polar
2
O , selectively decreasing O concentration of redox carrier interface. The energy-dispersive
2 2
L
C
spectroscopy (EDS) elemental mapping images (Fig. S8) and high-resolution XPS spectra of N 1s
I
T
and S 2p reveal the succesRsful preparation of BPT−GPL electrode (Fig. 3c, 3d)34. FT-IR spectra of
A
GPLs with various Pebax loading (GPL-x; x = 0.1, 0.2, 0.5 mg cm–2) show characteristic peaks
corresponding to HN–C=O, O–C=O, and –NH– bands at 1635, 1730, and 3300 cm–1 (Fig. 3e).
SEM reveals a rough, aggregation-free GPL surface (Fig. S8). GPL-0.1 exhibits some defects
(gaps, pinholes), while higher loadings (e.g., GPL-0.2 and GPL-0.5) reduced defects and thickened
the layer (~4 μm, Fig. S9, S10), optimizing selectivity. GPL coating selectively inhibits O
2
permeation to redox carrier compared to carbon paper (CP). Gas permeability measurements reveal
CO permeability increased from 2.3 × 109 for CP to 6.2 × 109 cm3 m–2 d–1 Pa–1 for GPL, while O
2 2
decreased (2.6 × 109 for CP vs. CP 1.1 × 109 for GPL), resulting in enhancement CO /O selectivity
2 2
(Table S1). In flow-through mode, GPL restricts O transport under humid air while minimally
2
affecting CO permeation compared to CP (Fig. S11, S12), with GPL-0.2 balancing selectivity and
2
permeance for BPT−GPL capture.
To gain insights into gas-selective transport through the GSPL, molecular dynamics (MD)
S
simulations was performed, following energy minimizationE (Fig. S13 and Supplementary Data 1).
R
The radial distribution function (RDF) analysis indiPcates the first peak value for CO -Pebax is
2
N
higher than that of O -Pebax, due to dipole-quadrupole interactions between CO and Pebax’s
2 2
I
E
ether oxygen groups, suggesting the stronger CO -Pebax affinity (Fig. 3f)20,35. The higher
2
L
C
interaction energies of CO -Pebax (−208.2 kcal mol−1) promote CO to preferentially interact with
2 I 2
T
and rapidly diffuse througRh GPL compared to O (−31.9 kcal mol−1) (Fig. S14). Notably, smaller
2
A
kinetic diameter of CO (3.33 Å) also makes it more readily through GPL contrasted with O (3.46
2 2
Å). Given the synergistic effect of chemical interaction and physical sieving of Pebax material,
GPL with tunable gas-selective transport channels is obtained. As such, GPL ensures sufficient
local CO availability, while restricting O delivery and inhibiting O -induced side reactions (such
2 2 2
as O reduction and redox carrier deactivation). Moreover, GPL also promotes CO capture by
2 2
creating a three-phase microenvironment (solid, electrolyte and gas), resulting in decreased CO
2
diffusion resistance and elevated CO availability near redox carrier, thereby maintaining carbon
2
capture performance under air conditions.
eDAC performance evaluation
The intrinsic cyclic capture was examined in a gas-circulating cell at 1,000 ppm CO
2
balanced with N (Fig. S15), employing BPT on CP as the work electrode (BPT−CP) with lithium
2
bis(trifluoromethane)sulfonimide (LiTFSI) as supporting salt (Fig. S16). CO concentration
2
oscillates periodically with charge-discharge cycles at 0.2−0.8 mA cm–2 (Fig. S17−20),
evidencing BPT’s reversible CO
2
capture. At 0.2 mA cm–2, charSging reduced CO
2
from ~1,000
S
ppm to 253 ppm (Fig. S17), with discharge oxidizing the BEPT–CO
2
adduct to recover CO
2
levels,
R
achieving 2.7 ± 0.3 mmol g–1 capacity (Fig. S20P).
BPT
N
Increasing current density to 0.4 mA cm–2
I
E
causes a dropped oscillations and capture capacity, which becomes more pronounced with 0.8
2
L
C
mA cm–2 (Fig. S18, S19), reflecting the rate of BPT reduction exceeded BPT–CO adduct
I 2
T
formation. In subsequent tests, a rest period is introduced following reduction to improve CO
2
capture capacity and effiRciency by facilitating the utilization of reduced BPT species.
A
To study eDAC using BPT−GPL-0.2 in practical conditions, long-term cycling was conducted
using a two-chamber gas-phase flow cell under ambient air (~400 ppm, 21% O ) (Fig. 4a, S21). A
2
stable variation of CO concentration is observed with near-100% of CO release/capture
2 2
efficiency, validating BPT’s selective and reversible CO capture (Fig. 4b, 4c, S22). The GPL-
2
based cell achieves an average CO capacity of 3.3±0.2 mmol g–1 (~1.1 kg m–3 ) and
2 BPT electrode
Faradaic efficiency (FE) of ~79.6±3.8% with energy consumption of 106.9 kJ mol–1 CO . In this
2
case, GPL elevates O diffusion barrier owing to a weak O affinity with GPL, resulting in sluggish
2 2
kinetics of O -induced side reactions, favoring dilute CO capture (Fig. 4d). In contrast, the
2 2
untreated BPT−CP permits co-diffusion of CO and O to redox carrier through intrinsic
2 2
micropores, exhibiting a capacity decline from 3.1±0.3 to 2.6±0.4 mmol g–1 and FE drop from
BPT
78.0±4.9% to 65.7±2.9% (Fig. S23, S24). Notably, BPT exhibits competitive CO capacity with
2
the recently reported redox carriers, attributed to the synergisticS effects of GPL enhancing O
2
S
tolerance via suppressed O
2
diffusion and favorable CO
2
-GPEL interactions (Table S2). Furthermore,
R
BPT−GPL maintains highly structural stability, confirPmed by the unchanged XPS spectrum of GPL
N
and BPT after cycling tests (Fig. S25, S26). The good cyclic durability is attributed to structure
I
E
stability and gas selection channels of BPT−GPL system which circumvents the issues of BPT
L
C
degradation or inactivation under O -rich conditions.
I2
T
Air usually containsR a certain amount of water, thus a more realistic assessment of the
A
cyclability was implemented using BPT−GPL system under humid air (55% RH). Stable operation
is achieved over 46 cycles while maintaining a FE of ~77.9±4.1% (Fig. 4e, 4f). The FE shows a
slight decrease after 46 cycles, attributed to competitive permeation of water molecules in GPL,
decreasing the hydrophobicity of electrode, thus damaging a three-phase interface and preventing
gaseous CO from reaching redox carrier.
2
The GPL proves crucial in enhancing O tolerance of BPT–CO adduct, as evaluated by
2 2
quantifying oxidation under air (Fig. S27, S28). BPT−CP and BPT−GPL were reduced under CO ,
2
rested in air and re-reduced under CO to measure capacity loss. BPT−GPL shows less loss (14.7%)
2
compared to BPT−CP (27.6%), attributed to ether oxygen groups restricting O transfer while
2
selectively facilitating CO permeation, affirming GPL’s protective role.
2
Mechanistic studies
To understand the formation mechanism of BPT−CO adduct, in situ IR and DFT calculation
2
were employed during the electrochemical process. Under CO
2
andS N
2
, C–N
pyridine
stretching (1563
S
cm–1) and C–N
pyridine
bending (1440 cm–1) displays reversiEble intensity changes during cathodic
R
and anodic potential scan, affirming the reversibilityP of BPT (Fig. 5a, S29, S30)36-38. Compared
N
with the spectroscopy under N
2
, a new peak under CO
2
at 1580 cm–1 belonging to C=O stretching
I
E
exhibits a gradual rise upon electro-reduction, and then decreases upon electro-oxidation,
L
C
corresponding to the dissociation and formation of BPT−CO adduct (Fig. 5a, 5b)33,39. The
I 2
T
characteristic C–N Rhas a slight redshift (from 1558 to 1563 cm–1) and the appearance of
pyridine
A
N –CO peak is at 1309 cm–1 under CO , indicating the sp2 N of pyridine units is more likely
pyridine 2 2
to bind CO (Fig. 5a) 36. DFT with SMD solvent model (Fig. 5c and Supplementary Data 1) further
2
suggests that pyridine nitrogen (−0.1 kcal mol–1) is more likely to interact with CO than
2
thiadiazole nitrogen (13.0 kcal mol–1).
The complexation of BPT with CO proceeds via an ECE mechanism (where E represents
2
electron transfer and C represents chemical reaction), as suggested by CV analysis. The current
intensity under CO is nearly doubled compared to that of the first reduction peak under N (Fig.
2 2
1c). The current intensity increases with increasing scan rate as steady is approached under CO
2
and N , suggesting a scan-rate dependence is consistent with a typical ECE mechanism (Fig.
2
S31)8,40,41. Overall, the reaction mechanism between CO and BPT is proposed (Fig. 5d). BPT
2
redox carrier first accepts one electron to form BPT•–, simultaneously binding to CO (BPT(CO )•–).
2 2
Due to decreased electron density for BPT•– via electron-withdraw effect of CO , BPT(CO )•– is
2 2
further reduced at the same potential for the following complexation with the second CO
2
(BPT(CO 2 ) 2 2–). S
S
E
R
P
N
The negative charge in reduced BPT species is delocalized through resonance effects,
I
E
enhancing the stability of reduced BPT and promoting a positive shift in the reduction potential.
L
C
This phenomenon is demonstrated by comparing BPT with 2,5-di(3-pyridyl)-1,3,4-thiadiazole
I
T
(2,5-DPT), which lacks reRsonance effects (Fig. 5e, 5f). In the reduced state, 2,5-DPT cannot switch
A
to resonance structure, leading to a negative shift in its reduction potential. Additionally, 2,5-DPT
exhibits a more positive oxidation potential, thus requiring more energy to release CO (Fig. 5g).
2
The resonance effects of BPT proves advantageous for CO release, highlighting its favorable
2
performance compared to 2,5-DPT.

Methods
Materials
All chemicals were purchased and not further purified. 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (CAS
No.: 15311-09-8, purity: >95%), 4,4’-bipyridine (CAS No.: 553-26-4, purity: >98%), 2,5-
di(pyridin-4-yl)thiophene (CAS No.: 141511-42-4, purity: >95%), 1,4-di(4-pyridyl)benzene (CAS
No.: 113682-56-7, purity: 97%) and 2,5-di(3-pyridyl)-1,3,4-thiadSiazole (CAS No.: 15362-52-4,
S
purity: 97%) were purchased from Pharmatech Ltd. DMSOE (anhydrous, ≥99.9%) and 1-butyl-3-
R
methylimidazolium bis(trifluoromethyl sulfonyl)imiPde (BmimTFSI) (purity: 98%) and LiTFSI
N
(purity: 99%) were purchased from Aladdin Industrial Co. Ltd. Pt wire and Ag+/Ag electrodes were
I
E
purchased from Ida Co. Ltd.
L
C
Material characterization
I
T
The morphology of all saRmples was characterized by a Hitachi Regulus 8100 scanning electron
A
microscopy. XPS spectra were collected by Thermo ESCALAB 250XI spectrometer. Single gas
permeability was recorded with Labthink BTY-B2P breathability tester. IR spectra were obtained
on Fourier transform infrared spectrometer (iS50).
UV-vis spectroscopy
BPT (5 mmol) in DMSO (7 mL) with TBAPF as supporting electrolyte was electrochemically
6
reduced at a constant current of 2 mA under N , then exposed to CO atmosphere for 30 min, and
2 2
finally put under N to be oxidized. For comparison, BPT (5 mmol) in DMSO (7 mL) with TBAPF
2 6
as supporting electrolyte was electrochemically reduced at a constant current of 2 mA under CO ,
2
and put under N to be oxidized. The optical absorption changes of BPT solution were examined
2
by a N5000Plus ultraviolet-visible spectrophotometer (wavelength from 190 to 1100 nm).
Preparation of BPT−GPL electrode
The ink was prepared by dispersing BPT (10 mg) and Vulcan XC 72R (2 mg) in a mixture of
isopropanol (0.5 mL) and chloroform (0.5 mL) containing Nafion (5 wt%, 50 μL). Then, the
mixture solution was ultrasonicated for 1 h to obtain a homogeneous ink. The ink was sprayed on
CP at a set temperature of 40 ℃. The loadings of the catalysts wSere controlled to be 1 mg cm–2.
S
The exposed geometrical surface area was maintained at 2×E2 cm2. The GPL layer was prepared by
R
a solvent evaporation method17,20. To obtain a polymPer solution (4 mg mL–1), Pebax 1657 was
N
dissolved in a mixture of ethanol and H O (v/v = 7/3) at 70 ℃. Then, the polymer solution was
2
I
E
sprayed on CP to form a GPL layer, with loading about 0.1, 0.2 and 0.5 mg cm–2. After spraying,
L
C
the electrode was dried in air for 12 h and then dried in vacuum oven for 24 h at 80 ℃.
I
T
Electrochemical charactRerization
A
Electrochemical characterization was carried out using a CHI 660E electrochemical workstation
(CHI660E, Chenhua) in a conventional single-compartment electrolytic cell (Ida, Tianjin) at 25 ℃.
A Pt wire and an Ag/Ag+ electrode (R0202, Ida) were used as counter electrode and quasi-reference
electrode with ferrocene as the internal reference, respectively. And, a glassy carbon electrode
(0.071 cm2) was applied as a work electrode.CV data were collected for the molecules (2 mmol)
in DMSO (10 mL) with TBAPF (0.1 mol L–1) under N and CO atmospheres, respectively. The
6 2 2
scan rate is 50 mV s−1, unless otherwise specified. To study the effects of cationic species on the
redox behavior of molecules, LiTFSI, NaTFSI and TBAPF were used as the supporting salt. The
6
Ag+/Ag quasi-reference electrode was calibrated by ferrocene as the internal reference. Ferrocene
(1 mmol L−1) was added to the electrolyte solution, and the oxidation (E ) and reduction (E )
Ox Red
peak potentials were collected. All potentials in this paper were provided without iR corrected and
converted to Fc+/Fc through the following equation:
potential (Fc+/Fc) = potential (Ag+/Ag)− 𝐸Ox+𝐸 Red (6)
2
Assemble of flow-based eDAC S
S
Electrochemical CO
2
capture and release tests were conducEted in a two-chamber flow cell (details
R
in Fig. 4a and S21). The system was assembled by uPsing BPT−GPL and LiFePO as CO capture
4 2
N
and counter electrodes, respectively, which were separated by a glass microfiber filter. BmimTFSI
I
E
was selected as the electrolyte due to its limited solubility of O compared to CO , which has a
2 2
L
C
quadrupole moment. LiTFSI was used as the supporting salt. The electrolyte solution was prepared
I
T
by dissolving LiTFSI (1 mRol L−1) in BmimTFSI ionic liquid before each test. 320 and 200 μL of
A
electrolyte were placed on the counter and working electrode, respectively. The BPT−GPL
electrode was continuously exposed to CO feed gas (~400 ppm CO , 21% O , balanced with N ).
2 2 2 2
The gas flow rate was 4 mL min−1 and was monitored by mass flow meter (Senvenstar, CS200,
±0.2%). The calculated Reynolds number (Re) was below 2000 (Equation 6). An online sensor
(COZIR-LP3, GSS) at gas exit was equipped to monitor CO concentration changes. The CO
2 2
sensor was calibrated with standard gas at the same flow rate (4 sccm). Charge-discharge cycles
were conducted on a battery test system (LAND, Wuhan). For CO capture, BPT−GPL (2 cm × 2
2
cm) was reduced under 0.2 mA cm–2, following a rest. For CO release, the CO -adducts were
2 2
oxidized at 0.2 mA cm–2, following a rest. The temperature of the flow cell was kept at ~25°C. No
hydrogen gas was produced, analyzed by using a gas chromatography (SRI 8610C, USA) equipped
with a helium ionization detector (HID) and a thermal conductivity detector (TCD).
𝜌𝑣𝑑
𝑅𝑒 = (7)
𝜇
Where ρ represents the gaseous density; v represents the flow rate; d is the diameter of pipe; μ is
the viscosity. S
S
The cell resistance was measured by electrochemical impeEdance spectroscopy (Interface 1010E,
R
Gamry), and all the cell voltages in our work wereP reported without IR compensation (non-IR
N
corrected). The open-circuit voltage (OCV) was first measured, followed by electrochemical
I
E
impedance spectroscopy at the OCV with a frequency range of 0.1 Hz to 10,000 Hz.
L
C
Treatment of the glass microfiber filter
I
T
The glass microfiber filterR (3×3 cm2, Whatman, GF/A) was dried in vacuo at 100 ℃ for 24 h before
A
use.
DFT calculations
DFT calculations have been carried out by ORCA quantum chemistry software42 (Version 6.0.0).
The B3LYP functional was adopted for all calculations. For geometry optimization and frequency
calculations, the 6-311+G(d,p) basis set43 was used, and the optimal geometry for each compound
was determined. The single point energy calculations were performed with a larger basis set the 6-
311++G (2df, 2pd) basis set. The DFT-D3 dispersion correction with BJ-damping44 was applied to
correct the weak interaction to improve the calculation accuracy. The SMD implicit solvation
model was used to account for the solvation effect. Finally, the single point energy of each
compound was added to the free energy correction terms calculated before to obtain the Gibbs free
energy. Then, the Gibbs free energy change of the binding process was calculated.
MD simulations
The MD simulations were performed in the GROMACS 2021 software package45-47, and the
simulations were performed 20 ns. In NPT simulations, the tempeSrature was maintained by the V-
S
rescale thermostat at 298.15 K and the pressure was maintaiEned at 1 bar by the Berendsen barostat
R
in an isotropic manner. The Particle-Mesh-Ewald (PPME) with a fourth-order interpolation was
N
used to evaluate the electrostatic interactions and whereas a cutoff of 1.4 nm was employed to
I
E
calculate the short-range van der Waals interactions48.
L
C
I
T
Data availability R
A
The data generated or analyzed in this study are provided in the manuscript and the Supplementary
Information. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a system for electrochemical direct air capture (eDAC) using a novel pyridine derivative, 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (BPT), in conjunction with a gas permeation layer (GPL) to mitigate oxygen-induced parasitic reactions. While the conceptual framework is timely and relevant for CO₂ separation, I cannot recommend the manuscript in its current form. The central claims—particularly regarding robustness and long-term electrochemical stability—are not adequately supported by the data. Below are my major revision points.\n1. Choice and Justification of BPT\nThe rationale for selecting BPT is underdeveloped. Many alternatives exist for electron-withdrawing substituents or conjugation-extended pyridines. The thiadiazole ring is relatively uncommon and should be justified with comparative data or literature precedent. A brief discussion of its electronic influence (e.g., quantified via Hammett σ constants or electrochemical comparisons) would help contextualize this choice.\n2. Insufficient Evidence for Oxygen Tolerance\nThe central claim of O₂ stability is primarily supported by a single CV trace (Fig. 1f), which only probes short timescales. No time-dependent electrochemical or spectroscopic data are provided to demonstrate long-term stability or suppression of oxidative degradation. Additional studies, such as repeated cycling under O₂, are necessary to substantiate robustness. Furthermore, Figure 1f visually obscures the blue traces under red ones, making it difficult to independently assess O₂ influence. Please clarify the visual presentation and provide quantifiable comparisons.\n3. Mechanistic Evidence for CO₂ Binding Site and Stability\nThe mechanistic proposal hinges on selective CO₂ binding at the pyridine nitrogen of BPT, yet this is supported only indirectly via in situ IR and DFT. No direct structural or spectroscopic evidence (e.g., NMR, HRMS, isotopic labeling) is presented to confirm the identity or reversibility of the CO₂ adduct.\n- Line 92: Equal anodic and cathodic peak currents do not imply thermodynamic stability; please revise this interpretation.\n- Line 144: Figure S1 shows diminished current over 50 cycles, contradicting the claim of 'high reversibility.'\n- Line 206: Please elaborate on how charge delocalization leads to reduced O₂ sensitivity—this concept is mentioned but not explained.\nIn addition, Figure 1h shows divergent UV–vis behavior depending on the CO₂ exposure sequence, suggesting additional pathways or side reactions not accounted for in the proposed mechanism. Further mechanistic validation would significantly strengthen the manuscript.\nSeveral other figures (e.g., S1, 1e, 1h) imply degradation or irreversible processes that contradict claims of 'sustained reversibility.' Please moderate language accordingly and re-evaluate your interpretation of electrochemical reversibility. I cannot be convinced its stability by the presented data.\n4. Discussion of Energy Metrics Lacks Clarity\nThe distinction between total energy consumption (kJ) and normalized energy consumption (kJ mol⁻¹ CO₂) is not clearly delineated in the main text (line 138). This leads to potential confusion when interpreting system performance. Please restructure this discussion to explicitly distinguish device-level and molecular-level energy metrics.\n5. Figure Organization and Captioning\nThe presentation of Figure 1 is confusing: panels c and d could be merged, and the caption order does not align with panel labels. Reorganizing these figures will improve clarity and prevent misinterpretation.\n6. No essential concentration was demonstrated.\nFigure 4d and f does not show any concentration than the feed gas. Additional experimental demonstration of “concentration” should be included.", "reviewer_2": "In this work, Hou et al. report a redox-swing direct-air-capture (eDAC) technology that uses an extended-conjugation bipyridine, 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (BPT), coupled with a Pebax-based gas-permeation layer (GPL). They start with different aromatic functionalization and narrow it down to BPT because of its superior performance compared to the other functionalized aromatics that they tested. The mechanism they highlight is the reduction of BPT binds CO2 to form a reversible di-anionic adduct reduces at –1.64 V vs Fc/Fc⁺, binds CO₂ to form a reversible BPT2- adduct. When spray-deposited (≈10 mg) on carbon paper, delivers 1.6 mmol g⁻¹BPT capture capacity at 1000 ppm CO₂ with 39 % electron efficiency and 162 kJ mol⁻¹ energy input after energy-recovery assumptions. Embedding BPT behind a 0.2 mg cm⁻² Pebax layer suppresses O₂ crossover (CO₂/O₂ permeability ratio 6.2/1.1= 5.6) and enables 36 cycles in ambient air (400 ppm CO₂, 21 % O₂) with 1.1 mmol g⁻¹BPT capacity, ~30 % electron utilization and 181 kJ mol⁻¹ energy input. In situ IR and DFT indicate that CO₂ binds preferentially to the pyridyl N’s, following an ECE mechanism. While this new chemistry is convincingly demonstrated, the practical relevance of the metrics, particularly under humid, O₂-rich conditions and over extended duty cycles, may be validated better or at least the conclusions around that could be bound a bit more. I have highlighted my concerns below that I believe the authors can address to make their work more robust:\nMajor points:\n1. True DAC operating window: There are limited testing at 400 ppm to call it a “true DAC” for the most part. All performance claims are made at 1000 ppm or in short (15 min) batch captures from static air. Can the authors comment on how their system could handle continuous flow tests at 400 ppm? Could the authors comment on the systems durability in such a case?\n2. Humidity tolerance: BPT was tested in DMSO with ≤10 vol % H₂O, yet atmospheric air can reach 30–80 % RH. Have the authors tested their system with CVs and cycling at ≥50 % RH (or with an aqueous/IL blend electrolyte) are necessary to validate stability. Or do the authors justify that they would use this technology by drying the feed gas at deployment?\n3. Balance faradaic efficiency: We see the electron utilization or faradaic efficiency of the process remains less than 40% even with pure CO2 feed. And it becomes lower when the CO2 concentration is lowered. What are the rest of the electrons doing? Do we have an account or estimate for the other parasitic reactions? What would the authors propose mitigating these other parasitic reactions?\n4. Material stability and lifetime: It would be nice to see if the authors can give a characterization (NMR, XPS etc if possible) of the materials, electrodes, electrolyte before and after long cycles to provide evidence that the Pebax and BPT don’t degrade significantly.\n5. Mass-normalized capacity reporting: Beauty is in the eye of the denominator. Right now the capacity is reported normalized to BPT mass. Typically, in an electrochemical process, it is always economically relevant to have these things reported normalized to electrode mass or area and specific the BPT loading/distribution. They already have a nice table S2 in the SI and perhaps these additional data points can be added there.\n6. Error bars: are these all one-off experiments? Or the data reported the average values of multiple runs. If they have error bars, I strongly encourage them to report it.\nMinor\n1. Computational methodology: are DFT free energies reported without ZPE corrections? Could the authors supply these details and confirm that implicit-solvent parameters reflect DMSO electrolyte?\n2. On page 15, do the authors mean Fig 4 when they mention Fig 3 (specifically 4d and e when they say 3d and e)\n3. Energy consumption reported as kJ/mol of CO2 is it per mol CO2 captured or released? Are they the same?\n4. Gas flow rate and Reynolds number in methods would be a nice addition. Are they the same conditions used to calibrate the CO2 sensor? Is the CO2 sensor dependent on flowrate?", "reviewer_3": "The authors investigate the electrochemical direct air capture (DAC) performance of 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (BPT) as a redox-active material. The inclusion of pyridine groups and extended conjugation appears to enhance the molecule’s oxidative stability and CO₂ uptake capacity. Additionally, the authors use a thin-film membrane layer to mitigate O₂ permeation into the bulk region, which was suggested to improve stability.\nHowever, while these modifications offer benefits, they may also introduce significant mass transport limitations for CO₂. The authors themselves acknowledge reduced sorption kinetics, which I believe are primarily due to the presence of the membrane layer, potentially acting as a barrier to CO₂ diffusion. Overall, while I find the work to be interesting and potentially valuable, it currently lacks sufficient depth for publication in Nature Communications. '\nI have a few specific concerns and suggestions:\nWere the gas permeation tests conducted under humid conditions? If not, the data may not accurately reflect real-world scenarios. The swelling of the membrane under humid conditions could significantly increase O₂ permeance, leading to misleading conclusions if this effect is not accounted for.\nWhile the authors examine the effect of H₂O, it appears that oxidative degradation under combined O₂ and humidity conditions was not evaluated. Given that real DAC systems operate in ambient air with varying humidity, it is essential to assess stability under all relevant conditions. Previous studies have suggested that humidity can accelerate degradation processes.\nThe conditions under which the data in Figures 4d and 4f were collected need to be clearly described. What gas mixtures, relative humidity (RH%), and flow rates were used? This information is critical for evaluating the relevance and reproducibility of the reported results."}, "author_response": "We thank the reviewer for this specific comment. The thiadiazole ring as a strong electron-withdrawing group has been reported in articles (Organic Letters 19, 3275-3278 (2017), J. Am. Chem. Soc. 146, 1318-1325 (2024)). Owing to the highly electronegative N (3.04) and S (2.58) atoms, the thiadiazole ring reduces the electron density of adjacent groups by resonance and inductive effects, exhibiting an electron-withdrawing effect. Compared with electron-withdrawing groups such as cyano (CN) and nitro (NO ), thiadiazole is an extended conjugated framework, enhancing electron delocalization, which is beneficial for the positive shift of BPT’s reduction potential and the enhancement of molecular stability.\nThe following sentences was added in manuscript to highlight the selection of BPT (page 4).\nThiadiazole has an extended π-conjugated framework that favors electron delocalization, which has been widely applied in organic semiconductors12. Due to high electronegativity of S (2.58) and N (3.04) centers, thiadiazole acts as an electron-withdrawing group in 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (BPT), inducing anodic shift in reduction potential relative to BPy13,14, making it suitable as a redox carrier.\nReference:\n12. Carrington, M. E. et al. Associative pyridinium electrolytes for air-tolerant redox flow batteries. Nature 623, 949-955 (2023).\n13. Hasegawa, T. et al. Thiadiazole-fused quinoxalineimide as an electron-deficient building block for N-type organic semiconductors. Organic Letters 19, 3275-3278 (2017).\n14. Fu, G. et al. Construction of thiadiazole-bridged sp2-carbon-conjugated covalent organic frameworks with diminished excitation binding energy toward superior photocatalysis. J. Am. Chem. Soc. 146, 1318-1325 (2024).\n2. Insufficient Evidence for Oxygen Tolerance\nThe central claim of O₂ stability is primarily supported by a single CV trace (Fig. 1f), which only probes short timescales. No time-dependent electrochemical or spectroscopic data are provided to demonstrate long-term stability or suppression of oxidative degradation. Additional studies, such as repeated cycling under O₂, are necessary to substantiate robustness. Furthermore, Figure 1f visually obscures the blue traces under red ones, making it difficult to independently assess O₂ influence. Please clarify the visual presentation and provide quantifiable comparisons.\nResponse:\nWe thank the reviewer for this important observation. We supplemented a continuous CV cycling in DMSO with 0.1 mol L⁻1 TBAPF under 12% CO + 20% O (Fig. S4).\n6 2 2\nThe peak of BPT-related after 300 cycles nearly overlaps with the first one, corroborating the exceptional O stability.\n2\nThe explanation has been added in our manuscript (page 11).\nO reduction operates as a distinct process, keeping BPT–CO adduct (e.g., BPT(CO )•–) stable as evidenced by the unchanged reversibility and position of the CO -related peak (Fig. S4).\n2 2 2\nSupporting information:\n2 2\nFig. 2 CV of BPT under different conditions. (a) CV curves under pure CO (Scan rate: 10~300 mV s–1). (b) Relationship between i and υ1/2. (c, d) CV curves under N (dot) and different CO concentration (solid). (e) CV curves with different mixture gas (15%~12% CO with 0~20% O ). (f) CV curves in DMSO with 0.1 M TBAPF and a certain amount of H O (0~10 vol%) under CO .\n2 2 2 6 2 2\n3. Mechanistic Evidence for CO₂ Binding Site and Stability\n3.1 The mechanistic proposal hinges on selective CO₂ binding at the pyridine nitrogen of BPT, yet this is supported only indirectly via in situ IR and DFT. No direct structural or spectroscopic evidence (e.g., NMR, HRMS, isotopic labeling) is presented to confirm the identity or reversibility of the CO₂ adduct.\nResponse:\nWe appreciate the reviewer’s thoughtful comments. We supplemented the 1H NMR spectra of BPT reduced under CO to validate the interaction between BPT and CO (Fig. S1). 1H NMR spectra after electrolysis under CO shifts upfield compared with BPT, confirming the generation of BPT−CO carbonate adduct. Bulk reduction of BPT often yields a mixture of adducts and radical species. The complexity of reaction system results in the challenging for traditional characterization methods to capture its dynamic behavior. Therefore, we indirectly prove the reaction mechanism by combining in situ infrared spectroscopy with theoretical calculations.\nThe following text is added to page 8 of the manuscript and the1H NMR spectrum are supplemented in supporting information (Fig. S1):\nThe proton nuclear magnetic resonance (1H NMR) spectrum after a capture electrolysis shifts upfield compared with BPT, reconfirming the CO binding with BPT (Fig. S1).\n3.2 - Line 92: Equal anodic and cathodic peak currents do not imply thermodynamic stability; please revise this interpretation.\nResponse:\nWe thank the reviewer for this insightful comment. We realize that the equal anode and cathode currents suggesting thermodynamic stability is an inappropriate expression.\nThe nearly equal peak currents of anode and cathode indicate that BPT−CO adduct remains relatively stable without significant decomposition or side reactions. The binding and dissociation process of BPT and CO is reversible and stable. To ensure accuracy in expression, we make some modifications in manuscript (page 6).\nThe quasi-reversibility, indicated by near-equal anodic and cathodic currents (i ≈ i ) (Fig. 1a), suggests a kinetically facile and stable process.\n3.3 - Line 144: Figure S1 shows diminished current over 50 cycles, contradicting the claim of 'high reversibility.'\nWe have made some modifications in manuscript (Page 8).\nPeak intensity recovers upon oxidation, suggesting reversible adduct formation and dissociation, showing its potential for stable capture-release performance in long-cycle tests.\n6 2\n(Scan rate: 50 mV s-1)\nFig. 4 CO capture-release performance during cycling charge-discharge. (a) Schematic illustration of a two-chamber gas-phase flow cell. (b) CO concentration and corresponding charge-discharge profiles under air. (c) Plot of Faradaic efficiency (blue) and release/capture (pink) versus cycle number under air. (d) CV curves of GC with and without GPL under N and O . (e) Comparison of redox-active materials for CO capture. (f) CO concentration and corresponding charge-discharge profiles under humid air. (g) Plot of electron utilization (orange), release/capture (blue) and coulombic efficiency (pink) versus cycle number under humid air.\n3.4 - Line 206: Please elaborate on how charge delocalization leads to reduced O₂ sensitivity—this concept is mentioned but not explained.\nResponse:\nWe thank the reviewer for raising this important suggestion. BPT exhibits reduced O sensitivity via resonance to disperse the negative charges. Reduced BPT is a three-ring conjugated system, where the negative charges uniformly distributed (Adv. Funct. Mater. 32 (2022)). Charge delocalization can reduce electron density of sp2 N, leading to a positive shift in reduction potential, making reduced BPT more difficult to provide electrons to oxygen and undergo oxidation reactions. To prove this point, we compare 2,5-di(3-pyridyl)-1,3,4-thiadiazole (2,5-DPT) having the same sp2 N but with partially disrupted conjugation (Fig. 5e, 5f). 2,5-DPT cannot undergo resonance to form a three-ring conjugated structure upon reduction, impeding charge delocalization, resulting in a more negative reduction potential. A more negative potential implies a higher local electron density, promoting the reaction of reduced 2,5-DPT with O .\n2\n2\nEncouragingly, reduced BPT displays a quinoidal-type skeleton29, facilitating charge delocalization within the three-ring conjugated system. The charge delocalization reduces the local electron density, inhibiting oxidative degradation and stabilizing the reduced species.\n3.5 In addition, Figure 1h shows divergent UV– (truncated for brevity, see full text)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I appreciate the authors' efforts in revising the manuscript and providing additional clarification. Based on the current revisions, I still cannot recommend the manuscript for publication.\n2. In the additional CV provided in Figure S4, the potential does not appear to be correctly referenced to Fc/Fc+. Similarly, Figure 4d should report potentials referenced properly to Fc/Fc+ for consistency and accuracy.\n3.1 For the NMR analysis, the explanation for the observed shifts still appears incomplete. A CO2 adduct that withdraws electron density from the ring would typically lead to a downfield shift, so the rationale for the reported upfield shift remains unclear. In addition, the BPT–CO2 peaks are relatively small compared to BPT itself, which makes it difficult to substantiate the central chemical hypothesis. The spectra of the BPT2−/BPT radical anion also do not show clear or significant peak shifts, raising further questions about the proposed redox processes.\n3.2 The interpretation of the CV peaks continues to be problematic. Equal peak sizes generally indicate a chemically reversible redox event with minimal side reactions; this does not, on its own, support claims about kinetic facility or a (ambigous) stable intermediate. These interpretations should be clearly distinguished.\n3.3 The explanation for the reduced peak intensity remains unconvincing. As noted earlier, if the system is chemically reversible, mass-transfer limitations should not account for the observed differences. This appears inconsistent with the authors' argument in point 3.2. Moreover, under stirring conditions, one would not typically expect peaks of such cleanliness. This suggests that either the experimental conditions or the interpretation requires further clarification.\n3.4 The response to point 3.4 is generally reasonable but still incomplete. The discussion focuses on comparing BPT with 2,5-BPT and noting the lower reduction potential of the latter, but it does not adequately address the oxygen stability of BPT itself despite its low reduction potential at around -1.7 V. Furthermore, no substantial new discussion addressing this issue was incorporated into the main text. The highlighted revision largely reiterates previous statements without providing deeper insight.", "reviewer_2": "The authors have made significant efforts to address my concerns satisfactorily."}, "author_response": "We sincerely appreciate your objective evaluation and constructive suggestions.\n1. In the additional CV provided in Figure S4, the potential does not appear to be correctly referenced to Fc/Fc+. Similarly, Figure 4d should report potentials referenced properly to Fc/Fc+ for consistency and accuracy.\nResponse: We sincerely appreciate your insightful feedback on the reference potential standard. We have corrected the data using Fc/Fc+ as the internal standard and revised Figure S4 and 4d.\nThese corrections have been incorporated into the revised version (Figure 4d) and the supplementary information (Figure S4).\n6\n2.1 For the NMR analysis, the explanation for the observed shifts still appears incomplete. A CO adduct that withdraws electron density from the ring would typically lead to a downfield shift, so the rationale for the reported upfield shift remains unclear. In addition, the BPT–CO peaks are relatively small compared to BPT itself, which makes it difficult to substantiate the central chemical hypothesis. The spectra of the BPT2−/BPT radical anion also do not show clear or significant peak shifts, raising further questions about the proposed redox processes.\nResponse: We thank the reviewer for the comment on NMR characterizations. We understand the concern that the introduction of an electron-withdrawing group, such as CO , can lead to a downfield shift. However, we would like to offer a more detailed explanation for the observed upfield shift. For example, the amide hydrogen is shifted upfield from 10.89 ppm (pristine isoindigos, IId) to 10.33 ppm (IId–CO ) (Nat. Commun. 2024, 15, 1175). This phenomenon is attributed to the two-electron-reduced IId–CO adduct increasing the global electron density of the molecule, resulting in an upfield shift of the NMR peak compared to that of IId. The 1H NMR spectra of 1,5-bis[3-(trimethylammonio)propyl]anthraquinones–CO (1,5-BTMAPAQs–CO ) also exhibits a upfield shift compared to 1,5-BTMAPAQs (ACS Energy Lett. 2024, 9, 3526-3535). Similarly, compared to BPT, although CO can induce a local deshielding effect, the injection of two electrons into the BPT–CO adduct significantly increases the overall electron density, resulting in an upfield shift in its NMR spectrum.\nThe relatively lower intensity of the BPT–CO signals compared to pristine BPT is attributed to the dynamic chemical equilibrium of the BPT–CO capture process under the experimental NMR conditions. The observed spectra is a steady-state concentration of the adduct, even if it is not the dominant species in the equilibrium mixture.\nThe absence of significant shifts for BPT•– is consistent with its paramagnetic nature. The unpaired electron typically induces significant line broadening (T2 relaxation) and localized field effects, which can mask subtle chemical shifts. We do not observe a clear shift for the radical anion, which actually aligns with the expected behavior of paramagnetic intermediates in NMR.\nWe have modified the relevant descriptions in the manuscript (page 7).\nThe structure change is reflected by the chemical shifts in the proton nuclear magnetic resonance (1H NMR) spectrum after capture electrolysis (Fig. S1)22-24."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The CV data are convincing and support reversible electrochemical behavior under the studied conditions. However, I remain unconvinced by the NMR analysis used to support the proposed molecular mechanism. In Figure S1, the BPT–CO₂ adduct shows only very weak signals and is described as a non-dominant species in equilibrium. If the CO₂ adduct is indeed minor or present only at negligible concentration, it becomes difficult to justify the proposed redox-mediated CO₂ capture mechanism at a fundamental level, as the central intermediate is not spectroscopically well supported. Given this, the mechanistic interpretation should be presented more cautiously, and the mechanism shown in the figure should be explicitly labeled as \"proposed mechanism\" on the figure in addition to the caption.\nRegarding Figure R1, the response does not clarify whether the CVs were recorded under stirring or not. This distinction is critical, particularly given that the authors invoke localized CO₂ depletion to explain changes in peak intensity. Please explicitly state whether these CV measurements were performed while stirring.\nFor consistency in the CV data under 20% O₂ conditions, the manuscript should clearly specify that the observed O₂ stability is demonstrated under CV cycling conditions. In addition, the statement that “charge delocalization promotes a uniform distribution of electron density, reducing accessible oxygen attack sites, thereby suppressing oxidative degradation and stabilizing the reduced species” is not fully accurate. While charge delocalization may slow electron-transfer kinetics to oxygen, it does not alter the thermodynamic driving force, which has already been shown in the CV to favor oxygen reduction at potentials more positive than the BPT reduction. This section should be revised to avoid leading to an incorrect mechanistic conclusion."}, "author_response": "We appreciate the Reviewer’s valuable feedback and suggestions. We have added a clear “Proposed mechanism” label in Fig. 5d for clear visibility. And, we have revised the related explanation in the manuscript (page 17 and 18).\nProposed mechanistic studies\nthe reaction mechanism between CO and BPT is proposed (Fig. 5d).\n2\n2\n2. Regarding Figure R1, the response does not clarify whether the CVs were recorded under stirring or not. This distinction is critical, particularly given that the authors invoke localized CO₂ depletion to explain changes in peak intensity. Please explicitly state whether these CV measurements were performed while stirring.\nResponse: We thank the Reviewer for pointing this out. We have supplemented the test conditions in the caption of Figure R1 and the “Methods” section of the manuscript.\nFig. R1. CV of 5 mM BPT dissolved in DMSO with 0.1 M TBAPF under CO . (test condition: CO was bubbled into the solution with stirring, followed by CV measurements without stirring. After 100 cycles, CO was bubbled into the solution again under stirring.)\n6 2 2\n(Page 23) CO was bubbled into the solution with stirring, followed by CV measurements without stirring."}, {"round": 4, "reviewer_comments": {"reviewer_1": "All of my concerns have been adequately addressed. I recommend publication."}, "author_response": ""}]

Decision letter