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"Ma, Lu"
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Heterogeneous Fe3 single-cluster catalyst for ammonia synthesis via an associative mechanism
The current industrial ammonia synthesis relies on Haber–Bosch process that is initiated by the dissociative mechanism, in which the adsorbed N
2
dissociates directly, and thus is limited by Brønsted–Evans–Polanyi (BEP) relation. Here we propose a new strategy that an anchored Fe
3
cluster on the θ-Al
2
O
3
(010) surface as a heterogeneous catalyst for ammonia synthesis from first-principles theoretical study and microkinetic analysis. We have studied the whole catalytic mechanism for conversion of N
2
to NH
3
on Fe
3
/θ-Al
2
O
3
(010), and find that an associative mechanism, in which the adsorbed N
2
is first hydrogenated to NNH, dominates over the dissociative mechanism, which we attribute to the large spin polarization, low oxidation state of iron, and multi-step redox capability of Fe
3
cluster. The associative mechanism liberates the turnover frequency (TOF) for ammonia production from the limitation due to the BEP relation, and the calculated TOF on Fe
3
/θ-Al
2
O
3
(010) is comparable to Ru B5 site.
The current industrial ammonia synthesis relies on the Haber-Bosch process that is limited by the Brønsted–Evans–Polanyi relation. Here, the authors propose a new strategy that an anchored Fe
3
on θ-Al
2
O
3
(010) surface serves as a heterogeneous single cluster catalyst for ammonia synthesis from first-principles calculations and microkinetic analysis.
Journal Article
Correlation between manganese dissolution and dynamic phase stability in spinel-based lithium-ion battery
Historically long accepted to be the singular root cause of capacity fading, transition metal dissolution has been reported to severely degrade the anode. However, its impact on the cathode behavior remains poorly understood. Here we show the correlation between capacity fading and phase/surface stability of an LiMn
2
O
4
cathode. It is revealed that a combination of structural transformation and transition metal dissolution dominates the cathode capacity fading. LiMn
2
O
4
exhibits irreversible phase transitions driven by manganese(III) disproportionation and Jahn-Teller distortion, which in conjunction with particle cracks results in serious manganese dissolution. Meanwhile, fast manganese dissolution in turn triggers irreversible structural evolution, and as such, forms a detrimental cycle constantly consuming active cathode components. Furthermore, lithium-rich LiMn
2
O
4
with lithium/manganese disorder and surface reconstruction could effectively suppress the irreversible phase transition and manganese dissolution. These findings close the loop of understanding capacity fading mechanisms and allow for development of longer life batteries.
To unlock the potential of Mn-based cathode materials, the fast capacity fading process has to be first understood. Here the authors utilize advanced characterization techniques to look at a spinel LiMn
2
O
4
system, revealing that a combination of irreversible structural transformations and Mn dissolution takes responsibility.
Journal Article
Manipulating the oxygen reduction reaction pathway on Pt-coordinated motifs
Electrochemical oxygen reduction could proceed via either 4e
−
-pathway toward maximum chemical-to-electric energy conversion or 2e
−
-pathway toward onsite H
2
O
2
production. Bulk Pt catalysts are known as the best monometallic materials catalyzing O
2
-to-H
2
O conversion, however, controversies on the reduction product selectivity are noted for atomic dispersed Pt catalysts. Here, we prepare a series of carbon supported Pt single atom catalyst with varied neighboring dopants and Pt site densities to investigate the local coordination environment effect on branching oxygen reduction pathway. Manipulation of 2e
−
or 4e
−
reduction pathways is demonstrated through modification of the Pt coordination environment from Pt-C to Pt-N-C and Pt-S-C, giving rise to a controlled H
2
O
2
selectivity from 23.3% to 81.4% and a turnover frequency ratio of H
2
O
2
/H
2
O from 0.30 to 2.67 at 0.4 V versus reversible hydrogen electrode. Energetic analysis suggests both 2e
−
and 4e
−
pathways share a common intermediate of *OOH, Pt-C motif favors its dissociative reduction while Pt-S and Pt-N motifs prefer its direct protonation into H
2
O
2
. By taking the Pt-N-C catalyst as a stereotype, we further demonstrate that the maximum H
2
O
2
selectivity can be manipulated from 70 to 20% with increasing Pt site density, providing hints for regulating the stepwise oxygen reduction in different application scenarios.
Controlling O
2
reduction pathways can help optimize catalytic activity and product selectivity. Here the authors report facile manipulation of 2e
‒
/4e
‒
pathways on Pt-coordinated motifs by varying the Pt site density or the coordination environment.
Journal Article
Chemisorption of polysulfides through redox reactions with organic molecules for lithium–sulfur batteries
Lithium–sulfur battery possesses high energy density but suffers from severe capacity fading due to the dissolution of lithium polysulfides. Novel design and mechanisms to encapsulate lithium polysulfides are greatly desired by high-performance lithium–sulfur batteries towards practical applications. Herein, we report a strategy of utilizing anthraquinone, a natural abundant organic molecule, to suppress dissolution and diffusion of polysulfides species through redox reactions during cycling. The keto groups of anthraquinone play a critical role in forming strong Lewis acid-based chemical bonding. This mechanism leads to a long cycling stability of sulfur-based electrodes. With a high sulfur content of ~73%, a low capacity decay of 0.019% per cycle for 300 cycles and retention of 81.7% over 500 cycles at 0.5 C rate can be achieved. This finding and understanding paves an alternative avenue for the future design of sulfur–based cathodes toward the practical application of lithium–sulfur batteries.
Novel cathode design holds the key to enabling high performance lithium-sulfur batteries. Here the authors utilize anthraquinone to chemically stabilize polysulfides, revealing that the keto groups of anthraquinone play a critical role in forming strong Lewis acid-based chemical bonding.
Journal Article
High temperature shockwave stabilized single atoms
The stability of single-atom catalysts is critical for their practical applications. Although a high temperature can promote the bond formation between metal atoms and the substrate with an enhanced stability, it often causes atom agglomeration and is incompatible with many temperature-sensitive substrates. Here, we report using controllable high-temperature shockwaves to synthesize and stabilize single atoms at very high temperatures (1,500–2,000 K), achieved by a periodic on–off heating that features a short on state (55 ms) and a ten-times longer off state. The high temperature provides the activation energy for atom dispersion by forming thermodynamically favourable metal–defect bonds and the off-state critically ensures the overall stability, especially for the substrate. The resultant high-temperature single atoms exhibit a superior thermal stability as durable catalysts. The reported shockwave method is facile, ultrafast and universal (for example, Pt, Ru and Co single atoms, and carbon, C3N4 and TiO2 substrates), which opens a general route for single-atom manufacturing that is conventionally challenging.
Journal Article
Elucidating anionic oxygen activity in lithium-rich layered oxides
Recent research has explored combining conventional transition-metal redox with anionic lattice oxygen redox as a new and exciting direction to search for high-capacity lithium-ion cathodes. Here, we probe the poorly understood electrochemical activity of anionic oxygen from a material perspective by elucidating the effect of the transition metal on oxygen redox activity. We study two lithium-rich layered oxides, specifically lithium nickel metal oxides where metal is either manganese or ruthenium, which possess a similar structure and discharge characteristics, but exhibit distinctly different charge profiles. By combining X-ray spectroscopy with operando differential electrochemical mass spectrometry, we reveal completely different oxygen redox activity in each material, likely resulting from the different interaction between the lattice oxygen and transition metals. This work provides additional insights into the complex mechanism of oxygen redox and development of advanced high-capacity lithium-ion cathodes.
A reversible oxygen redox process contributes extra capacity and understanding this behavior is of high importance. Here, aided by resonant inelastic X-ray scattering, the authors reveal the distinctive anionic oxygen activity of battery electrodes with different transition metals.
Journal Article
Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper–molecular solid catalyst
Ammonia (NH
3
) is essential for modern agriculture and industry and is a potential energy carrier. NH
3
is traditionally synthesized by the Haber–Bosch process at high temperature and pressure. The high-energy input of this process has motivated research into electrochemical NH
3
synthesis via nitrogen (N
2
)–water reactions under ambient conditions. However, the future of this low-cost process is compromised by the low yield rate and poor selectivity, ascribed to the inert N≡N bond and ultralow solubility of N
2
. Obtaining NH
3
directly from non-N
2
sources could circumvent these challenges. Here we report the eight-electron direct electroreduction of nitrate to NH
3
catalysed by copper-incorporated crystalline 3,4,9,10-perylenetetracarboxylic dianhydride. The catalyst exhibits an NH
3
production rate of 436 ± 85 μg h
−1
cm
−2
and a maximum Faradaic efficiency of 85.9% at −0.4 V versus a reversible hydrogen electrode. This notable performance is achieved by the catalyst regulating the transfer of protons and/or electrons to the copper centres and suppressing hydrogen production.
Electrochemically reducing nitrogen-containing molecules could provide less energy-intense routes to produce ammonia than the traditional Haber–Bosh process. Here the authors use a catalyst comprising Cu embedded in an organic molecular solid to synthesize ammonia from nitrate ions.
Journal Article