Shuttling protons in ammonia synthesis
An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One ...relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto
et al.
report the use of a tetraalkyl phosphonium salt in place of ethanol (see the Perspective by Westhead
et al.
). This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium.
Science
, abg2371, this issue p.
1187
; see also abi8329, p.
1149
A phosphonium cation acts as a stable proton shuttle during electrochemical ammonia synthesis.
Ammonia (NH
3
) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH
3
synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH
3
production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.
In addition to its use in the fertilizer and chemical industries1, ammonia is currently seen as a potential replacement for carbon-based fuels and as a carrier for worldwide transportation of ...renewable energy2. Implementation ofthis vision requires transformation of the existing fossil-fuel-based technology for NH3 production3 to a simpler, scale-flexible technology, such as the electrochemical lithium-mediated nitrogen-reduction reaction3,4. This provides a genuine pathway from N2 to ammonia, but it is currently hampered by limited yield rates and low efficiencies4-12. Here we investigate the role of the electrolyte in this reaction and present a high-efficiency, robust process that is enabled by compact ionic layering in the electrode-electrolyte interface region. The interface is generated by a high-concentration imide-based lithium-salt electrolyte, providing stabilized ammonia yield rates of 150 ± 20 nmol s-1 cm-2 and a current-to-ammonia efficiency that is close to 100%. The ionic assembly formed at the electrode surface suppresses the electrolyte decomposition and supports stable N2 reduction. Our study highlights the interrelation between the performance of the lithium-mediated nitrogen-reduction reaction and the physicochemical properties of the electrode-electrolyte interface. We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia production.
Stable metal anode cycling for high energy density batteries can be realized through modification of electrolyte composition and optimization of formation protocols, i.e., electrode interphase ...preconditioning conditions. However, the relationship between these and the electrochemical performance is still unclear due to a lack of molecular level understanding of electric double layer (EDL) changes with modification of these two parameters. Herein, we examine the impact of ionic liquid (IL) electrolyte composition (salt concentration and cosolvent) and preconditioning cycling conditions on Li anode performance through EDL changes affecting both the solid–electrolyte interphase (SEI) and deposition morphology. Each electrolyte composition results in a particular interfacial Li-ion solvation environment, which controls the reductive stability, Li deposition potential, and ultimately the composition of properties of the SEI. The latter is dependent on the EDL composition such as the IL cation/Li-anion ratio or the presence of other surface active additives. It is found that in a superconcentrated electrolyte, a high current density (≥10.0 mA cm–2/1.0 mAh cm–2) is beneficial during the metal anode preconditioning step, compared with the case of low Li salt-containing IL. This correlates with a predominance of Li x (anion) y (x > y) at a highly negatively charged interface, which is present when higher current densities are used for preconditioning, as suggested by molecular dynamics simulations. In contrast, for the lower viscosity superconcentrated electrolyte containing 20 wt % of ether cosolvent, a more moderate preconditioning step current density (6.0 mA cm–2/1.0 mAh cm–2) leads to an optimized deposition morphology and improved cycling performance. This is a consequence of the competing processes of ion transport at the interface, which controls the Li+ ion flux and the intrinsic reduction kinetics occurring at the more negative electrode.
The lithium mediated reduction of N2 is one of the only available approaches to electrochemical ammonia production at significant yields under ambient conditions. However, much remains to be ...investigated about the various electrochemical processes and side reactions that are involved. Herein, we have examined the effects of parameters including electrode potential, convection, N2 pressure, and water content to refine and control the process. We demonstrate that a closely linear ammonia yield can be maintained during experiments up to 60 h in length, with approximately constant faradaic efficiency. This steady state operation appears to be preceded by a coating of the electrode surface with the products of the reductive electrolyte decomposition, such as LiF. We demonstrate ammonia yield rates above 1 nmol s–1 cm–2 and faradaic efficiencies as high as 60% through the improved control of the reaction conditions.
Membranes are ubiquitous in nature with primary functions that include adaptive filtering and selective transport of chemical/molecular species. Being critical to cellular functions, they are also ...fundamental in many areas of science and technology. Of particular importance are the adaptive and programmable membranes that can change their permeability or selectivity depending on the environment. Here, we explore implementation of such biological functions in artificial membranes and demonstrate two-dimensional self-assembled heterostructures of graphene oxide and polyamine macromolecules, forming a network of ionic channels that exhibit regulated permeability of water and monovalent ions. This permeability can be tuned by a change of pH or the presence of certain ions. Unlike traditional membranes, the regulation mechanism reported here relies on specific interactions between the membranes' internal components and ions. This allows fabrication of membranes with programmable, predetermined permeability and selectivity, governed by the choice of components, their conformation and their charging state.
The instability and expense of anodes for water electrolyzers with acidic electrolytes can be overcome through the implementation of a cobalt‐iron‐lead oxide electrocatalyst, Co–Fe–PbOx, that is ...self‐healing in the presence of dissolved metal precursors. However, the latter requirement is pernicious for the membrane and especially the cathode half‐reaction since Pb2+ and Fe3+ precursors poison the state‐of‐the‐art platinum H2 evolving catalyst. To address this, we demonstrate the invariably stable operation of Co–Fe–PbOx in acidic solutions through a cobalt‐selective self‐healing mechanism without the addition of Pb2+ and Fe3+ and investigate the kinetics of the process. Soft X‐ray absorption spectroscopy reveals that low concentrations of Co2+ in the solution stabilize the catalytically active Co(Fe) sites. The highly promising performance of this system is showcased by steady water electrooxidation at 80±1 °C and 10 mA cm−2, using a flat electrode, at an overpotential of 0.56±0.01 V on a one‐week timescale.
The Co–Fe–PbOx water oxidation anode catalyst facilitates long‐term O2 evolution reaction in acidic electrolytes at elevated temperatures. Through a cobalt‐selective self‐healing mechanism, this catalyst operates in the absence of dissolved Pb2+ and Fe3+ precursors deeming it a prospective anode material for low‐cost water electrolyzer systems.
Interfacial self‐assembly is a powerful organizational force for fabricating functional nanomaterials, including nanocarriers, for imaging and drug delivery. Herein, the interfacial self‐assembly of ...pH‐responsive metal–phenolic networks (MPNs) on the liquid–liquid interface of oil‐in‐water emulsions is reported. Oleic acid emulsions of 100–250 nm in diameter are generated by ultrasonication, to which poly(ethylene glycol) (PEG)‐based polyphenolic ligands are assembled with simultaneous crosslinking by metal ions, thus forming an interfacial MPN. PEG provides a protective barrier on the emulsion phase and renders the emulsion low fouling. The MPN‐coated emulsions have a similar size and dispersity, but an enhanced stability when compared with the uncoated emulsions, and exhibit a low cell association in vitro, a blood circulation half‐life of ≈50 min in vivo, and are nontoxic to healthy mice. Furthermore, a model anticancer drug, doxorubicin, can be encapsulated within the emulsion phase at a high loading capacity (≈5 fg of doxorubicin per emulsion particle). The MPN coating imparts pH‐responsiveness to the drug‐loaded emulsions, leading to drug release at cell internalization pH and a potent cell cytotoxicity. The results highlight a straightforward strategy for the interfacial nanofabrication of pH‐responsive emulsion–MPN systems with potential use in biomedical applications.
Low‐fouling, biodegradable oil‐in‐water emulsions with diameters of 100–250 nm are produced by the self‐assembly of metal–phenolic networks at the interface of emulsions. The coated emulsions are stable and exhibit a high drug‐loading capacity and offer promise as a therapeutic nanocarrier.
Nanostructured materials have potential as platforms for analytical assays and catalytic reactions. Herein, we report the synthesis of electrocatalytically active cobalt phosphate nanostructures ...(CPNs) using a simple, low-cost, and scalable preparation method. The electrocatalytic properties of CPNs toward the electrooxidation of glucose (Glu) were studied by cyclic voltammetry and chronoamperometry in relevant biological electrolytes, such as phosphate-buffered saline (PBS), at physiological pH (7.4). Using CPNs, Glu detection could be achieved over a wide range of biologically relevant concentrations, from 1 to 30 mM Glu in PBS, with a sensitivity of 7.90 nA/mM cm2 and a limit of detection of 0.3 mM, thus fulfilling the necessary requirements for human blood Glu detection. In addition, CPNs showed a high structural and functional stability over time at physiological pH. The CPN-coated electrodes could also be used for Glu detection in the presence of interfering agents (e.g., ascorbic acid and dopamine) and in human serum. Density functional theory calculations were performed to evaluate the interaction of Glu with different faceted cobalt phosphate surfaces; the results revealed that specific surface presentations of under-coordinated cobalt led to the strongest interaction with Glu, suggesting that enhanced detection of Glu by CPNs can be achieved by lowering the surface coordination of cobalt. Our results highlight the potential use of phosphate-based nanostructures as catalysts for electrochemical sensing of biochemical analytes.
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