Efficient photoelectrocatalytic ammonia synthesis over graphene oxide-modified Fe-based metal-organic frameworks with high conductivity and photoresponsivity.

Electrochemical ammonia synthesis: Mechanistic understanding and catalyst design

The rational design of photocatalysts for efficient nitrogen (N2) fixation at ambient conditions is important for revolutionizing ammonia production and quite challenging because the great difficulty lies in the adsorption and activation of the inert N2. Inspired by a biological molecule, chlorophyll, featuring a porphyrin structure as the photosensitizer and enzyme nitrogenase featuring an iron (Fe) atom as a favorable binding site for N2via π-backbonding, here we developed a porphyrin-based metal-organic framework (PMOF) with Fe as the active center as an artificial photocatalyst for N2 reduction reaction (NRR) under ambient conditions. The PMOF features aluminum (Al) as metal node imparting high stability and Fe incorporated and atomically dispersed by residing at each porphyrin ring promoting the adsorption and the activation of N2, termed Al-PMOF(Fe). Compared with the pristine Al-PMOF, Al-PMOF(Fe) exhibits a substantial enhancement in NH3 yield (635 μg g-1cat.) and production rate (127 μg h-1 g-1cat.) of 82% and 50%, respectively, on par with the best-performing MOF-based NRR catalysts. Three cycles of photocatalytic NRR experimental results corroborate a stable photocatalytic activity of Al-PMOF(Fe). The combined experimental and theoretical results reveal that the Fe-N site in Al-PMOF(Fe) is the active photocatalytic center that can mitigate the difficulty of the rate-determining step in photocatalytic NRR. The possible reaction pathways of NRR on Al-PMOF(Fe) were established. Our study of porphyrin-based MOF for the photocatalytic NRR will provide insight into the rational design of catalysts for artificial photosynthesis.

Atomic-level insights into the activation of nitrogen via hydrogen-bond interaction toward nitrogen photofixation

The central motivation of the thesis was to provide possible solutions and concepts to improve the performance (e.g. activity and selectivity) of electrochemical N2 reduction reaction (NRR). Given that porous carbon-based materials usually exhibit a broad range of structural properties, they could be promising NRR catalysts. Therefore, the advanced design of novel porous carbon-based materials and the investigation of their application in electrocatalytic NRR including the particular reaction mechanisms are the most crucial points to be addressed. In this regard, three main topics were investigated. All of them are related to the functionalization of porous carbon for electrochemical NRR or other electrocatalytic reactions. In chapter 3, a novel C-TixOy/C nanocomposite has been described that has been obtained via simple pyrolysis of MIL-125(Ti). A novel mode for N2 activation is achieved by doping carbon atoms from nearby porous carbon into the anion lattice of TixOy. By comparing the NRR performance of M-Ts and by carrying out DFT calculations, it is found that the existence of (O-)Ti-C bonds in C-doped TixOy can largely improve the ability to activate and reduce N2 as compared to unoccupied OVs in TiO2. The strategy of rationally doping heteroatoms into the anion lattice of transition metal oxides to create active centers may open many new opportunities beyond the use of noble metal-based catalysts also for other reactions that require the activation of small molecules as well. In chapter 4, a novel catalyst construction composed of Au single atoms decorated on the surface of NDPCs was reported. The introduction of Au single atoms leads to active reaction sites, which are stabilized by the N species present in NDPCs. Thus, the interaction within as-prepared AuSAs-NDPCs catalysts enabled promising performance for electrochemical NRR. For the reaction mechanism, Au single sites and N or C species can act as Frustrated Lewis pairs (FLPs) to enhance the electron donation and back-donation process to activate N2 molecules. This work provides new opportunities for catalyst design in order to achieve efficient N2 fixation at ambient conditions by utilizing recycled electric energy. The last topic described in chapter 5 mainly focused on the synthesis of dual heteroatom-doped porous carbon from simple precursors. The introduction of N and B heteroatoms leads to the construction of N-B motives and Frustrated Lewis pairs in a microporous architecture which is also rich in point defects. This can improve the strength of adsorption of different reactants (N2 and HMF) and thus their activation. As a result, BNC-2 exhibits a desirable electrochemical NRR and HMF oxidation performance. Gas adsorption experiments have been used as a simple tool to elucidate the relationship between the structure and catalytic activity. This work provides novel and deep insights into the rational design and the origin of activity in metal-free electrocatalysts and enables a physically viable discussion of the active motives, as well as the search for their further applications. Throughout this thesis, the ubiquitous problems of low selectivity and activity of electrochemical NRR are tackled by designing porous carbon-based catalysts with high efficiency and exploring their catalytic mechanisms. The structure-performance relationships and mechanisms of activation of the relatively inert N2 molecules are revealed by either experimental results or DFT calculations. These fundamental understandings pave way for a future optimal design and targeted promotion of NRR catalysts with porous carbon-based structure, as well as study of new N2 activation modes.

Developing efficient noble-metal-free catalysts for the electrochemical N2 reduction reaction (NRR) under ambient conditions shows promise in fertilizer production and hydrogen storage. Here, as a proof-of-concept prototype, we design and implement an Fe–N/C–carbon nanotube (CNT) catalyst derived from a metal–organic framework and carbon-nanotube-based composite with built-in Fe–N3 active sites. This catalyst exhibits enhanced NRR activity with NH3 production (34.83 μg·h–1·mg–1cat.), faradaic efficiency (9.28% at −0.2 V vs RHE), selectivity, and stability in 0.1 M KOH aqueous media under mild conditions. Experimental and theoretical results both reveal that Fe–N3 species are the primary catalytically active centers for the NRR. This work provides insight into precise construction of more efficient and stable NRR electrocatalysts and further expands the possibilities of transition metal–nitrogen–carbon (M–N–C)-based nanomaterials in NRR fields.

Nitrogen (N2) fixation at ambient condition by electrochemical N2 reduction reaction (NRR) is energy‐efficient and eco‐friendly as compared to the traditional Harber–Bosch process, but it is extremely challenging. Development and design of high‐performance NRR electrocatalysts are indispensable to achieve the goal. In this work, a strongly coupled hybrid of nano‐Fe3O4 with reduced graphene oxide (rGO) is synthesized via an in situ redox hydrothermal approach, and the synthesized Fe3O4@rGO hybrid has excellent activity, selectivity, and stability as an NRR catalyst. The NH3 yield rate of 28.01 μg h‐1 mg‐1 at −0.3 V and the Faradaic efficiency (FE) of 19.12% at −0.1 V are obtained in 0.1 M Na2SO4 solutions at ambient conditions. The superior NRR performance is attributed to the chemical coupling effect between rGO and nano‐Fe3O4 particles, which leads to the enhancement of the binding affinity to N2 molecules, improvement of the conductivity, and lowering the free energy of reaction for the limiting reaction step. This work provides a facile route in fabricating hybrid NRR catalysts with superior performance and shed lights on the reaction mechanism with theoretical mechanistic calculations.

Vacancy engineering of oxidized Nb2CTx MXenes for a biased nitrogen fixation

Interface and defect engineer of titanium dioxide supported palladium or platinum for tuning the activity and selectivity of electrocatalytic nitrogen reduction reaction

The global NH3 production is dominated by Haber–Bosch process, requiring high temperature and pressure. Electrochemical N2 reduction reaction (NRR) under ambient conditions is a greener path for artificial N2 fixation to NH3 but calling for efficient catalyst to increase activity and selectivity. Herein, we report the iron-based metal–organic frameworks (MOFs), i.e., MIL-88B–Fe and amine-functionalized MIL-88B–Fe (NH2–MIL-88B–Fe) as efficient catalysts for electrochemical NRR under ambient temperature and pressure in neutral electrolyte. NH2–MIL-88B–Fe shows higher NH3 yield rate of 1.205 × 10–10 mol s−1 cm−2 than MIL-88B–Fe (3.575 × 10–11 mol s−1 cm−2). Furthermore, NH2–MIL-88B–Fe exhibits the highest Faradaic efficiency of 12.45% at 0.05 V versus RHE. The control experiments prove that NH3 is produced through electrocatalytic NRR. This work may trigger the interest of using MOFs as highly efficient catalysts for electrochemical NH3 production.

Electrochemical reduction of N2 to NH3 under ambient conditions may provide an alternative to the Haber−Bosch process for sustainable NH3 synthesis when powered by solar- or wind-generated electricity. However, the development of this process has been hindered by the lack of efficient electrocatalysts for the N2 reduction reaction (N2RR), due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). Here we present our recent studies of electrochemical ammonia synthesis on Pd and Fe/Fe-oxide catalysts. First, we found that Pd nanoparticles can catalyze the electrohydrogenation of N2 to NH3 in 0.1 M phosphate buffer solution (PBS) with a high yield rate and a Faradaic efficiency of 8.2% for NH3 production at 0.1 V vs the reversible hydrogen electrode (RHE). In operando X-ray absorption spectroscopy (XAS) study revealed the formation of α-phase Pd hydride during the reaction. The unique N2RR activity of Pd at low overpotentials outperforms that of Au and Pt nanoparticle catalysts, which is attributed to a new electrohydrogenation mechanism that involves an electrochemical formation of PdHx and a chemical hydride transfer from PdHx to *N2 to form *N2H, thus to lower the energy barrier for the rate-limiting step of N2RR. Second, we developed an Fe/Fe3O4 catalyst for N2RR, which was prepared by oxidizing an Fe foil at 300 oC in O2 atmosphere followed by in situ electrochemical reduction until a stable state was reached. The derived Fe/Fe3O4 catalyst shows greatly enhanced activity and selectivity for N2RR under ambient conditions than the original Fe foil, achieving a Faradaic efficiency of 8.29% for NH3 production at −0.3 V vs RHE in 0.1 M PBS electrolyte, which is around 120 times higher than that of the Fe foil. The high selectivity is enabled by an enhancement of the intrinsic (surface-area-normalized) N2RR activity as well as an effective suppression of the undesired HER. Comparative studies indicate that the N2RR activity may depend on the Fe/Fe-oxide ratio. Furthermore, the N2RR selectivity of the Fe/Fe3O4 catalyst is superior to that of Fe, Fe3O4, and Fe2O3 nanoparticle catalysts, which may provide new insights into the understanding and development of efficient electrocatalysts for ambient NH3 synthesis.

The electrochemical N2 reduction reaction (NRR) under ambient conditions is attractive in replacing the current Haber-Bosch process toward sustainable ammonia production. Metal-heteroatom-doped carbon-rich materials have emerged as the most promising NRR electrocatalysts. However, simultaneously boosting their NRR activity and selectivity remains a grand challenge, while the principle for precisely tailoring the active sites has been elusive. Herein, we report the first case of crystalline two-dimensional conjugated covalent organic frameworks (2D c-COFs) incorporated with M-N4-C centers as novel, defined, and effective catalysts, achieving simultaneously enhanced activity and selectivity of electrocatalytic NRR to ammonia. Such 2D c-COFs are synthesized based on metal-phthalocyanine (M = Fe, Co, Ni, Mn, Zn, and Cu) and pyrene units bonded by pyrazine linkages. Significantly, the 2D c-COFs with Fe-N4-C center exhibit higher ammonia yield rate (33.6 μg h-1 mgcat-1) and Faradaic efficiency (FE, 31.9%) at -0.1 V vs reversible hydrogen electrode than those with other M-N4-C centers, making them among the best NRR electrocatalysts (yield rate >30 μg h-1 mgcat-1 and FE > 30%). In situ X-ray absorption spectroscopy, Raman spectroelectrochemistry, and theoretical calculations unveil that Fe-N4-C centers act as catalytic sites. They show a unique electronic structure with localized electronic states at Fermi level, allowing for stronger interaction with N2 and thus faster N2 activation and NRR kinetics than other M-N4-C centers. Our work opens the possibility of developing metal-nitrogen-doped carbon-rich 2D c-COFs as superior NRR electrocatalyst and provides an atomic understanding of the NRR process on M-Nx-C based electrocatalysts for designing high-performance NRR catalysts.

p-Block element-based electrocatalysts featuring a tunable electronic structure to achieve exceptional N2 activation and proton suppression have garnered extensive interest for the electrochemical N2 reduction reaction (NRR). Albeit various reaction mechanisms were proposed to understand and optimize the NRR performance, methods to effectively design and rapidly screen potential candidates are still elusive. Herein, a couple of explicit and interpretable descriptors on the entire p-block element-based electrocatalysts are put forward to predict NRR activity and selectivity via high-throughput theoretical simulations and a symbolic regression algorithm, taking two-dimensional (2D) bismuthine with p-block elements doped in or adsorbed as an example. The descriptors are merely composed of inherent atomic properties (p orbital electron number, electron affinity, electronegativity, atomic radius, etc.) combined with algebraic operators, independent of the intricate DFT calculations. Multi-task regression results demonstrate that the doped and adsorbed bismuthine systems possess the same descriptors, namely, the descriptors of doped-Bi can accurately forecast the NRR performance of adsorbed-Bi, and vice versa. Five potential candidates (5/40) with outstanding NRR activity, selectivity and stability are screened. C-doped and Si-doped bismuthine possess less negative limiting potentials of the NRR [UL(NRR)] of -0.46 and -0.68 V and positive [UL(NRR) - UL(HER)] values of 1.15 and 0.13 V, respectively, superior to those of the majority of reported p-block element-based electrocatalysts, which are expected to be verified by the experimental research. This work offers a feasible solution for developing promising electrocatalysts for the NRR and potentially other electrochemical reactions on the basis of explainable descriptors using geometric information and intrinsic atomic quantities.

Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4-ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmolNH3 s−1 cm−2 at a Faradaic efficiency of 70 ± 24% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.

The electrochemical dinitrogen (N2 ) reduction reaction (NRR) under ambient conditions has gained significant interest as an environmentally friendly alternative to the traditional Haber-Bosch process for the synthesis of ammonia (NH3 ). However, up to now, most of the reported NRR electrocatalysts with satisfactory catalytic activities have been hindered by the large overpotential in N2 activation. The preparation of highly efficient Mo-based NRR electrocatalyst in acidic electrolytes under ambient conditions is demonstrated here, consisting of stabilized single Mo atoms anchored on holey nitrogen-doped graphene synthesized through a convenient potassium-salt-assisted activation method. At -0.05V versus a reversible hydrogen electrode (RHE), an electrode consisting of the resultant electrocatalyst immobilized on carbon fiber paper can attain an exceptional Faradaic efficiency of 50.2% and a NH3 yield rate of 3.6µg h-1 mgcat-1 with low overpotentials. Density functional theory calculations further unveil that compared to the original graphene without holes, the edge coordinated Mo atoms and the existence of vacancies on holey graphene lower the overpotential of N2 reduction, thereby promoting the NRR catalytic activity. This work could provide new guidelines for future designs in single-atom catalysis that would be beneficial to ambient N2 fixation, and replacement of classical synthesis processes that are very energy-intensive.

High spin polarization ultrafine Rh nanoparticles on CNT for efficient electrochemical N2 fixation to ammonia