Metastable Phase Cu with Optimized Local Electronic State for Efficient Electrocatalytic Production of Ammonia from Nitrate

Due to the extremely high bond energy of N≡N (∼941 kJ/mol), the traditional Haber-Bosch process of ammonia synthesis is known as an energy-intensive and high CO2-emission industry. In this paper, a cascade N2 reduction process with dielectric barrier discharge (DBD) plasma oxidation and electrocatalytic reduction as an alternative route is first proposed. N2 is oxidized to be reactive nitrogen species (RNS) by nonthermal plasma, which would then be absorbed by KOH solution and electroreduced to NH4+. It is found that the production of NOx is a function of discharge length, discharge power, and gas flow rate. Afterward, the cobalt catalyst is used in the process of electrocatalytic reduction of ammonia, which shows high selectivity (Faradic efficiency (FE) above 90%) and high yield of ammonia (45.45 mg/h). Finally, the cascade plasma oxidation and electrocatalytic reduction for ammonia synthesis is performed. Also, the performance of the reaction system is evaluated. It is worth mentioning that a stable and sustainable ammonia production efficiency of 16.21 mg/h is achieved, and 22.16% of NOx obtained by air activation is converted into NH4+. This work provides a demonstration for further industrial application of ammonia production with DBD plasma oxidation and electrocatalytic reduction techniques.

The rapid capacity degradation and poor rate capability hinder the application of Rich-Ni layered LiNix Coy Mnz O2 (NCM) as cathode materials for high-energy lithium-ion batteries. In this study, density functional theory (DFT) calculations, combined with conventional electrochemical measurements, reveal from the atomic view that the dual improvements in electronic and ionic conductivities are the main facts for the property enhancement. The bandgap of the cathode material is reduced to 1.1623eV due to the increased number of electrons near the Fermi level after W intercalation. Such improved electronic conductivity subsequently leads to a suppressed polarization and reduced resistance, enabling an improved cycle life of up to 93.97% after 100 cycles at 0.5 C. Furthermore, the doping with W6+ also introduced a strong WO bond into the layered structure so that the thickness of the Li slab is expanded to 2.6476 Å, which reduces the energy barrier from 0.355 to 0.308eV for the migration of Li+ within the Li slab, as confirmed by the DFT calculation. Consequently, the rate performance is greatly improved due to the reduced diffusion energy, with a specific capacity of 159.11 mAg-1 even at 5 C rate, indicating high potential for future applications.

Open AccessCCS ChemistryRESEARCH ARTICLES22 Oct 2022Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N5 Site for Enhanced Electrocatalytic Reaction Bingyu Huang†, Senhe Huang†, Chenbao Lu, Longbin Li, Judan Chen, Ting Hu, Dirk Lützenkirchen-Hecht, Kai Yuan, Xiaodong Zhuang and Yiwang Chen Bingyu Huang† Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Senhe Huang† Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Chenbao Lu Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Longbin Li Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Judan Chen Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Ting Hu School of Materials Science and Engineering, Nanchang University, Nanchang 330031 , Dirk Lützenkirchen-Hecht Faculty of Mathematics and Natural Sciences-Physics Department, Bergische Universität Wuppertal, Wuppertal D-42119 , Kai Yuan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Xiaodong Zhuang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 and Yiwang Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 Institute of Advanced Scientific Research (iASR), Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University, Nanchang 330022 https://doi.org/10.31635/ccschem.022.202202241 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal porphyrins are star molecules that possess well-defined coordination metal centers for versatile catalytic reactions. However, most previous work has focused on the correlations between in-plane symmetric configuration of metal-N4 sites and their catalytic performance. Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge. Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin (Co-N4) to construct penta-coordinated cobalt porphyrin (Co-N5) renders cobalt a higher electron density, thereby favoring the rate-determining O2 adsorption/activation and reducing the oxygen electroreduction barrier. Therefore, a well-defined Co-N5 site is rationally introduced into the azo-linked polymer framework for a fundamental structure–catalytic performance correlation study. As-prepared Co-N5 catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N4 counterpart, discloses a markedly higher power density (141.4 mW cm−2), and possesses better long-term durability (over 160 h cycles) in a Zn-air battery. Moreover, such a Co-N5 catalyst also showcases potential applications for CO2 reduction with high CO2-to-CO conversion faradic efficiency and better selectivity than the Co-N4 counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction. This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metal-porphyrin-based catalysts and offers atomic understanding of the structure–performance correlation on single atom catalysts for future catalyst design. Download figure Download PowerPoint Introduction Currently, many advanced electrocatalysts have been developed to facilitate the sluggish kinetics of the multiple-electron transfer process in energy conversion reactions, such as the oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR).1–4 Due to maximized atom utilization, mainstream research has focused on the isolated transition metal/nitrogen coordinated (M-N-C) single-atom catalysts (SACs) with high catalytic activity and selectivity.5–9 Despite extensive investigations, the rational design and controllable and precise synthesis of M-N-C catalysts continues to be the main obstacle to multiple-electron transfer processes.10–12 The preparation of M-N-C catalysts normally needs high-temperature pyrolysis to increase the graphitization degree for better conductivity.13–17 Unfortunately, the pyrolysis process inevitably evokes metal aggregation due to the thermal decomposition of metal precursors and high surface energy of single metal atoms, posing challenges for maintaining atomic-metal isolation.13,18,19 The inhomogeneity and indistinction of the catalytic environment of carbonaceous materials give rise to tremendous difficulties in simultaneously enhancing the catalysts' activity and selectivity. In addition, the inherently less-defined active sites formed after pyrolysis leaves an ambiguous structure–performance relationship and seriously precludes us from exploring the in-depth mechanism for different electrocatalytic reactions.20–24 Therefore, the above-mentioned challenges stimulate the vigorous search for developing cost-effective and high-performance pyrolysis-free electrocatalysts with well-defined active sites. Transition-metal macrocycles, such as cobalt porphyrins, which possess the distinct tetra-coordinated cobalt porphyrin (Co-N4) site, have been heavily studied.25–28 Benefitting from the production of fewer radical oxygen species during the electrocatalytic process, the Co-N4 structure is more robust and advantageous for electrocatalysis of ORR.16,20,29–31 However, the representative plane-symmetric electron configuration of Co-N4 is not the optimal structure for the chemisorption and activation of reactants.32–34 Breaking the structure symmetry with penta-coordination to regulate the charge redistribution of Co-N4 sites could promote the electrocatalytic process, but the accurate synthesis of this structural motif is still a noteworthy challenge. Besides, the role of penta-coordination and the exact local microenvironment of such active sites have not yet been ascertained.35,36 In view of this, offering a model catalyst as an ideal platform is urgently required to elucidate the contribution of axial penta-coordination on catalytic activity, selectivity, and durability of Co-N4 sites. In this work, we probed the positive effect of axial pyridinic penta-coordinated cobalt porphyrin (Co-N5) to achieve optimized electronic localization on the Co-N5 site for boosting ORR through density functional theory (DFT) calculations as well as electrochemical analysis. DFT calculations indicate Co-N5 with axial pyridinic coordination possesses obviously higher electronic density on the Co center in comparison with Co-N4. Taking advantage of the electronic-push effect of penta-coordination, the ORR rate-determining step of O2 adsorption/activation can be significantly promoted, ensuring better ORR performance of Co-N5 than the Co-N4 model. Hence, we innovatively designed and synthesized an azo-linked polymer framework with atomically dispersed electron-rich Co-N5 sites for ORR through a pyrolysis-free axial-pyridinic coordination strategy. The Co-N5 catalyst displays impressive ORR performance with a more positive half-wave potential of 0.811 V and lower Tafel slope of 39 mV dec−1 than the Co-N4 counterpart (0.785 V and 50 mV dec−1), which support the theoretical predictions. Encouragingly, such a penta-coordination-induced electronic localization strategy also promises the potential for improving the CO2-to-CO conversion faradic efficiency and selectivity of electrocatalytic CO2 reduction. This work provides new design strategies toward well-defined single-atom electrocatalysts with axial coordination and offers new models for fundamentally understanding the catalytic mechanism of asymmetric coordination systems. Experimental Methods Synthesis of [email protected] catalysts First, 20 mg (27.3 μmol) of CoTAPP and 80 mg of G-py were redispersed in 20 mL of dimethyl sulfoxide (DMSO). Then, the mixture was stirred for 15 h to ensure CoTAPP was fully coordinated to G-py. Immediately after the coordination, 35.2 mg (0.11 mmol, 4 equiv to CoTAPP) of PhI(OAc)2 was added to the mixed solution, and the mixture was stirred for 24 h. The resultant precipitate was filtered and washed with DMSO, methanol, deionized water, and ethanol for three times, and dried at 60 °C overnight. [email protected] was harvested as a black powder (86.8 mg, 87% yield). [email protected] and [email protected] were prepared with a procedure similar to [email protected] Experimental details, materials characterization methods, and synthesis of other samples are available in the Supporting Information. Results and Discussion Theoretical calculations and catalytic mechanism Based on cobalt porphyrin, two models with different coordination environments, axial pyridinic Co-N5 model and Co-N4 model were designed (Figure 1a and Supporting Information Figure S1). The elementary steps and corresponding ORR adsorption configurations on the two models are presented in Supporting Information Figure S2. The Gibbs free energies at different potentials for all elementary steps were evaluated to illustrate how the penta-coordination affects intrinsic ORR activity. As given in Figure 1b, all elementary steps are distinctly downhill at U = 0 V, thus the reaction is exothermic and able to proceed spontaneously. When the potential rises to 1.23 V, the rate-determining step of Co-N4 is the first step (* + O2 + H2O + e− → *OOH + OH−) with a high energy barrier of 0.34 eV. In contrast, the final release step (*OH + e− → OH− + *) of the Co-N5 model is the rate-determining step, and its free energy can be distinctly reduced to only 0.26 eV. The thermodynamic limiting potentials, which represent the maximum potential to ensure all steps downhill are 0.97 and 0.89 V for Co-N5 and Co-N4 models, respectively (Figure 1c and Supporting Information Figure S3), revealed the Co-N5 model requires the minimum overpotential to drive the oxygen reduction. This result implies that the axial pyridine coordination plays a crucial role in regulating the ORR intermediates adsorption strength and decreasing the reaction barrier of the rate-determining step. Similarly, axial pyridine coordination can also alter the Fe electronic microenvironment, thereby enhancing the catalytic activity of the Fe-N5 model ( Supporting Information Figures S4–S7), further validating the universality of additional axial coordination. Figure 1 | (a) DFT calculation models of Co-N5 and Co-N4 for the electrochemically catalyzed ORR. (b) Free energy diagrams at U = 0 V and U = 1.23 V, and (c) free energy diagrams for the thermodynamic limiting potentials of Co-N5 model and Co-N4 model. (d) PDOS of Co atom for Co-N5 model (top) and Co-N4 model (bottom); the d-band center is denoted by the dashed gray line. Differential charge density distribution after O2 absorption on (e) Co-N5 model and (f) Co-N4 model. Download figure Download PowerPoint To further study axial coordination induced changes in the Co center's electronic configuration and interaction with oxygen-containing intermediates, projected density of states (PDOS) was conducted. The axial coordination obviously tunes the Co 3d orbital according to the PDOS in Figure 1d. The d-band center of the Co-N5 model at −1.59 eV is closer to the Fermi level than that of Co-N4 at −1.76 eV, thereby leading to an increase in O-containing intermediates' adsorption.37 The enhanced adsorption ensures subsequent ORR steps proceed through a more efficient four-electron pathway. Hence, the thermodynamic onset potentials improvement can be ascribed to fine-tuned adsorption strength of the ORR intermediates, which directly determines the activity and selectivity of catalysts. In addition, the higher PDOS near the Fermi level represents more abundant charge carriers and better electronic conductivity for the Co-N5 model. During the ORR process, the activated Co d orbitals and their hybridization with O p orbitals co-determine the adsorption strength for oxygen-containing adsorbates.38–42 After O2 adsorption and *OH formation, the increased overlapping degree of the strong σ-bond that originates from the Co dz2 orbital and O p orbital, along with the decreased O2 antibonding orbital filling degree that appears above the Fermi level for Co-N5 model, can be observed. It theoretically suggests that the extra fifth pyridine coordination assures a tighter connection between Co centers and O2 ( Supporting Information Figures S8 and S9), thus enabling higher ORR selectivity towards the four-electron pathway.43 As verified by charge density differences ( Supporting Information Figure S10), obvious asymmetrical charge distribution caused by axial coordination can be found for the Co-N5 model in comparison with the symmetric Co-N4 model. An apparent charge accumulation on the Co center is found to form the electron-rich Co-N5 site due to the electronic-push effect of axial pyridine. As expected, the O2 only absorbs on the individual Co center (Figure 1e,f), which can provide superior ORR electrocatalytic sites. Furthermore, the axial pyridine can construct an electronic pathway that renders adequate charge transfer to oxygen molecules from the conductive graphene layer. In general, the electron-rich Co-N5 site promises stable chemisorption and activation of O2, which facilitates O–O bond cleavage, thus offering better selectivity for the four-electron reduction pathway. Therefore, the axial-pyridine coordination-induced electronic localization strategy is viable to efficaciously enhance the ORR kinetics. Synthesis and structural characterization To experimentally confirm the calculation results and demonstrate the significance of penta-coordination architecture in ORR, an azo-linked penta-coordinated cobalt porphyrin-based polymer (CoTAPP-Azo) anchored on pyridine functionalized graphene (G-py) ([email protected]) was synthesized. The synthesis route and structure of [email protected] and the counterpart [email protected] (directly grown CoTAPP-Azo on pristine graphene) are schematically revealed in Figure 2a. Tetrakis(4-aminophenyl) porphyrin (TAPP) was first chelated with cobalt cation to obtain CoTAPP ( Supporting Information Figure S11). TAPP coordinating with Fe (FeTAPP) and Ni (NiTAPP) were also successfully obtained using the same procedure. The formation of CoTAPP, FeTAPP, and NiTAPP were confirmed by mass spectrometry ( Supporting Information Figure S12) and Fourier-transform infrared (FTIR) spectroscopy ( Supporting Information Figure S13). The successful pyridine functionalization in G-py was confirmed by thermogravimetric analysis ( Supporting Information Figure S14), Raman spectroscopy ( Supporting Information Figure S15), and X-ray photoelectron spectroscopy (XPS) ( Supporting Information Figure S16). Subsequently, CoTAPP was anchored on G-py through axial pyridine coordination; following an azo-coupling reaction, [email protected] was finally obtained (see details in Supporting Information Scheme S1). For comparison, FeTAPP-based polymer hybridized with G-py ([email protected]) and graphene ([email protected]) and NiTAPP-based polymer hybridized with G-py ([email protected]), were synthesized by similar routes. Figure 2 | (a) Synthetic route of CoTA[email protected]y and [email protected] (b) HAADF-STEM image of [email protected] (c) High-resolution Co 2p XPS spectra of [email protected] and [email protected] Download figure Download PowerPoint The morphology of [email protected] was first identified by scanning electron microscopy (SEM). Compared with the thin-layered sheet structure of G-py, a thicker layer was observed, indicating G-py was covered by CoTAPP-Azo ( Supporting Information Figures S17 and 18). The transmission electron microscopy (TEM) image of [email protected] also presented representative lamellar plate morphology ( Supporting Information Figure S18c). To further discern the structural features at the atomic level, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out. HAADF-STEM images and the corresponding energy dispersive spectroscopy (EDS) of [email protected] indicated homogeneous spatial distribution of C, N, Co elements ( Supporting Information Figure S19). The abundant bright points (highlighted by red circles) offer direct evidence for the uniform distribution of single Co atoms (Figure 2b). Similarly, [email protected], [email protected], and [email protected] all exhibited similar lamella morphology with homogeneously distributed elements ( Supporting Information Figures S20–S22). The chemical structures of [email protected] were preliminarily examined by FTIR spectroscopy ( Supporting Information Figure S23). Compared with CoTAPP, the corresponding FTIR spectrum of [email protected] displayed a significant intensity increase of N=N stretching signals (1570 cm−1, 1215 cm−1) along with an obviously weakened N–H peak at 3400 cm−1, proving the successful polymerization of CoTAPP. The increased ID/IG ratio of 0.94 for [email protected] to 1.07 for [email protected] in the Raman spectra ( Supporting Information Figure S24) verifies surface lattice interference caused by penta-coordination. X-ray diffraction (XRD) confirmed that G-py was coated by CoTAPP-Azo, and the absence of crystalline cobalt species diffraction peaks demonstrated the highly dispersed state of Co atoms ( Supporting Information Figure S25), agreeing well with the HAADF-STEM observations. [email protected] and [email protected] both show similar XRD patterns ( Supporting Information Figures S26 and S27). Furthermore, XPS was extracted to unravel the nature of the chemical bonding of [email protected] and [email protected] ( Supporting Information Figures S28–S31). Compared with [email protected], the N 1s XPS spectrum for [email protected] displayed an obvious pyridinic N peak (398.3 eV, Supporting Information Figure S29). The C 1s XPS spectrum of [email protected] showed that the C–N peak shifts to higher binding energy (BE) than that of [email protected] (ΔBE = 0.4 eV), revealing the decreased electron density of C atom. In the Co 2p spectra (Figure 2c), [email protected] underwent a shift to lower BE for Co 2p3/2 (780.3 eV) and Co 2p1/2 (795.6 eV) peaks, relative to that of [email protected] (780.8 and 796.0 eV for Co 2p3/2 and Co 2p1/2, respectively), providing evidence for the electronic localization on the Co with penta-coordination.44,45 This result confirms that axial pyridine ligands act as channels between G-py and CoTAPP-Azo and boost the charge transfer from graphene to Co centers, which is consistent with the electron-rich Co-N5 model in theoretical calculations. Importantly, similar phenomena were found in [email protected] and [email protected], suggesting the universality of such an electron localization approach. The UV–vis spectrum of [email protected] revealed a clear porphyrin Soret band at 448 nm ( Supporting Information Figure S32). Interestingly, an appreciable peak change to 438 nm in [email protected] was observed due to the penta-coordination-induced electron transfer. Meanwhile, photoluminescence spectroscopy was conducted to explore the charge separation behaviors ( Supporting Information Figure S33). Significantly increased quenching occurred in [email protected] compared with [email protected], further demonstrating that axial pyridine can enhance the charge transfer between G-py and CoTAPP-Azo. The work function, which represents the minimum energy needed to draw one inner electron from the nucleus, was obtained by ultraviolet photoelectron spectroscopy. [email protected] showed a 0.51 eV shift to higher BE than [email protected] in the second electron cut-off edge ( Supporting Information Figure S34). Hence, with respect to 4.49 eV for [email protected], the smaller work function for [email protected] (3.98 eV) indicated that the electrons are more likely to be activated and transferred outward.46,47 We assessed the porosity of the samples by nitrogen physisorption isotherms ( Supporting Information Figure S35). [email protected] and [email protected] both displayed the typical type-IV isotherms with an evident hysteresis loop, indicative of the coexistence of micropores and mesopores. The Brunauer–Emmett–Teller specific surface areas for [email protected] and [email protected] were calculated to be 506.6 and 440.3 m²/g, respectively. Pore size distribution demonstrated by nonlocalized DFT showed the pore widths centered at 1.9 and 4.8 nm for [email protected] ( Supporting Information Figure S36). These results demonstrate the improved surface area and hierarchical porous structure of [email protected], which are conducive to increase the quantity of accessible active sites and enhance the mass transport. To investigate the local coordination environment of the Co center, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were examined. The stronger white-line peak intensity of [email protected] (7727 eV) compared with [email protected] was assigned to the larger coordination number of the Co center for [email protected] than [email protected] (Figure 3a).48 The quantitative fits ( Supporting Information Table S1) showed a Co atom in [email protected] was straightforwardly connected by N atoms with a coordination number of 5.1 and the average bond length of 1.98 Å. From the Co K-edge Fourier-transformed EXAFS spectra (Figure 3b), the first shell peak for [email protected] around 1.43 Å corresponded to the Co–N scattering path, accompanying the absent Co–Co peak at 2.18 Å, jointly signifying the isolated Co atoms' configuration. [email protected] displayed an obviously positive shift for the Co–N first shell in than [email protected], suggesting an Co–N bond length caused by the to the EXAFS results (Figure and Supporting Information Figures and in to an of for [email protected], the first shell intensity for [email protected] was enhanced with an to revealing the larger number of N coordination The was also for the coordination In Figure the intensity maximum at for [email protected] was assigned to Co–N Co–Co and corresponding coordination was demonstrating the of the Co The shift of [email protected] maximum compared with that of CoTAPP is caused by the in [email protected] Hence, on and EXAFS as well as the XPS we that [email protected] by Co-N5 sites was successfully synthesized. Figure | (a) Co K-edge spectra and (b) the of Co K-edge EXAFS spectra of [email protected] with Co and CoTAPP as (c) The corresponding EXAFS of [email protected] model with Co N and C (d) of [email protected] in comparison with Co and CoTAPP. Download figure Download PowerPoint Electrocatalytic ORR performance To how axial penta-coordination affects ORR [email protected] and [email protected] were first evaluated with a In ( Supporting Information Figure significant peaks in for [email protected], but a peak was observed in The (Figure and Supporting Information Figure that [email protected] the ORR performance with the most positive half-wave potential of 0.811 V which is 26 mV more positive than that of [email protected] (0.785 V These results indicate that [email protected] possesses superior catalytic performance that from the electron localization caused by pyridine penta-coordination. in comparison with [email protected], the density for [email protected] was enhanced because the axial pyridine as between G-py and CoTAPP-Azo, thus boosting the charge transfer from graphene to CoTAPP layer. Figure 4 | (a) ORR of [email protected] and [email protected] catalysts at (b) Tafel (c) density by electrochemically active surface and (d) comparison of Tafel and for [email protected] and [email protected] catalysts. (e) The of [email protected] and [email protected] and after (f) and power density specific for Zn-air at and long-term performance of Zn-air at a density of using [email protected], [email protected], and as catalysts. Download figure Download PowerPoint To further into the reaction kinetics of [email protected], the at from to were ( Supporting Information Figure The ( Supporting Information Figure which were from at different potentials, revealed the toward the of Based on the the number for [email protected] was which with the This proves that the Co-N5 site a four-electron pathway towards ORR, [email protected] has a smaller of ( Supporting Information Figure We the enhanced ORR selectivity to the Co-N5 configuration that can be with because of the effect between Co and pyridinic the resultant O–O bond with stronger stretching is thus providing higher selectivity for four-electron ORR. [email protected] has a more positive than [email protected] ( Supporting Information Figures that ORR activity can be improved by microenvironment through extra axial penta-coordination on different metal Moreover, the Tafel slope of 39 mV dec−1 the process of [email protected] for oxygen reduction (Figure and Supporting Information Figure by the of electrochemical spectroscopy ( Supporting Information Figure the decreased in the for [email protected], indicating the optimized and electron toward ORR kinetics. In addition, [email protected] exhibited the electrochemically active surface area ( Supporting Information Figure which is conducive to more active sites. density with an in the ORR can be found (Figure the intrinsic activity for [email protected] For further comparison, the performance density Tafel density and for different electrocatalysts are (Figure and

The system Y1−xSrxMnO3 (x = 0.000, 0.005, 0.010, 0.050, and 0.100) was studied as a potential cathode material for solid oxide fuel cells. Powders were prepared using an organometallic precursor; however, achieving homogeneous compositions was complicated due to the presence of intermediate, metastable phases. The desired hexagonal Y1−xSrxMnO3 phase formed from the precursor at 800 °C, while small amounts of a metastable orthorhombic (Y,Sr)MnO3 phase formed in the temperature range between 850°and 960 °C, and another orthorhombic YMn2O5 phase between 840°and 1200 °C. The metastable (Y, Sr)MnO3 phase readily transformed into the stable hexagonal phase at about 960 °C. The other metastable intermediate phase, YMn2O5, was formed as a decomposition product of a portion of the major hexagonal YMnO3 at 840 °C, and subsequently reacted with Y2O3 back to the hexagonal YMnO3 at 1200 °C. For the studied compositions, densities higher than 95% theoretical could be obtained by sintering in air at temperatures above 1400 °C for 2 h. The investigated system was comparable in electrical conductivity with the current cathode material La1−xSrxMnO3, and had an average apparent thermal expansion coefficient between 5 and 7 ppm/°C in the temperature range between 200°and 1000 °C. Unfortunately microcracking was observed in all sintered specimens, possibly caused by a high-temperature phase transition between the hexagonal and cubic polymorphs of Y1−xSrxMnO3. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.

Highly efficient electrochemical carbon dioxide reduction to syngas with tunable ratios over pyridinic- nitrogen rich ultrathin carbon nanosheets

Precisely regulating surface reconstruction of copper (Cu) chalcogenides‐based catalysts to promote the multicarbon (C2+)selectivity of the electrochemical CO2 reduction reaction (CO2RR) is hampered by the challenging control of the intractable anions and the optimal Cuδ+ reduction (0 < δ < 1). Herein, a porous carbon‐supported copper selenides electrocatalyst that can remarkably improve the C2‐product yield and especially unveil the time‐revolved electrochemical CO2RR reconstruction process to enable the high C2‐selectivity, most notably for ethanol is constructed. The Faradic efficiency (FE) of C2‐products achieved is as high as ≈85.2% with a partial current density of 229.5 mA cm−2. Operando infrared spectroscopy and density functional theory (DFT) calculations unravel that the surface Se vacancies (VSe) formation brings closer the neighboring Cu+ atoms and activates the Cu sites, thereby rendering efficient generation of the key intermediates (*CO and *CHO) and lowering the C–C coupling barrier for C2 production. The appearance of metallic Cu can shorten the next‐nearest Cu0–Cu+ distance for O atom to bridge in, leading to the preferential formation of *OC2H4 towards ethanol instead of C–O bond cleavage to form ethylene. This work opens the avenue of designing suitable local atomic structures catalysts to engage the intermediates for targeted CO2RR products.

Preparation of 0D/2D ZnFe2O4/Fe-doped g-C3N4 hybrid photocatalysts for visible light N2 fixation

Combined experimental and DFT studies of Co82Zr12V6-xBx melt-spun ribbons to investigate structure and magnetic properties

Molecular dynamics (MD) simulation and density functional theory (DFT) calculations were used to predict the material properties and explore the improvement on the surface corrosion resistance for the Mg66Zn30Ca3Sr1 bulk metallic glass (BMG). The Mg66Zn30Ca4 BMG was also investigated to realize the influence of the addition of Sr element on the material behaviors of Mg66Zn30Ca4. The Mg-Zn-Ca-Sr parameters of the next nearest-neighbor modified embedded atom method (2NN MEAM) potential were first determined by the guaranteed convergence particle swarm optimization (GCPSO) method based on the reference data from the density functional theory (DFT) calculation. Besides, using the 2NN MEAM parameters of the Mg-Zn-Ca-Sr system, the structures of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 were predicted by the simulated-annealing basin-hopping (SABH) method. The local atomic arrangements of the predicted BMG structures are almost the same as those measured in some related experiments from a comparison with the calculated and experimental X-ray diffraction (XRD) profiles. Furthermore, the HA index analysis shows that the fractions of icosahedra-like local structures are about 72.20% and 72.73% for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively, indicating that these two BMG structures are entirely amorphous. The uniaxial tensile MD simulation was conducted to obtain the stress-strain relationship as well as the related mechanical properties of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Consequently, the predicted Young’s moduli of both BMGs are about 46.4 GPa, which are very close to the experimental values of 48.8 ± 0.2 and 49.1 ± 0.1 GPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively. However, the predicted strengths of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 are about 850 and 900 MPa, both are slightly higher than the measured experimental values about 747 ± 22 and 848 ± 21 MPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Regarding the thermal properties, the predicted melting temperature of Mg66Zn30Ca3Sr1 by the square displacement (SD) profile is about 620 K, which is very close to the experimental melting temperature of about 613 K. The self-diffusion coefficients of Mg, Zn, Ca, and Sr elements were also calculated for temperatures near their melting points by means of the Einstein equation. The methodology can determine the diffusion barriers for different elements by utilizing these diffusion coefficients resulting in a fact that the diffusion barriers of Ca and Sr elements of Mg66Zn30Ca3Sr1 are relatively high. For the electronic properties predicted by the DFT calculation, the projected density of states (PDOS) profiles of surface Mg, Zn, Ca, and Sr elements clearly show that the addition of Sr into Mg66Zn30Ca4 effectively reduces the s and p orbital states of surface Mg and Zn elements near the Fermi level, particularly the p orbits, which suppresses the electron transfer as well as increases the surface corrosion resistance of Mg66Zn30Ca4. Consequently, this study has provided excellent 2NN MEAM parameters for the Mg, Zn, Ca, and Sr system by the GCPSO method to predict real BMG structures as well as by means of the DFT calculation to explore the electronic properties. Eventually, through our developed numerical processes the material properties of BMGs with different compositions can be predicted accurately for the new BMG design.

Structural transitions at high pressure and metastable phase in Si0.8Ge0.2

Porous LaFeO3 nanofiber with oxygen vacancies as an efficient electrocatalyst for N2 conversion to NH3 under ambient conditions

Recycling materials back to supply chain not only mitigates the shortage of critical materials such as the metals of Li, Ni, Co, etc, but also protects global environment. Direct recycling of lithium nickel manganese cobalt oxides (NMC) mainly relies on a relithiation process based on purified spent NMC cathode materials. How to efficiently get rid of chemically bonded surface impurities on spent NMC and obtain a clean surface is critical for relithiation. One of the unavoidable impurities is fluorine, which exists in the form of either carbon fluoride (CFx) or metal fluoride (M-F) and can be eliminated by either thermal treatment or hydrothermal procedure under alkaline environments depending on the state of the F.1,2 Generally the hydrothermal method needs relative high temperature (> 200 °C ) and long reaction time (> 16 hours) due to the slow kinetics. A redox mediator such as H2O2, ethanol or ethylene glycol has been reported to be effective to lower the temperature and pressure for relithiation of recycled NMC111 and NMC622, which was pretreated with N-Methyl-2-pyrrolidone solvent.3 However the mechanism is still not fully understood.This work combines material characterizations and electrochemistry and density functional theory (DFT) calculations to study the role of surface oxygen in eliminating fluorine species on recycled NMC532 cathode material. The recycled NMC532 powders were separated from graphite by froth flotation method and there were significant amount of fluorine impurity species on the surface confirmed by XPS. Initial trial of the addition of small amount of H2O2 additive to LiOH aqueous solution during the hydrothermal process was able to decrease the reaction time significantly to fully eliminate both CFx and M-F species compared to the reaction without H2O2. Further calcination steps with the addition of 5% excessive lithium source using LiOH successfully recovered the recycled NMC532 and the half coin cell using the rejuvenated cathode materials shows a discharge capacity of 154 mAh/g at 0.2 C within 2.7 to 4.2 V voltage range. DFT calculations was conducted on a NMC (012) surface and both lithium hydroxide and sodium hydroxide were selected to understand the role of OH- group and surface oxygen species. The kinetics study was conducted with transition state theory4 and the activation energy barriers were calculated by climbing image nudged elastic band (cNEB) method.5,6 The removal of surface F- takes place with the displacement of F- with OH- (*-(M)F(s) +XOH(s) à XF(s) + *-(M)OH(s), X= Li, Na). F- displacement reactions require more than one basic molecule per F- removal with the formation of either LiF or NaF, which is soluble in water and can be removed by a filtration process. Depending on the surface ‘foreign’ oxygen species, the reaction can either go with a hydroxide pathway or a peroxide pathway, with the latter involving the formation of a peroxide intermediate with a ‘foreign’ oxygen atom leading to much smaller barrier energy compared to that in the hydroxide pathway. On a delithiated surface, the barrier energy for F- removal is predicted to be higher than pristine surfaces as the delithiated surface is less reactive to bind ‘foreign’ oxygen. This is consistent with the experimental results in which the additive of H2O2 can provide ‘foreign’ oxygen atoms and favors peroxide pathway leading to shorter time to remove surface fluorine. Additionally, the reaction with NaOH has a lower barrier than that with LiOH for the removal of fluoride species. Future work will investigate alternative additives as well as fluorine surface-bulk transfer which potentially happen if small amount of fluorine residues exist during calcination step.

Topological nodal line semimetal PdSn4 is investigated through high-pressure Raman spectroscopy, corroborated by first principles density functional theory (DFT) calculations. At ambient conditions, the Raman spectrum shows a broad peak structure at around 130 cm−1 together with several weak peaks. DFT calculations within the Ccce orthorhombic structure identify the 12 possible Raman active modes in this system below 140 cm−1, correlating well with the experimental observation. A comparison of the Raman spectra of RSn4 (R = Pd, Pt, and Au) showed a similar broad peak-like structure with slightly differing wavenumbers in agreement with a chemical pressure description of these systems. Indeed, phonon dispersions from DFT studies within the Ccce orthorhombic structure provided similar phonon band structures for both PdSn4 and PtSn4 and demonstrate that the orthorhombic lattice of PdSn4 to be stable at 25 GPa. In this pressure range, DFT as well as experiments showed a regular hardening of the Raman phonon modes. A careful analysis indicates that the pressure dependence of the mode positions has a different rate in the high- and low-pressure regions. This is an indication of a pressure-dependent isostructural phase transition in PdSn4, similar to that observed in PtSn4.

There is significant interest in improving the performance of batteries to increase electrification of transportation and aviation. Recently, performance improvements have been in large part due to changes in the composition of the cathode material family, LiNixMnyCo(1-x-y)O2 (e.g., 111-622-811). Despite the importance of these materials and tremendous progress with density functional theory (DFT) calculations in understanding basic design principles, it is computationally prohibitively expensive to make this problem tractable. Specifically, predicting the open circuit voltage for any cathode material in this family requires evaluation of stability in a quaternary phase space. In this work, we develop machine-learning potentials using fingerprinting based on atom-centered symmetry functions, used with a neural network model, trained on DFT calculations with a prediction accuracy of 3.7 meV/atom and 0.13 eV/Å for energy and force, respectively. We perform hyperparameter optimization of the fingerprinting parameters using Bayesian optimization through the Dragonfly package. Using this ML calculator, we first test its performance in predicting thermodynamic properties within the Debye-Grüneisen model and find good agreement for most thermodynamic properties, including the Gibbs free energy and entropy. Then, we use this to calculate the Li-vacancy ordering as a function of Li composition to simulate the process of discharging/charging of the cathode using grand canonical Monte Carlo simulations. The predicted voltage profiles are in good agreement with the experimental ones and provide an approach to rapidly perform design optimization in this phase space. This study serves as a proof-point of machine-learned DFT surrogates to enable battery materials optimization.

The development of acidic oxygen evolution reaction (OER) electrocatalysts with high activity, extended durability, and lower costs furthers the development and utilization of proton-exchange membrane (PEM) water electrolyzers. Although iridium-based catalysts are currently the primary OER catalysts used within PEM electrolyzers, ruthenium has a higher activity, lower cost, and higher global supply compared to iridium; however, ruthenium-based catalysts generally have lower stability compared to iridium-based catalysts. Within acidic OER catalysts, stability remains a significant challenge for non-iridium materials, particularly under the highly corrosive environment of highly acidic conditions and high anodic potentials encountered during PEM water electrolysis. As an approach to obtain high OER activity and improved stability, we investigated the effect of titanium substitution within rutile ruthenium oxide as a model system of oxide structure that combines a highly OER active, unstable element (Ru) with a highly stable but OER-inactive element (Ti). Titanium-substituted ruthenium oxides, Ru1-xTixO2, at different Ti substitution ratios were synthesized via wet-chemistry and subsequent thermal treatments. X-ray diffraction analysis and high resolution transmission electron microscopy images support Ti is substituted within the RuO2 structure. X-ray photoelectron spectroscopy of core and valence bands shows Ti substitution alters the surface electronic structure. From rotating disk electrode measurements, Ti substitution lowers the OER mass activity, OER specific activity, and electrochemical surface area. In addition to affecting OER activity, Ti substitution increases the OER stability and lowers Ru dissolution. Density functional theory (DFT) calculations of the titanium-substituted ruthenium oxides show that effects of Ti substitution on the reaction energies and activation energies are highly dependent on the site. Theoretical analysis supports that specific sites may predominately act as catalytic sites for the OER, while other sites influence metal dissolution. Further understanding how the structure of bimetallic oxides influences OER activity and stability provides a pathway to electrocatalysts with higher activity, improved stability, and lower cost.