Two-Dimensional Biphenylene: A Graphene Allotrope with Superior Activity toward Electrochemical Oxygen Reduction Reaction

Selective CO2 Reduction over γ-Graphyne Supported Single-Atom Catalysts: Crucial Role of Strain Regulation

The two-electron CO2 reduction reaction (2e-CO2RR) is the most promising process for realizing industrial utilization of CO2, but it is hindered by the competitive hydrogen evolution reaction (HER) because of the comparable equilibrium potential. Strategies to enhance 2e-CO2RR activity and selectivity by suppressing HER are highly demanded. Inspired by the low in-plane Young's modulus of the recently synthesized γ-graphyne (GY), we propose tensile-strain regulation as an effective method to improve the selectivity of the CO2RR against HER. By means of constant-potential calculations and constrained ab initio molecular dynamics simulations, we demonstrate the good stability and high CO2RR activity of GY-supported Co (Co-GY) single-atom catalysts (SACs). The change in potential of zero charges of *COOH is revealed to be more sensitive to tensile strain than that of *H species on Co-GY SACs, resulting in a slower change of its adsorption energy than that of *H species under working potentials and consequently enhanced CO2RR selectivity toward CO production. Besides, the strain-dependent regulation mechanism also applies to other M-GY SACs, demonstrating strain regulation as an effective strategy for designing and manipulating SACs for the selective 2e-CO2RR.

Bifunctional oxygen reduction/evolution reaction electrocatalysts achieved by axial ligand modulation on two-dimensional porphyrin frameworks

Exploring efficient and low-cost oxygen reduction and oxygen evolution reaction (ORR/OER) bifunctional catalysts is essential for the development of energy storage and conversion devices. Herein, enlightened by the experimentally synthesized cobalt(II) meso-tetraethynylporphyrins (Co-TEP) molecule, we designed a novel 2D covalent organic framework (COF), namely a 2D Co-TEP monolayer, by dimensional expansion. The 2D Co-TEP monolayer, with Co atoms distributed separately and stabilized by uniform pyrrolic-N coordination, features metal-nitrogen-carbon single-atom catalyst activity and shows tunable catalytic activity for the electrochemical ORR/OER by axial ligand (O, OH, Cl, CN, CH3, NO, F) modulation. By means of the state-of-the-art constant-potential first-principles computations and microkinetic simulations, we demonstrated that 2D Co-TEP-CN exhibits good ORR/OER performance in both acidic and alkaline conditions. The difference between the onset-potential for the OER and the half-wave potential for the ORR is only 0.85 V at pH = 1, smaller than that of Pt/IrO2 electrocatalysts. The good electrocatalytic performance is maintained by replacing the center metal atoms with Mn, Fe and/or Ni. Our investigation highlights the role of the pyrrolic-N coordination and the ligands in improving the catalytic activity of 2D COFs and provides new insights into the rational design of efficient bifunctional ORR/OER catalysts.

Engineering Two-Dimensional Nodal Semimetals in Functionalized Biphenylene by Fluorine Adatoms

We propose a band engineering scheme on the biphenylene network, a newly synthesized carbon allotrope. We illustrate that the electronic structure of the biphenylene network can be significantly altered by controlling conditions affecting the symmetry and destructive interference of wave functions through periodic fluorination. First, we investigate the mechanism for the appearance of a type-II Dirac fermion in a pristine biphenylene network. We show that the essential ingredients are mirror symmetries and stabilization of the compact localized eigenstates via destructive interference. While the former is used for the band-crossing point along high symmetry lines, the latter induces highly inclined Dirac dispersions. Subsequently, we demonstrate the transformation of the biphenylene network's type-II Dirac semimetal phase into various Dirac phases such as type-I Dirac, gapped type-II Dirac, and nodal line semimetals through the deliberate disruption of mirror symmetry or modulation of destructive interference by varying the concentration of fluorine atoms.

Transition metal small clusters anchored on biphenylene for effective electrocatalytic nitrogen reduction

The synthesis of ammonia via an electrochemical nitrogen reduction reaction (NRR, N2 + 6H+ + 6e- → 2NH3), which can weaken but not directly break an inert NN bond under mild conditions via multiple progressive protonation steps, has been proposed as one of the most attractive alternatives for the production of NH3. However, the development of appropriate catalyst materials is a major challenge in the application of NRRs. Recently, single- or multi-metal atoms anchored on two-dimensional (2D) substrates have been demonstrated as ideal candidates for facilitating NRRs. In this work, by applying spin-polarized density functional theory and ab initio molecular dynamic simulations, we systematically explored the performances of nine types of transition metal multi-atoms anchored on a recently developed 2D biphenylene (BPN) sheet in nitrogen reduction. Structural stability and NRR performance catalyzed by TMn (TM = V, Fe, Ni, Mo, Ru, Rh, W, Re, Ir; n = 1-4) clusters anchored on BPN sheets were systematically explored. After a strict six-step screening strategy, it was found that W2, Ru2 and Mo4 clusters loaded on BPN demonstrate superior potential for nitrogen reduction with extremely low onset potentials of -0.26, -0.36 and -0.17 V, respectively. Electronic structure analysis revealed that the enhanced ability of these multi-atom catalysts to effectively capture and reduce the N2 molecule can be attributed to bidirectional charge transfer between the d orbitals of transition metal atoms and molecular orbitals of the adsorbed N2 through a "donation-back donation" mechanism. Our findings highlight the value of BPN sheets as a substrate for designing multi-atom nitrogen reduction reaction catalysts.

Biphenylite as Anode Materials for Alkali Metal Ion Batteries with Self-Enhanced Storage Mechanism

In this work, we theoretically investigate the feasibility of biphenylite, the van der Waals layered bulk structure from experimental biphenylene network monolayers, as an anode material for alkali metal ions. The results indicate that the theoretical properties of Li, Na, and K in biphenylite are generally beyond those in graphite. Li-biphenylite exhibits a high specific capacity of 744 mAh·g-1, with a corresponding voltage range of 0.90-0.36 V, low diffusion barrier (<0.30 eV), and small volume change (∼9.9%), far exceeding those of Li-graphite. Moreover, a novel self-enhanced storage mechanism is observed and unveiled, in which the heavy intercalation of Li atoms (i.e., electron doping) induces puckered distortion of the nonhoneycomb carbon frameworks to enhance the intercalation ability and capacity of Li ion via a chemical activation of carbon frameworks. Possessing excellent anode performance beyond graphite, biphenylite is a promising "all-around" anode material candidate for alkali metal ion batteries, especially for lithium ion batteries.

Anisotropic thermal expansion of silicon monolayer in biphenylene network

Materials with a negative thermal expansion property are of great importance in the emerging family of two-dimensional materials. For example, mixing two materials with negative and positive coefficients of thermal expansion avoids volume changing with temperature. In this work, based on first-principles calculations and Grüneisen's theory, we investigated the thermal expansion properties of a silicon monolayer in biphenylene networks. Our results show that the thermal expansion is greatly negative and anisotropic, as the linear thermal expansion coefficient along the a-direction is significantly smaller than the one along the b-direction, even at high temperatures. At 300 K, the thermal expansion coefficients along the two lattice directions are -17.010 × 10-6 K-1 and -2.907 × 10-6 K-1, respectively. By analyzing the Grüneisen parameters and the elastic compliance, we obtained an understanding of the giant negative thermal expansion of the material. Rigid unit modes are also responsible for the negative thermal expansion behavior. Our work provides fundamental insights into the thermal expansion of silicon monolayer in biphenylene networks and should stimulate the further exploration of the possible thermoelectric and thermal management applications of the material.

Graphene oxide-based large-area dynamic covalent interfaces

Dynamic materials, being capable of reversible structural adaptation in response to the variation of external surroundings, have experienced significant advancements in the past several decades. In particular, dynamic covalent materials (DCMs), where the dynamic covalent bonds (DCBs) can reversibly break and reform under defined conditions, present superior dynamic characteristics, such as self-adaptivity, self-healing and shape memory. However, the dynamic characteristics of DCBs are mainly limited within the length scale of covalent bonds, due to the local position exchange or the inter-distance variation between the chemical compositions involved in the reversible covalent reactions. In this minireview, a discussion regarding the realization of long-range migration of chemical compositions along the interfaces of graphene oxide (GO)-based materials via the spatially connected and consecutive occurrence of DCB-based reversible covalent reactions is presented, and the interfaces are termed "large-area dynamic covalent interfaces (LDCIs)". The effective strategies, including water adsorption, interfacial curvature and metal-substrate support, as well as the potential applications of LDCIs in water dissociation and humidity sensing are summarized. Additionally, we also give an outlook on potential strategies to realize LDCIs on other 2D carbon-based materials, including the interfacial morphology and periodic element doping. This minireview provides insights into the realization of LDCIs on a wider range of 2D materials, and offers a theoretical perspective for advancing materials with long-range dynamic characteristics and improved performance, including controlled drug delivery/release and high-efficiency (bio)sensing.

Two-Dimensional Biphenylene-Based Carbon Allotrope Family with High Potassium Storage Ability

The development of new carbon materials with novel properties and excellent applications is essential and urgent in many fields, such as potassium-ion batteries (PIBs). In this study, a family of 30 two-dimensional biphenylene carbon allotropes (2D-BCAs) have been systematically extended in theory. The energies of these allotropes are slightly higher than that of graphene, which can be well described by a quantitative energy equation. The 2D-BCAs show high synthesizability consistent with the experimental biphenylene network via "HF-zipping" reactions. The 2D-BCAs are metallic or semimetallic. Six representative 2D-BCAs exhibit good lattice dynamical and thermal stability, excellent anisotropic mechanical properties, and ORR catalytic activity. Moreover, the selected 2D-BCAs demonstrate ultrahigh theoretical potassium-storage capacities of 1116-1489 mAh·g-1, low migration barriers of 0.03-0.22 eV, and low open-circuit voltages of 1.10-0.02 V. The remarkable properties render 2D-BCAs as promising anode materials in PIBs, electrocatalysts, and conductors in electronics and iontronics.

Interface engineering of transition metal-nitrogen-carbon by graphdiyne for boosting the oxygen reduction/evolution reactions: A computational study

Exploring high-efficiency electrocatalysts to boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is pivotal to the large-scale applications for clean and renewable energy technologies, such as fuel cells, water splitting, and metal-air batteries. Herein, by means of density functional theory (DFT) computations, we proposed a strategy to modulate the catalytic activity of transition metal-nitrogen-carbon catalysts through their interface engineering with graphdiyne (TMNC/GDY). Our results revealed that these hybrid structures exhibit good stability and excellent electrical conductivity. Especially, CoNC/GDY was identified as a promising bifunctional catalyst for ORR/OER with rather low overpotentials in acidic conditions according to the constant-potential energy analysis. Moreover, the volcano plots were established to describe the activity trend of the ORR/OER on TMNC/GDY using the adsorption strength of the oxygenated intermediates. Remarkably, the d-band center and charge transfer of the TM active sites can be utilized to correlate the ORR/OER catalytic activity and their electronic properties. Our findings not only suggested an ideal bifunctional oxygen electrocatalyst, but also provided a useful strategy to obtain highly efficient catalysts by interface engineering of two-dimensional heterostructures.

Theoretical prediction of 2D biphenylene as a potential anchoring material for lithium-sulfur batteries

Searching for good anchoring materials that can suppress the shuttle effect is critical to large-scale commercialization of lithium-sulfur (Li-S) batteries. In this work, the adsorption behavior of lithium polysulfides (LiPSs, such as S8 and Li2Sn, n = 1, 2, 4, 6, and 8), the sulfur reduction reaction (SRR), the decomposition processes of Li2S and the diffusion behavior of Li atoms on intrinsic and doped 2D biphenylene (BIP) are systematically investigated by employing the first-principles calculation method. Calculations show that the adsorption energies of LiPSs on the electrolyte (DOL and DME) are smaller than those on the intrinsic/B doped BIP. The moderate anchoring strength (0.8-2.0 eV) between LiPSs and the BIP can effectively suppress the shuttle effect. Moreover, the Gibbs free energy barrier for SRR is 0.72/0.64 eV on intrinsic/B doped BIP. The dissociation energy barrier of Li2S on intrinsic/B doped BIP is 1.35 eV, while the diffusion energy barrier of Li atoms on intrinsic/B doped BIP is 0.18 eV/0.30 eV. Lower energy barriers are conducive to enhancing the discharging and charging efficiency. Therefore, intrinsic and B doped BIP are predicted as good anchoring materials for Li-S batteries.

Proton penetration mechanism and selective hydrogen isotope separation through two-dimensional biphenylene

Hydrogen isotope separation is of prime significance in various scientific and industrial applications. Nevertheless, the existing technologies are often expensive and energy demanding. Two-dimensional carbon materials are regarded as promising candidates for cost-effective separation of different hydrogen isotopes. Herein, based on theoretical calculations, we have systematically investigated the proton penetration mechanism and the associated isotope separation behavior through two-dimensional biphenylene, a novel graphene allotrope. The unique non-uniform rings with different sizes in the biphenylene layer resemble the topological defects of graphene, serving as proton transmission channels. We found that a proton can readily pass through biphenylene with a low energy barrier in some specific patterns. Furthermore, large kinetic isotope effect ratios for proton-deuteron (13.58) and proton-triton (53.10) were observed in an aqueous environment. We thus conclude that biphenylene would be a potential carbon material used for hydrogen isotope separation. This subtle exploitation of the natural structural specificity of biphenylene provides new insight into the search for materials for hydrogen isotope separation.

Edge-Site-Free and Topological-Defect-Rich Carbon Cathode for High-Performance Lithium-Oxygen Batteries

The rational design of a stable and catalytic carbon cathode is crucial for the development of rechargeable lithium-oxygen (LiO2 ) batteries. An edge-site-free and topological-defect-rich graphene-based material is proposed as a pure carbon cathode that drastically improves LiO2 battery performance, even in the absence of extra catalysts and mediators. The proposed graphene-based material is synthesized using the advanced template technique coupled with high-temperature annealing at 1800 °C. The material possesses an edge-site-free framework and mesoporosity, which is crucial to achieve excellent electrochemical stability and an ultra-large capacity (>6700 mAh g-1 ). Moreover, both experimental and theoretical structural characterization demonstrates the presence of a significant number of topological defects, which are non-hexagonal carbon rings in the graphene framework. In situ isotopic electrochemical mass spectrometry and theoretical calculations reveal the unique catalysis of topological defects in the formation of amorphous Li2 O2 , which may be decomposed at low potential (∼ 3.6 V versus Li/Li+ ) and leads to improved cycle performance. Furthermore, a flexible electrode sheet that excludes organic binders exhibits an extremely long lifetime of up to 307 cycles (>1535 h), in the absence of solid or soluble catalysts. These findings may be used to design robust carbon cathodes for LiO2 batteries.

Locally spontaneous dynamic oxygen migration on biphenylene: a DFT study

The dynamic oxygen migration at the interface of carbon allotropes dominated by the periodic hexagonal rings, including graphene and carbon nanotubes, has opened up a new avenue to realize dynamic covalent materials. However, for the carbon materials with hybrid carbon rings, such as biphenylene, whether the dynamic oxygen migration at its interface can still be found remains unknown. Using both density functional theory calculations and machine-learning-based molecular dynamics (MLMD) simulations, we found that the oxygen migration departing away from the four-membered carbon (C₄) ring is hindered, and the oxygen atom prefers to spontaneously migrate toward/around the C₄ ring. This locally spontaneous dynamic oxygen migration on the biphenylene is attributed to a high barrier of about 1.5 eV for the former process and a relatively low barrier of about 0.3 eV for the latter one, originating from the enhanced activity of the C-O bond near/around the C₄ ring due to the hybrid carbon ring structure. Moreover, the locally spontaneous dynamic oxygen migration is further confirmed by MLMD simulations. This work sheds light on the potential of biphenylene as a catalyst for spatially controlled energy conversion and provides the guidance for realizing the dynamic covalent interface at other carbon-based or two-dimensional materials.

Biphenylene: A Two-Dimensional Graphene-Based Coating with Superior Anti-Corrosion Performance

Metal corrosion can potentially cause catastrophic engineering accidents threatening personal safety; thus, coating protection is a tremendously valuable anti-corrosion initiative. Recently, biphenylene, a novel two-dimensional (2D) graphene-based material, has achieved a remarkable synthetic breakthrough; the anti-corrosion properties of biphenylene, with its specific pore structure, are predicted to be beneficial in applications of metal corrosion resistance. In this study, the anti-corrosion mechanism of biphenylene is deciphered utilizing first principles and molecular dynamics. The results suggest that biphenylene with tetragonal, hexagonal, and octagonal carbon rings supplies adequate sites for stable O atom adsorption. The charge transfer amounts of +0.477 and +0.420 e facilitate the formation of a compact oxygen-rich layer on the material surface to acquire outstanding anti-corrosion properties. The moderate wettability of biphenylene prevents the water-based solution from encroaching on the biphenylene coating and substrate. In addition, the intensive binding between biphenylene and the aluminum substrate strengthens the integration of the two heterogeneous structures with -413.7 and 415.5 eV, which guarantees the durable application of biphenylene coating.

Why heterogeneous single-atom catalysts preferentially produce CO in the electrochemical CO2 reduction reaction

Formate and CO are competing products in the two-electron CO2 reduction reaction (2e CO2RR), and they are produced via *OCHO and *COOH intermediates, respectively. However, the factors governing CO/formate selectivity remain elusive, especially for metal-carbon-nitrogen (M-N-C) single-atom catalysts (SACs), most of which produce CO as their main product. Herein, we show computationally that the selectivity of M-N-C SACs is intrinsically associated with the CO2 adsorption mode by using bismuth (Bi) nanosheets and the Bi-N-C SAC as model catalysts. According to our results, the Bi-N-C SAC exhibits a strong thermodynamic preference toward *OCHO, but under working potentials, CO2 is preferentially chemisorbed first due to a charge accumulation effect, and subsequent protonation of chemisorbed CO2 to *COOH is kinetically much more favorable than formation of *OCHO. Consequently, the Bi-N-C SAC preferentially produces CO rather than formate. In contrast, the physisorption preference of CO2 on Bi nanosheets contributes to high formate selectivity. Remarkably, this CO2 adsorption-based mechanism also applies to other typical M-N-C SACs. This work not only resolves a long-standing puzzle in M-N-C SACs, but also presents simple, solid criteria (i.e., CO2 adsorption modes) for indicating CO/formate selectivity, which help strategic development of high-performance CO2RR catalysts.

Biphenylene monolayer: a novel nonbenzenoid carbon allotrope with potential application as an anode material for high-performance sodium-ion batteries

Allotrope metal structures composed of carbon as anode materials for metal-ion batteries are a current research hotspot. In this work, the recently synthesized graphene allotrope, two-dimensional (2D) biphenylene, consisting of tetragonal, hexagonal and octagonal carbon rings, was explored theoretically. Our first-principles calculations verified that 2D biphenylene has dynamical, mechanical and thermal stability and exhibits metallic features. Its novel structure can provide multiple adsorption sites for Na ions, a fast charge-discharge rate (low Na migration barriers of <0.2 eV) and high theoretical capacity (1075.37 mA h g^-1). These superior properties, combined with its carbon abundance and light mass, make the biphenylene monolayer a promising high-performance anode for sodium-ion batteries (SIBs).

The reduction reaction of carbon dioxide on a precise number of Fe atoms anchored on two-dimensional biphenylene

The reduction reaction of carbon dioxide on a precise number of Fe atoms anchored on two-dimensional biphenylene.