Bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for rechargeable metal-air batteries and regenerative fuel cells. However, the commercial oxygen electrocatalysts (mainly noble metal based) can only exhibit either ORR or OER activity, and also suffer from inherent cost and stability issues. It remains challenging to achieve efficient ORR and OER bifunctionality on a single catalyst. Metal-free structures offer relatively large scope for such bifunctionality to be enginnered within one catalyst, together with improved cost-effectiveness and durablility. Herein, by closely coupled computational design and experimental development, highly effective bifunctionality is achieved in a phosphorus and nitrogen co-doped graphene framework (PNGF) - with both ORR and OER activities reaching the theoretical limits of metal-free catalysts, superior to the noble metal counterparts in both (bi)functionality and durability. In particular, with the identification of active P-N sites for OER and N-doped sites for ORR , we successfully intensified such sites by one-pot synthesis to tailor the PNGF. The resulting catalyst reaches an ORR potential of 0.845 V vs. RHE at 3 mA cm-2 and an OER potential of 1.55 V vs. RHE at 10 mA cm-2, respectively. Its combined ORR and OER overpotential of 705 mV is significantly lower than those reported previously for metal-free bifunctional catalysts

Two-dimensional (2D) Xenes of a single type of element can offer fascinating electronic properties, such as massless Dirac fermions for extremely high charge-carrier mobility and topological insulators for dissipationless electron transport. However, the realization of either the massless Dirac fermions or the topological insulator in a same element system via a simple physical method has rarely been reported, which is of great importance for the development of next-generation electronic devices. Here, by using first-principles calculations, we identify that a 2D square tellurium system can be effectively tuned to realize either the massless Dirac fermions or the topological insulator phase. The 2D square tellurium system shows three structural phases via strain effect, i.e., buckled square, buckled rectangular, and planar square phases, which exhibit extraordinary topological properties. There are four anisotropic Dirac points in the buckled square phase, in which the Fermi velocity can be as high as 9.44 × 10 5 m/s. The buckled rectangular phase can behave as a quantum spin Hall insulator with a band gap of 0.24 eV, pointing towards promising applications for room-temperature devices. There also exist nodal lines in buckled square/planar square structures in the non-spin-orbit-coupling case. These findings extend the knowledge on single-layer materials and promote future applications of the 2D tellurium systems.