Metal–Organic Frameworks Derived Cobalt Phosphide Architecture Encapsulated into B/N Co‐Doped Graphene Nanotubes for All pH Value Electrochemical Hydrogen Evolution

DOI: 10.1002/aenm.201601671 easy for the agglomeration under high current and long-term testing for HER, which showed poor stability and exhibited low HER activity irrespective of pH value. [7] Recently, researchers employed carbon shell to protect the metal or their compounds NPs from acidic atmospheric degradation and agglomeration with neighboring NPs. [8] These new metal/carbon composites not only enabled the catalytic applications of metal NPs which are also not stable as the naked in ambient atmosphere, but also improved their electron transport to certain extent. However, if the carbon layers on active species are too thick or with low porosity, it could hinder the desired access of mass transport, and as a result catalytic activity of metal is significantly decreased. [9] In this regard, recently developed in situ technology to prepare metal@carbon by using metal–organic frameworks (MOFs) as precursors has highlighted new functionality for such application. [10] MOFs are crystalline regular porous materials prepared by the coupling of metal ions with organic linkers,[11] and their derived composites, like nitrogendoped porous carbon, exhibit large surface area and hierarchical pore structures, which play important roles to ample the various catalytic reactions, such as HER, oxygen reduction, and evolution reaction. [12] However, such kind of materials consisted of some low degree of graphitization and poor bonding interactions between active metal NPs and derived carbon. [13] To improve the electron transport and valid mass diffusion path for HER, we apply this concept by using B/N co-doped graphene (BCN) nanotubes to confine MOF-derived CoP NPs. Furthermore, it is well documented that chemical replacement of carbon atom by nitrogen (N) and boron (B) can modulate the charge polarization of carbon nanotubes. [14] In particular, the two elements are reverse in electronegativity to that of carbon as B and N co-doping could activate the electron spin density between heteroatom and adjacent carbon atom. Thus, synergistic effect of ternary system of BCN nanotubes greatly enlarges the catalytic surface area. [14a,15] Meanwhile, it is anticipated that co-doping of B and N in graphene nanotubes encapsulated metal phosphide NPs will not only prevent the agglomeration of metal but also produce the additional active sites to enhance the HER performance. To the best of our knowledge, encapsulation of metal phosphide NPs in heteroatom graphitic nanotubes is a challenging task and has rarely been reported. Herein, we introduce a bottom-up strategy for synthesis of CoP architecture encapsulated into BCN nanotubes (CoP@BCN) through pyrolysis and phosphidation as shown in Owing to the increasing worldwide concern over energy crisis and environmental issues, great attention has been triggered for the development of clean and highly efficient energy conversion and storage techniques by electrocatalytic reaction in recent years. [1] As a promising candidate for the future energy supply, molecular hydrogen (H2) possess the highest gravimetric energy density and produced from electrocatalytic water splitting. The highly efficient electrocatalysts are crucial for lower overpotential to improve the energy transfer efficiency in hydrogen evolution reaction (HER). [2] Traditionally, rare earth metal Pt has been regarded as the best electrocatalyst to accelerate the kinetics in HER which is a prerequisite. [3] Unfortunately, their commercial applications are impeded by high cost and scarcity. [4] A key strategy to replace the precious Pt catalyst with earth abundant metals would promote global scalability of such potential clean energy applications. [5] Thus motivated by these several challenges, the search for a cost-effective, large earth abundant, highly efficient, and long stability has been made of immense significance toward earth-enriched metal for HER electrocatalysts. Recently, transition metal phosphide (TMP) based electrocatalysts have been broadly applied for HER activity. [5] Among the existing myriad of TMP, cobalt phosphide (CoP) has been identified as a promising candidate for HER as compared to iron, copper, nickel, and tungsten phosphides. [6] It has been identified that phosphorus (P) atom insertion into cobalt crystal lattice plays a crucial role for HER performance, because P atoms with more electronegativity can grab electron from metal atoms and act as a proton carrier. [2c] Nevertheless, the higher concentration of P in TMP restricts the delocalization of the metal atoms and impede the electron transportation which discards the HER activity. [6f ] Besides this, these electrocatalysts with certain morphologies of nanoparticles (NPs) or nanowires (NWs) are