Self-Organized Single-Atom Tungsten Supported on the N-Doped Carbon Matrix for Durable Oxygen Reduction

Abstract

Rational engineering of atomically scaled metalnitrogen-carbon (M-N-C) moieties has been the topic of recent research interest because of their potential application as an electrochemical oxygen reduction reaction (ORR) catalyst. Despite numerous efforts on M-N-Cs, attaining both adequate activity and a satisfactory stability simultaneously is a principal issue. Herein, we demonstrated the synthesis of a single-atom tungsten catalyst supported on the N-doped carbon matrix (W-N-C) and its application as an ORR catalyst. W-N-C was synthesized using the economically viable, simple, one-step pyrolysis of dicyandiamide and tungsten(VI) chloride at moderate temperature (700 °C). The synthesis of W-N-C avoids any post acid treatment as it does not require any subsidiary sacrificial metal like Zn and, hence, does not induce any burden associated with chemical waste management. The atomic dispersion of W atoms stabilized by N-doped porous carbon and the formation of WN 2 C 2 were confirmed by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy. Interestingly, as-synthesized WN 2 C 2 exhibited unprecedented electrocatalytic activity with a half-wave potential of 764 mV vs reversible hydrogen electrode (RHE) as well as significantly enhanced stability (retaining >99% diffusion-limited current density and the loss in activity is 10.5% at 0.84 V after 10,000 potential cycles), which is much better than the stability limit set by the US Department of Energy in an alkaline medium. Overall, the activity of W-N-C surpasses that of Pt/C after 5000 cycles. The excellent stability is believed to be due to the symmetric coordination of the metal active site (W 2 N 2 C 2 ).

Metal–Nitrogen–Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions

“…The EXAFS fitted results confirm that the coordination number of the centered W atom in the first shell was about four, coordinated with two C atoms and two N atoms at bond lengths of 1.86 ± 0.01 and 2.07 ± 0.01 Å, respectively. [109] While the FT-EXAFS spectra of Fe-N-C-900 display a strong and a weak peak at around 1.5 and 2.5 Å, assigned to the bonds of Fe-C and Fe-N, respectively. Another curve fitting using Demeter was performed to obtain the structure information of Fe-N-C-900, and the Fe-N coordination number was about 5 (FeN 5 ).…”

“…The atomically dispersed W species are stabilized by nitrogen-doped carbon, and the generation of WN 2 C 2 has been confirmed, which is the main active site during the catalytic reaction. [109] By directly grafting Cu-N complexes onto the carbon support at a certain temperature, a Cu-N-C catalyst with two kinds of coordinative configurations of Cu II -N 4 and Cu I -N 2 has been successfully synthesized by Wang's group, which respectively exist as a center defect and an edge defect in carbon network. [110] In addition, bimetallic Co/Zn ZIFs were used as the precursor owing to its isolated intrinsic metal nodes connected with linkers, and the N numbers surrounding central Co sites could be modulated through controlling the selective breakage of CN bonds at varied at varied calcining temperatures (Figure 6).…”

“…H 2 O 2 by 2e − reaction is less stable and tends to decompose into OH* intermediates, which are chemisorbed on the surface, indicating that the ORR with a 2e − pathway is undesirable on CoN 2 -G. [214] Besides, W-N-C has been investigated as an highly efficient electrocatalyst for ORR with outstanding stability in alkaline solutions, attributable to the symmetrically coordinated WN 2 C 2 active site. [109]…”

Modulating the Local Coordination Environment of Single‐Atom Catalysts for Enhanced Catalytic Performance in Hydrogen/Oxygen Evolution Reaction

“…In this regard, several alternative synthetic strategies have been proposed. [38,40,93,[126][127][128] Li et al proposed a secondary-atom-assisted strategy for preparing porous N-doped carbon nanowires supported-Fe SACs (denoted as Fe-NCNWs). [124] Following a typical synthetic strategy, a CTAB (cetyltrimethylammonium bromide) template was initially dissolved in an HCl solution, followed by oxidization using ammonium peroxydisulfate (APS).…”

Design of Hierarchical Oxide‐Carbon Nanostructures for Trifunctional Electrocatalytic Applications

“…[29][30][31] Reports suggest that metal-nitrogen coordination (M-N x ) centers are highly beneficial for ORR and HER. [32][33][34][35][36][37] Transition metal nanoparticles supported on N-doped carbon have been extensively studied for HER. [38][39][40] Metal nanoparticles optimize the surface states of M-N x thereby reducing the energy barrier for water dissociation.…”