Rational Design Of Several Novel Tungsten-based Functional Materials For Efficient Electrocatalysis

Efficient energy storage and conversion is one of the greatest challenges of the modern age,because it can provide advanced solutions for reducing the use of fossil fuels and meeting the sustainability imperatives.Hydrogen has been considered as a promising energy carrier for future clean-energy demands,owing to its high energy density and environmentally friendly properties.One of the most economical and sustainable ways to generate hydrogen is electrocatalytic hydrogen evolution from water(HER),which has the ability to store energy without the emission of carbon dioxide.Among various hydrogen-based energy conversion systems,fuel cells,which are driven by the most significant cathodic process-oxygen reduction reaction(ORR),since the behavior of the devices is highly dependent on the ORR process.Unfortunately,although platinum-based catalysts still remain as the benchmark HER and ORR electrocatalysts,the high cost,limited supply,and poor long-term durability largely impede their widespread applications.Therefore,exploring the non-noble materials for efficiently catalyzing the HER and ORR process with superior activity and stability is still a tremendous challenge to overcome.Tungsten carbide-based materials have long been advocated and investigated as potential replacements of platinum for HER and ORR catalysts,owing to the intercalation of C into the W lattice gives rise to a "Pt-like" d-band electronic density states.while they cannot present desirable activity for HER and ORR process in alkaline solutions,because the tungsten active sites have strong affinity for H*,OH*,O*and OOH*intermediates in the process of HER and ORR,which is likely to lead to the easy poisoning of electrocatalysts,especially the gradually oxidation of W sites into inert WxOy species that greatly hampers their long-term durability.Thus,our work mainly focused on the synthesis of novel tungsten-based materials as highly efficient HER and ORR electrocatalysts,and the main results are listed as follows:(1)A unique eutectoid-structured WC/W2C heterostructure(ES-WC/W2C)that can serve as a highly active electrocatalyst for alkaline HER has been fabricated via calcination of a special two-dimensional organic-inorganic tungsten precursor(WOx/W-PDA).This novel ES-WC/W2C catalyst exhibits high alkaline HER activity with an ultra-low onset-potential of 17 mV and a low overpotential of 75 mV at 10 mA/cm2(?10).It yields an ultra-high exchange current density of 0.58 mA/cm2,an enhancement of nearly 14-and 12-fold in comparison with the phase-pure WC and W2C,respectively.Even when normalized to the electrochemically active surface area(ECSA),the normalized current density(J0,normalized)is still significantly higher than the J0,normalized for phase-pure WC and W2C,demonstrating the substantial improvement of intrinsic activity by constructing such heterostructures.Moreover,it also exhibits an exceptionally stability in alkaline solutions,showing no evidence of significant degradation over 480 h(>20 days)of H2 production,far exceeding the stability of other tungsten carbide-based electrocatalysts.To the best of our knowledge,this is the first time such a eutectoid-structured materials has been reported to efficiently catalyze the HER in alkaline solution.(2)Various atomically dispersed tungsten on nitrogen-doped carbon nanosheets with controlled W-N coordination numbers as efficient catalysts for ORRs have been synthesized through the deliberate modulation of the synthesis parameters,such as the pyrolysis atmosphere,temperature,and time,within a very narrow range(700-750?).Instead of being considered to be almost inactive towards ORR,the single-atom tungsten electrocatalysts show remarkable,durable and coordination number-sensitive ORR catalytic ability.It is shown that single-atom tungsten with a W-N coordination number of 5 exhibits markedly high ORR catalytic activity in 0.1 M KOH with onset potential(?1.01 V),half-wave potential(0.88 V)and a mass activity of 0.63 A/mg(at 0.9 V versus RHE),which even surpasses those of commercial Pt/C.Meanwhile,the WNs catalyst catalyzes the ORR with a onset potential of 0.87 V and a half-wave potential of 0.77 V in 0.1 M HClO4,both of which are nearly comparable to the benchmark Pt/C.In contrast,the single-atom tungsten electrocatalysts with W-N coordination numbers of 3 and 4 exhibit relatively poor ORR activity in both acidic and alkaline electrolytes.The DFT calculations suggest that the sharp increase in the ORR activity of the single-atom tungsten catalysts can be attributed to the moderate interaction between OH-and the single W atoms,which is probably caused by the optimal dz2-pz orbital hybridization and re-distribution of the charges.