The Design Of Highly Efficient Electrocatalysts For Water Splitting Based On Noble Metal

To overcome the energy crisis and environmental issues,developing clean and renewable energy sources is an effective way.Hydrogen has drawn great attention due to the advantages of high energy density,low pollutant emission,abundant reserves and easy storage.Among various advanced technology,electrochemical water splitting has been studied extensively because of its ease and generality.Currently,the electrocatalysts for water splitting include two groups:non-noble metals and noble metals.Nevertheless,the catalytic activities of non-metals are not satisfactory.So,the design of highly efficient and stable noble metal-based catalysts is of paramount significance.Up to now,there are two strategies to improve the activity of an electrocatalyst system for water splitting:?i?increasing the number of active sites on catalysts through improving catalyst structuring,choosing the supports and tuning the shape;?ii?increasing the intrinsic activity of catalysts via the methods of polymorph,confinement,adsorbate,intercalation,multi-metal and core-shell structure.In this paper,to design highly efficient and stable electrocatalysts for water splitting based on raw materials of SiNWs,nitrogen doped graphene,nitrogen doped carbon and noble metals,the nanotechnology,supports,shape turning,confinement and mutli-metal strategies were employed.The main results are as follows:?1?To design highly efficient catalysts for hydrogen evolution reaction?HER?,the stepwise-design concept for the production of hydrogen was described based on the first principles calculations?the Volmer reaction and Heyrovsky reaction occur at different component on the catalysts?.The theoretical prediction has been verified for the Ru-Au-SiNW ternary nanocomposite.The electrochemical results suggested that the optimal Rh-Au-SiNW?Rh:Au:Si=2.2:28.5:69.3 mass ratio?exhibited lower Tafel slope(24 mV·dec^-1)than that of 40 wt%Pt/C catalyst(30 mV·dec^-1).More importantly,its current density surpassed that of 40 wt%Pt/C catalyst at overpotential larger than 0.19 V.Furthermore,at the overpotential of 0.3 V,the turnover frequency of Rh-Au-Si was 11 times larger than that of 40 wt%Pt/C catalyst.The above results indicated that the stepwise-desige concept for highly efficient electrocatalyst was verified for the Rh-Au-Si NW nanocomposite.?2?In order to achieve low Pt usage,highly efficient and stable catalysts for HER,the Pt-Au-SiNW nanocomposite were synthesized.The electrochemical results indicated that the optimal composition of Pt-Au-SiNW?Pt:Au:Si=7.2:31.3:61.5mass ratio?with the Tafel slope of only 24 mV·dec^-1.At overpotential of 60 mV,the mass activity of Pt-Au-SiNW was 6.5 times larger than that of 40 wt%Pt/C catalyst.More importantly,the Pt-Au-SiNW catalysts exhibited better stability than that of that of 40 wt%Pt/C catalyst at high overpotential.At the overpotential above 0.17 V,its electrocatalytic current density surpassed the 40 wt%Pt/C catalyst.The outstanding catalytic performance was verified by theoretical calculation:the fast hydrogen adsorption rate on the Pt,the quick migration of the adsorbed hydrogen atoms on Au and hydrogen evolution on SiNW.?3?Achieving high activity electrocatalyst for HER based on Rh-Ag-SiNW nanocomposite.The activity of Rh-Ag-SiNW catalyst was predicted through the theoretical calculation.The experimental results suggested that Rh-Ag-SiNW exhibited lower Tafel slope(51 mV·dec^-1)and larger exchange current density(87.1?A·cm^-2)than that of Pt-Ag-SiNW(the Tafel slope and exchange current density was 56mV·dec^-1 and 9.1?A·cm^-2,respectively).In addition,at the overpotential of 0.2 V,the mass activity of Rh-Ag-SiNW(11.5 mA·?g_Rh^-1)was 5 and 3.3 folds higher than that of Pt-Ag-SiNW(2.3 mA·?g_Pt^-1)and 40 wt%Pt/C(3.5 mA·?g_Pt^-1),respectively.More importantly,the Rh-Ag-SiNW nanocomposite had good stability in acidic media.?4?The follower-like Pt nanocrystals loaded on nitrogen doped graphene?Pt NC/N-graphene?were synthesized by using Si nanowires as the sacrificial template,which were employed as highly efficient and stable electrocatalyst for water splitting.The experimental results showed that the Pt NC/N-graphene catalyst exhibited superior HER activity and stability with only a low Pt loading?5.0 wt%?.To be specific,the mass activity of the Pt NC/N-graphene catalyst was 5 times higher than that of 20 wt%Pt/C catalyst.Furthermore,compared with the 20 wt%Pt/C catalyst,the Pt NC/N-graphene catalyst also achieved a lower Tafel slope(28.0 mV·dec^-1),larger exchange current density(0.989 mA·cm^-2)and higher turnover frequency(2.05 s^-1).More importantly,the catalyst also displayed better stability than that of 20 wt%Pt/C catalyst.?5?Ru@RuO₂ nanoparticles encapsulated in nitrogen doped carbon?Ru@RuO₂@CN_x?was successfully synthesized by a facile method,which was developed as a highly efficient and bifunctional electrocatalyst for water splitting in the neutral electrolyte.As an OER catalyst,the Ru@RuO₂@CN_x exhibited low Tafel slope(92 mV·dec^-1)and afforded the current density of 10 mA·cm^-2 at the overpotential of369 mV,which was lower than that of commercial RuO₂ catalyst(the Tafel slope and overpotential at 10 mA·cm^-2 was 157 mV·dec^-1 and 620 mV,respectively).As an HER catalyst,this catalyst also exhibited excellent HER activity with Tafel slope of only 61mV·dec^-1,which was lower than that of 20 wt%Pt/C(67 mV·dec^-1)and commercial RuO₂(92 mV·dec^-1)catalysts.More significantly,in the overall water splitting,Ru@RuO₂@CN_x catalyst reached 10 mA·cm^-2 at a cell voltage of only 1.75 V in 1 M PBS?pH=7.0?electrolyte and exhibited excellent stability.