The Synthesis and Performance of Water Splitting Electrocatalysts

Author:Zhang Shan

Supervisor:wang er kang


Degree Year:2019





The energy demand increases as the rapid economic development and the rising global population,and much attention has been paid on the climate change and environmental issues if we still rely on the limited fossil fuels.People come to realize the importance of seeking clean and renewable energy such as solar and wind for the sustainable development of society.Among these,hydrogen has been considered as the most promising energy carrier due to its high energy density and zero carbon emission.Electrocatalytic water splitting plays a crucial role in not only providing the high-purity hydrogen,but also storing the intermittent sunlight and wind into the form of chemical energy.It consists of two half reactions:cathodic hydrogen evolution reaction(HER)and the oxygen evolution reaction(OER)at the anode.The energy conversion efficiency of water splitting electrolyzer is very limited owing to the sluggish kinetics of both reactions,especially the anodic OER.Therefore,fabricating the effective electrocatalysts is highly desirable for minimizing the overpotential and maximizing the electricity utilization efficiency.Currently,the state-of-art electrocatalysts for HER and OER are commercial Pt/C and Ru/Ir-based materials,respectively.But their widespread deployment is largely hindered by the high cost and natural scarcity.Keeping these in mind,our research mainly focus on how to reduce the dosage of noble metals without compromising the catalytic performance and develop the earth-abundant catalysts as alternatives.Much efforts have been devoted to improve the catalytic activity and stability through manipulating the composition,morphology and electronic structure of the materials.The main progress are outlined as below:1.A facile wet chemical approach has been developed to prepare a series of Ni@RuM(M=Ni or Co)core-shell nanocrystals(NCs),which featured Ni as core with component-tunable alloy shell(RuNi or RuxCoy),for effective water splitting catalysts.The core-shell structure could reduce the usage of Ru,and the electronic effect induced by the thin tunable alloy shell made great contribution to the enhanced performance,thus leading to the high mass activity of the products.Specifically,at the overpotential of 70 mV vs.reversible hydrogen electrode(RHE),the Ni@RuNi NCs possessed the mass activity of 1590 mA mgRu-1 for HER,which was almost twice than that of commercial Pt/C(950 mA mgPt-1).As for OER,the mass activity of the optimal Ni@Ru0.4Co0.6 NCs was 270 mA mgRu-1(330 mV vs.RHE),which was three times higher than that of commercial RuO2(89 mA mgRu-1).An alkaline water electrolyzer was fabricated with Ni@RuNi NCs as cathode and Ni@Ru0.4Co0.6 NCs as anode,respectively,and rather good activity and stability were achieved.2.Mo,S-codoped NiSe nanosheets assemblies were in-situ grown on nickel foam(NF)through one-step hydrothermal reaction(Mo,S-codoped NiSe/NF-160 NSs),which functioned as efficient HER catalysts under alkaline condition.It was revealed that the codoping of Mo and S could realize morphological and electronic dual modulation simultaneously.The presence of Mo induced the formation of nanosheets assemblies,and the electronic structure was further modulated by partial substitution of Se by S for favorable HER kinetics.Therefore,the obtained Mo,S-codoped NiSe/NF-160 NSs displayed much enhanced HER activity in 1 M KOH,which only needed 88 mV vs.RHE to deliver the current density of 10 mA cm-2.Moreover,it exhibited small Tafel slope of 82 mV dec-1 and excellent durability.3.A facile in-situ electrochemical tuning procedure was been demonstrated to introduce atomic level iron into cobalt carbonate hydroxide hydrate(CCHH)which was grown on NF(Fe-CCHH/NF).It was proved that the extremely low content of Fe(≤0.28 wt.%)was existed in the form of "single atom".And the subsequent experiments confirmed that the "Fe single atom" could adjust the electronic structure of the CCHH for beneficial OER routes with the morphology and crystal phase preserved.The best-performing Fe-CCHH/NF-30 exhibited greatly enhanced OER activity,which could afford the current density of 50 mA cm-2 at the overpotential of 230 mV vs.RHE.Noteworthy,the resulting Fe-CCHH/NF-30 was able to maintain the constant large current density of~55 mA cm-2 as long for 130 h,reflecting its extra-long time stability4.During the investigation of the non-noble metal electrocatalysts,some ultrasmall transition metal dichalcogenides(TMDs)nanodots such as MoS2 and MoSe2 were also obtained,and the optical property and light to thermal conversion ability were further explored.The resulting MoS2 displayed distinct and pH-sensitive fluorescence,which could applied as pH analysis platform.Additionally,the polyvinyl-pyrrolidone(PVP)protected ultrasmall MoSe2 nanodots were prepared and served as efficient light-absorbing agent for effective tumor destruction.