Structural Design Of Cobalt-based Nanocatalysts Toward Efficient Electrocatalysis

Electrochemical water splitting and its reverse reaction,i.e.,H₂-O₂ fuel cell,are the key issues of hydrogen economy.The former includes hydrogen evolution reactions?HER?and oxygen evolution reaction?OER?,and the latters includes hydrogen oxidation reactions?HOR?and oxygen reduction reactions?ORR?.So far,noble metals hold the benchmark of electrocatalysts.For example,platium?Pt?is the art of state for HER,HOR and ORR,and iridium?Ir?and ruthenium?Ru?based materials are for OER.However,they are serverly limited by the high cost,low earth-abundance,and poor long-term durability.It is urgent to explore the efficient electrocatalysts with element-abundance and economic-cost.Cobalt and its phosphides are highlighted as efficient candinates due to the noble-metal-like catalytic properties.In this dissertation,rational design on nanostructured metallic cobalt and its phosphides is conducted to modulate their surface/interface textures and electronic configuration,resulting in optimization on abundance and strength of active-sites.As a result,the efficient HER,OER and ORR is well accomplished.The main content of this dissertation is as follows:1.Controlled pyrolysis of MOFs materials is introduced to control the thickness of carbon shells in ZIF-67 derived Co@NC,and further to expose active-sites.Meanwhile,the ultrathin carbon shells can improve the electronic interactions with metallic Co cores.Via varying temperature and flow rate during ZIF-67 pyrolysis,a series of Co@NC with vaired thickness of carbon shells can be facily obtained,which enables the good accessibility and the electronic optimization of active Co cores.With moderated temperature and flow-rate,the resulting ultrathin carbon-shell,on the one hand,renders Co cores easily accessible to electrolytes and,on the other hand,promotes the electronic penetration to optimize metallic Co active sites.As expected,the optimal Co@NC affords the benchmarking performance of noble-metal-free electrocatalysts in HER and ORR.For example,the Co@NC-775 displays a low overpotential(?₁₀=82and 96 mV for reaching a current density of-10 mA cm^-2),a small Tafel slope(35 mV dec^-1 and 80 mV dec^-1)for HER in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.Meanwhile,Co@NC-775 affords a large half-wave potential for ORR(E_1/2=0.85 V and0.78 V),nearly four transferred electron numbers in 0.5 M H₂SO₄ and 1.0 KOH.2.Industry-viable electrospinning technology followed by pyrolysis introduced is introduced to develop composition-tunable Ni_2-xCo_xP embedded within N-doped carbon nanofibers(denoted as Ni_2-xCo_xP/N-C NFs)as efficient electrocatalysts for water splitting.The designed NFs are featured by the ultrafine Ni_2-xCo_xP?2³ nm?within conducting and nanoporous N-C matrix,the 1D hierarchical nanostructures beneficial for active-sites exposure and charge transfer,and more importantly the effective electronic modulation associated with tunable alloy composition.The variation of x in Ni_2-x-x Co_x P leads to obvious alteration of d state in solid-solution alloys,accomplishing an optimal binding with hydrogen intermediate.As a result,the optimal Ni_0.7Co_1.3P/N-C NFs afford high activity for HER,which is featured by low?₁₀(overpotential required to reach a current density of-10 mA cm^-2)of 100,98 and 130mV at pH=0,7,14,respectively.Furthermore,the Ni_0.7Co_1.3P/N-C NFs also deliver efficient OER(?₁₀=280 mV)in an alkaline electrolyte,achieving the high efficiency for overall water splitting.It presents a comparable cell voltage(1.56 V@10 mA cm^-2)to commercial IrO₂/C-Pt/C coupled electrocatalysts(1.5 V@10 mA cm^-2),but remarkably better stability in a long term durability tests?⁴8 hours?.3.Pyrophosphate decoration on the surface of CoP can be used to concentrate H⁺,and at the meantime generate rich oxygen defects.The oxygen defects will promote water dissociation in an alkaline solution,and thereby boost HER activity.Via facily heating CoP in air and consequently H₂/Ar,rich oxygen defect is fabricated on the surface of Co₂P?PO₇?_x/CoP NPs,and followed by PH₃,P atoms are back-filled into the oxygen defects,in which the P sites can stabilize the oxygen defect for efficient HER.The resulting composites afford an excellent HER activity,featured by a low?₁₀ of 92and 105 mV in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.In summary,we proposed serveal strategies to optimize the structure of Co@NC and cobalt phosiphides toward efficient electrocatalysis.This work will shed some light on the development of cost-efficient electrocatalyst in energy-related fields.