Developments of high performance electrocatalysts for polymer electrolyte membrane devices and Mg alloys for fast hydrogen generation

Abstract

Hydrogen energy, which features numerous benefits, including abundant, environmentally friendly properties and high energy density, is an excellent candidate for future energy source. Currently, hydrogen economy was composed of hydrogen generarion, storage and fuel cell operation. For hydrogen generarion methode, environmentally friendly hydrogen generation method has recieved great attention such as electrolyzer and hydrolysis of metallic fuel. Currently, however, electrolyzer and hydrolysis of metallic fuel have cost problem manily caused by using expensive electrocatalyst and low hydrogen generation rate for commercialization, respectively. Therefore, it is nessessary to develop high performance electrocatalysts with low cost and metallic fuel for fast hydrogen generarion. For fuel cell operarion, high cost fuel cell still retard commercialization. To reduce fuel cell cost, it is important to development low cost electrocatalysts with high performance. In this thesis, Development of high performance electrocatalysts for polymer electrolyte membrane device and Mg alloys for fast hydrogen generarion was explored. The major findings of this work are summarized below

1.Synergetic effects of edge formation and sulfur doping in graphene based catalyst for an effeicient ORR catalyst

The present work is to enhance oxygen reduction reaction (ORR) activity of transition metal？nitrogen？graphene catalysts through formation of edge site and doping S by facile ball milling method. The edge-activated S-doped Fe-N-graphene (EA-SFeNG) was successfully synthesized by simply mixing graphene oxide (GO), melamine as a nitrogen source, sulfur and iron acetate using ball milling, and then followed by pyrolysis treatment for 2 h at 700 ˚C, and then optimized to exhibit the best ORR catalytic performance. The ORR performance of the EA-SFeNG was measured to have the onset potential ($V_{onset}$: 1.02 $V_{RHE}$) and the half wave potential ($V_{1/2}$: 0.848 $V_{RHE}$) that is comparable to those ($V_{onset}$: 1.05V, $V_{1/2}: 0.865V) of the commercial 20wt.% Pt/C. The S in the catalyst exists in the form of $SO_x$, and the ball milling process increase edge sites in the catalysts, which acted as a highly ORR active site. It was demonstrated from the measurement of nanoscale work function by kelvin probe force microscopy that the work function of the EA-SFeNG catalyst is significantly reduced by the S doped and the active edge sites formed in the catalyst. Hence, the excellent ORR performance of the EA-SFeNG catalyst can be attributed to the increase in the defect density such as the doping S and the edge site in the graphene.

2. Design of Mg-Ni alloys for fast hydrogen generation from seawater and their application in PEMFCs

Mg and its alloys are very attractive for the on-board hydrogen production via hydrolysis because their hydrolysis reaction occurs in neutral seawater instead of the alkaline water necessary for the hydrolysis of Al and its alloys. Mg powder, showin a high hydrolysis reaction rate due to its large reaction area, is dangerous when in contact with moisture or air and is also expensive. However bulk Mg in forms of plate and bar, though economical and safe, exhibits low hydrogen generation rate due to oxide film formed on its surface. Based on the fact that the hydrogen production rate form the hydrolysis of Mg is proportional to the corrosion rate of Mg to $Mg^{2+}$ in water the Mg-Ni alloys was designed to form an electrochemically noble phase ($Mg_2Ni$) along grain boundaries so as to accelerate the corrosion rate of Mg by combined action of galvanic and intergranular corrosion between the $Mg_2Ni$ preciptates acting as cathode and Mg matrix phase acting as anode. The $Mg_2Ni$ precipitate was 0.7 V electrochemically noble to Mg matrix, and hence acts as a cathode in the Mg-Ni alloy. It has been demonstrated that the designed Mg-3Ni alloy exhibits the highest hydrogen generation rate (23.8 ml.$min^{-1}.g^{-1}$) among the Mg-Ni alloys that is 1300 times faster than that of pure Mg in 3.5 % NaCl solution. In fact, the surface morphology of the Mg-3Ni alloy changes gradually to porous structure with hydrolysis of Mg due to corrosion reaction mentioned above. The PEMFC operated with hydrogen generated from the hydrolysis of 2g Mg-3Ni alloy produced a power of 7.3 W for 20 min that is equivalent to 1.215 KWh/Kg-Mg-3Ni.

3. Design of Mg-Ni-Sn alloys for fast H2 generation via hydrolysis in seawater

Small amount of Sn was added to Mg-3Ni alloy to cause pitting corrosion by locally breaking the surface oxide film on Sn dispersed in grains and $Mg_2Sn$ precipitated at grain boundaries. It is evident from surface elemental analysis that the Sn was uniformly dissolved in Mg matrix whereas $Mg_2Ni$ precipitated at grain boundaries in Mg-3Ni-1Sn alloy. Hence, Mg-3Ni-1Sn alloy showed an excellent hydrogen generation rate of 28.71 ml/ming that is 1700 times faster than that of Mg due primarily to the combined action of galvanic and intergranular corrosion as well as pitting corrosion in 3.5 wt.% NaCl solution. As the solution temperature increased from 30 to 70 °C, the hydrogen generation rate from the hydrolysis of Mg-3Ni-1Sn alloy was dramatically increased from 34 to 257.3 ml/min.$g^{-1}$. The activation energy for the hydrolysis of Mg was calculated to be 43.13 KJ/mol.

4. Porous Co-P foam fabricated by electrodeposition as an efficient electrochemical catalyst for hydrogen evolution reaction

Developing an electrocatalyst with a low cost and high performance for an efficient hydrogen evolution reaction (HER) is desirable to replace the Pt catalyst currently used. A highly porous Co-P foam is synthesized by electrodeposition of Co on a nodular copper sheet at a high cathodic current density from a solution containing 0.1 M $CoCl_2$, 0.5 M $NaH_2PO_2.H_2O$, 1.2 M $(NH_4)_2SO_4$, 0.7 M $H_2SO_4$. The CoP foam containing 6 wt. % P has been optimized at -3A/$cm^2$ so as to exhibit a best HER catalytic performance in 0.5 M $H_2SO_4$. Its overpotential at 10 mA and tafel slope was 50 mV and 58 mV $dec^{-1}$, respectively, were comparable to those ($\eta_{10mA}$: 45 mV, tafel slope: 62 mV $dec^{-1}$) of commercial Pt/C. XPS and surface analyses suggest that the best HER performance of CoP electrodeposited at -3 A/$cm^2$ can be attributed to the existence of and the amorphous phase of CoP foam as well as the porous structure providing large electrochemical active surface area.