SeKwon Oh

Self-Powered Cardiac Pacemaker Enabled by Flexible Single Crystalline PMN-PT Piezoelectric Energy Harvester

A flexible single-crystalline PMN-PT piezoelectric energy harvester is demonstrated to achieve a self-powered artificial cardiac pacemaker. The energy-harvesting device generates a short-circuit current of 0.223 mA and an open-circuit voltage of 8.2 V, which are enough not only to meet the standard for charging commercial batteries but also for stimulating the heart without an external power source.

Self-powered deep brain stimulation via a flexible PIMNT energy harvester

Deep brain stimulation (DBS) is widely used for neural prosthetics and brain–computer interfacing. Thus far in vivo implantation of a battery has been a prerequisite to supply the necessary power. Although flexible energy harvesters have recently emerged as alternatives to batteries, they generate insufficient energy for operating brain stimulation. Herein, we report a high performance flexible piezoelectric energy harvester by enabling self-powered DBS in mice. This device adopts an indium modified crystalline Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIMNT) thin film on a plastic substrate to transform tiny mechanical motions to electricity. With slight bending, it generates an extremely high current reaching 0.57 mA, which satisfies the high threshold current for real-time DBS of the motor cortex and thereby could efficiently induce forearm movements in mice. The PIMNT based flexible energy harvester could open a new avenue for future in vivo healthcare technology using self-powered biomedical devices.

Porous Co–P foam as an efficient bifunctional electrocatalyst for hydrogen and oxygen evolution reactions

A high-performance bifunctional Co–P foam catalyst was successfully synthesized by facile one-step electrodeposition at a high cathodic current density. The synthetic approach includes fast generation of hydrogen bubbles as well as fast deposition of Co–P, which played a key role in forming a porous Co–P foam structure. The Co–P foam exhibits remarkable electrocatalytic activity and stability in both acidic and alkaline solution. Its HER activity was recorded with an overpotential of 50 mV in 0.5 M H2SO4 and 131 mV in 1 M KOH at 10 mA cm−2, which is comparable to that of commercial Pt/C (η@10 mA cm−20.5 M H2SO4: 33 mV, η10 mA, 1 M KOH: 80 mV). The Co–P foam (η@10 mA cm−2: 300 mV) exhibits better OER activities than Ir/C (η@10 mA cm−2: 345 mV) and RuO2 (η@10 mA cm−2: 359 mV) in 1 M KOH solution. The excellent performance of the Co–P foam as an HER and OER catalyst can be attributed to the charge separation between Co and P in Co–P foam as well as the porous foam structure providing a large electrochemically active surface area (ECSA). The ECSA of the Co–P foam was calculated to be 118 cm2, which was 2.4 times higher than that of a Co–P film (49 cm2).

Synergetic effects of edge formation and sulfur doping on the catalytic activity of a graphene-based catalyst for the oxygen reduction reaction

An edge activated S doped Fe-N-graphene (EA-SFeNG) was successfully synthesized via facile ball milling followed by a pyrolysis process. The oxygen reduction reaction (ORR) performance of EA-SFeNG was dramatically improved by doping S and forming edge sites in Fe-N-graphene; the onset potential was shifted from 0.91 VRHE to 1.0 VRHE with the half-wave potential increased from 0.77 VRHE to 0.848 VRHE. The EA-SFeNG exhibited catalytic performances that are comparable to those of commercial 20 wt% Pt/C (Vonset: 1.05 V, V1/2: 0.865 V); however, its durability was better than that of the Pt catalyst in alkaline media. The excellent ORR activity can be attributed to the increase in defect density and SOx bonding in the EA-SFeNG. Furthermore, we experimentally demonstrate that the work function of the Fe-N-graphene is significantly reduced from 4.06 eV to 4.01 eV by the increase in edge density and doping S, thereby improving the ORR kinetics of EA-SFeNG.

Enhanced exchange coupling constant and thermal stability of antiferromagnetically coupled media with thin Co interlayers

The magnetic properties and thermal stability of antiferromagnetically coupled (AFC) media with thin Co interlayers are investigated. Since the thermal stability is strongly dependent on the exchange coupling constant Jex in the AFC media, the thin Co interlayers were inserted on both interfaces of the Ru layer to obtain higher values of Jex. The Jex above 0.6 erg/cm2 was obtained in the AFC media with the Co interlayers above 1 nm in comparison with about 0.1 erg/cm2 in AFC media without the Co interlayers. The thermal stability of the AFC media with Co interlayers was greatly improved over those of the AFC media without Co interlayers.

Galvanic corrosion of Cu coupled to Au on a print circuit board; Effects of pretreatment solution and etchant concentration in organic solderability preservatives soft etching solution

In present study, we quantitatively define the galvanic corrosion phenomenon of Cu electrically coupled to Au on Print Circuit Board in Organic Solderability Preservatives (OSP) pretreatment (pickling and soft etching) solutions. As a result of polarization and ZRA test, galvanic corrosion rate of Cu in soft etching solution was about 3000 times higher than that of pickling solution. The oxone in OSP soft etching solution was acted as strong oxidant for Cu on PCB substrate. And the galvanic corrosion of Cu in OSP soft etching solution was examined with the change of etchants (oxone (KHSO5), sulfuric acid (H2SO4)) concentration. The galvanic corrosion rate of Cu was increased by the increase of the oxone and sulfuric acid concentrations, which lead to the increase of cathodic reactant such as HSO5- and H+ ions. And the degree of galvanic corrosion rate of Cu (Δisoft etching = icouple, (Cu-Au) - icorr, Cu) decreased with the decrease of the oxone and sulfuric acid concentrations.

Effects of temperature and operation parameters on the galvanic corrosion of Cu coupled to Au in organic solderability preservatives process

In this work, we quantitatively examined the effects of temperature and operation parameters such as anode (Cu) to cathode (Au) area ratio, stirring speed, and Cu ion concentration on the galvanic corrosion kinetics of Cu coupled to Au (icouple (Cu-Au)) on print circuit board in organic solderability preservative (OSP) soft etching solution. With the increase of temperature, galvanic corrosion rate (icouple (Cu-Au) was increased; however, the degree of galvanic corrosion rate (icouple (Cu-Au) - icorr (Cu)) was decreased owing to the lower activation energy of Cu coupled to Au, than that of Cu alone. With the increase of area ratio (cathode/anode), stirring speed of the system, icouple (Cu-Au) was increased by the increase of cathodic reaction kinetics. And icouple (Cu-Au) was decreased by the increase of the Cu-ion concentration in the OSP soft etching solution.

Ga–Doped Pt–Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mgPt-1) and a 17.3-fold improvement in the specific activity (2.53 mA cm-2) compared to the commercial Pt/C (0.106 A mgPt-1 and 0.146 mA cm-2). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

Correction to Ga–Doped Pt–Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction JeongHoon Lim, Hyeyoung Shin, MinJoong Kim, Hoin Lee, Kug-Seung Lee, YongKeun Kwon, DongHoon Song, SeKwon Oh, Hyungjun Kim, and EunAe Cho* Nano Lett., 2018, 18 (4) 2450−2458. DOI: 10.1021/acs.nanolett.8b00028. I Acknowledgments, the statement “Korea Institute of Science and Technology (KIST) under Contract No. 2017081254” should be “National Research Foundation of Korea under Contract No. NRF-2015M1A2A2056558”. Addition/Correction

Galvanic corrosion behaviors of Cu connected to Au on a printed circuit board in ammonia solution

During etching treatments of printed circuit board (PCB) with ammnioa solution, galvanic corrosion occurs between electrically connected gold and copper, and resulting in unexpected over-etching problems. Herein, we determine corrosion of galvanic coupled Cu to Au quantitatively in ammonia solutions, and evaluate factors influencing corrosion of galvanic coupled Cu to Au (i.e., area ratio of anode to cathode and stirring speed). The difference of the corrosion rate (Δi = icouple, (Cu-Au)–icorr, Cu) of Cu connected to Au (117 μA/cm2) and of single Cu (86 μA/cm2) infers the amount of over-etching of Cu resulting from galvanic corrosion in ammonia solution (Δi = 0.31 μA/cm2). As the stirring speed increases from 0 to 400 rpm, the corrosion rate of galvanic coupled Cu to Au increases from 36 to 191 μA/cm2. Furthermore, we confirm that an increase in the area ratio (Au/Cu) from 0.5 to 25 results in a higher rate of corrosion of Cu connected to Au. The corrosion rate of galvanic coupled Cu to Au is approximately 20 times higher when the area ratio of Au to Cu is 25 (1360 μA/cm2) than when the ratio is 0.5 (67 μA/cm2).