We demonstrate an electrochemical method-which we term oxidative decoupling transfer (ODT)for transferring chemical vapor deposited graphene from physically deposited copper catalyst layers. This copper oxidation-based transfer technique is generally applicable to copper surfaces, and is particularly suitable where the copper is adhered to a substrate such as oxidized silicon. Graphene devices produced via this technique demonstrate 30% higher mobility than similar devices produced by standard catalyst etching techniques. The transferred graphene films cover more than 94% of target substrates-up to 100 mm diameter films are demonstrated here-and exhibit a low Raman D:G peak ratio and a homogenous and continuous distribution of sheet conductance mapped by THz time-domain spectroscopy. By applying a fixed potential of-0.4V vs. an Ag/AgCl reference electrode-significantly below the threshold for hydrogen production by electrolysis of water-we avoid the formation of hydrogen bubbles at the graphene-copper interface, preventing delamination of thin sputtered catalyst layers from their supporting substrates. We demonstrate the reuse of the same growth substrate for five growth and transfer cycles and prove that this number is limited by the evaporation of Cu during growth of graphene. This technique therefore enables the repeated use of the highest crystallinity and purity substrates without undue increase in cost.
In this study, an impedance model based on electrochemical theory considering hydrogen peroxide formation during a two-step oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) has been developed. To validate the theoretical treatment, electrochemical impedance spectroscopy (EIS) measurements were carried out in an open-cathode 16 cm 2 H 2 /air PEFC stack. The results show that inductive loops at low frequencies of the impedance spectra are attributed to mechanisms related to hydrogen peroxide formation during ORR. The results also demonstrate that the mechanisms during consumption of hydrogen peroxide to form water (second-step in ORR) can be the dominating process for losses in the PEFC compared to the mechanisms during oxygen consumption to form hydrogen peroxide (first-step in ORR). Oxygen transport limitations can be a result of hydrogen peroxide adsorbed onto the surface of the electrode which reduces the number of active sites in the cathode catalyst layer for oxygen to react. This study could support results from other experimental techniques to identify hydrogen peroxide formation during the ORR that limit the performance of PEFCs.
In this study, an impedance model based on electrochemical theory of platinum oxide formation has been developed and combined with the impedance model based on hydrogen peroxide formation during the oxygen reduction reaction (ORR) and reported in a previous study to charaterise inductive loops in impedance spectra of polymer electrolyte fuel cells (PEFCs). To validate the theoretical treatment, the simulated frequency response predicted by the theoretical model is compared against electrochemical impedance spectroscopy (EIS) measurements carried out in an open-cathode 16 cm 2 H 2 /air PEFC stack at three different current densities. The results show that neither model in isolation (hydrogen peroxide nor platinum oxide models) can accurately reproduce the inductive loops in the EIS measurements at low frequencies. By deriving a model considering kinetics of hydrogen peroxide and platinum oxide formation, it is possible to reproduce the inductive loops at low frequencies and to estimate the DC polarisation resistance related to the slope of the polarisation curve as frequency reaches zero during EIS. This study demonstrates that different mechanisms that cause PEFC degradation and low performance could be manifested in EIS measurements simultaneously. The resulting model could support other electrochemical techniques to quantify the rates of hydrogen peroxide and platinum oxide formation during the ORR that limit the performance of PEFCs.
We have successfully fabricated a pH-sensor array based on chemical-vapor-deposition (CVD)-synthesized graphene. As large-scale monolayer graphene is synthesized by this method, the size and the position of graphene can be controlled. Therefore, after transferring graphene onto SiO2/Si substrates, a graphene field-effect transistor (FET) array was produced. The sensing characteristics of the CVD-synthesized graphene-based device were investigated using three buffer solutions with different pH values (pHs 4.0, 6.8, and 9.3). The electrical measurements reveal that for most of the graphene FETs in the array, a similar stepwise increment in drain current was observed upon the introduction of each buffer solution with increasing pH value sequence. This will lead to the realization of the fabrication of multiplex hand-held chemical and biological sensors based on CVD-synthesized graphene.
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