Reports: ND1050630-ND10: Carbon Nanoelectrodes: Generating Hydrogen from Water

B. C. Regan, PhD, University of California (Los Angeles)

Our project has been focused on using transmission electron microscopy (TEM) to achieve a better fundamental understanding of electrochemistry inside the Debye layer. Progress in this direction will lead to more efficient solar cells, higher capacity batteries, cleaner cars and a healthier economy.  This investigation is timely because of a technological innovation, pioneered by Frances Ross at IBM and coworkers, which allows the TEM of wet samples.  The basic idea involves building a chamber that sandwiches a thin environmental region between two electron-transparent windows.   If the entire system is constructed so as to be sufficiently thin – say, less than a few hundred nanometers thick – then it is possible to acquire atomic-resolution images of materials inside the environmental chamber, as various groups have now demonstrated.  This technological advance opens up whole new areas for applications of the high-resolution imaging offered by electron microscopy.  Whereas previously investigators interested in aqueous electrochemical or biological systems were limited to post-mortem analysis of desiccated or frozen samples, now they can study active electrochemical devices or biological cells in vivo.

As discussed in last year’s annual report, funding from the ACS PRF has lead directly to three major publications from our group.  In the first, published in Applied Physics Express, we studied nanobubble dynamics in water with TEM.  Here the nanobubble formation was initiated by Joule heating a microfabricated platinum nanowire that was immersed in the liquid.  During this study we also noticed that nanobubbles could be initiated by focusing the electron beam on the sample.  In the second publication we further explored beam-induced effects, discovering that even relatively mild dose-rates can produce dramatic effects that dominate the dynamics of the system under observation.  While viewing platinum nanoparticles in water, we saw that the electron beam can cause them to charge up and be expelled from the field of view.  This work was published in Langmuir.  The third major publication, which appeared in ACS Nano, built on the previous two in the sense that dose effects were minimized to the point of being negligible, and externally-controlled electrodes were used to initiate and control the system dynamics.  Here we worked with a saturated aqueous solution of lead (II) nitrate, and demonstrated reversible plating (both as a compact layer and as dendrites) and stripping of the lead on gold electrodes.  We also demonstrated direct imaging of the cation concentration, and direct correlation between the measured electrochemical transport (current-voltage) relationship and the time variation seen in the TEM images.

In the last year we have been further investigating the electrochemistry of aqueous and non-aqueous solutions.   Having demonstrated that we can observe the movement of Pb2+ ions in solution, we have sought to image other cations.  We have been particularly interested in cations such as Cs+ which have more negative standard electrode potentials and are thus less likely to plate.  With such non-plating ions we hope to show that in situ TEM can be used to image the formation and structure of the electrical double layer in aqueous solutions.

As an intermediate step we have also been investigating intercalation compounds of graphite.  For instance, using concentrated sulfuric acid as the electrolyte we have electrochemically intercalated sulfate ions into single crystals of graphite prepared by micromechanical cleavage.  As seen in the captured scanning TEM images, the sulfate ions cause the graphite to swell.  As for the Pb2+ in the lead nitrate solution, local changes of the sulfate ion concentration in the sulfuric acid can be directly observed through the concomitant changes in the scattered electron intensity.  Ions can be “seen” intercalating and de-intercalating the graphite.  We have also observed the controllable evolution of hydrogen gas from the graphite (or multi-layer graphene) nanoelectrodes at sufficiently large applied potentials.  We presented these results at the August 2013 Microscopy & Microanalysis meeting in Indianapolis.

Funding provided by the American Chemical Society’s Petroleum Research Fund over the life of this New Directions grant has supported graduate student Edward White, who has been responsible for the body of work summarized above.  More detailed descriptions of the research have been provided in the three refereed journal publications mentioned, as well as at the American Physical Society’s March Meeting, the 2012 and 2013 Microscopy and Microanalysis Meetings, the Electron Microscopy and Analysis Group Conference (EMAG) 2013, and in invited seminars at various academic institutions.  This fluid cell work represents a completely new and successful research direction for the PI, and efforts to continue this work are ongoing.