Here is the synthesis from Simon's Notes: 20ml H2O with 1.2g of KMnO4 dissolved in it is first mixed with 40ml of GO suspension at 1mg/ml to produce the oxidising solution. This is put into a 250ml beaker and magnetically stirred with a flea. Another 40ml of H2O with 2.2g of MnSO4 dissolved in it is then added drop wise to the stirred solution. The reaction is almost immediate, even at room temperature. For the substituted MnO2, I just substituted 25% of the MnSO4 with ZnSO4. Filter, wash with distilled water 3 times to remove the residual H2SO4 and the result is a lovely smooth chocolate brown powder which is way more conductive than the usual MnO2. Makes a nice ink for printing. It's so conductive you don't really need to add carbon or graphite to it, although a little graphite (like 2 or 3%) does improve it somewhat. Beyond that, no further improvement is found, suggesting the bulk conductivity is already about as good as it's going to get, and the graphite is just adding a percolation network which reduces surface resistivity between the particles. It even works when mixed with PVAc glue and dried at 80 degrees into a plastic film, which makes a nice flexible electrode. I think the graphene is helping to keep the brittle MnO2 from breaking up during cycling, cos I'm getting much better lifetime of these cells than with the old MnO2 without graphene. I made a proper test cell with the newly synthesised rGO:MnO2 powder over the weekend. It's working much better than I expected. Only problem so far is the new powder (weird purple-black colour) once dried, is so fine that it literally climbs out of the containers with the slightest movement. Talk about nano powder! It's got everywhere. :-( Still, it works great in a zinc cell. I've started doing pulsed forming of the cells, which seems to improve their current handling ability greatly. After construction (cell voltage about 1.4V vs Zn), I just pulse the cell from a 3.5V supply without current limiting; maybe 10 pulses at 100mS each, with 5 seconds rest in-between each pulse. The latest cells have all been made by mixing a premixed 9:1 graphite:carbon black conductive mixture with the active materials in mass proportion 2:1 active:conductive. So the final powders have 33% conductive carbon/graphite and 66% active material. I used 400 mesh powdered zinc as the anode active material. Once made, I mix the powders with a small amount of electrolyte which has 5% PVA (commercial builder's PVAc glue) added to it, until it just forms a liquid slurry. This is dropped onto the pre-knurled graphite foil current collectors to coat the 1cm2 area required, rubbed against the surface to create a uniform surface tension, and then dried on a hotplate at 80 degrees. I assemble the cell with an electrolyte-wetted disc of 1.2um glass fibre filter in-between the electrodes, sandwiched between two transparent 50x33mm self-adhesive PET labels. To make connections easier, the end portion of each graphite foil electrode strip is wrapped by folding around it pieces of fine copper mesh (300hpi) so that a few mm gets trapped between the labels. That way, simple croc clips can be used to connect the cell. So far, I've achieved 2mAh per square centimetre this way, when cycled between 1.85V and 1.05V. Self-discharge is almost absent. Leakage at full charge is tiny too; I get about 15uA charge current when the cell voltage is at 1.85V after 60 minutes charging. It would probably drop lower, but I've not left it on charge more than an hour. Electrolyte is pH 6, 2.0M ZnSO4, 0.25M ZnO, 0.4M MnSO4, 20mM Bi2O3 with about 0.01mM EDTA and 0.5% neutralised Carbopol 940.

• published: 03 Dec 2013
• views: 246