Try mixture of epoxy glue with as much as possible graphite powder a

Try mixture of epoxy glue with as much as possible graphite powder a

As Yurii said expoxy or "super glue" is a great solution . You can also look into silver epoxy if you want it to be conductive .

Thanks, Silver would be the last thing I want in there :)

Sorry, for some reason I missed the fact you wanted a metal-free one. So I guess an epoxy glue (as long as you use aqueous solutions (if it is exposed to the solution) is the best option.

Also, some ready to use, low cost[1], commercial[1,2,3] (near[1,2] DC, and DC[3]) conductive Carbon epoxies might fit to your objectives.

1. Conductive Carbon Epoxy Air Dry Electrically Conductive, Carbon 61, 100gm https://www.ebay.com/itm/Conductive-Carbon-Epoxy-Air-Dry-Electrically-Conductive-Carbon-61-100gm/161125511870?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49564%26meid%3D809acbee730845089394339ecbe62561%26pid%3D100005%26rk%3D2%26rkt%3D2%26sd%3D202096762747&_trksid=p2047675.c100005.m1851

2. Conductive Inks & Coatings http://www.henkel-adhesives.com/conductive-inks-coatings-27433.htm https://www.globalhenkelelectronics.com/electronics_literature/get_file.php?file_id=102

3. CNTs static conductive/conductive water-based coating http://www.timesnano.com/en/view.php?prt=3,35,115,220

You could use carbon ink. You can also use a carbon fiber, then glue it down with an epoxy. That has the added benefit of insulating the connection