Direct measurement of proton conductivity of a single ionomer nanofiber

The capability of measuring the ion conductivity of ionomer nanofibers, enabling bottom-up improvement of many clean energy technologies, e.g., polymer electrolyte fuel cells, remains a great challenge due to the difficulties in fixing the sample nondestructively and eliminating the interference from contact impedance and substrate. Herein, we developed a measurement system to tackle this challenge by employing a microelectrode chip and a novel sample welding strategy. The microelectrode chip consists of four Au probes and a cavity in the center. The four-probe setup eliminates the contact impedance and any interfacial effect from the current-carrying electrodes. The central cavity enables the suspension of the tested nanofiber, thereby excludes the substrate interference. The suspended fiber is welded to the probes by the same solution of tested nanofiber under optical microscopy, so that intimate and tight junctions can be formed between the sample and Au probes. Compared to commonly used nano-welding method using electron-beam-induced deposition, this self-welding method can avoid the electron irradiation damage to the delicate sample under test. The measured proton conductivity with the developed system possesses high accuracy and gives insights into the proton conduction mechanism of ionomer nanofibers. Furthermore, the proposed method can be applied to other ionic conductive nanofibers or nanofilms.