A bipolar electrode (BPE) is an electrically conductive material that promotes electrochemical reactions at its extremities (poles) even in the absence of a direct ohmic contact. More specifically, when sufficient voltage is applied to an electrolyte solution in which a BPE is immersed, the potential difference between the BPE and the solution drives oxidation and reduction reactions. Because no direct electrical connection is required to activate redox reactions, large arrays of electrodes can be controlled with just a single DC power supply or even a battery. The wireless aspect of BPEs also makes it possible to electrosynthesize and screen novel materials for a wide variety of applications. Finally, bipolar electrochemistry enables mobile electrodes, dubbed microswimmers, that are able to move freely in solution.

Bipolar electrode (BPE) focusing locally enriches charged analytes in a microchannel along an electric field gradient that opposes a counter-flow. This electric field gradient forms at the boundary of an ion depletion zone generated by the BPE. Here, we demonstrate concentration enrichment of a fluorescent tracer by up to 500,000-fold. The use of a dual-channel microfluidic configuration, composed of two microchannels electrochemically connected by a BPE, enhances the rate of enrichment (up to 71-fold/s). Faradaic reactions at the ends of the BPE generate ion depletion and enrichment zones in the two, separated channels. This type of device is equivalent to previously reported micro/nanochannel junction arrangements used for ion concentration polarization, but it is experimentally more flexible and much simpler to construct.

Membraneless desalination: A simple power supply is used to apply a 3.0 V potential bias across a microelectrochemical cell comprising two microchannels spanned by a single bipolar electrode (BPE) to drive chloride oxidation and water electrolysis at the BPE poles. The resulting ion depletion zone and associated electric field gradient direct ions into a branching microchannel, consequently producing desalted water. Gnd=ground.

In this paper we show that a microelectrochemical cell comprising two parallel microchannels spanned by a single bipolar electrode can be used to simultaneously enrich and separate both anions and cations within a single microchannel. This is possible because reduction and oxidation of water at the cathodic and anodic poles of the bipolar electrode, respectively, lead to ion depletion zones. Specifically, TrisH(+) is neutralized by OH(-) at the cathodic pole, while acetate buffer is neutralized by H(+) at the anodic pole. This action creates a local electric field gradient having both positive and negative components, and hence positive and negative ions follow their respective field gradients leading to separation. In the presence of an opposing counter-flow (pressure driven flow in this case), enrichment also occurs. In addition to separation and enrichment in a single channel, it is also possible to simultaneously enrich cations in one microchannel and anions in the other. Enrichment is achieved by controlling experimental parameters, including the type of buffer and the direction and magnitude of the opposing counter-flow.

We report a method for removing ions from aqueous solutions without the use of a membrane. The approach, which we call bipolar electrode depletion (BED), is based on the formation of an asymmetric electric field profile in a microchannel containing a bipolar electrode (BPE). The asymmetric field arises from local increases in conductivity caused by faradaic reactions at the BPE. We show how the asymmetric field can be used to deplete anions from a microchannel via a combination of electrophoresis and electroosmosis. We also apply this approach to filter an anionic species from a mixture of charged and neutral species being transported through a microchannel via electroosmosis. This technique could be utilized for desalination or filtration of any species possessing a net charge (e.g. heavy-metals, bacteria, proteins, or functionalized-nanoparticles).