Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)
The Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) is a process by which, when electrical potential differences are imposed on two catalyst electrodes separated by an electrolyte, the catalytic rate of the formation of a product at least one of the catalysts is enhanced above that which could possibly be attained were the reaction purely electrochemical. The effect which appears to defy Faraday’s laws of electrolysis can be justified by firstly considering an electrochemical cell with two electrodes made of Au and Pt which are respectively the cathode and the anode and exposed to the same bulk of a gas mixture consisting of CH4, O2, and an inert gas. The generation of CO2 from the conventional combustion reaction and its electrochemical analogue at the anode are respectively:
CH4 + 2O2 CO2 + 2H2O E1
CH4 + 4O2- CO2 + 2H2O + 8e- E2
O2- in reaction E2 is the charge carrier in solid oxide electrolytes frequently employed for high temperature electrochemical reactions and hence its net consumption is proportional to the electrical current observed during electrolysis. By stoichiometry, the formation of CO2 by reaction E2 must also be linearly dependent on the current generated by this reaction and thus can be at most as high as the equivalent rate governed by the observed current. If only reaction E2 is active, an increase in the current always results in a predictable and linear increase in the rate of CO2 formation. However, with both E1 and E2 simultaneously active, an increase in the current may lead to a tremendous and non-linear increase in the rate of CO2 formation which exceeds that possibly achieved solely electrochemically. The excess increase in the rate is attributed to the enhancement of the conventional combustion reaction E1.
The enhancement of the conventional combustion reaction is explained in terms of formation of a promoter species, i.e. O atoms strongly adsorbed on the catalyst working electrode. This species originates from O2- migration in the solid electrolyte due to the applied electromotive force showed in figure 2 to the anode-electrolyte-gas three-phase boundaries.
There, O2- is oxidized to O and then back-spills to the metal surface, thereby forming an effective electronic double layer as shown in Figure 3 and increasing the work function of the catalyst anode. This favors the dissociative adsorption of electron donor molecules, which are generally hydrocarbons for combustion reactions and hence CH4 in this example, but disfavors the dissociative adsorption of electron acceptor molecules, which are O2 in this case. These induced changes in the adsorption characteristics can actually lead to both acceleration and inhibition of the catalytic rate of a reaction but for methane combustion they result in rate enhancement because the adsorption coverage of alkane at catalyst surfaces is known to be relatively minimal compared to that of oxygen. Depending on reactions, the rates can be increased, depending on reactions, 100 to 1 000-fold of the base rates at open circuit conditions.
The migration of O2- necessary for NEMCA effects is generated by the ‘oxygen pumping’ reaction:
O2 + 4e- 2O2- E3
The reaction proceeds to the right side of the equation at the three-phase boundaries of the cathode and to the left side at those of the anode, thus forcing a net movement of O2- from the cathode to the anode. Ideally, the ions are desired to stay at the catalyst surfaces and not to be spent. In reality, they are consumed in both reaction E2 and E3, so the ions must be constantly supplied and this leads to a minimal electrical energy requirement in this kind of systems.