  Microfluidic fuel cells (MFCs) are microfluidic electrochemical conversion
devices that are used to power small pieces of electrical equipment. Their
performance relies on the improvement of the mass transfer of the reactants at
the electrode interface. {In this work, a MFC is developed to implement a novel
imaging technique that allows the measurement of reactant concentration fields,
featuring formic acid as the fuel and potassium permanganate as the oxidant.
The concentration fields were imaged } based on transmitted visible
spectroscopy, which links the light intensity passing through the MFC to its
local reactant concentration. {An analytical model was developed to estimate
the mass diffusivity and kinetic reaction rate coefficient. For the first time,
mass transport and transfer coefficient were simultaneously measured during
operation. These parameters estimated using the proposed technique can be
implemented in a numerical model to predict the MFC performance and
concentration distribution. This work paves the way toward advanced imaging
tools for operando mass transfer characterizations in microfluidics and Tafel
kinetic characterization in many electrochemical devices.
