![]() a The exergonic glucose oxidation does not provide enough energy to directly drive the electrolysis of water, because the difference in ΔG of the two reactions is positive. In particular, we show that it is possible to convert the energy of the electrooxidation of glucose, coupled to the 4-electron oxygen reduction reaction ( ΔG 1 = −223 kJ mol −1), to power the formation of dihydrogen by electrolysis of water ( ΔG 2 = +239 kJ mol −1) in the same reaction vessel.Įnergetics of the uphill reaction principle. Therefore, we demonstrate the general idea of the proposed up-conversion with the electroenzymatic generation of hydrogen as a final high energy product, because its formation can be readily tracked (and quantified) in this model system. In order to illustrate the philosophy of this concept in a proof-of-concept experiment, reaction products such as the ones mentioned above are not suitable, because detection and quantification of all processes should be easy and straight-forward. The concept might also be used for autonomous sensing devices or electrolysis cells to locally generate for example specific drug metabolites, by converting a low energy educt molecule to form a higher energy product molecule. The mechanism we describe herein could allow e.g., the in situ generation of reactive oxygen species (ROS), needed for the regulation of several biological functions in the body 1, to suppress tumor growth and induce cell death at specific locations 2, even though their electrochemical synthesis needs rather high potentials 3. Such a concept is crucial if both reactions have to take place in a single medium, and is therefore of general importance from a thermodynamic point of view, but also more specifically for example in all in vivo applications, where both reactions must proceed in the same solution or environment (i.e., in blood). ![]() Here we demonstrate that it is nevertheless possible to drive a chemical reaction with an overall positive free energy ΔG > 0 in the same medium, by temporarily storing energy of a first reaction as electromagnetic energy, which is then used to raise the potential of electrons that subsequently participate in a second reaction. ![]() For the same reason it is also not possible to connect several electricity delivering cells in series in the same medium in order to add their individual potential differences to obtain a globally higher value. However, combining both reactions in one-and-the-same environment typically leads to a chemical short-circuit, because both electrochemical systems share the potential of the same electrical ground, with the result that thermodynamics forbids such an uphill reaction. This is obviously possible if the two reactions occur in physically separated reservoirs, like in the case of a battery acting as a first reaction reservoir, driving an electrochemical reaction in a second reservoir. It is therefore necessary to find means to transform energy from a first reaction and then use it to drive a second reaction where a higher energy product is formed. While it is straightforward to multiply the work in mechanical systems to raise the energy of a subsystem, this is far from trivial in a closed chemical system. One might therefore ask the very fundamental question whether energies released in an exergonic reaction can be added up to drive the formation of a higher energy endergonic reaction product of a smaller mole fraction in the one and the same medium. However, for the vast majority of reactions, which proceed in a closed system, that means in the same reaction medium, this is not possible. For instance, an external battery can raise the potential of an electron in an electrochemical process, or light can promote an electron in a reaction to a molecular state with a higher energy. If the sum of the free enthalpies of a reaction system is positive, then the reaction can normally only proceed if an external energy source is provided. The free enthalpy, or Gibbs energy G, determines whether a chemical reaction can proceed spontaneously or whether an external energy input is needed.
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