Design and Development of pH Differential Fuel Cells

Abstract

It is estimated that the global annual acids/alkaline wastes are equivalent to 100 million tons. The disposal process of these waste acid/alkaline solution is the direct neutralization process wherein a lot of heat (~1.11 × 1014 kJ/100 million tons) and salts are expelled to the environment, thus causing serious threats to the environment. If the acid/alkali wastes are neutralized in an electrochemical pathway, energy of about 44 TW h can be harvested in the form of electrical energy which offers a unique platform for the simultaneous treatment of industrial acid and alkaline wastes. Therefore, the development of an electrochemical neutralization device may open a new avenue to design novel aqueous energy storage and conversion devices including fuel cells, supercapacitors and batteries to meet future energy demand. The overall neutralization reaction which involves the formation of water and salt does not lead to change in the oxidation state of the participating species which is a major challenge to perform this reaction electrochemically. How to develop a pH driven fuel cell utilizing the energy of neutralization remains a major challenge? How the neutralization driven fuel cells could be used for other applications such as Alcohol reformation and water desalination?By exploiting the pH dependent nature of Hydrogen redox reaction we for the first demonstrated the concept of direct conversion of Neutralization energy into electrical output without consuming any fuel. We named it as Fuel exhaling fuel wherein neutralization energy is converted to electrical energy with simultaneous regeneration of hydrogen fuel at the cathode. This electrochemical neutralization cell employs H+/H2 redox couple to perform the neutralization reaction in an electrochemical pathway and the positive entropy change of the reaction allows nearly 30% of electrical energy to be harvested from the surroundings. The electrochemical neutralization cell (ENC) delivered a peak power density of ~70 mW/cm2 at a peak current density of ~160 mA/cm2 with a cathodic H2 output of ~80 mL in 1 hour. By using electrochemical neutralization as driving force, We could incorporate the additional functionalities of desalination and alcohol reformation into the fuel cell that too with simultaneous generation of electrical energy. By designing a 3-compartment electrochemical cell simultaneous water desalination during electricity production could be achieved by exploiting the electrochemical neutralization reaction as the driving force. This pathway is unprecedented because desalination is achieved in the electrochemical neutralization cell without irreversibly consuming free energy stored in expensive metals as in redox flow batteries and metal ion batteries but by just interconverting the energy of neutralization as an electrical driving force. The device uses acid and alkali as fuels for desalination by performing reversible redox reactions involving only gases, water, H+ and OH- such that the products and reactants of the redox reactions do not poison the desalinated water as in the state-of-the-art electrodialysis process. The electrochemical desalination cell driven by the electrochemical neutralization demonstrates performance metrics comparable to the state-of-the-art desalination processes reported in the literature. Taking it a step further we show the design and performance of an alcohol reforming fuel cell (ARFC) wherein alcohol reformation is coupled with the electricity production. The thermodynamic calculation showed that alcohol reformation which is otherwise a high temperature/pressure catalytic process can be driven at room temperature and pressure during electricity production with the simultaneous generation of pure hydrogen by utilizing the electrochemical neutralization reaction as the driving force. ARFC chemistry is unusual because of its distinctly positive entropic heat which allows ~56 % of the total available energy to be harvested from the surroundings leveraging a thermodynamic efficiency as high as 2.67. This ARFC demonstrates an energy density of ~253 Wh/kg with ~62 % of methanol to hydrogen conversion, Figure 2. Since the hydrogen fuel produced is free from carbon containing impurities, ARFC can be directly utilized as a fuel reservoir for a H2-O2 fuel cell in a tandem configuration.Although the electrochemical neutralization has many promising applications, it is still in the early stages of development. The devices work on the principle of asymmetric electrolyte configuration and as such the low-cost membranes which are stable in highly acidic and alkaline pHs have to be designed for sustainable operation. Different redox couples have to be identified to explore new possibilities and stable and durable electrocatalysts have to be developed to drive the reaction efficiently in respective pH solutions.