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Abstract
Light-driven water-splitting to generate hydrogen and oxygen from water is typically carried out in an electrochemical cell with an external voltage greater than 1.23 V applied between the electrodes. In this work, we examined the use of a concentration/chemical bias as a means of facilitating water-splitting under light illumination without the need for such an externally applied voltage. Such a concentration bias was created by employing a pH differential in the liquid electrolytes within the O2-generating anode half-cell and the H2-generating cathode half-cell. A novel, stretchable, highly ion-conductive polyacrylamide CsCl hydrogel was developed to connect the two half-cells. The key feature of the cell was the half-cell electrodes, which comprised thin-film conducting polymer composites that were previously designed to maximize light-driven catalysis at moderate pH. Upon being connected with the hydrogel in the presence of light irradiation (0.25 sun intensity on each electrode), the half-cells spontaneously produced hydrogen and oxygen from water, without the need for an externally applied voltage bias greater than 1.23 V. The cell operated reliably and efficiently for 14 h of continuous testing. These results demonstrate the fundamental feasibility of light-driven water-splitting in a photoelectrochemical concentration cell when employing electrodes that operate efficiently at moderate pH, even with low levels of light illumination. The designed conducting polymer composites proved ideal in that regard.
Highlights
The generation of hydrogen (H2) and oxygen (O2) from water (H2O) requires a minimum of 1.23 V of electrical energy (E◦) and 0.25 V of heat energy, giving a total minimum theoretical energy requirement of 1.48 V [1]
Except for a few impractically elaborate and expensive multi-junction semiconductor cells [3], all water-splitting photoelectrochemical cell (PEC) developed to date require the application of additional energy, beyond that already present in the irradiating sunlight
This additional energy is typically supplied in the form of an external electrical voltage that is applied between the O2-generating anode and the H2-generating cathode of the cell, which exceeds 1.23 V
Summary
The generation of hydrogen (H2) and oxygen (O2) from water (H2O) requires a minimum of 1.23 V of electrical energy (E◦) and 0.25 V of heat energy (which is supplied in the form of electricity), giving a total minimum theoretical energy requirement of 1.48 V [1]. Except for a few impractically elaborate and expensive multi-junction semiconductor cells (known popularly as “artificial leaves”) [3], all water-splitting PECs developed to date require the application of additional energy, beyond that already present in the irradiating sunlight This additional energy is typically supplied in the form of an external electrical voltage that is applied between the O2-generating anode and the H2-generating cathode of the cell, which exceeds 1.23 V. At the above molar ratios, the conducting polymer electrically connected the largest number of catalytic sites (thereby maximizing the catalytically active area) by the shortest, most conductive pathway (thereby maximizing the catalytic activity of each site) Given these electrodes, we wanted to see whether they could be deployed to create a long-lived PEC concentration cell. We report here such a water-splitting photoelectrochemical concentration cell (PECC) employing the above conducting polymer composite catalysts (at the above pH conditions) at the anode and and continuously for >14 h of testing
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