Decoupled Water Splitting via Redox Mediators
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Solution Overview
Problem
Current electrochemical systems for hydrogen and oxygen production face challenges in achieving efficient generation at low voltages, with existing mediators being incompatible with polyelectrolyte membranes and causing light attenuation, leading to tightly coupled hydrogen and oxygen evolution reactions with low rates and high energy requirements.
Innovation Solution
The use of polyoxometallates and quinone compounds as mediators that can reversibly accept and donate electrons and protons, decoupling the oxygen and hydrogen evolution reactions, allowing for generation at lower voltages and increased production rates, and are stable in both oxidized and reduced forms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional mediators are used in electrochemical water splitting, then hydrogen and oxygen evolution reactions can proceed, but the mediators are incompatible with polyelectrolyte membranes and cause light attenuation, resulting in tightly coupled reactions with low production rates
Solution Approach 1:
The patent employs a redox-active chemical mediator that shuttles electrons between the photoanode and photocathode, enabling decoupled water oxidation and reduction reactions. This mediator operates in a Z-scheme photoelectrochemical system, accepting electrons from the photoanode and donating them to the photocathode, thereby facilitating hydrogen and oxygen evolution while being compatible with polyelectrolyte membranes and transparent to light
Solution Approach 2:
The patent divides the water splitting process into two separate half-reactions occurring at different locations and times: oxygen evolution at the photoanode and hydrogen evolution at the photocathode. This segmentation allows independent optimization of each reaction and enables the use of a mediator to connect the two processes, improving overall productivity while avoiding the limitations of conventional tightly-coupled systems
2Productivity
If high voltage is applied to drive water splitting, then hydrogen and oxygen can be produced, but component degradation increases and energy efficiency decreases
Solution Approach 1:
The redox mediator enables water splitting at lower applied voltages by facilitating electron transfer between electrodes through chemical reactions. The mediator's redox couples operate at potentials that reduce the overall voltage requirement compared to direct electrochemical water splitting, thereby decreasing overpotential and improving energy efficiency while maintaining productive hydrogen and oxygen evolution rates
Solution Approach 2:
The patent utilizes photoexcitation to change the energy state of the mediator, enabling it to accept and donate electrons at favorable potentials. This parameter change from ground state to excited state allows the system to overcome kinetic barriers and proceed with water splitting at lower effective voltages, reducing energy loss and component degradation
3Device complexity
If a single photo catalyst is used for both oxygen and hydrogen evolution, then the system is simplified, but the large voltage gap between OER and HER onsets cannot be traversed efficiently
Solution Approach 1:
The redox mediator acts as an intermediary that bridges the voltage gap between oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). By accepting electrons from the photoanode at OER potentials and donating them to the photocathode at HER potentials, the mediator enables efficient electron transfer across the voltage gap, allowing separate optimization of photo catalysts for each half-reaction while maintaining system efficiency
Solution Approach 2:
The patent segments the photo catalyst system into separate photoanode and photocathode components, each optimized for its specific half-reaction. This segmentation allows independent selection of materials with appropriate band structures and catalytic properties for oxygen and hydrogen evolution, respectively, while the mediator connects these separated functions, resolving the voltage gap issue without requiring a single complex multi-functional catalyst
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed method achieves high Faradaic efficiencies of at least 90% and allows for the production of hydrogen and oxygen at modest voltages, reducing component degradation and increasing production rates, making it suitable for sustainable energy storage.
Implementation Method 1
The mediator has a reversible redox wave lying between the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER)
Implementation Method 2
a photo responsive material is present within the cell itself, and its photochemical response drives the electrochemistry within the cell
Implementation Method 3
the electrochemical splitting of water to produce oxygen (O2) and hydrogen (H2)
Data Source
AI summary
The invention provides methods for producing hydrogen and oxygen, comprising the steps of: (i) oxidising a mediator at a working electrode to yield an oxidised mediator, and reducing protons at a counter electrode to yield hydrogen; and (ii) reducing an oxidised mediator at a working electrode to yield a mediator, and oxidising water at a counter electrode to yield oxygen, wherein the oxygen generation step is performed non-simultaneously to the hydrogen generation step, and the oxidised mediator of step (i) is used as the oxidised mediator of step (ii), or the mediator of step (ii) is used as the mediator of step (i), and the mediator has a reversible redox wave lying between the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).