Entry for:The Peer Prize for Climate
Scientists around the world have been searching for ways to make fuel using renewable energy, both to displace fossil fuels and to efficiently store solar and wind energy. However, economic competition with low-cost fossil fuels has never been accomplished.
This forces us to look for new ways to make our fuel-generating technology work in the real world. In this research, we make hydrogen fuel out of environmentally harmful dairy wastewater: acid whey. We propose to use renewable electricity to form fuel while simultaneously cleaning wastewater, solving both the storage problem and cleaning water - a twofold climate impact.
The primary contaminant in acid whey dairy wastewater is lactic acid. In this research, we use an electrochemical cell to oxidize lactic acid to carbon dioxide at an anode, removing it from water, connected to a cathode that produces hydrogen gas (the simplest fuel to make in this system). Our focus is an understanding of the mechanism at the anode that decomposes lactic acid, and how different catalysts affect the efficiency and selectivity of this process.
We use electrochemistry, focusing on methods such as cyclic voltammetry and chronoamperometry, to study the how electrons are removed from lactic acid leading to its decomposition. This lets us look at the oxidation state of our catalysts, to determine how lactic acid coordinates to it. Kinetic isotope effect studies with deuterated water show us how hydroxyl radicals play a role. Electron microscopy shows us electrode morphology, while product detection by nuclear magnetic resonance spectroscopy and gas chromatography help us quantify reaction products.
We later applied this research to a real dairy plant wastewater at Stonyfield Farms, building a polymer electrolyte membrane electrolyzer that removes lactic acid from their wastewater, allowing its reuse.
Our study was initiated by a surprising voltammetry result, that showed the presence of lactic acid in water changed the oxidation states accessible by iridium-based catalysts. This is unusual, since electrochemical wastewater treatment typically involves partial oxidation of water to form a hydroxyl radical, which has little to no effect on the first coordination sphere of the catalyst.
In the case of iridium, it turns out that this is not the case - the organic substrate coordinates to the metal center and is oxidized through an inner-sphere electron transfer pathway. Our proposed mechanisms predict an anodic shift in the Ir(IV)/Ir(V) reversible potential if this is the case, which we see in cyclic voltammograms (Figure 1).
To confirm this result, kinetic isotope effect (KIE) studies (Figure 3) show that catalysts that proceed by classical hydroxyl radical decomposition (Pt) exhibit a high H/D KIE, while inner-sphere electron transfer catalysts (Ir) do not.
Product detection (Table 1 and Figure 4) shows that the reaction intermediates vary between catalysts that use hydroxyl radicals to oxidize lactic acid version inner-sphere electron transfer catalysts. These results are critical, since they determine how these catalysts can be integrated into practical systems to treat wastewater.
Wastewater treatment is one of the most carbon-intensive, environmentally harmful, and energy inefficient processes on our planet. The use of fossil fuels, rather than renewable energy, makes this only worse - but is necessitated by our inability to store wind and solar energy on a large scale. We need energy on-demand.
In this work, we show that this problem can be solved by combining energy-efficient wastewater treatment with fuel generation and exploring the mechanism behind how catalysts for this process work. By understanding these mechanisms, we can improve this process's efficiency to make it viable on a large scale. If we power this using sunlight or wind, we can simultaneously clean up wastewater and store renewable energy in the form of fuels.
Ultimately, we can build a process that oxidizes organics into pure carbon dioxide at an anode (that can be used as a product rather than emitted into the atmosphere), and reduces atmospheric carbon dioxide at a cathode for truly carbon-negative renewable energy conversion and storage on top of wastewater treatment, taking two of the most carbon-intensive processes on the planet and making them carbon-negative.
5. Future ideas/collaborators needed to further research?
As a start-up company and industrial laboratory, we're continuing this work by embedding catalysts on membrane surfaces, and building a demonstration facility to show that this is feasible. However, we would like to extend this work with universities to develop a more fundamental understanding of organic/contaminant oxidation on metal surfaces and how inner sphere/outer sphere mechanisms play a role, to bring this research to new types of waste water and carbon emissions.