Towards Rational Optimization of a Wastewater Treatment Electrocatalyst for the Chlorine Evolution Reaction Using a Method for Finding Active Sites on Brush Coated Samples
Authors: Finke Cody Enslin , Omelchenko Stefan R , Hoffmann Michael R 
Published: February 2020

Electrocatalytic systems show promise for addressing global environmental problems by offering waste free mechanisms for processes like fuel production and modular wastewater treatment in developing countries. Although several catalysts have been developed for these tasks their catalytic mechanisms are poorly understood which makes rational optimization and design of a given electrocatalyst a very difficult undertaking. This problem is universal, but it is crucially important for wastewater treatment in the developing world.

Currently, 4.2 billion people lack access to adequate sanitation; poor sanitation is now the largest killer of children under five. Human waste pollution in natural waters will, according to UNESCO, lead to two thirds of the world’s population entering a state of water scarcity by 2025. Most water pollution issues are localized in the developing world where human wastewater treatment infrastructure is poor or nonexistent. For practical reasons, developed world style solutions to wastewater problems are largely infeasible in the developing world. For example, it is an enormous challenge to lay sewer lines in cities like Delhi where millions of people live in buildings often constructed without rebar. It is therefore attractive to consider a technological solution to this problem where water is treated and recycled or discharged on site without sewer lines. This would reduce the demand for water and eliminate the largest source of water pollution without displacing millions of people to dig sewer lines.

One convenient option for onsite wastewater treatment is electrochemical chlorine evolution from chloride naturally found in urine. This technology is especially beneficial because in addition to providing oxidation power to mineralize organics, chlorine can also remove nitrogen compounds through breakpoint chlorination and phosphorous compounds through cathodic electroplating of hydroxyapatite. All of this can be accomplished while recycling water with no external input of chemicals, with off grid solar energy, and a sanitary flush toilet and hand-wash station.

Unfortunately, good chlorine evolution reaction (CER) electrocatalysts for wastewater treatment need to be highly stable to withstand the harsh wastewater environment, and the only catalyst so far discovered that has met this requirement is TiO2 coated IrO2 which is far too expensive for applications in the developing world. One obvious way to make these electrodes affordable is to optimize their efficiency such that much less material is needed to treat the same amount of wastewater.

In recent years, active site based optimization has proved to be an attractive strategy for catalytic optimization. Catalytic “active sites” are regions of a catalyst that have a “sweet spot” of topography and/or chemical composition which leads to heightened catalytic activity. This sweet spot has the chemical and physical characteristics to bind neither reactant nor product neither too tightly nor too loosely and can stabilize intermediates. This method of optimization involves first finding the structure or chemical composition that best catalyzes the reaction of interest and then optimizing the catalyst to maximize the active sites. Unfortunately, there are not currently methods of active site discovery for the general catalyst and complexity in identifying an active site becomes high when multiple sites work together or where both structure and chemical composition play strong roles in the observed activity.

In our work, we have used a scanning electrochemical microscope to identify electrocatalytic hotspots on a brush coated sample and then used electron microscopy and energy dispersive x-ray spectroscopy to determine what is unique about structure and function of the hotspots. With this information we were able to determine that the thickness of the overlying TiO2 layers play a crucial role in the CER active site formation. By tuning the TiO2 thickness we then rationally engineered a catalyst that is as much as 20 times more active than the state of the art IrO2 CER catalyst. We believe that this increase in activity has to do with the strain that the underlying IrO2 puts on the overlying TiO2 layer. This theory is consistent with computational results that indicate the Metal-O bond strength is a crucial indicator of CER activity.

This innovation could lead to a significant decrease in the amount Ir needed to treat wastewater and therefore the cost of electrocatalytic wastewater treatment could become affordable in the developing world. Furthermore, using this method, we were able to increase the activity of a catalyst that has been around for decades by 20 fold by interrogating the brush coated version of the catalyst. It is therefore possible that this active site based optimization method could be used to optimize the numerous other catalysts that could be prepared by brush coating.