Earth Abundant Catalysts for Water Electrolysis at Low Overpotentials
University of Kansas
Chemical & Petroleum Engineering
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Water electrolysis via renewable electricity (e.g., wind, solar) provides a carbon neutral route for pure hydrogen production to fuel zero emission power generation devices, such as fuel cells. Deployment of fuel cell vehicles is growing in the US and around the world, coupled with the construction of hydrogen fueling stations, of which there are expected to be 200 hydrogen refueling stations operating in California by 2025. However, 96% of hydrogen is still produced by steam reformation of natural gas or sourced from other petroleum feedstocks. As a result, the fuel cell vehicles produce zero emissions at the tailpipe but the production of the hydrogen fuel still contributes to greenhouse gas emissions. Moreover, mature technologies essential to the sustenance of the human population, specifically the Haber-Bosch process for the production of ca. 187 million tonnes of ammonia fertilizer per year, account for the majority of hydrogen consumption. Thus, a carbon neutral route for hydrogen production from renewable feedstocks is an exigency for Earth’s inhabitants to avoid the worsening of the catastrophic effects of anthropogenic climate change. Water splitting is a technology that is hundreds of years old, yet it has still not become commercially relevant due to high capital costs and high operating costs of the electrolyzers available today. A major driver of these costs are the hydrogen evolution and oxygen evolution electrocatalysts, which are typically composed of platinum and iridium for operation in acidic electrolyte. The main objective of this doctoral work is to decrease the overpotential of the water splitting reactions by tuning the morphology of earth abundant metal catalysts. Platinum is known to catalyze the hydrogen evolution reaction (HER) at the thermodynamic equilibrium potential, and it is most active in acidic media. However, the corrosive nature of acidic electrolyte causes degradation of non-precious metal catalysts as well as other components that the electrolyte comes into contact with. Various transition metal dichalcogenides have been investigated as alternatives to platinum, but here we report the first nanostructured FeS2 HER catalyst. By utilizing a hot sulfur injection synthesis and adjusting the sulfur concentration, we were able to create one-dimensional (1D) FeS2 wires and two-dimensional (2D) FeS2 discs, unique from the previously known three-dimensional (3D) FeS2 cubes. We found that the catalytic activity of the nanostructured FeS2 was highly dependent on the morphology, and the 2D FeS2 discs had an exchange current density of the same order of magnitude as Pt in neutral electrolyte. The 2D morphology of the FeS2 discs bears resemblance to the 2D MoS2 catalyst which is known to have HER activity in acidic media, with the edges of the 2D nanosheet structure being the active sites. The oxygen evolution reaction (OER) is the largest source of inefficiency in overall water splitting reaction because it is a four electron/four proton transfer reaction. NiFe oxides have emerged as highly active OER catalysts in alkaline media, surpassing the activity of even the precious metal- based catalysts, IrOx and RuOx. Specifically, Ni0.8:Fe0.2 has been shown to be the most active ratio for NiFe catalysts because above ca. 25% iron, segregation of the metals occurs. Utilizing surface interrogation mode of scanning electrochemical microscopy (SI-SECM), we measured the kinetic rate constant(s) of the active sites on crystalline Ni0.8:Fe0.2 and amorphous Ni0.8:Fe0.2. We found that the crystalline Ni0.8:Fe0.2, synthesized at high temperatures, had both ”fast” and ”slow” sites with a rate constant of 1.3 s-1 and 0.05 s-1, respectively, while the amorphous Ni0.8:Fe0.2, synthesized by a low temperature (microwave-assisted) method, only had ”fast” sites with a rate constant of 1.9 s-1. High resolution transmission electron microscopy (HRTEM) and electron diffraction showed that the microwave-assisted Ni0.8:Fe0.2 was amorphous down to the five nanometer scale, indicating that low temperature synthesis is crucial for homogeneous dispersion of the metals and maximization of bimetallic active sites. Further SI-SECM studies revealed that the Ni0.8Fe0.2OOH layered double hydroxide (LDH), a structure known to have electrolyte permeability, had an anomalously high active site density (4500 sites nm-2) compared to that of the crystalline Ni0.8:Fe0.2 (500 sites nm-2), from which it was synthesized by electrochemical conditioning. This gives evidence that electrolyte permeability allows for catalysis via subsurface sites. The final objective of this work was to lower the overall water splitting overpotential with earth abundant metal catalysts. Current water electrolyzers utilize acidic (PEM) or alkaline configurations. However, neither of these configurations are amenable to using the FeS2 discs and nanoamorphous Ni0.8:Fe0.2 oxide for overall water splitting because the former is the most active in neutral media and the latter is most active in alkaline media. By analogy with the chlor-alkali membrane process, we utilized the K+ form of the Nafion 117 membrane to enable water splitting with the anode in alkaline media and the cathode in neutral media, or dual-pH water splitting. With this system, we were able to achieve water splitting at 10 mA cm-2 under 1.6 V—a total overpotential of less than 270 mV—for a 24 h period.
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