A precious-metal-free regenerative fuel cell for storing renewable electricity [electronic resource] : fundamental catalysis studies and device development
Keywords: Chemical Engineering.

The ever-increasing global demand for energy coupled with a growing awareness of the deleterious effects of fossil fuels have led to an explosive growth in the development of renewable energy technologies. Solar photovoltaics and wind turbines are attractive options; however, they are intermittent and localized in nature, which necessitates the use of energy storage devices to achieve energy time-shifting and allow for maximum market penetration of renewables into the grid. Unitized regenerative fuel cells (URFCs), based on H2O-H2-O2 interconversion chemistries, are an interesting class of energy storage devices that could potentially enable such a future. However, commercial URFCs typically utilize expensive platinum group metals as catalysts or operate at extremely high temperatures. The development of an alkaline anion exchange membrane (AEM) coupled with earth-abundant catalysts could enable a pathway towards a low-temperature, cost-effective URFC. In order to demonstrate the viability of the AEM-URFC concept, we first developed catalysts for each of the four reactions involved, namely the hydrogen evolution and oxidation reactions (HER, HOR) and the oxygen reduction and evolution reactions (ORR, OER). We adapted a synthesis procedure from the literature to produce a carbon-supported Ni catalyst that displays high activities for the HER, the HOR and the OER. Also, we modified the synthesis procedure of an O2-bifunctional MnOx catalyst previously developed in our lab such that the catalyst retained its high activity yet can be readily integrated into commercial URFC setups. Next, we integrated the Ni and MnOx catalysts with a commercial AEM into an existing cell setup, and the resultant prototype device obtained round trip efficiencies of 34-40 % at 10 mA/cm2. This first report of a precious-metal free AEM-URFC opens up new possibilities for enabling cost-effective and widespread deployment of renewable electricity. We pinpointed the degradation of carbon on the O2 electrode as one of the factors leading to a loss of device performance upon repeated cycling. Therefore, we developed a carbon-free, precious-metal-free O2 electrode by electrodepositing MnOx onto a stainless steel substrate followed by high-temperature calcination to achieve the desired MnOx phase. Fundamental electrochemical testing in addition to device testing reveal that this electrode exhibits superior stability compared to a carbon-containing O2 electrode, thereby revealing a pathway towards longer-lasting AEM-URFCs. Next, the knowledge gained from developing the AEM-URFC prototype was extended to polymer electrolyte membrane electrolyzers. Precious-metal Pt is typically used at the cathode to drive the HER; however, molybdenum sulfides have shown promise as an active and stable class of non-precious HER catalysts. Despite recent efforts to develop high-performance molybdenum sulfides, there has been limited work showing the effectiveness of these catalysts in operating devices. Hence, we synthesized three distinct molybdenum sulfides via facile routes specially designed for catalyst-device compatibility and demonstrated that these catalysts might potentially replace Pt as the cathode catalyst in commercial electrolyzers. The AEM-URFC device can serve as a platform for incorporating catalysts that display even better performance for any of the four reactions mentioned above. To that end, we investigated two distinct classes of catalysts, namely heteroatom-doped carbons and Ni-based mixed oxides. Heteroatom-doped carbon-based catalysts have received enormous attention due to their low costs and high activities for the ORR. However, there has been limited work on their performance for the OER and their compatibility with existing commercial setups. Therefore, we developed an NH3-activated N-doped carbon catalyst via a facile synthesis route and showed that this catalyst displays high performance both in a three-electrode electrochemical cell and also in an operating RFC device, thereby demonstrating the feasibility of N-doped carbon replacing Pt and Ir as the O2 catalysts in commercial RFCs. Also, literature reports have shown that substrates play a crucial role in the OER activity of Co and Mn oxides. We looked at the case of NiCeOx, which displays low activity for the OER when supported on a typical glassy carbon substrate. However, we discovered that the presence of a thin film of Au results in a significant boost in the OER performance of NiCeOx. The resultant geometric and specific activities are superior to those of the best catalysts reported in the literature, and is an interesting potential candidate to drive the OER in an AEM-URFC. Metal alloys typically deliver higher performance oxygen catalysis compared to their pure metal counterparts. However, metal alloys are typically more complicated and a fundamental understanding of how these catalysts operate in-situ is critical in the development of next-generation catalysts that deliver better performance. As such, we developed a bifunctional MnNiOx catalyst and utilized a newly-developed synchrotron technique to simultaneously detect changes in the Mn and Ni electronic structures via in-situ via X-ray emission spectroscopy. The obtained data highlights the effects of adding a second element to the chemical state and the resultant catalytic activity of the original catalyst. In summary, this dissertation discusses a broad spectrum of issues with AEM-URFCs, from fundamental catalysis to actual device operation. This work provides an important step towards a potentially commercial-level, precious-metal-free URFC for cost-effective energy storage to help scale the use of intermittent renewable energy.