Novel PGM-Free Electrocatalysts for Alkaline Electrolyzers

Abstract

N Introduction Fuel cell technology has been extensively developed in order to create new clean sources of energy. In order to have a wide implementation of fuel cell devices powered by hydrogen (H2), an inexpensive source of clean H2 should be created. One of the technologies that produces hydrogen gas is the splitting of water through electrolysis. The majority of electrolyzers available on the market use proton exchange membrane (PEM) technology, which requires catalysts consisting of expensive and rare platinum group metals (PGMs). To increase penetration into the water electrolyzer and fuel cell markets, abundant and inexpensive materials should be incorporated. Alkaline exchange membrane water electrolysis (AEMWE) is an alternative technology that does not require PGM catalysts and, therefore, enables the use of base metals, alloys and oxides. The goal of this research has been focused on the creation of a catalyst that is stable in alkaline media, has a high surface area, is easy to synthesize, and can be produced at a lower cost while maintaining a high oxygen evolution reaction (OER) activity. Experimental Unsupported ternary Ni-Mo-Cu materials were synthesized by the Sacrificial Support Method (SSM) using a high surface area silica [1-3]. The following process was followed, and produced a NiMoCo OER catalyst achieving a surface area (SA) of 25 m2 g-1. 15g of EH-5 (SA=400 m2 g-1) silica, 2.5g of nickel nitrate, 2.5g of copper nitrate and 3g of ammonium molybdate tetrahydrate were mixed by grinding them in a porcelain crucible. The crucible was placed into a tube furnace and heated to 100 °C with a ramp rate of 10ºC/min. The 100°C temperature was held for 50 minutes before an additional temperature ramp was conducted at an interval of 5ºC/min until 550°C was reached and held for four hours.. During this time, a reducing atmosphere was maintained using 7% H2. The silica was later removed by rinsing with 7M potassium hydroxide (KOH) for 24 hours, followed by washing and drying. Cell testing was performed using a Fuel Cell Technologies 25 cm2 fuel cell stack, modified for electrolysis operation. The carbon anode flow field was replaced with an anode piece fabricated at Proton OnSite, based on 316L SS, which was cleaned and passivated in a nitric solution prior test. ASTM Type II DI water was fed to the anode side of the cell via a diaphragm pump. A submersible heater, placed in the water reservoir, was used to control stack temperature at 50ºC. An image of the test station is shown below in Figure 1. Figure 1. 25 cm2 Test Stand Results and Discussion SEM images show that the NiMoCu material has a well-developed porous structure, as well as primary particles on the nanometer scale (Figure 2). The surface area was measured at 25±3 m2 g-1 via BET analysis. Based on these results, the team decided to proceed with high surface area silica as the sacrificial support.Figure 2. SEM on Ni-Mo-Cu catalyst Electrochemical activity of the OER is shown in Figure 3. Figure 3. ORR activity of different Ni-Mo-Cu electrocatalysts in alkaline media. Operational testing was conducted on the NiMoCo OER catalyst provided by UNM in the 25 cm2 cell previously described. As shown below in Figure 4, the UNM supplied material experienced relative stability at the steady-state current density of 200 mA/cm2, as compared to the PGM based reference plot. Both tests used the same membrane and electrode binders. Figure 4. Steady-state operation graph of non-PGM anode versus PGM reference Conclusion Synthesis of spinel-based catalysts by SSM was investigated. Upon examination of the physical characteristics, it can be assumed that further modification to the design of the electrocatalysts is necessary. The electrochemical characteristics from the RDE experimentation proved that the catalysts are active in OER and can act as a replacement for conventional Pt-group catalysts in the appropriate conditions. Short duration operational testing has shown some translation of the RDE measurements to in-cell performance. Additional testing is planned for evaluation of repeatability and longer term stability.