THE DEVELOPMENT OF HIGH MASS-SPECIFIC ACTIVITY RHODIUM SULFIDE CATALYSTS FOR HOR/HER IN HYDROGEN-BROMINE FUEL CELL

Abstract

The integration of intermittent energies into the electrical grid like wind and solar power demands the participation of efficient and cost-effective energy storage systoms. H2-Br2 fuel cell is one of the promising technologies due to the advantage from the fast kinetics of Br2/Br- and low price of HBr. The corrosive nature of Br2/Br- determines that the metallic Pt is not a good choice for the HOR/HER catalysis on the negative electrode in the long-term operation of the fuel cell. The RhxSy is free from the corrosion of the bromine and bromide. The low mass-specific surface area and activity of existing RhxSy catalyst for HOR/HER, however, obstruct the deployment of the H2-Br2 fuel cell. In this dissertation, a clear roadmap to solve those problems has been demonstrated. Core-shell structure was the first attempt to increase the mass-specific surface area of rhodium sulfide catalyst. Catalysts with RhxSy as shell and different percentages (5%, 10%, and 20%) of platinum on carbon as core material were synthesized. The TEM and EDX results confirm the existence of the core-shell structure. Cyclic voltammetry was used to evaluate the Pt-equivalent mass specific ECSA and durability of these catalysts. Cycling test and polarization curve measurement in the H2-Br2 fuel cell were used to assess the catalyst stability and performance in a real fuel cell. The results show that the catalyst with core-shell structure has higher mass-specific ECSA (50 m2/gm-Rh) compared to a commercial catalyst RhxSy/C catalyst from BASF, 6.9 m2/gm-Rh). It also shows better HOR/HER performance in the fuel cell. Compared to the platinum catalyst, the core-shell catalysts show more stable performance in the fuel cell cycling test. The cheap metal core material was also explored to replace the expensive Pt material. However, the CV test suggests that the cheap metal catalyst was dissolved in the acidic RhCl3 solution and then precipitated by the sulfide ion during the synthesis, which results in a lower ECSA/mass compared to the commercial catalyst. Supporting carbon materials were functionalized to create more suitable nucleation sites for the precipitation of rhodium sulfide nanoparticles resulting in a catalyst with smaller particle size, better particle distribution, and higher hydrogen oxidation and evolution reaction performance. XPS and FT-IR confirm that the dominant functional group on the carbon is the ketone group, which is more suitable group for rhodium sulfide particles formation than the carboxylic group. TEM and cyclic voltammetry results confirm that the catalysts with treated carbon have a smaller average particle size (7.2 nm vs. 13.2 nm) and higher mass-specific ECSA (21.8 m2/g-Rh vs. 9.1 m2/g-Rh) than those with the untreated carbon. The H2-Br2 fuel cell test results show that the catalysts with functionalized carbon have better performance in the kinetic region but poorer performance in the mass transfer dominant region. The issue was identified to be due to the weak affinity of the ketone group on the carbon surface with the Nafion ionomer in the catalyst ink. The mass-specific surface area of RhxSy was further increased by the selection of a more effective nanoparticle growth mechanism. The diffusion-controlled nanoparticle growth mechanism was created by controlling the concentration of Rh2S3 monomer in the synthesis of rhodium sulfide catalyst, which decreases the average particle size from 13.2 nm to 3.2 nm by the TEM measurements. The mass-specific ECSA is improved from 9.1 m2/g-Rh to 43 m2/g-Rh with this approach. Moreover, the crystal phase composition in the mixed RhxSy was modified by the usage of a new sulfur source, Na2S, which results in an increase in the active area specific exchange current (0.59 A/cm2 vs 0.51 A/cm2). The mass-specific exchange current density increases from 0.58 A/mg-Rh of RhxSy/untreated carbon synthesized by the traditional sulfur source) to 2.8 A/mg-Rh with the combination of the usage of functionalized carbon material, the diffusion-controlled of nanoparticle growth mechanism, and the new sulfur source. The affinity issue between the ketone functional group on the carbon surface and the Nafion ionomer in the catalyst ink was resolved by using the Baeyer-Villiger reaction and ester hydrolysis to convert the Nafion-unfriendly ketone group to the Nafion-friendly carboxylic group. After the organic reactions, the modification of the surface functional group was validated by the FT-IR method. Furthermore, the TEM, XRD, and cyclic voltammetry methods confirm that the organic reactions have no negative effect on the catalyst and carbon surface in the process of organic reactions. The H2-Br2 fuel cell tests confirm that the mass transfer resistance observed in the fuel cell with the RhxSy on the ketone-dominated pretreated carbon was significantly reduced by this approach. The discharge performance of hydrogen-bromine fuel cell with the RhxSy /pretreated carbon with surface functional groups conversion is improved by 2.8 times compared to that of the fuel cell with the commercial RhxSy catalyst.