The penetration and coating of nanoparticles by electrophoretic deposition (EPD) into three-dimensional substrates was studied. Specifically, EPD of cobalt ferrite nanoparticles (20 nm) was investigated as a possible electrocatalyst to improve the kinetically slow anodic reaction of the oxidation of ammonium sulfite to ammonium sulfate in the hydrogen production sub-cycle of the solar sulfur-ammonia thermochemical water-splitting process. The primary goals were to uniformly penetrate and coat nanoparticles into 3D substrates by electrophoretic deposition and then to test their electrocatalytic activity by linear sweep voltammetry in 2 M ammonium sulfite.

Electrophoretic deposition of cobalt ferrite nanoparticles from five bath compositions was performed on the following substrates: aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt. The bath compositions were 2 g/L of cobalt ferrite nanoparticles in 90/10 vol. % water/isopropanol with 1 mM (CHigh) or 0.05 mM (CLow) of hexadecyltrimethlyammonium bromide (CTAB), in 100% ethanol (E), in 100% acetylacetone (AA), and 100% acetylacetone with 0.2 wt. % polyethylenimine (AA-PEI). For electrophoretic deposition, a constant voltage was applied from 20 V to 128 V for a deposition time from 1 to 13 minutes. The zeta potential was measured to be 23  1 mV at pH of 6 for CHigh bath and 13  2 mV at pH 5 for E bath.

Dry and wet adhesion of EPD deposits on aluminum was performed to determine adherence of nanoparticles to the substrate both after deposition and during the electrochemical process. Nanoparticles had good adherence to the substrate.

To determine if nanoparticles could penetrate into 3D felt substrates by EPD, the ratio of Peclet number to Damköhler number (ratio of electrophoretic velocity to local deposition rate) was calculated to be greater than 1000, which indicated full penetration. Scanning electron microscopy (SEM) of the top and middle of the EPD deposits was used to examine the nanoparticle penetration and deposit morphology. It was confirmed by SEM images that there was full penetration and coating of nanoparticles into the 3 mm carbon felt and the 6 mm graphite felt from the 100% ethanol bath.

The electrocatalytic activity of the EPD deposits on aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt substrates were analyzed by linear sweep voltammetry at 50 mV/s for the oxidation of 2 M ammonium sulfite. For the 3 mm carbon felt substrates, the highest electrochemical activity occurred on EPD deposits from the AA-PEI bath and then E bath. The highest electrochemical activity of the EPD deposits on 6 mm graphite felt occurred from the bath chemistries in decreasing order: CLow, AA-PEI, and E. The EPD deposit on 6 mm graphite felt from CLow bath obtained the highest electrochemical activity of all the deposits. For the 3 mm carbon felt, the ratio of current density of deposit compared to blank substrate ranged from 4 to 8 from E bath. The ratio of current density of EPD deposit on graphite paper and 6 mm graphite felt from E bath compared to blank substrate was about 1.6. The 3 mm carbon felt EPD deposits from AA-PEI bath ranged from 9 to 14, for the ratio of current density of the deposit compared to the blank substrate. EPD deposits from AA-PEI bath had a ratio of current density of deposit to blank substrate of 1.9 for deposits on aluminum foil, graphite paper, and 6 mm graphite felt.

Overall, EPD was successful as a method to penetrate and coat cobalt ferrite nanoparticles up to 6 mm thick 3D felt. EPD deposits from AA-PEI bath improved the electrochemical activity compared to the blank substrate more than EPD deposits from E bath. But the EPD deposits from E bath were thin and uniform as well as deposited at much lower voltages compared to deposits from AA-PEI bath. EPD deposits from 3 mm carbon felt resulted in the largest improvement in electrochemical activity compared to the blank substrate from both E and AA-PEI bath. However, EPD deposits into 6 mm graphite felt had the highest electrochemical activity from all of the deposits. The EPD deposits with the highest electrochemical activity into 6 mm graphite felt in decreasing order were CLow, AA-PEI and E. The EPD conditions for these deposits were 40 V for 1 min from CLow resulting in current density of 75 mA/cm², 127 V for 4 min from AA-PEI resulting in current density of 60 mA/cm², and 40 V for 1 min from E resulting in current density of 38 mA/cm².