Our research group works to gain a fundamental understanding of how to program and process nanoscale building blocks into functional structures, and the structure-property relationships of the resulting nanostructured materials. We seek to develop new nano-materials and methods for batteries, supercapacitors, fuel cells, and printable electronics. For building blocks we mainly utilize monodisperse nanoparticles made through colloidal chemistry. One issue that has yet to be adequately addressed for colloidal nanoparticles is their adherence  to each other and to the electrically conductive support.  In this talk I will discuss our recent results overcoming critical challenges to create functional assemblies from nanoparticles using electrophoretic deposition (EPD).

For batteries, we have found that EPD can be used to form additive-free battery electrodes from colloidal nanoparticles. Typically, polymeric binders are necessary to adhere the particles to each other and the substrate, and carbon additives are necessary to increase conduction. These additives, however, generate a weight increase of 10−40%. We have found that EPD creates a strong electrical and mechanical bond for nanoparticles adhesion, enabling the batteries to perform at maximum capacity.  This innovation increases the power density by reducing the overall volume. We have demonstrated high-performing anodes of cobalt oxide, Ge, and metal sulfides. The Co₃O₄films show high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm3) even after 50 cycles. The additive-free Ge nanoparticle films show stable capacities past 50 cycles at 750 mAh/g.

Using EPD methods we have made additive-free nanoparticle electrodes for supercapacitors. We synthesized a series of Co-Mn-O stoichiometries to understand how cationic substitution affects performance. We find that the highest-performing Co−Mn mixture has a 1:1 ratio of Co to Mn and shows an energy density of 26.6 Wh/kg with a specific capacitance of 173.6 F/g. This nanoparticle electrode delivers the highest power density of 3.8 kW/Kg at a 5 A/g constant current discharge. The energy density and power density delivered by the optimal mixture is ~6× and ~3× higher, respectively, than that of pure Co₃O₄ electrodes.  The specific capacitance for the Co-Mn mixture is also ~4× better than the pure Co₃O₄supercapacitor.

For electrocatalysis, we compare EPD-deposited nanoparticle films to the conventional dropcast method. We study cobalt oxide thin films for the oxygen reduction/oxygen evolution reactions (ORR and OER).  In examining the film’s catalytic properties we find that the electrophoretically deposited nanoparticles outperform the dropcast films by as much as 2.5 for the oxygen reduction reaction and 2.6 and the oxygen evolution reaction when accounting for both surface area and mass.  This process results in this significant catalytic improvement in oxygen reduction/evolution performance which cannot be duplicated by dropcast films with the same loading.

We have applied these techniques to make printable electronics: Using our surface treatment methods to link the nanoparticles, and the EPD method for deposition, we make copper sulfide films with high conductivity and high mobilities.  We show that our nanoparticle films have conductivities that are on par with many bulk copper sulfide films (~75 S·cm^-1), without the need for heat-treatments.

D.-H. Ha*, T. Ly*, J.M. Caron, H. Zhang and R.D. Robinson, “A General Method for High-Performance, Additive-Free Li-ion Battery Electrodes from Colloidal Nanoparticles: The Case of MnS, Cu_2-xS, and Ge,” ACS Appl. Mater. Inter. 7, 25053-25060 (2015) DOI: 10.1021/acsami.5b03398

D.H. Ha, M.A. Islam, and R.D. Robinson, “Binder-free and Carbon-free Nanoparticle Batteries: A Method for Nanoparticle Electrodes without Polymeric Binders or Carbon Black,” Nano Letters 12, 5122-5130 (2012). http://dx.doi.org/10.1021/nl3019559

S.D. Perera, X. Ding, A. Bhargava, R. Hovden, A. Nelson, L.F. Kourkoutis, R.D. Robinson, “Enhanced supercapacitor performance for equal Co-Mn stoichiometry in colloidal Co_3-xMn_xO₄ nanoparticles, in additive-free electrodes,” accepted Chem. Mater. (2015) 10.1021/acs.chemmater.5b02106

M. Fayette, A. Nelson, and R.D. Robinson, “Electrophoretic Deposition Improves Catalytic Performance of Co₃O₄ Nanoparticles for Oxygen Reduction/Oxygen Evolution Reactions,” Journal of Materials Chemistry A 3, 4274–4283 (2015).  DOI: 10.1039/C4TA04189E

O.O. Otelaja, D.-H. Ha, T. Ly, H. Zhang, and R.D. Robinson, “Highly Conductive Cu_2-xS Nanoparticle Films through Room Temperature Processing, and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition,” ACS Applied Materials and Interfaces 6, 18911–18920 (2014). http://dx.doi.org/10.1021/am504785f