Heterogeneous and Nanostructured Electrocatalysts

There is considerable interest in development of solar fuels devices based on nanoscaled photovoltaic devices, integrated with electrocatalysts that utilize oxidizing equivalents, at a minimum overpotential, for the splitting of water to protons and O2 (oxygen evolution reaction, OER). Reducing equivalents (electrons) generated by the photovoltaic are concomitantly used to reduce the protons to hydrogen, again with a negligible overpotential.

Recent Projects

In collaboration with Alex Bell in Chemical Engineering, an investigation of electrocatalytic Co3O4 nanoparticles for water oxidation demonstrated increasing electrocatalytic activity as the particle size decreases.1 Additionally, 10 nm nanoparticles of Co3O4, CoO, and Co metal were found to exhibit small differences in performance, and each appears to access the same surface-active species under the catalytic conditions.2 The thermal molecular precursor (TMP) approach was used to prepare nanoparticles of cobalt metaphosphate, Co(PO3)2, for construction of nanostructured anodes for OER. This OER catalyst operates at an onset overpotential (310 mV; pH 6.4), that is substantially lower than that associated with Co3O4 and CoPi, two well-known cobalt-based catalysts for this reaction.3 Various surface-bound cobalt species, obtained by treatment of silica with Co[N(SiMe3)2]2, ranged from single-atom cobalt centers to small clusters, and interestingly the greatest activity was observed for the sample dominated by single-cobalt centers.4 Related studies probed the nature of the supporting oxides on cobalt catalysts for OER,5 and physical studies reveal mechanistic aspects to the function of surface species in OER.6

Other contributions in this area involve semiconductor surface modifications that allow covalent attachment of a molecular catalyst. The Seyferth reagent (PhHgCCl2Br) was found to functionalize a hydrogen-terminated Si(111) surface with introduction of surface, Si–CCl2H groups.7,8 Also, glassy carbon electrodes were modified with a sterically-controlled electrochemical reduction using the diazonium salt iPr3SiOCH2C6H3N2+BF4-. The bulky silyl protecting group prevents the uncontrolled growth of structurally ill-defined and electronically blocking polyphenylene multilayers, and separates the –C6H4CH2OH groups of the resulting monolayer to allow better accommodation of sizable molecules. 

References

1. "Size-Dependent Activity of Co3O4 Anodes for Alkaline Water Electrolysis." A. J. Esswein, M. J. McMurdo, P. N. Ross, A. T. Bell and T. D. Tilley. J. Phys. Chem. C 2009, 113, 15068-15072. DOI: 10.1021/jp904022e2.

2. "Comparison of Cobalt-based Nanoparticles as Electrocatalysts for Water Oxidation." N. H. Chou, P. N. Ross, A. T. Bell and T. D. Tilley, ChemSusChem 2011, 4, 1566-1569. DOI: 10.1002/cssc.201100075

3. "Electrocatalytic Water Oxidation at Neutral pH by a Nanostructured Co(PO3)2 Anode." H. S. Ahn and T. D. Tilley, Adv. Func. Mater. 2013, 23, 227-233. DOI: 10.1002/adfm.201200920

4. "Photocatalytic Water Oxidation by Very Small Cobalt Domains on a Silica Surface." H. S. Ahn, J. Yano and T. D. Tilley, Energy Environ. Sci. 2013, 6, 3080–3087. DOI: 10.1039/C3EE42067A

5. "Water Oxidation by Cobalt Centers on Various Oxide Surfaces: The Effects of Oxide Surface Acidity and Oxygen Atom Affinity on Catalysis." H. S. Ahn, J. Yano and T. D. Tilley, ACS Catal. 2015, 5, 2573-2576. DOI: 10.1021/cs502120f

6. "An Electrochemical Study of the Energetics of the Oxygen Evolution Reaction at Nickel Iron (Oxy)hydroxide Catalysts." J. Swierk, S. Klaus, L. Trotochaud, A. Bell, T. D. Tilley, J. Phys. Chem. C 2015, 119, 19022-19029. DOI: 10.1021/acs.jpcc.5b05861

7. "Multi-Functional Silicon Surfaces: Reaction of Dichlorocarbene Generated from the Seyferth Reagent with Hydrogen-Terminated Silicon (111) Surfaces." W. Liu, I. D. Sharp and T. D. Tilley, Langmuir 2014, 30, 172-178. DOI: 10.1021/la403789a

8. "Sterically Controlled Functionalization of Carbon Surfaces with –C6H4CH2X (X = OSO2Me or N3) Groups for Surface Attachment of Redox-Active Molecules." W. Liu and T. D. Tilley, Langmuir 2015, 31, 1189-1195. DOI: 10.1021/cla503796z