Despite their advantages over traditional solid-state solar cells, photoelectrochemical solar cells (PECs) have yet to enter the market place due to the absence of electrode materials that are efficient, low-cost, and stable in aqueous environments [1]. In the majority of PEC systems Platinum is used as a stable counter electrode material exhibiting low overpotentials, but its high cost and limited supply make it impractical for large-scale production of PECs. In this work, we explore the use of alternatives to replace Pt, including transition metal carbides and bimetallic alloys, as counter electrode materials in PECs. Numerous transition metal carbide [2] and bimetallic [3] materials are known to exhibit catalytic activity comparable to that of Pt-group metals in a variety of reaction systems, making them promising candidates for the replacement of Pt in those systems.

Research completed to date has focused on the use of tungsten monocarbide (WC) as a counter electrode material in an aqueous polysulfide PEC system. PECs based on polysulfide electrolytes and II-VI semiconductors have attracted much attention owing to their potential as low-cost photovoltaic systems [4]. Characterization of the stability and activity of WC and Pt electrodes has been performed using combined surface science (X-ray photoelectron spectroscopy) and electroanalytical (linear sweep voltammetry, chronoamperametry, cyclic voltammetry) methods. Both WC and Pt electrodes were seen to be stable at all potentials cathodic to their rest potential, but WC displayed a significant advantage in terms of activity relative to Pt in the polysulfide electrolyte. In this work we also explore the use of WC electrodes modified with submonolayer Pt. In previous work, our group has shown that the deposition of submonolayer Pt onto WC can have synergistic effects on the electrochemical stability and activity of the electrode [5].

An integrated PEC device comprised of a polysulfide electrolyte, thin-film CdSe photoelectrode, and WC or Pt counterelectrode has been constructed. The device was tested under AM 1.5 illumination for two hours, with no degradation in the short-circuit current being observed during this time. The PEC employing WC as the counterelectrode had a larger short-circuit current that that utilizing Pt, a result which is consistent with the electroanalytical experiments performed on the individual counter electrodes.

[1] T. Bak, J. Nowotny, and C.C. Sorrell, Int. J. of Hydrogen Energy, 27 (2002) 994-995.

[2] H. Hwu and J.G. Chen, Chemical Reviews, 105 (2005).

[3] J.G. Chen, C.A. Menning, and M.B. Zellner, Surface Science Reports, 63 (2008).

[4] J. Winnick, P.M. Lessner, and E.J. Cairns. Appl. Phys. Lett., 53 (1988) 1985.

[5] E.C. Weigert, A.L. Stottlemyer, M.B Zellner, and J.G. Chen, J. Phys. Chem. C, 111 (2007) 14617.