Hydrogen: the Fuel of our Future Marie Mayer Stanford Synchrotron Radiation Laboratory University of Illinois at Urbana-Champaign

Overview Environmental relevance Photoelectrochemical hydrogen production X-Ray Photoelectron Spectroscopy (XPS) Results

Carbon-free Energy Sunlight that hits earth for 1 hour = planet’s power for 1 year Equivalent of 1.3 trillion barrels of oil from sun in 1 year How do we capture it? Solar cells Hydrogen production Storage of solar energy

Photoelectrochemical Hydrogen Production current O2 H2 Pt Anode Conduction Band H2O/H2 2H2O + 4h+  4H+ + O2 Eg 2H2O + 2e-  2OH- + H2 H2O/O2 Valence Band Hydrogen Electrode (Semiconductor Cathode) Thank you, Theanne, for providing me with most of this slide.

Experimental GaPxN1-x for PEC cells Dilute (x < 0.02) nitrogen decreases PEC corrosion ~20 times1 As grown & corroded samples GaP GaP0.98N0.02 GaP0.998N0.002 Pt Counter Electrode Ag/AgCl Reference Electrode H2SO4 electrolyte GaP(N) sample Optical lamp: 100 mW-cm-2 (1 sun) 1. T. Deutsch. Sunlight, Water and III-V Nitrides for Fueling the Future, PhD Thesis, Boulder, Colorado, 2006. Thank you to NREL for lending the cell.

Synchrotron XPS Theory: the watered-down version 3. Detector records kinetic energies (depths) of escaped electrons—radius of curvature depends on velocity Detector 2. Core electron is excited to free electron e- Yay! I’ve escaped! 1. Synchrotron radiation as X-rays e- e- e- e- e- e- e- 4. Identify using chemical binding energy

Synchrotron XPS (BL 5-1) Chemical Environment Tunable h! Pick kinetic energy by picking h Depth profile: Ga3d, Ga3p, P2p, N1s, O1s, C1s Kinetic Energy (eV) Escape Depth (Å) 200 9 450 14.85 750 21.25 900 24.3

Result 1: Ga surface depletion during corrosion More dramatic with N Possible Mechanism: GaP + 3H+ +  Ga3+ + PH3 Electrolyte analysis DEPTH

Result 2: Bulk nitrogen migration to surface N + XO  NOx N  surface during corrosion Oxidizes with air contact 396 394 392 390 388 386 BE/eV N (bulk) KE=900 hν=1310 NOx, NHx Intensity [arb. Units] N N N N N N N Nitrogen-containing sample

Result 2: Bulk nitrogen migration to surface (2) Possible formation of GaN (before oxidizing)? Corrosion stability of wider band gap GaP: 2.26 eV GaN: 3.2 eV

Recap: GaP1-xNx Observed that N content assists H2 production Formation of oxides during H2 production Surface Ga depletion Bulk N to surface Future work Mechanisms Electrochemical measurements SEM

A moment of gratitude… Theanne Schiros Jen Leisch Lars-Åke Näslund Hirohito Ogasawara Anton Nikitin Anders Nilsson Dennis Nordlund Mike Toney Robert Kirby, Surface & Materials Science Sarah Kurtz, National Renewable Energy Laboratory (NREL) Friendly supply room fellows Anyone who even so much as made me smile.

Questions?

Yes, my friends, I believe that water will one day serve as our fuel, that the hydrogen and oxygen which compose it will supply an inexhaustible source of heat and light. I believe that when the coal mines have been exhausted, we will both heat and be heated with water. Water is the coal of the future.” -Jules Verne The Mysterious Island

Fossil fuels produce CO2… Data from Energy Information Administration Database. 2006, US Department of Energy, www.eia.doe.gov.