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Staff information


Aaron Appel

Physical Sciences
Chemist, Team Lead
Pacific Northwest National Laboratory
PO Box 999
MSIN: K2-57
Richland, WA 99352


Aaron Appel is an organometallic chemist with a focus on basic energy research related to the production and use of fuels through catalysis and electrocatalysis. He is currently leading the molecular catalysis effort on CO2 reduction and fuel utilization, and is also working as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center. For the reduction of CO2 and subsequent fuel utilization, the goal of the work is the design of new catalysts with specific emphasis on the use of bifunctional interactions and thermochemical considerations. Dr. Appel's role in the Center for Molecular Electrocatalysis is in thermochemical measurements, especially as they pertain to the production and utilization of H2.

Dr. Appel's primary areas of expertise are thermochemical measurements, electrochemistry, catalyst design and synthetic chemistry, small molecule spectroscopy, and mass spectrometry. Dr. Appel also has a background in CO2 capture research, particularly in developing new methods with higher energy efficiency, such as electrochemically switchable CO2 capture.

Research Interests

  • Electrochemistry: Utilization of electrochemistry for the conversion of electrical energy to chemical energy through electrocatalysis. Controlled substrate binding using electrochemical switching, including CO2 capture and concentration. Hydrogen production and utilization and CO2 capture, concentration, and conversion are all of primary interest.
  • Fundamental thermodynamic studies of bond strengths: Determination of bond strengths using thermodynamic cycles through the study of equilibria and electrochemical processes, particularly as related to catalysis.
  • Synthetic inorganic and organometallic chemistry: Synthesis of relevant species to catalysis and electrocatalysis, particularly for fuel production and utilization. Characterization of synthetic species by electrochemistry, mass spectrometry, NMR, EPR, and other applicable techniques.

Education and Credentials

  • B.S. in Chemistry from Washington State University, Pullman
  • Ph.D. in Inorganic Chemistry from University of Colorado, Boulder

Affiliations and Professional Service

  • American Chemical Society

PNNL Publications


  • Burgess S.A., A.J. Kendall, D.R. Tyler, D.R. Tyler, J.C. Linehan, and A.M. Appel. 2017. "Hydrogenation of CO2 in Water Using a Bis(Diphosphine) Ni-H Complex." ACS Catalysis 7, no. 4:3089-3096. PNNL-SA-123128. doi:10.1021/acscatal.7b00350
  • Burgess S.A., A.M. Appel, J.C. Linehan, and E.S. Wiedner. 2017. "Changing the Mechanism for CO2 Hydrogenation Using Solvent-Dependent Thermodynamics." Angewandte Chemie International Edition 56, no. 47:15002-15005. PNNL-SA-128948. doi:10.1002/anie.201709319
  • Cammarota R.C., M.V. Vollmer, J. Xie, J. Ye, J.C. Linehan, S.A. Burgess, and A.M. Appel, et al. 2017. "A bimetallic nickel-gallium complex catalyses CO2 hydrogenation via the intermediacy of an anionic d10 nickel-hydride." Journal of the American Chemical Society 139, no. 40:14244-14250. PNNL-SA-127919. doi:10.1021/jacs.7b07911
  • Jeletic M.S., E. Hulley, M.L. Helm, M.T. Mock, A.M. Appel, E.S. Wiedner, and J.C. Linehan. 2017. "Understanding the Relationship Between Kinetics and Thermodynamics in CO2 Hydrogenation Catalysis." ACS Catalysis 7, no. 9:6008-6017. PNNL-SA-125045. doi:10.1021/acscatal.7b01673


  • Lindley B.M., A.M. Appel, K. Krogh-Jespersen, J.M. Mayer, and A.J. Miller. 2016. "Evaluating the Thermodynamics of Electrocatalytic N2 Reduction in Acetonitrile." ACS Energy Letters 1, no. 4:698-704. PNNL-SA-119958. doi:10.1021/acsenergylett.6b00319


  • Connelly S.J., E.S. Wiedner, and A.M. Appel. 2015. "Predicting the Reactivity of Hydride Donors in Water: Thermodynamic Constants for Hydrogen." Dalton Transactions 44, no. 13:5933-5938. PNNL-SA-105710. doi:10.1039/C4DT03841J
  • Miller D.L., B.J. Boro, K. Grubel, M.L. Helm, and A.M. Appel. 2015. "Synthesis and Characterization of a New TRIPHOS Ligand Derivative and the Corresponding Pd(II) Complexes." European Journal of Inorganic Chemistry 2015, no. 35:5781-5785. PNNL-SA-111376. doi:10.1002/ejic.201500791
  • Pegis M.L., J.A. Roberts, D.J. Wasylenko, E.A. Mader, A.M. Appel, and J.M. Mayer. 2015. "Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide." Inorganic Chemistry 54, no. 24:11883-118/88. PNNL-SA-111283. doi:10.1021/acs.inorgchem.5b02136
  • Peterson S.M., M.L. Helm, and A.M. Appel. 2015. "Nickel Complexes of a Binucleating Ligand Derived from an SCS Pincer." Dalton Transactions 44, no. 2:747-752. PNNL-SA-104617. doi:10.1039/c4dt02718c
  • Weiss C.J., E.S. Wiedner, J.A. Roberts, and A.M. Appel. 2015. "Nickel Phosphine Catalysts with Pendant Amines for Electrocatalytic Oxidation of Alcohols." Chemical Communications 51, no. 28:6172-6174. PNNL-SA-107999. doi:10.1039/C5CC01107H
  • Zall C.M., J.C. Linehan, and A.M. Appel. 2015. "A Molecular Copper Catalyst for Hydrogenation of CO-2 to Formate." ACS Catalysis 5, no. 9:5301-5305. PNNL-SA-110891. doi:10.1021/acscatal.5b01646


  • Appel A.M. 2014. "Carbon-Fuelled Future." Chemistry and Industry 78, no. 4:36-39. PNNL-SA-100705. doi:10.1002/cind.784_11.x
  • Appel A.M. 2014. "Electrochemistry: Catalysis at the boundaries." Nature 508, no. 7497:460-461. PNNL-SA-101718. doi:10.1038/nature13226
  • Appel A.M., and M.L. Helm. 2014. "Determining the Overpotential for a Molecular Electrocatalyst." ACS Catalysis 4, no. 2:630-633. PNNL-SA-99198. doi:10.1021/cs401013v
  • Bullock R.M., A.M. Appel, and M.L. Helm. 2014. "Production of Hydrogen by Electrocatalysis: Making the H-H Bond by Combining Protons and Hydrides." Chemical Communications 50, no. 24:3125-3143. PNNL-SA-97704. doi:10.1039/c3cc46135a
  • Galan B.R., E.S. Wiedner, M.L. Helm, J.C. Linehan, and A.M. Appel. 2014. "Effects of Phosphine-Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes." Organometallics 33, no. 9:2287-2294. PNNL-SA-97196. doi:10.1021/om500206e
  • Jeletic M.S., M.L. Helm, E.B. Hulley, M.T. Mock, A.M. Appel, and J.C. Linehan. 2014. "A Cobalt Hydride Catalyst for the Hydrogenation of CO2: Pathways for Catalysis and Deactivation." ACS Catalysis 4, no. 10:3755-3762. PNNL-SA-103938. doi:10.1021/cs5009927
  • Kumar N., D.M. Camaioni, M. Dupuis, S. Raugei, and A.M. Appel. 2014. "Mechanistic Insights into Hydride Transfer for Catalytic Hydrogenation of CO2 with Cobalt Complexes." Dalton Transactions 43, no. 31:11803-11806. PNNL-SA-101141. doi:10.1039/c4dt01551g
  • Lercher J.A., A.M. Appel, T. Autrey, R.M. Bullock, D.M. Camaioni, H.M. Cho, and D.A. Dixon, et al. 2014. "Multifunctional Catalysts to Synthesize and Utilize Energy Carriers." In Frontiers at the Interface between Homogeneous and Heterogeneous Catalysis, II, 207-219. Washington, District Of Columbia:U.S. Department of Energy, Office of Science. PNNL-SA-103068.
  • Weiss C.J., P. Das, D. Higgins, M.L. Helm, and A.M. Appel. 2014. "Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines." ACS Catalysis 4, no. 9:2951-2958. PNNL-SA-101717. doi:10.1021/cs500853f
  • Wiedner E.S., and A.M. Appel. 2014. "Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes." Journal of the American Chemical Society 136, no. 24:8661-8668. PNNL-SA-101312. doi:10.1021/ja502316e


  • Appel A.M., J.E. Bercaw, A.B. Bocarsly, H. Dobbek, D.L. DuBois, M. Dupuis, and J.G. Ferry, et al. 2013. "Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2." Chemical Reviews 113, no. 8:6621-6658. PNNL-SA-92184. doi:10.1021/cr300463y
  • Franz J.A., M.J. O'Hagan, M. Ho, T.L. Liu, M.L. Helm, S. Lense, and D.L. DuBois, et al. 2013. "Conformational Dynamics and Proton Relay Positioning in Nickel Catalysts for Hydrogen Production and Oxidation." Organometallics 32, no. 23:7034-7042. PNNL-SA-97203. doi:10.1021/om400695w
  • Galan B.R., M.L. Reback, A. Jain, A.M. Appel, and W.J. Shaw. 2013. "Electrocatalytic Oxidation of Formate with Nickel Diphosphane Dipeptide Complexes: Effect of Ligands Modified with Amino Acids." European Journal of Inorganic Chemistry 2013, no. 30:5366-5371. PNNL-SA-96292. doi:10.1002/ejic.201300751
  • Hoffert W.A., M.T. Mock, A.M. Appel, and J.Y. Yang. 2013. "Incorporation of Hydrogen Bonding Functionalities into the Second Coordination Sphere of Iron-Based Water Oxidation Catalysts." European Journal of Inorganic Chemistry 2013, no. 22-23:3846-3857. PNNL-SA-89420. doi:10.1002/ejic.201201499
  • Horvath S., L. Fernandez, A.M. Appel, and S. Hammes-Schiffer. 2013. "pH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts." Inorganic Chemistry 52, no. 7:3643-3652. PNNL-SA-89871. doi:10.1021/ic302056j
  • Hulley E.B., K.D. Welch, A.M. Appel, D.L. DuBois, and R.M. Bullock. 2013. "Rapid, Reversible Heterolytic Cleavage of Bound H2." Journal of the American Chemical Society 135, no. 32:11736-11739. PNNL-SA-94418. doi:10.1021/ja405755j
  • Jeletic M.S., M.T. Mock, A.M. Appel, and J.C. Linehan. 2013. "A Cobalt-based Catalyst for CO2 Hydrogenation Under Ambient Conditions." Journal of the American Chemical Society 135, no. 31:11533-11536. PNNL-SA-94778. doi:10.1021/ja406601v
  • Wiedner E.S., A.M. Appel, D.L. DuBois, and R.M. Bullock. 2013. "Thermochemical and Mechanistic Studies of Electrocatalytic Hydrogen Production by Cobalt Complexes Containing Pendant Amines." Inorganic Chemistry 52, no. 24:14391-14403. PNNL-SA-98467. doi:10.1021/ic4025475


  • Jain A., M.L. Reback, M.L. Lindstrom, C.E. Thogerson, M.L. Helm, A.M. Appel, and W.J. Shaw. 2012. "Investigating the Role of the Outer-Coordination Sphere in [Ni(PPh2NPh-R2)2]2+ Hydrogenase Mimics." Inorganic Chemistry 51, no. 12:6592-6602. PNNL-SA-85188. doi:10.1021/ic300149x
  • Lense S., M. Ho, S. Chen, A. Jain, S. Raugei, J.C. Linehan, and J.A. Roberts, et al. 2012. "Incorporating Amino Acid Esters into Catalysts for Hydrogen Oxidation: Steric and Electronic Effects and the Role of Water as a Base." Organometallics 31, no. 19:6719-6731. PNNL-SA-87812. doi:10.1021/om300409y
  • O'Hagan M.J., M. Ho, J.Y. Yang, A.M. Appel, M. Rakowski DuBois, S. Raugei, and W.J. Shaw, et al. 2012. "Proton Delivery and Removal in [Ni(PR2NR'2)2]2+ Hydrogen Production and Oxidation Catalysts." Journal of the American Chemical Society 134, no. 47:19409-19424. PNNL-SA-88737. doi:10.1021/ja307413x
  • Seu C., A.M. Appel, M.D. Doud, D.L. DuBois, and C. Kubiak. 2012. "Formate oxidation via ß-deprotonation in [Ni(PR2NR'2)2(CH3CN)]2+ complexes." Energy & Environmental Science 5, no. 4:6480-6490. PNNL-SA-84203. doi:10.1039/c2ee03341k


  • Appel A.M., D.H. Pool, M.J. O'Hagan, W.J. Shaw, J.Y. Yang, M. Rakowski DuBois, and D.L. DuBois, et al. 2011. "[Ni(PPh2NBn2)2(CH3CN)]2+ as an Electrocatalyst for H2 Production: Dependence on Acid Strength and Isomer Distribution." ACS Catalysis 1, no. 7:777-785. PNNL-SA-77921. doi:10.1021/cs2000939
  • Galan B.R., J. Schoffel, J.C. Linehan, C. Seu, A.M. Appel, J.A. Roberts, and M.L. Helm, et al. 2011. "Electrocatalytic Oxidation of Formate by [Ni(PR2NR`2)2(CH3CN)]2+ Complexes." Journal of the American Chemical Society 113, no. 35:12767-12779. PNNL-SA-77812. doi:10.1021/ja204489e
  • Kilgore U.J., J.A. Roberts, D.H. Pool, A.M. Appel, M.P. Stewart, M. Rakowski DuBois, and W.G. Dougherty, et al. 2011. "[Ni(PPh2NC6H4X2)2]2+ Complexes as Electrocatalysts for H2 Production: Effect of Substituents, Acids, and Water on Catalytic Rates." Journal of the American Chemical Society 133, no. 15:5861-5872. PNNL-SA-76108. doi:10.1021/ja109755f
  • Roberts J.A., A.M. Appel, D.L. DuBois, and R.M. Bullock. 2011. "Comprehensive Thermochemistry of W-H Bonding in the Metal Hydrides CpW(CO)2(IMes)H, [CpW(CO)2(IMes)H]·+, and [CpW(CO)2(IMes)(H)2]+. Influence of an N-Heterocyclic Carbene Ligand on Metal Hydride Bond Energies." Journal of the American Chemical Society 133, no. 37:14604-14613. PNNL-SA-78855. doi:10.1021/ja202830w


  • Appel A.M., S. Lee, J.A. Franz, D.L. DuBois, and M. Rakowski DuBois. 2009. "Free Energy Landscapes for S-H Bonds in (Cp2Mo2S4)-Mo-star Complexes." Journal of the American Chemical Society 131, no. 14:5224-5232. PNNL-SA-63520. doi:10.1021/ja8093179
  • Appel A.M., S. Lee, J.A. Franz, D.L. DuBois, M. Rakowski DuBois, and B. Twamley. 2009. "Determination of S-H Bond Strengths in Dimolybdenum Tetrasulfide Complexes." Organometallics 28, no. 3:749-754. PNNL-SA-61722.
  • Franz J.A., S. Lee, T.A. Bowden, M.S. Alnajjar, A.M. Appel, J.C. Birnbaum, and T.E. Bitterwolf, et al. 2009. "Activation of the S-H Group in Fe(µ2-SH)Fe Clusters: S-H Bond Strengths and Free Radical Reactivity of the Fe(µ2-SH)Fe Cluster." Journal of the American Chemical Society 131, no. 42:15212-15224. PNNL-SA-66979.


  • Appel A.M., S. Lee, J.A. Franz, D.L. DuBois, M. Rakowski DuBois, J.C. Birnbaum, and B. Twamley. 2008. "Formation and Reactivity of a Persistent Radical in a Dinuclear Molybdenum Complex." Journal of the American Chemical Society 130, no. 28:8940-8951. PNNL-SA-57391. doi:10.1021/ja078115r
  • Holladay J.E., H.M. Brown, A.M. Appel, and Z.C. Zhang. 2008. "Novel Hydride Transfer Catalysis for Carbohydrate Conversions." In Proceedings of the 22nd Conference on Catalysis of Organic Reactions, edited by ML Prunier, 123, 411-418. Boca Raton, Florida:CRC Press. PNNL-SA-58413.


  • Fraze K., K. Fraze, A. Wilson, A. Wilson, A.M. Appel, M. Rakowski DuBois, and D.L. DuBois. 2007. "Thermodynamic Properties of the Ni-H Bond in Complexes of the Type [HNi(P2RN2R')2](BF4) and Evaluation of Factors that Control Catalytic Activity for Hydrogen Oxidation/Production." Organometallics 26, no. 16:3918-3924. PNNL-SA-53931. doi:10.1021/om070143v

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