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Staff information


Aaron Appel

Physical Sciences
Scientist, 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 SA, AJ Kendall, DR Tyler, JC Linehan, and AM Appel. 2017. "Hydrogenation of CO2 in Water Using a Bis(Diphosphine) Ni-H Complex." ACS Catalysis 7(4):3089-3096.  doi:10.1021/acscatal.7b00350
  • Klug C, MJ O'Hagan, RM Bullock, AM Appel, and ES Wiedner. 2017. "Impact of Weak Agostic Interactions in Nickel Electrocatalysts for Hydrogen Oxidation." Organometallics 36(12):2275-2284.  doi:10.1021/acs.organomet.7b00103
  • Bullock RM, AK Das, and AM Appel. 2017. "Surface Immobilization of Molecular Electrocatalysts for Energy Conversions." Chemistry - A European Journal 23(32):7626 - 7641.  doi:10.1002/chem.201605066
  • Burgess SA, AM Appel, JC Linehan, and ES Wiedner. 2017. "Changing the Mechanism for CO2 Hydrogenation Using Solvent-Dependent Thermodynamics." Angewandte Chemie International Edition 56(47):15002-15005.  doi:10.1002/anie.201709319
  • Cammarota RC, MV Vollmer, J Xie, J Ye, JC Linehan, SA Burgess, AM Appel, L Gagliardi, and CC Lu. 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(40):14244-14250.  doi:10.1021/jacs.7b07911
  • Jeletic MS, E Hulley, ML Helm, MT Mock, AM Appel, ES Wiedner, and JC Linehan. 2017. "Understanding the Relationship Between Kinetics and Thermodynamics in CO2 Hydrogenation Catalysis." ACS Catalysis 7(9):6008-6017.  doi:10.1021/acscatal.7b01673
  • Zhang S, AM Appel, and RM Bullock. 2017. "Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride." Journal of the American Chemical Society 139(21):7376-7387.  doi:10.1021/jacs.7b03053


  • Cardenas AJ, B Ginovska-Pangovska, N Kumar, J Hou, S Raugei, ML Helm, AM Appel, RM Bullock, and MJ O'Hagan. 2016. "Controlling Proton Delivery with Catalyst Structural Dynamics." Angewandte Chemie International Edition 55(43):13509-13513.  doi:10.1002/anie.201607460
  • Lindley BM, AM Appel, K Krogh-Jespersen, JM Mayer, and AJ Miller. 2016. "Evaluating the Thermodynamics of Electrocatalytic N2 Reduction in Acetonitrile." ACS Energy Letters 1(4):698-704.  doi:10.1021/acsenergylett.6b00319
  • Raugei S, ML Helm, S Hammes-Schiffer, AM Appel, MJ O'Hagan, ES Wiedner, and RM Bullock. 2016. "Experimental and Computational Mechanistic Studies Guiding the Rational Design of Molecular Electrocatalysts for Production and Oxidation of Hydrogen." Inorganic Chemistry 55(2):445-460.  doi:10.1021/acs.inorgchem.5b02262
  • Zhang S, H Li, AM Appel, MB Hall, and RM Bullock. 2016. "Facile P-C/C-H Bond Cleavage Reactivity of Nickel Bis(diphosphine) Complexes." Chemistry - A European Journal 22(28):9493-9497.  doi:10.1002/chem.201601469


  • Zall CM, JC Linehan, and AM Appel. 2015. "A Molecular Copper Catalyst for Hydrogenation of CO-2 to Formate." ACS Catalysis 5(9):5301-5305.  doi:10.1021/acscatal.5b01646
  • Connelly SJ, ES Wiedner, and AM Appel. 2015. "Predicting the Reactivity of Hydride Donors in Water: Thermodynamic Constants for Hydrogen." Dalton Transactions 44(13):5933-5938.  doi:10.1039/C4DT03841J
  • Miller DL, BJ Boro, K Grubel, ML Helm, and AM Appel. 2015. "Synthesis and Characterization of a New TRIPHOS Ligand Derivative and the Corresponding Pd(II) Complexes." European Journal of Inorganic Chemistry 2015(35):5781-5785.  doi:10.1002/ejic.201500791
  • Pegis ML, JA Roberts, DJ Wasylenko, EA Mader, AM Appel, and JM Mayer. 2015. "Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide." Inorganic Chemistry 54(24):11883-118/88.  doi:10.1021/acs.inorgchem.5b02136
  • Peterson SM, ML Helm, and AM Appel. 2015. "Nickel Complexes of a Binucleating Ligand Derived from an SCS Pincer." Dalton Transactions 44(2):747-752.  doi:10.1039/c4dt02718c
  • Weiss CJ, ES Wiedner, JA Roberts, and AM Appel. 2015. "Nickel Phosphine Catalysts with Pendant Amines for Electrocatalytic Oxidation of Alcohols." Chemical Communications 51(28):6172-6174.  doi:10.1039/C5CC01107H


  • Appel AM. 2014. "Carbon-Fuelled Future." Chemistry and Industry 78(4):36-39.  doi:10.1002/cind.784_11.x
  • Appel AM. 2014. "Electrochemistry: Catalysis at the boundaries." Nature 508(7497):460-461.  doi:10.1038/nature13226
  • Wiedner ES, and AM 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(24):8661-8668.  doi:10.1021/ja502316e
  • Appel AM, and ML Helm. 2014. "Determining the Overpotential for a Molecular Electrocatalyst." ACS Catalysis 4(2):630-633.  doi:10.1021/cs401013v
  • Bullock RM, AM Appel, and ML Helm. 2014. "Production of Hydrogen by Electrocatalysis: Making the H-H Bond by Combining Protons and Hydrides." Chemical Communications 50(24):3125-3143.  doi:10.1039/c3cc46135a
  • Galan BR, ES Wiedner, ML Helm, JC Linehan, and AM Appel. 2014. "Effects of Phosphine-Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes." Organometallics 33(9):2287-2294.  doi:10.1021/om500206e
  • Jeletic MS, ML Helm, EB Hulley, MT Mock, AM Appel, and JC Linehan. 2014. "A Cobalt Hydride Catalyst for the Hydrogenation of CO2: Pathways for Catalysis and Deactivation." ACS Catalysis 4(10):3755-3762.  doi:10.1021/cs5009927
  • Kumar N, DM Camaioni, M Dupuis, S Raugei, and AM Appel. 2014. "Mechanistic Insights into Hydride Transfer for Catalytic Hydrogenation of CO2 with Cobalt Complexes." Dalton Transactions 43(31):11803-11806.  doi:10.1039/c4dt01551g
  • Lercher JA, AM Appel, T Autrey, RM Bullock, DM Camaioni, HM Cho, DA Dixon, Z Dohnalek, F Gao, VA Glezakou, MA Henderson, JZ Hu, E Iglesia, AJ Karkamkar, BD Kay, GA Kimmel, JC Linehan, J Liu, I Lyubinetsky, D Mei, CHF Peden, RJ Rousseau, GK Schenter, WJ Shaw, J Szanyi, H Wang, Y Wang, and RS Weber. 2014. "Multifunctional Catalysts to Synthesize and Utilize Energy Carriers." In Frontiers at the Interface between Homogeneous and Heterogeneous Catalysis, II, pp. 207-219.  U.S. Department of Energy, Office of Science, Washington, DC. 
  • Weiss CJ, PP Das, DLM Higgins, ML Helm, and AM Appel. 2014. "Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines." ACS Catalysis 4(9):2951-2958.  doi:10.1021/cs500853f


  • Hoffert WA, MT Mock, AM Appel, and JY Yang. 2013. "Incorporation of Hydrogen Bonding Functionalities into the Second Coordination Sphere of Iron-Based Water Oxidation Catalysts." European Journal of Inorganic Chemistry 2013(22-23):3846-3857.  doi:10.1002/ejic.201201499
  • Appel AM, JE Bercaw, AB Bocarsly, H Dobbek, DL DuBois, M Dupuis, JG Ferry, E Fujita, R Hille, P Kenis, CA Kerfeld, RH Morris, CHF Peden, A Portis, S Ragsdale, TB Rauchfuss, J Reek, LC Seefeldt, RK Thauer, and GL Waldrop. 2013. "Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2." Chemical Reviews 113(8):6621-6658.  doi:10.1021/cr300463y
  • Franz JA, MJ O'Hagan, MH Ho, TL Liu, ML Helm, S Lense, DL DuBois, WJ Shaw, AM Appel, S Raugei, and RM Bullock. 2013. "Conformational Dynamics and Proton Relay Positioning in Nickel Catalysts for Hydrogen Production and Oxidation." Organometallics 32(23):7034-7042.  doi:10.1021/om400695w
  • Galan BR, ML Reback, A Jain, AM Appel, and WJ Shaw. 2013. "Electrocatalytic Oxidation of Formate with Nickel Diphosphane Dipeptide Complexes: Effect of Ligands Modified with Amino Acids." European Journal of Inorganic Chemistry 2013(30):5366-5371.  doi:10.1002/ejic.201300751
  • Horvath S, L Fernandez, AM 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(7):3643-3652.  doi:10.1021/ic302056j
  • Hulley EB, KD Welch, AM Appel, DL DuBois, and RM Bullock. 2013. "Rapid, Reversible Heterolytic Cleavage of Bound H2." Journal of the American Chemical Society 135(32):11736-11739.  doi:10.1021/ja405755j
  • Jeletic MS, MT Mock, AM Appel, and JC Linehan. 2013. "A Cobalt-based Catalyst for CO2 Hydrogenation Under Ambient Conditions." Journal of the American Chemical Society 135(31):11533-11536.  doi:10.1021/ja406601v
  • Wiedner ES, AM Appel, DL DuBois, and RM Bullock. 2013. "Thermochemical and Mechanistic Studies of Electrocatalytic Hydrogen Production by Cobalt Complexes Containing Pendant Amines." Inorganic Chemistry 52(24):14391-14403.  doi:10.1021/ic4025475


  • Jain A, ML Reback, ML Lindstrom, CE Thogerson, ML Helm, AM Appel, and WJ Shaw. 2012. "Investigating the Role of the Outer-Coordination Sphere in [Ni(PPh2NPh-R2)2]2+ Hydrogenase Mimics." Inorganic Chemistry 51(12):6592-6602.  doi:10.1021/ic300149x
  • Lense S, MH Ho, S Chen, A Jain, S Raugei, JC Linehan, JA Roberts, AM Appel, and WJ Shaw. 2012. "Incorporating Amino Acid Esters into Catalysts for Hydrogen Oxidation: Steric and Electronic Effects and the Role of Water as a Base." Organometallics 31(19):6719-6731.  doi:10.1021/om300409y
  • O'Hagan MJ, MH Ho, JY Yang, AM Appel, M Rakowski DuBois, S Raugei, WJ Shaw, DL DuBois, and RM Bullock. 2012. "Proton Delivery and Removal in [Ni(PR2NR'2)2]2+ Hydrogen Production and Oxidation Catalysts." Journal of the American Chemical Society 134(47):19409-19424.  doi:10.1021/ja307413x
  • Seu C, AM Appel, MD Doud, DL DuBois, and C Kubiak. 2012. "Formate oxidation via ??-deprotonation in [Ni(PR2NR'2)2(CH3CN)]2+ complexes." Energy & Environmental Science 5(4):6480-6490.  doi:10.1039/c2ee03341k


  • Appel AM, DH Pool, MJ O'Hagan, WJ Shaw, JY Yang, M Rakowski DuBois, DL DuBois, and RM Bullock. 2011. "[Ni(PPh2NBn2)2(CH3CN)]2+ as an Electrocatalyst for H2 Production: Dependence on Acid Strength and Isomer Distribution." ACS Catalysis 1(7):777-785.  doi:10.1021/cs2000939
  • Galan BR, J Schoffel, JC Linehan, C Seu, AM Appel, JA Roberts, ML Helm, UJ Kilgore, JY Yang, DL DuBois, and C Kubiak. 2011. "Electrocatalytic Oxidation of Formate by [Ni(PR2NR`2)2(CH3CN)]2+ Complexes." Journal of the American Chemical Society 113(35):12767-12779.  doi:10.1021/ja204489e
  • Kilgore UJ, JA Roberts, DH Pool, AM Appel, MP Stewart, II, M Rakowski DuBois, WG Dougherty, WS Kassel, RM Bullock, and DL DuBois. 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(15):5861-5872.  doi:10.1021/ja109755f
  • Roberts JA, AM Appel, DL DuBois, and RM 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(37):14604-14613.  doi:10.1021/ja202830w


  • Appel AM, SJ Lee, JA Franz, DL 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(14):5224-5232.  doi:10.1021/ja8093179
  • Appel AM, SJ Lee, JA Franz, DL DuBois, M Rakowski DuBois, and B Twamley. 2009. "Determination of S-H Bond Strengths in Dimolybdenum Tetrasulfide Complexes." Organometallics 28(3):749-754. 
  • Franz JA, SJ Lee, TA Bowden, MS Alnajjar, AM Appel, JC Birnbaum, TE Bitterwolf, and M Dupuis. 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(42):15212-15224. 


  • Appel AM, SJ Lee, JA Franz, DL DuBois, M Rakowski DuBois, JC Birnbaum, and B Twamley. 2008. "Formation and Reactivity of a Persistent Radical in a Dinuclear Molybdenum Complex." Journal of the American Chemical Society 130(28):8940-8951.  doi:10.1021/ja078115r
  • Holladay JE, HM Brown, AM Appel, and ZC Zhang. 2008. "Novel Hydride Transfer Catalysis for Carbohydrate Conversions." In Proceedings of the 22nd Conference on Catalysis of Organic Reactions, vol. 123, ed. ML Prunier, pp. 411-418.  CRC Press, Boca Raton, FL. 


  • Fraze K, A Wilson, AM Appel, M Rakowski DuBois, and DL 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(16):3918-3924.  doi:10.1021/om070143v

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