Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

Abstract

The synthesis of renewable fuels from abundant water or the greenhouse gas CO₂ is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO₂ reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H₂ evolution and CO₂ reduction to C₁ products, by summarizing cases for higher-value products from N₂ reduction, C _x>1 products from CO₂ utilization, and other reductive organic transformations.

Figure 1

Schematic representation of (a) natural and (b) artificial photosynthesis.

Figure 2

Structures of (a) [NiFe]-H₂ase active site, (b) [FeFe]-H₂ase active site (Fe_p = “proximal” iron, Fe_d = “distal” iron), and (c) synthetic cofactors used to reconstitute [FeFe]-H₂ases.

Figure 3

Structure of [NiFe]-CODH active site with (a) reduced nickel center in the absence of substrate and (b) bound CO₂ after incubation with NaHCO₃.

Figure 4

Catalytic cycle of galactose oxidase showing a noninnocent tyrosine ligand that cooperates with a copper ion to achieve a two-electron substrate transformation.

Figure 5

Simplified mechanistic pathways for H₂ evolution by a molecular catalyst (M) with oxidation state n (M⁽ⁿ⁾).

Figure 6

Competing pathways for CO and HCO₂H formation from CO₂, showing the pivotal role of the metal-hydride in determining product distribution. Either of the two hydrides shown can undergo side reactions to form H₂ as in Figure 5. Oxide (O^2–, above right) is produced as either CO₃^2– or H₂O/OH^–.

Figure 7

Photosensitizers used in photocatalytic fuel synthesis (counterions have been omitted for clarity).

Figure 8

General mechanism of H₂ generation by Co-based molecular catalysts (HA = acid).

Figure 9

H₂ evolution mechanism for DuBois-type HECs. P and N substituents omitted for clarity (B = base). Weak Brønsted acids promote the center route, whereas stronger ones favor the right pathway.

Figure 10

Reaction pathways for [MnBr(bpy-R)(CO)₃]-type CRCs. Different substituents (R-groups) and experimental conditions influence the mechanistic details.

Figure 11

General mechanism for iron porphyrin CRCs. Porphyrin charge state shown as a dianion. R = substituents that do not intramolecularly interact with bound substrate (e.g., phenyl groups).

Figure 12

CO₂ reduction to CO mediated by Ni1. Hg electrodes are thought to destabilize the product-inhibited Ni⁺-CO species, avoiding CRC deactivation. N-bound H atoms omitted for clarity.

Figure 13

Schematic representation of three different configurations for the production of solar fuels. Left, electrocatalysis: electrode surface modified with a molecular electrocatalyst. Center, photocatalysis: molecular electrocatalyst anchored onto a light-harvesting colloidal material. Right, photoelectrocatalysis: light-harvesting SC-based photocathode modified with a molecular catalyst.

Figure 14

Schematic representation of a heterogenized molecular catalyst on a metal oxide surface, highlighting the main features contributing to efficient electronic interaction between the catalyst and the surface.

Figure 15

Schematic diagram of (a) a vesicle formed from cylindrically shaped amphiphiles and (b) a micelle formed from conical surfactants (Reprinted with permission from ref (546). Copyright 2015 John Wiley and Sons).

Figure 16

Selected examples of commonly used methods to covalently introduce functional groups onto π-conjugated carbon surfaces that subsequently enable functionalization with a catalyst: (a) chemical or electrochemical surface oxidation, (b) chemical or electrochemical reduction of in situ generated diazonium salts, (c) 1,3-dipolar cycloaddition of azomethine ylides, and (d) direct grafting of azide groups.

Figure 17

Schematic representation of noncovalent modification of carbon π-conjugated surfaces with (a) compounds bearing polyaromatic moieties enabling π-π interactions or (b) hydrophobic polymeric chains.

Figure 18

Surface binding motifs of different anchors used for grafting of dyes and catalysts onto MO_x materials: (a) carboxylate moieties through a chelating, unidentate, or bridging binding mode; (b) phosphonate anchors through mono-, bi-, and tridentate motifs; (c) silane functions through a mono-, bi-, or tridentate binding mode; (d) hydroxamates in unidentate, chelating, or bridging binding modes; and (e) acetylacetonate chelating or bridging binding modes. The potentially dominant binding modes are highlighted by a blue frame.^,

Figure 19

(a) Schematic representation of a semiconducting CdS nanoparticle modified with a molecular catalyst through the anchoring group R_A (left) or an enzyme (right). (b) Molecular modification of p-silicon surfaces using Grignard reagents, UV-induced grafting of alkenes, or reduction of diazonium salts.

Figure 20

(a) Schematic representation of a CV experiment for an electrode modified with a molecular catalyst showing nonturnover redox waves (red dashed trace) and a catalytic wave (red trace) in the presence of substrate. (b) Schematic representation of chronoamperometry (or CPE) at E_appl showing the current value (red trace) and the corresponding charge passed (blue trace).

Figure 21

(a) [FeFe]-H₂ase mimic immobilized on an amine modified GC electrode through peptide coupling, (b) Co(tpy)L₂ complex immobilized on a GC electrode through electrochemical reduction of the tpy diazonium salt, followed by metalation, (c) Ni DuBois-type catalyst Ni97 immobilized onto an amine-functionalized CNT electrode through peptide coupling, (d) Succinamide analogue of Ni97 immobilized onto GC initially modified with an azide group, and (e) cobalt diimine-dioxime catalyst Co77 covalently grafted onto an amine functionalized CNT surface through an amide linker.

Figure 22

Molecular structure of HECs immobilized on carbon-based electrodes.

Figure 23

(a) Entrapment of Co porphyrins Co13 or Co54 in Nafion films deposited onto a glassy carbon electrode^, (b) [Fe-Fe]-H₂ase mimic immobilized onto a pyrrole modified glassy carbon electrode.

Figure 24

Immobilization of molecular catalysts onto CNT surfaces through π- π-stacking: (a) Ni DuBois catalyst Ni98, (b) cobaloxime with an axial pyridine linked to a pyrene anchor, Co80, and (c) cobaloxime ligated via axial pyridine units within a polymer matrix, Co81.

Figure 25

(a) Co dithiolate derivatives adsorbed onto a graphite surface. (b) 2D Co coordination polymer Co84 immobilized onto glassy carbon. (c) Ni or Co 1D coordination polymer immobilized onto glassy carbon.

Figure 26

Molecular structures of HECs immobilized on metal oxide electrodes.

Figure 27

Immobilization of (a) phosphonated Co diimine-dioxime Co87 onto mesoITO, (b) a cobaloxime derivative through coordination to a polymerized polypyridine (Co90) deposited on mesoITO, and (c) a Ni DuBois catalyst onto mesoTiO₂ using phosphonate anchoring Ni35.

Figure 28

(a) [NiFeSe]-H₂ase immobilized within an IO-ITO electrode. (b) Protein film voltammetry (pH 6.0) of a [NiFeSe]-H₂ase immobilized within an IO-ITO electrode showing reversible electrocatalytic H₂ conversion (proton reduction and H₂ oxidation at the thermodynamic potential).

Figure 29

(a) Immobilization of Co38 onto graphite electrodes through impregnation or drop-casting. (b) Attachment of Co91 through coordination to a pyridine moiety grafted onto a glassy carbon electrode. (c) Encapsulation of a CoPc within a polypyridine polymer (Co92) acting as an immobilization matrix as well as first and secondary coordination spheres.

Figure 30

Structures of CRCs immobilized on carbon-based electrodes.

Figure 31

(a) Co chlorin Co94 encapsulated in a Nafion membrane on CNTs. (b) Mn-bpy catalyst Mn1 entrapped in a Nafion matrix on glassy carbon. (c) Co protoporphyrin IX polymer Co96 electropolymerized onto a glassy carbon electrode.

Figure 32

(a) Co porphyrin MOF (Co97) based on Co53(803) and (b) Co porphyrin based COFs (Co98 and Co99)^, immobilized onto a thin ALD layer of alumina on a carbon disk electrode.

Figure 33

Different Co macrocycles immobilized on π-conjugated surfaces: (a) Co38 onto a CNT, (b) Co100 onto carbon cloth, and (c) Co54 onto a CNT.

Figure 34

Schematic representation of concentration-dependent mechanism toward CO, HCO₂^–, and H₂ with [Mn(bpy)(CO)₃]-type CRC immobilized on a MWCNT-electrode.

Figure 35

Covalent grafting of (a) a Co porphyrin onto an azide-modified diamond electrode surface using “click” chemistry, and (b) Fe33 onto an amine-modified CNT using peptide coupling.

Figure 36

(a) Structure of Fe porphyrin Fe₂34 bearing a phosphonate anchor. Immobilization of (b) a Mn bpy catalyst modified with phosphonate groups (Mn29) onto mesoTiO₂ and (c) a carboxylic acid modified Ni cyclam (Ni69) onto a mesoTiO₂ electrode.

Figure 37

Schematic representation of two classes of supported colloidal photocatalysis, where the colloidal material is used either as (a) a light absober or (b) a scaffolding agent.

Figure 38

Carbon nitride-based architectures.

Figure 39

Molecular structures of HEC combined with carbon colloids.

Figure 40

Schematic representation of photocatalytic TPP|GO|Fe₃1 assembly.

Figure 41

Molecular structure of Co-based catalysts employed with QDs toward H₂ evolution.

Figure 42

Molecular structure of HECs used with QDs in photocatalysis.

Figure 43

(a) Schematic representation of CdSe|β-CD|Fe₂40 assembly and related potentials. (b) Schematic representation of CdS-DETA|Ni101 assembly and related potentials. The first reduction potential of each catalyst is shown as E_cat.

Figure 44

Schematic representation of (a) “on particle” and (b) “through particle” systems in DSP.

Figure 45

Molecular structure of Co-based HECs employed toward H₂ evolution in DSP systems.

Figure 46

Schematic representation of the anchored catalyst Co88, Co87, and Co89 and the corresponding Co-phosphonate distance, as well as charge transfer kinetics of the [Co^3+/2+] couple.

Figure 47

Structures of the PSs used in DSP systems.

Figure 48

Schematic representation of the PS32|f-SiO₂-C₁₈|Co112 assembly for H₂ evolution.

Figure 49

Schematic of Co113 and PS33 embedded into a lipid membrane for H₂ evolution (not to scale).

Figure 50

Structure of (a) HECs associated with micelles and vesicles and (b) the accompanying molecular photosentizer.

Figure 51

Molecular structure of CRCs employed in colloidal systems.

Figure 52

Diagram of a syngas-evolving colloidal DSP system using Co88 and a CO₂-reducing Re catalyst in MeCN:H₂O with BIH as a SED.

Figure 53

Band diagram depicting energy level requirements for a functional DSPC system.

Figure 54

Assembly approaches that have been adopted to construct DSPCs: (a) coimmobilization, (b) layer-by-layer coassembly, and (c) PS-catalyst dyad.

Figure 55

Molecular structures of PSs used in DSPC.

Figure 56

Molecular structures of HECs used in DSPC.

Figure 57

Layer-by-layer coassembly of PS and catalyst via Zr⁴⁺-bridge approach on MO_x electrode.

Figure 58

Schematic representation of Zr⁴⁺ bridge-based photocathodes; (a) NiO|PS39-Zr⁴⁺-Ni35, and (b) IO-ITO|DA-Zr⁴⁺-PS40-Zr⁴⁺-Ni35.

Figure 59

Structures of photosensitizer-molecular catalyst dyads.

Figure 60

Possible DSPC assembly where ET between the photosensitizer and catalyst is mediated by a SC, and the assembly is attached to a conductive surface (C).

Figure 61

Band diagram depicting energy level requirements for a successful LAPC system.

Figure 62

Valence (red) and conduction (green) band positions for inorganic p-type light-absorbing SC materials discussed in this section.

Figure 63

General assembly approach taken to construct LAPCs, where a protection layer and porous scaffold are optional elements.

Figure 64

Molecular structures of HEC immobilized on narrow-bandgap photocathodes.

Figure 65

Molecular catalysts that transform CO₂ to highly reduced hydrocarbon products.

Scheme 1. Catalytic Reactions Leading to the Reduction of Carbon−Carbon Multiple Bonds

Figure 66

Photoreduction of styrene derivatives (via metal-hydride formation) with TiO₂-Co125.

Scheme 2. Product Distribution of Dehalogenation Reactions with Co125 (B12)

Scheme 3. Possible Products in the Reduction of Amides with H₂

Figure 67

Structures of catalysts used for the hydrogenation of amides or aldehydes/ketones.

Scheme 4. Reaction Conditions for the Catalytic Reduction of Amides and Imines

Scheme 5. Reduction of Esters by Iron Catalyst Fe37

Scheme 6. Reduction of Aldehydes and Ketones by Complex Mn32

Scheme 7. Photocatalytic Aldehyde and Ketone Reduction Mediated by PS7 and Co126

Figure 68

Structures of NAD⁺ and NADH cofactors.

Figure 69

Reaction cascade through coimmobilized enzymes.

Figure 70

Artificial photosynthetic system for the enzyme-free light-induced regeneration of NADH coupled to enzymatic reduction of glutamate to α-ketoglutarate.

Scheme 8. Reduction of N₂, via Haber–Bosch Process and Natural Process

Figure 71

Reduction of N₂ by CdS-MoFe protein hybrid system.

Figure 72

Enzymatic fuel cell for the reduction of N₂ to ammonia.

Figure 73

Structure of iron complex Fe38 used as a N₂ reduction catalyst and general conditions used for the reduction of N₂ to ammonia catalyzed by this complex.

Figure 74

Iron complexes applied in the catalytic reduction of dinitrogen.

Scheme 9. Reaction Conditions Used for the Catalytic Reduction of N₂ with Iron Complexes

Scheme 10. Proposed Mechanism for the Titanocene Mediated Reduction of Dinitrogen

Scheme 11. Reaction Conditions Used for the Reduction of N₂ to Silyl Amine

Figure 75

Structures of complexes applied for the catalytic reduction of N₂ to silyl amine.