Effects of Peptide-Functionalized Surfaces on the Electrochemical Hydrogen Evolution Reaction

With the increasing availability of low-cost renewable electricity, electrocatalytic reactions are poised to play a key role in alleviating dependence on fossil energy. The hydrogen evolution reaction (HER) is a crucial electrochemical reaction. Gaseous hydrogen generated in water electrolyzers can be used as a portable source of energy for hydrogen fuel cells. Contrarily, HER can be a nuisance in other electrochemical reduction reactions, such as when discharging lead (Pb)-acid batteries [1], in the electroreduction of nitrogen to ammonia [2], and in reducing carbon dioxide to fuels and chemicals [3]. Thus, the ability to modulate HER activity and selectivity, either by promotion or by inhibition, is of practical importance and is pivotal to developing electrochemical technologies that can help transition away from a fossil-based energy economy.

Conventional methods for modulating an electrocatalyst’s HER activity include tuning the combinations of metals, metal alloys, and substrates [4]. Functionalizing the electrode surface with peptides, i.e., short chains of amino acids, provides a novel approach to manipulating HER activity. Peptides have the advantage of being inexpensive and highly tunable, where elements of their design such as secondary structure motifs, sequence, length, and functional groups can be easily altered. For example, peptides can be modified to have affinity for gold and platinum metals by introducing cysteine or a Pt-binding sequence, respectively [5,6]. Challenges to using peptides on electrodes are similar to challenges associated with enzyme-based electrodes, which include potential issues with stability [7] and characterization [8]. Other perceived issues may be cost, and the slow pace associated with synthesizing peptides. However, peptide synthesis speeds and costs have considerably increased and decreased, respectively, within the past several years. For instance, in 2017, a fully automated, flow-based approach to solid-phase polypeptide synthesis was reported that can create amide bonds in 7 s and has total synthesis times of 40 s per amino acid [9], compared with synthesis times of ∼1 h for similar systems in the 1990s. The advancements in manufacturing make peptides potentially promising tools for catalytic modulation.

Insights into how peptides can modify catalytic materials can be gained from prior work in which peptides have been used in non-electrochemical catalytic systems. Excellent reviews have been written describing how peptides modulate the surface chemistry of synthesized catalytic particles, particularly for noble metals, and thus will not be reviewed here. Generally, peptides have been shown to impact the size, shape, facet, structure, disorder, and composition of catalytic materials which in turn has been shown to affect the catalytic activity (e.g., turnover frequency) of organic reactions, photochemical reactions, and more generally, of wide classes of reactions performed in aqueous solutions [10–12]. Peptides can modulate catalytic activity by controlling surface exposure to the solution. In one elegant example, a non-natural azobenzene photoswitch was coupled to a peptide, which, when capped on gold nano-particle, could assume a cis or trans configuration when exposed to specific wavelengths of light [13]. Particles with capping peptides in the trans configuration demonstrated higher activity, which was attributed to higher metal exposure to reagents in solution. Surface exposure can also be controlled by varying the degree of peptide binding to the catalyst surface (e.g., strength of the binding residues, and grafting density, or spacing between bound residues). Higher activity is observed in systems with surface-bound peptides that have less binding or fewer pinning sites [14,15]. These studies demonstrate the various mechanisms through which peptides can alter metal particle catalytic activity for non-electrochemical reactions, such as altering the surface chemistry, or the exposure of the surface to reactants.

Similar to non-electrochemical catalytic surfaces, peptides have been shown to alter surface chemistry or reactant exposure to affect the electrochemical reaction activity. In a study of palladium nanoparticles featuring surface-bound peptides, specific residue binding and surface exposure seem to influence electrochemical oxygen reduction. Modulating the anchoring strength at the end of the peptide was suspected to allow flexibility in the rest of the peptide chain and allow for high surface exposure and oxygen gas adsorption [16]. Additionally, by utilizing peptides with high affinity for gold or palladium, surface chemistries could be altered in bimetallic gold/palladium particles [11,16]. These peptides preferentially pulled either gold or palladium atoms to the surface of the particles, and the different resulting surface compositions were shown to affect electrochemical methanol oxidation.

Overall, the growing field of peptide-capped and peptide-functionalized catalytic materials has shown that sequence-defined peptides can be a powerful modulator of catalytic and electrocatalytic reactions. Specifically, two main approaches, using peptides to control reactant exposure and surface composition, are common to both non-electrocatalytic and electrocatalytic reactions (Fig. 1). This review will specifically highlight how amino acids and peptides have played a similar role in modulating HER electrocatalysis when attached to solid electrodes. We will discuss prior work that featured electrocatalytic surfaces such as iron, lead, gold, and platinum, as well as possible mechanisms peptides could play in modulating HER, with special attention to impacts on proton transfer rates at the interface.

Extensive research has been carried out, both experimental and computational, to elucidate the elementary electrochemical HER mechanism [17–22]. The electrochemical HER mechanism is generally considered to consist of three elementary steps:

1. Volmer step—Electrochemical adsorption of a proton on the surface (where an open surface site is denoted as “*” and a surface adsorbed H atom as “H*”)

   $H++e−+*→H*(acidicmedia)$

   $H2O+e−+*→H*+OH−(basicmedia)$
2. Tafel step—Chemical desorption to evolve H₂

   $H*+H*→H2+2*$
3. Heyrovsky step—Electrochemical desorption to evolve H₂

   $H*+H++e−→H2+*(acidicmedia)$

   $H*+H2O+e−+→H2+OH−+*(basicmedia)$

Figure 2 illustrates the possible HER pathways in acidic and basic conditions. A near-surface or surface-bound amino acid or peptide can affect all of the steps of HER, acting as either a promoter or an inhibitor. Peptides are particularly attractive for potentially controlling the amount of surface site blockage because both the binding sites and the molecular weight (bulkiness or flexibility) can be tuned. Adsorption of an amino acid or peptide may inhibit HER by blocking the active sites on the solid surface (*), limiting the Volmer step. Similar deterrence to hydrogen adsorption has been shown due to the presence of organic ammonium cations [23]. Additionally, the Tafel or Heyrovsky step is rate-limiting on many catalysts of interest, and are of second order in surface-bound or near-surface hydrogen [24,25]. If peptides were to block active sites on the catalyst surface, it would result in lowering of hydrogen coverage on the surface or the local concentration of near-surface hydrogen, resulting in significant inhibition of the Tafel or Heyrovsky steps. Peptides may also inhibit HER by making proton transport to the surface more difficult and slowing the Volmer or Heyrovsky step.

Peptides or amino acids can also promote HER. For example, peptides may increase proton shuttling or proton transfer kinetics to enhance the Heyrovsky and Volmer steps. Peptides can also stabilize active surface compositions of catalyst materials to enhance HER kinetics. The following sections provide examples of how peptide- or amino acid-functionalized surfaces enhance or inhibit HER.

Single amino acids have been used to create materials which inhibit electrochemical HER. These simpler amino acids can provide mechanistic insights without the complications of peptide length, secondary structure, or bulkiness. Many of the examples of HER inhibition by surface-bound amino acids are in the context of corrosion prevention in strong acids, where hydrogen gas is evolved as part of the corrosion reaction. One study examined the iron corrosion inhibition efficiency of alanine (Ala), cysteine (Cys), and S-methyl cysteine (S-MCys) [26]. In this work, the authors were able to show that all the amino acids were able to mitigate HER. Surface blocking resulting from cathodic adsorption of cationic amino acid species (formed due to protonation in acidic medium) was proposed to be the reason for reduced HER. Samples with amino acids containing sulfur (Cys and S-MCys) inhibited corrosion more because the sulfur moiety is strongly adsorbed. Interestingly, the authors attributed the lower inhibition efficiency of S-MCys compared to Cys to the bulky methyl side chain, which sterically hinders adsorption. This study demonstrates the range of HER inhibition that can be achieved with just a few amino acids [26].

In another example, L-serine (Ser) was recently found to inhibit HER on the negative electrode of a lead (Pb)-acid battery [1]. The ability of Ser to inhibit HER reached a maximum at 10 mM and resulted in an 88% decrease in HER current compared with having no Ser; however, the inhibition did not increase with larger concentrations of the amino acid [1]. Mechanistically, the protonated Ser found in acidic solutions adsorbed to the Pb working electrode surface via the NH₃⁺ group and formed a protective layer preventing HER [1].

Taken together, these studies demonstrate some of the same trends seen in the literature where non-electrochemical and electrochemical peptide-bound particle catalytic activity is modulated by strength of binding and the resulting coverage of the peptide on the surface. Parallels are also observed in hydrogen oxidation reactions, where polarization curves show phenyl group adsorption on the surface of Pt negatively impacts performance in anion-exchange membrane fuel cells (AEMFCs) [27]. Peptides can be designed to contain amino acids with phenyl side chains, and thus, phenyl group adsorption is a potentially relevant design consideration for HER as well.

Amino acids have also been used to promote HER by tuning the surface compositions of electrocatalysts. The amino acid tryptophan (Trp) was used to assemble a stabilized core–shell catalytic material for HER. Specifically, Trp polymerized-gold (Au) cores served as a templates for Pt deposition, forming bimetallic core–shell nanoparticles [28]. When the core–shell materials were compared to pure Au or Pt particles on a glassy carbon electrode, the core–shell materials demonstrated the lowest HER overpotential. The authors pointed out that the hybrid nanospheres had a high porosity structure, which potentially increased the utilization of Pt.

The ability of peptides and amino acids to tune the surface hydrophobicity and chemical nature is another attractive way to affect HER. One study found that an optimal glycine coverage on copper (Cu) nanowires reduced HER rates and promoted the selectivity of the desired electroreduction of CO₂ into hydrocarbons [29]. Density functional theory (DFT) calculations revealed that the positively charged amine (NH₃⁺) present in the glycine stabilizes an intermediate of the CO₂ reduction reaction, contributing to the enhanced selectivity for that reaction. Additionally, impedance measurements depicted the development of charge transfer resistance with the addition of glycine on the Cu nanowires, indicating that glycine was occupying the surface sites of this working electrode, reducing the rate of HER. The glycine coverage may have had a larger inhibitory effect on HER than on CO₂ reduction, and an optimal coverage seemed to emerge for selectivity. Furthermore, with contact angle measurements, it was shown that the electrode surface still maintains its hydrophilic nature after moderate glycine functionalization. This hydrophilicity was advantageous for the electroreduction of CO₂ because it provided more contact area between the electrode and electrolyte. With a large amount of glycine, however, the Cu working electrode had a hydrophobic surface, which blocked the transfer of dissolved CO₂ during electrochemical reduction. Therefore, the amino acid in this work served a multifunctional role and was used to tune surface coverage, tune hydrophobicity, and stabilize desired intermediates to repress HER and achieve high selectivity for a desired electrochemical reaction.

Electrode functionalization can have a significant role in altering the near-surface environment of the catalyst, and in turn can affect the proton transfer process. In one specific example of electrode modification with biological molecules, a hybrid bilayer membrane consisting of lipid molecules allowed modulation of the proton transfer rate [30]. Controlling the proton transfer rate had significant electrochemical impact and allowed modulation of the electrochemical oxygen reduction reaction to achieve high selectivity for water formation versus formation of undesired products such as hydrogen peroxide [30]. Likewise, peptides may also play a role in modulating proton conductivity and have an impact on HER.

Peptide functionalization can affect the proton transfer process of the Volmer and Heyrovsky steps of HER by altering the proton transfer kinetics. Proton transfer kinetics can be affected differently with the inclusion of specific functional groups in the electrode/electrolyte interfacial region, through either steric hindrance or the chemical functionality introduced. For example, a recent study showed that the size of proton donor molecules with equivalent chemical functionality can be tuned to enhance the selectivity of interfacial reactions, especially in aprotic solvents [31]. Specifically, less bulky trialkyl ammonium cations were found to lead to higher HER activity than bulkier groups with equivalent proton affinity in acetonitrile.

Amino acids could also act as proton donors in the near-surface electrode environment. One study demonstrated that aromatic amino acids promote proton conduction in self-assembled peptide nanotube networks [32]. In dehydrated samples, the conductivity was largely influenced by the long-range ordering of the aromatic amino acid networks. In hydrated samples, the presence of carboxylic acids as proton donors was more dominant in promoting proton transfer [32]. Overall, these studies provide some design considerations for using peptides to promote proton transfer in the near-surface environment of an electrode, particularly by incorporating acidic or aromatic moieties and by using less bulky molecules.

Peptides could also play a role in proton conductivity by reorganizing thin films of ion-conducting polymer (ionomer) structures near the surface of electrodes, as illustrated by Fig. 3. In hydrogen fuel cells with low loadings of Pt, experiments and simulations suggest reactant transport limitations at or near the Pt surface manifest in voltage losses and that ionomer interaction with Pt is a likely cause. Industrial electrolyzers, which feature an ion-exchange membrane, also contain ionomer in the electrode layers. For Nafion^®, the most common commercial ionomer, a thin (5–10 nm) film forms on the catalyst surfaces. Studies have shown that the thin-film confinement of Nafion^® impedes the transport of protons and gases that are involved in electrochemical reactions at the catalyst surface [33–40]. Efforts to find ways to tune ionomer thin film structure and optimize transport at the electrode surface have been of interest in the perfluorinated sulfonic acid polymer electrolyte community [33,41–45]. Recently, it has been demonstrated that proteins and peptides are capable of organizing thin layers of Nafion^® and anion-exchange ionomer (an advanced material for low-cost electrolyzers and fuel cells) [46–48]. In the studies with peptides, the sequence Cys-Val-Pro-Gly-Lys-Gly, featuring a positively charged lysine (Lys) residue and a gold-binding Cys residue, was able to attract sulfonic groups of the Nafion^® via an electrostatic interaction and bind the ionomer to the solid surface. Likewise, the sequence Cys-Val-Pro-Gly-Glu-Gly, featuring a negatively charged glutamic acid (Glu), was able to attract and change the morphology of a positively charged anion-exchange ionomer layer on gold [46,47]. Ionic conductivity measurements indicated that when anion-exchange ionomer was assembled on a Cys-Val-Pro-Gly-Glu-Gly elastin peptide layer on an interdigitated electrode, it facilitated a favorable microstructure for conductivity compared to the anion-exchange ionomer assembled without the peptide. All together, these studies provide a foundation for future work to tune the peptide structure and modulate ionomer thin film microstructure. This ionomer control will be useful in gaining structure-function understanding, and it will eventually be useful in promoting the transport of gasses and ions to and from the active sites of HER catalysts in solid-polymer electrolyte devices.

The above examples show how peptides can be used to modulate proton transfer rates and proton conductivity and represent a potential area of future research as peptides play a growing role in the control of electrochemical reactions.

Other potential mechanisms for promoting HER may be operable using peptides, including enhancing electron conductivity or serving as antifouling agents. Electron conductivity enhancement may be useful when peptides are used to bind enzymes to surfaces, with the peptide linkers serving as electron transfer conduits [49]. Peptide linkers can be used with enzymes that catalyze HER, such as hydrogenases [50–52]. The mechanisms of electron conduction in peptides are a widely explored fields and recently reviewed elsewhere [53,54]. Peptides could also serve as metal-binding sites, thereby creating a conductive conduit from an electrode surface to a bound enzyme [55].

Another mechanism where peptides may play a role in controlling HER is through antifouling. Antifouling properties are important for electrode integrity, particularly in wastewater systems where undesired deposits form, or complex biological media where nonspecific protein adsorption is undesired [56,57]. Peptide systems have been designed to limit biofouling, and they have advantages over other materials, such as being biodegradable, nontoxic, and biocompatible [56]. Peptides can be designed to kill organisms or prevent binding of biological material and cells to a surface.

Finally, while not the subject of this review, five amino acids are known to be electrochemically active [58,59]. These amino acids include L-tryptophan (Trp), L-tyrosine (Tyr), L-histidine (His), L-methionine (Met), and L-cysteine (Cys), and special consideration should be given to these amino acids when designing peptides for electrochemical systems.

While the control of electrochemical HER using surface-bound peptides and amino acids is a young field, this review highlights a multitude of ways peptides could modulate this important reaction. Often, peptide-induced catalytic inhibition is shown to correlate with the degree of binding and degree of surface exposure. Peptides have been shown to inhibit electrochemical HER via these routes, albeit in the context of corrosion inhibition which releases hydrogen gas as part of the reaction. Peptides have also been shown to promote reactivity by controlling and stabilizing active surface compositions of catalytic materials, including materials for electrochemical HER. Finally, peptides have been observed to promote ionic transport and a promising area of future research would be to explore ways to enhance HER via this mechanism. Peptides also have the potential to benefit emerging HER systems, such as those driven by enzymes or those utilizing wastewater. Collectively, carefully designed peptide systems have the potential to modulate electrochemical HER through a variety of mechanisms, including by modulating surface coverage, surface composition, and ionic transfer. As selectivity for more complex electrochemical reactions become desirable, the multifunctional role peptides can play should be considered as new materials are developed for these applications.

We extend our gratitude to the U.S. Department of Energy (DOE) Basic Energy Sciences program as well as to the U.S. DOE Small Business Innovation Research (SBIR) program.

This work was primarily supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, under Award # DE-SC0016529. Some of the initial ideas were conceived during prior support by a U.S. Department of Energy Small Business Innovation Research (SBIR) grant (Grant No. DE-SC0015956).