• Energy Research

The bifunctional mechanism that involves adsorbed hydroxide in the alkaline hydrogen oxidation and evolution reactions, important in hydrogen fuel cells and water electrolysers, is hotly debated. Hydroxide binding has been suggested to impact activity, but the exact role of adsorbed hydroxide in the reaction mechanism is unknown. Here, by selectively decorating steps on a Pt single crystal with other metal atoms, we show that the rate of alkaline hydrogen evolution exhibits a volcano-type relationship with the hydroxide binding strength. We find that Pt decorated with Ru at the step edge is 65 times more active for the hydrogen evolution reaction (HER) than is the bare Pt step. Simulations of electrochemical water dissociation show that the activation energy correlates with the OH* adsorption strength, even when the adsorbed hydroxide is not a product, which leads to a simulated volcano curve that matches the experimental curve. This work not only illustrates the alkaline HER mechanism but also provides a goal for catalyst design in targeting an optimum hydroxide binding strength to yield the highest rate for the alkaline HER. A three-dimensional (H and OH adsorbed species) HER activity volcano and the implications for hydrogen oxidation are discussed. The appropriate descriptors for a catalyst’s hydrogen evolution activity in alkaline electrolyte are debated. Combining simulations and single-crystal studies of metal-decorated Pt surfaces, McCrum and Koper show that activity exhibits a volcano-type relationship with the hydroxide binding strength of the catalyst, providing a target for catalyst design.

The electrocatalytic reduction of carbon dioxide is a promising approach for storing (excess) renewable electricity as chemical energy in fuels. Here, we review recent advances and challenges in the understanding of electrochemical CO2 reduction. We discuss existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon–carbon bond formation is also a key mechanistic step in CO2 electroreduction to high-density and high-value fuels. We show that both the initial CO2 activation and C–C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood. In addition, we discuss recent progress in in situ spectroscopic techniques and computational techniques for mechanistic work. Finally, we identify some challenges in furthering our understanding of these themes.

With the increased interest in the electrochemical conversion of renewable electricity, water, and carbon dioxide to fuels, there is an ever-growing number of papers reporting new electrocatalysts for the oxygen evolution reaction (OER).1−5 The OER is the anode reaction in the electrolysis of water and CO2 and a major source of efficiency loss, because of its high overpotential.6 To meaningfully compare the activity of the many new materials that are currently synthesized and tested, it is important that the research community agrees on proper standardization, benchmarking, and best practices7,8 Several papers reporting on the activity of OER state that it is necessary to saturate the electrolyte with oxygen gas before measurement, in order for the electrode “to reach its rest potential” or “to fix the equilibrium potential”,2,9−14 and saturating the solution with oxygen seems to have become an often-employed practice (see, e.g., refs (15−18)). A recent paper claimed that oxygen in the electrolyte may reduce the OER activity on nickel (supported on graphene) and change the Tafel slope via a van der Waals-type interaction of molecular oxygen with the active site, hampering access by hydroxide ions.11 The argument to fix the equilibrium potential is based on the idea that, in the absence of oxygen, the driving force for oxygen evolution should be higher than in its presence. However, at a given electrode potential, the rate of oxygen evolution itself must be independent of whether O2 is present in solution or not. What potentially changes in the presence of oxygen is the rate of the back reaction, i.e., the oxygen reduction reaction (ORR), and therefore the net production rate of oxygen may be dependent on whether O2 is present or not. This could play a role for a reversible reaction near its equilibrium potential, but it should be irrelevant for an irreversible reaction such as OER (ORR rates can safely be neglected above 1.3 V). The rate of an electrode reaction at a given applied potential is more accurately measured in the absence of the product, regardless of whether, under the experimental conditions, the equilibrium potential is well-defined or not. This also implies that the overpotential, when defined as the difference between the applied potential and the equilibrium potential, is not well-defined in the absence of oxygen in solution, but this has no (theoretical) effect on the rate of the OER at a given applied potential. Of course, close to equilibrium, it may still be necessary to correct for any current due to the back reaction if one wants to know the rate of the forward reaction only. The notion that the presence of O2 in solution may have an effect on the state of the surface (and thereby influence its chemistry and “rest potential”) has been studied for platinum electrodes.19,20 It was found that O2 in solution may indeed have an effect on the activity and stability of platinum electrodes, but apparently only under potential cycling conditions, or at relatively negative potentials, suggesting that oxygen reduction may play a role. Kongkanand and Ziegelbauer concluded that the effect of O2 in solution on the oxide coverage on platinum is negligible.19 To clarify the experimental role of oxygen saturation of the electrolyte on OER activity, we decided to study the effect of electrolyte oxygen on the OER on Pt and Ni-oxyhydroxide electrodes. In this Viewpoint, we show that (i) oxygen dissolved in the electrolyte has no significant effect on the OER activity (at least not on Pt-oxide and Ni-oxyhydroxide surfaces) and (ii) in the standardization of best practices for OER studies, care should be taken in employing a proper (placement of the) reference electrode, and in taking measures that small oxygen bubbles are efficiently removed from the electrocatalyst surface (for instance, by rotation).

Abstract Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO2 reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field.