Modeling integrated photovoltaic–electrochemical devices using steady-state equivalent circuits

Abstract

We describe a framework for efficiently coupling the power output of a series-connected string of single-band-gap solar cells to an electrochemical process that produces storable fuels. We identify the fundamental efficiency limitations that arise from using solar cells with a single band gap, an arrangement that describes the use of currently economic solar cell technologies such as Si or CdTe. Steady-state equivalent circuit analysis permits modeling of practical systems. For the water-splitting reaction, modeling defines parameters that enable a solar-to-fuels efficiency exceeding 18% using laboratory GaAs cells and 16% using all earth-abundant components, including commercial Si solar cells and Co-or Ni-based oxygen evolving catalysts. Circuit analysis also provides a predictive tool: given the performance of the separate photovoltaic and electrochemical systems, the behavior of the coupled photovoltaic-electrochemical system can be anticipated. This predictive utility is demonstrated in the case of water oxidation at the surface of a Si solar cell, using a Co-borate catalyst. P owering electrochemical reactions with photovoltaic devices to produce fuels provides an appealing solution to the societal need for clean energy (1). Although the deployment of photovoltaic modules has expanded rapidly over the last decade as costs have dropped (2), utilization of solar power is constrained by its local intermittency, thus providing an imperative for storage by the direct conversion of solar energy to chemical fuels. Photosynthetic organisms directly convert solar energy into chemical fuels by splitting water to produce molecular oxygen and hydrogen equivalents, which are fixed by their combination with carbon dioxide to produce carbohydrates. The technological imitation of photosynthesis-an "artificial leaf"-can be realized by integrating oxygen and hydrogen evolution catalysts to a semiconductor in a buried junction configuration (3). Most buried junction devices have relied on expensive solar cell architectures and/or catalysts (4), including those demonstrating solar-to-fuel efficiencies (SFEs) exceeding 18% (5-7). More economical artificial leaves have been realized with earthabundant catalysts and solar cell materials but at reduced SFE (8, 9). We now seek to provide an analytical framework for the construction of higher SFE architectures comprising earthabundant materials.The efficiency of converting solar energy to stored chemical fuel has been considered (10-13) for a variety of configurations, including the specific treatment of buried junction devices (10). A primary result of these analyses is that singlejunction solar cells are limited in powering water splitting with high SFE because solar cell materials that are well matched to the solar spectrum do not produce sufficient voltages to drive water splitting. Multijunction devices overcome this limitation, enabling high SFE by integrating several materials into a single multijunction device (5-7, 12). Despite their higher limiting SFEs, however, multijun...

Simulations of the irradiation and temperature dependence of the efficiency of tandem photoelectrochemical water-splitting systems

“…Equivalent circuit models have been developed to assess the inuence of the properties of different components, such as the choice of light absorber combinations or electrocatalysts, on the overall system performance. [5][6][7][8] Recently, models that solve for the governing coupled conservation equations have been developed in 1D, 9 2D, 10 and 3D, 11 allowing for a better understanding of the interactions between the properties of the components and the design choices of the system. Except for the 2-D model, these models do not account for the detailed characteristics of the solar absorbers, and to date, the models have not been used to examine the performance of the system in response to variations in operational conditions such as variations in the irradiation, changes in temperature, or the use of concentrated solar irradiation.…”

Ten-percent solar-to-fuel conversion with nonprecious materials

“…However, this is only the case if all resistive losses are negligible; if resistive losses are present, the operating point can occur to the right of V MPP , reducing J OP and SFE. Modeling indicates that using a four-cell c-Si module overcomes the impact of resistive losses on SFE (52).…”

“…Because a single c-Si solar cell is unable to provide enough voltage to drive the watersplitting reaction, we use multiple single-junction solar cells series connected into minimodules. Although this approach does not result in a monolithic structure in which catalysts are directly deposited on the PV device in a buried-junction configuration (e.g., an artificial leaf), the equivalent circuit for both constructs is identical (52). This approach allows for modular independent optimization, after which the components could be integrated into a monolithic design.…”

Unusual stoichiometry control in the atomic layer deposition of manganese borate films from manganese bis(tris(pyrazolyl)borate) and ozone

“…24 For many applications, metal borates must be grown as thin films. Cobalt borate and nickel borate films were electrodeposited from solutions of the metal ions in the presence of borate buffers, [5][6][7][8][9][10][11][12][13][14][15][16] while nanowhiskers of Ni 3 (BO 3 ) 2 were grown on nickel substrates by a molten salt method. 24 Metal borate film growth by gas phase processes is complicated by the high vapor pressure of B 2 O 3 , which makes it generally difficult to control the metal to boron ratios in the thin films at the high temperatures required for growth.…”

“…[1][2][3][4] Recently, first row transition metal borates have emerged as dioxygen (O 2 ) evolution catalysts, [5][6][7][8][9][10][11][12][13][14][15][16] cathode materials in lithium ion batteries, [17][18][19][20][21][22] catalysts for H 2 production by hydrolysis of sodium borohydride, 23 and superhydrophilic Ni 3 (BO 3 ) 2 layers. 24 For many applications, metal borates must be grown as thin films.…”